8 Navigation – Military Avionics Systems


8.1 Navigation Principles

8.1.1 Introduction

Navigation has been an ever-present component of humankind’s exploitation of the capability of flight. While the principles of navigation have not changed since the early days of sail, the increased speed of flight, particularly with the advent of the jet age, has placed an increased emphasis upon accurate navigation. The increasingly busy skies, together with rapid technology developments, have emphasised the need for higher-accuracy navigation and the means to accomplish it. Navigation is no longer a matter of merely getting from A to B safely, it is about doing this in a fuel-efficient manner, keeping to tight airline schedules, and avoiding other air traffic – commercial, general aviation, leisure and military. Navigation of military aircraft has to comply with the same regulations as civil traffic when operating in controlled airspace. Platforms adopted from civil aircraft will retain the civil navigation systems as described in the companion volume ’Civil Avionics Systems’ (Moir and Seabridge, 2003), some of which are described here for ease of reference. More than likely, legacy military platforms will be fitted with a bespoke system meeting most but possibly not all the latest requirements specified for controlled airspace and may on occasion need to operate with certain limitations until the necessary upgrades are embodied.

Outside controlled airspace in operational theatres the navigational accuracy will be determined by the accuracy provided by the platform mission, weapons system and possibly the weapons being carried. Operational mission navigation constraints include the optimisation of routing to avoid hazardous surface-to-air missile (SAM) and anti-aircraft artillery (AAA). Routing may also take into account the need to maximise the aircraft stealth or low-observability characteristics. Therefore, while military aircraft need to adopt all the necessary features to enable them to operate safely alongside civil aircraft in today’s crowded airspace, they also add a further layer of complexity to the mission management function.

This section summarises some of the modern methods of navigation, leading to more detailed descriptions of how each technique operates. A later section in the chapter relates to future air navigation system (FANS) requirements, also known within military circles as global air transport management (GATM).

The main methods of navigation as practised today may be summarised and simplified as follows:

  • Classic dead-reckoning navigation using air data and magnetic, together with Doppler or LORAN-C;
  • Radio navigation using navigation aids – ground-based radio-frequency beacons and airborne receiving and processing equipment;
  • Barometric inertial navigation using a combination of air data and inertial navigations (IN) or Doppler;
  • Satellite navigation using a global navigation satellite system (GNSS), more usually a global positioning system (GPS);
  • Multiple-sensor navigation using a combination of all the above.

The more recent the pedigree of the aircraft platform, the more advanced the navigational capabilities are likely to be. However, it is common for many legacy platforms to be retrofitted with inertial and GNSS navigation, the accuracy of which far outshines the capability of the original system.

8.1.2 Basic Navigation

The basic navigation parameters are shown in Figure 8.1 and may be briefly summarised as follows:

  1. An aircraft will be flying at a certain height or altitude relative to a barometric datum (barometric altitude) or terrain (radar altitude).
  2. The aircraft may be moving with velocity components in the aircraft X (Vx), Y (Vy) and Z (Vz) axes. Its speed through the air may be characterised as indicated airspeed (IAS) or Mach number (M). Its speed relative to the ground is determined by true airspeed (TAS) in still air conditions.
  3. The aircraft will be flying on a certain heading; however, the prevailing wind speed and direction will modify this to the aircraft track. The aircraft track represents the aircraft path across the terrain and will lead to the destination or next waypoint of the aircraft. Wind speed and direction will modify the aircraft speed over the ground to ground speed.
  4. The aircraft heading will be defined by a bearing to magnetic (compass) north or to true north relating to earth-related geographic coordinates.
  5. The aircraft will be flying from its present position – defined by latitude and longitude to a waypoint also characterised by latitude and longitude.
  6. A series of flight legs – defined by way points – will determine the aircraft designated flight path from the departure airfield to the destination airfield.

As has already briefly been described, there are sensors and navigation techniques that may be used solely or in combination to navigate the aircraft throughout the mission.

The relationship of the different axis sets is shown in Figure 8.2. These may be characterised as follows:

Figure 8.1 Basic navigation parameters.

  1. Earth datum set. As shown in Figure 8.2, the earth axis reference set comprises the orthogonal set Ex, Ey, Ez, where:
    • Ex represents true north;
    • Ey represents east;
    • Ez represents the local gravity vector.
  2. The orthogonal aircraft axis set where:
    • Ax is the aircraft longitudinal axis (corresponding to the aircraft heading);
    • Ay is the aircraft lateral axis;
    • Az is the aircraft vertical axis (corresponding to Ez).

Figure 8.2 Earth-related coordinates.

For navigation purposes, the accuracy with which the aircraft attitude may be determined is a key variable for Doppler navigation systems in which the Doppler velocity components need to be resolved into aircraft axes. Similarly, attitude is used for IN axis transformations.

The navigation function therefore performs the task of manoeuvring the aircraft from a known starting point to the intended destination, using a variety of sensors and navigation aids.

The classic method of navigation which has been in used for many years is to use a combination of magnetic and inertial directional gyros used together with airspeed information derived from the air data sensors to navigate in accordance with the parameters shown in Figure 8.1. This is subject to errors in both the heading system and the effects of en-route winds which can cause along-track and across-track errors. In the 1930s it was recognised that the use of radio beacons and navigation aids could significantly reduce these errors by providing the flight crew with navigation assistance related to precise points on the ground.

8.2 Radio Navigation

For many years the primary means of navigation over land, at least in continental Europe and the North American continent, was by means of radio navigation routes defined by VHF omniranging/distance measuring equipment (VOR/DME) beacons as shown in Figure 8.3. By arranging the location of these beacons at major navigation or crossing points, and in some cases airfields, it was possible to construct an entire airway network that could be used by the flight crew to define the aircraft flight from take-off to touchdown. Other radio frequency aids include distance measuring equipment (DME) and non-distance beacons (NDB). The operation of the radio navigation and approach aids is described elsewhere in this chapter.

Figure 8.3 Radio navigation using VOR, DME and automatic direction finding (ADF).

Figure 8.3 shows:

  1. Three VOR/DME beacon pairs: VOR 1/DME 1, VOR 2/DME 2 and VOR 3/DME 3 which define waypoints 1 to 3. These beacons represent the intended waypoints 1, 2 and 3 as the aircraft proceeds down the intended flight plan route – most likely an identified airway. When correctly tuned, the VOR/DME pairs succesively present the flight crew with bearing to and distance from the next waypoint.
  2. Off-route DME beacons, DME 4 and DME 5, may be used as additional means to locate the aircraft position by means of the DME fix obtained where the two DME 4 and DME 5 range circles intersect. As will be seen, DME/DME fixes are a key attribute in the modern navigation system.
  3. Off-route NDB beacons may be used as an additional means to determine the aircraft position by obtaining a cross-fix from the intersection of the bearings from NBD 1 and NDB 2. These bearings are derived using the aircraft ADF system.
  4. In the military context: as well as these beacons TACAN and VORTAC beacons may be used specifically. TACAN has the particular advantage that it may be used in an offset mode where a navigational point may be specified in terms of a range and bearing offset from the TACAN beacon itself. TACAN also has certain features whereby it may be used to determine the range and bearing to other formations, eg a tanker aircraft, thereby facilitating airborne rendezvous operations.

Thus, in addition to using navigation information from the ‘paired’ VOR/DME or TACAN beacons that define the main navigation route, position fix, cross-fix, range or bearing information may also be derived from DME or NDB beacons in the vicinity of the planned route by using automatic direction-finding techniques. As has already been described in Chapter 7, a major limitation of the radio beacon navigation technique results from line-of-sight propagation limitations at the frequencies at which both VOR and DME operate. As well as the line-of-sight and terrain-masking deficiencies, the reliability and accuracy of the radio beacons can also be severely affected by electrical storms. Over longer ranges, LORAN-C could be used if fitted.

Owing to the line-of-sight limitations of these radio beacons, these navigation techniques were only usable overland where the beacon coverage was sufficiently comprehensive or for close off-shore routes where the beacons could be relied upon.

8.2.1 Oceanic Crossings

In 1969 the requirements were already specified for self-contained long-range commercial navigation by advisory circular AC 121-13. The appropriate document specified that self-contained navigation systems should be capable of maintaining a maximum error of ±20 nm across track and ±25 nm along track for 95% of the flights completed. Two systems were addressed in the specification: one using Doppler radar and the other using an inertial navigation system (INS).

In June 1977, the North Atlantic (NAT) minimum navigation performance specifications (MNPS) were altered to reflect the improved navigation sensors – see advisory circular AC 120-33. This defined the separation requirements for long-range navigation over the North Atlantic. The lateral separation was reduced from 120 to 60 nm while retaining the previous vertical separation of 2000 ft. Statistical limits were specified as to how long an aircraft was allowed to spend 30 nm off-track and between 50 and 70 nm off-track – the latter actually representing an overlap with an adjacent track. The standard deviation of lateral track errors was specified as 6.3 nm.

The Doppler radar system was specified as being an acceptable navigation means applying within certain geographical boundaries. Eastern and western entry points or ‘gateways’ were specified as entry and departure points into and out of the North Atlantic area. These gateways were identified as a number of specific named NDB or VOR beacons on both sides of the ocean. The North Atlantic transit area was specified as being the oceanic area bounded by the eastern and western gateways and lying between the latitude of 35°N and 65°N. By the standards of the allowable navigation routes available to today’s aviators, this represented a very restricted envelope.

The aircraft equipment requirements were also carefully specified:

  • Dual Doppler and computer systems;
  • Dual polar path compasses;
  • ADF;
  • VOR;
  • One LORAN receiver capable of being operated from either pilot’s station.

8.3 Inertial Navigation Fundamentals

The availability of inertial navigation systems (INS) to the military aviation community during the early 1960s added another dimension to the navigation equation. Now flight crew were able to navigate by autonomous means using an on-board INS with inertial sensors. By aligning the platform to earth-referenced coordinates and present position during initialisation, it was now possible to fly for long distances without relying upon LORAN, VOR/DME or TACAN beacons. Waypoints could be specified in terms of latitude and longitude as arbitrary points on the globe, more suited to the aircraft’s intended flight path rather than a specific geographic feature or point in a radio beacon network (Figure 8.4). The operation of inertial platforms is described in detail in section 8.8.

The specifications in force at this time also offered an INS solution to North Atlantic crossings as well as the dual-Doppler solution previously described. The inertial solution required serviceable dual INS and associated computers to be able to undertake the crossing. There were also limitations on the latitudes at which the ground alignment could be performed – 76° north or south – as attaining satisfactory alignment becomes progressively more difficult the nearer to the poles the INS becomes.

Figure 8.4 Fundamentals of inertial navigation.

Advisory circular AC 25-4 set forth requirements for operating an INS as a sole means of navigation for a significant portion of the flight. These requirements may be summarised as follows:

  1. The ability to provide the following functions:
    • Valid ground alignment at all latitudes appropriate for the intended use of the INS;
    • The display of alignment status to the crew;
    • Provision of the present position of the aircraft in suitable coordinates: usually latitude from +90° (north) to −90° (south) and longitude from +180° (east) to −180° (west);
    • Provision of information on destinations or waypoints;
    • Provision of data to acquire and maintain the desired track and the ability to determine deviation from the desired track (across-track error);
    • Provision of information needed to determine the estimated time of arrival (ETA).
  2. The ability to comply with the following requirements:
    • ± 20 nm across track and ±25 nm along track;
    • Maintainenance of this accuracy on a 95% probability basis at representative speeds and altitudes and over the desired latitude range;
    • The capacity to compare the INS position with visual fixes or by using LORAN, TACAN, VOR, DME or ground radar (air traffic control).
  3. The provision of a memory or in-flight alignment means. Alternatively, the provision of a separate electrical power source – usually a dedicated stand-alone battery – able to support the INS with full capability for at least 5 min in the event of an interruption of the normal power supply.

8.4 Satellite Navigation

The foregoing techniques were prevalent from the 1960s to the 1990s when satellite navigation became commonly available. The use of global navigation satellite systems (GNSS), to use the generic name, offers a cheap and accurate navigational means to anyone who possesses a suitable receiver. Although the former Soviet Union developed a system called GLONASS, it is the US global positioning system (GPS) that is the most widely used. The principles of satellite navigation using GPS will be described in detail later in the chapter.

GPS receivers may be provided for the airborne equipment in a number of ways:

  1. Stand-alone GPS receivers, most likely to be used for GPS upgrades to an existing system. These are multichannel (typically, 12-channel) global navigation satellite system (GNSS) receivers – the B777 utilises this approach.
  2. GPS receivers integrated into a multifunction receiver unit called a multimode receiver (MMR). Here, the GPS receiver function is integrated into one LRU along with VOR and ILS receivers.

8.4.1 Differential GPS

One way of overcoming the problems of selective availability is to employ a technique called differential GPS (DGPS). Differential techniques involve the transmission of a corrected message derived from users located on the ground. The correction information is sent to the user who can apply the corrections and reduce the satellite ranging error. The two main techniques are:

  1. Local-area DGPS. The corrections are derived locally at a ground reference site. As the position of the site is accurately known, the satellite inaccuracies can be determined and transmitted locally to the user, in this case by line-of-sight VHF data link. The local-area DGPS system under development in the United States is called the local-area augmentation system (LAAS) and is described below.
  2. Wide-area DGPS. The wide-area correction technique involves networks of data collection ground stations. Information is collected at several ground stations which are usually located more than 500 miles apart. The correction information derived by each station is transmitted to a central location where the satellite corrections are determined. Corrections are sent to the user by geostationary satellites or other appropriate means. The wide-area augmentation system (WAAS) being developed in the United States is outlined below.

Note that differential techniques may be applied to any satellite system. For GPS the basic accuracy without selective availability is about ±100 m as opposed to ±8 m when the full system is available. The DGPS developments under way in the United States are intended to improve the accuracy available to civil users.

8.4.2 Wide-area Augmentation System (WAAS)

The operation of WAAS, shown in Figure 8.5, is described as follows:

  1. WAAS is a safety-critical system that augments basic GPS and will be deployed in the contiguous United States, Hawaii, Alaska and parts of Canada.
  2. WAAS has multiple wide-area reference stations which are precisely surveyed and monitor the outputs from the GPS constellation.
  3. These reference stations are linked to wide-area master stations where corrections are calculated and the system integrity assessed. Correction messages are uplinked to geostationary earth orbit (GEO) satellites that transmit the corrected data on the communications L1 band to aircraft flying within the WAAS area of coverage. Effectively, the GEO satellites act as surrogate GPS satellites.
  4. WAAS improves the GPS accuracy to around ±7 m which is a considerable improvement over the ‘raw’ signal. This level of accuracy is sufficient for Cat I approach guidance.

Some problems were experienced in initial system tests during 2000. Commissioning of the WAAS requires extensive testing to ensure integrity levels, accuracy, etc., and the system was finally commissioned during 2004.

Figure 8.5 Wide-area augmentation system.

8.4.3 Local-area Augmentation System (LAAS)

The operation of LAAS is shown in Figure 8.6 and described below:

  1. LAAS is intended to complement WAAS but at a local level.
  2. LAAS works on similar principles except that local reference stations transmit correction data direct to user aircraft on VHF. As such, the LAAS coverage is restricted by VHF line-of-sight and terrain-masking limitations.
  3. LAAS improves the GPS accuracy to about ±1 m, close to the higher GPS level of accuracy. This level of accuracy is sufficient to permit Cat II and Cat III approaches which are described more fully in Chapter 9.

Figure 8.6 Local-area augmentation system.

Implementation is as before, with final deployment in 2006, although these timescales are apt to slip, as has the implementation of WAAS. According to present plans it is expected that LAAS will be deployed at up to 143 airfields throughout the United States. It was the anticipation of LAAS implementation that caused the United States to modify its stance upon the implementation of MLS as an approach aid successor to ILS, the space-based GPS system being seen as more flexible than ground-based MLS.

8.5 Integrated Navigation

Integrated navigation, as the name suggests, employs all the features and systems described so far. An integrated navigation solution using a multisensor approach blends the performance of all the navigation techniques already described together with GPS to form a totally integrated system. In this case the benefits of the GPS and IN derived data are blended to provide more accurate data fusion, in the same way as barometric and IN data are fused (Figure 8.7).

Such an integrated system is a precursor to the introduction of the advanced navigation capabilities that will comprise the future air navigation system (FANS). FANS is designed to make more efficient use of the existing airspace such that future air traffic increases may be accommodated. Some elements of FANS have already been implemented, others will take several years to attain maturity. A key prerequisite to achieving a multisensor system is the installation of a high-grade flight management system (FMS) to perform the integration of all the necessary functions and provide a suitable interface with the flight crew.

Figure 8.7 Integrated GPS and inertial navigation.

8.5.1 Sensor Usage – Phases of Flight

When assessing which navigation sensors to use for various phases of flight, the navigation accuracy, equipment availability and reliability and operational constraints all need to be taken into account. The advent of GPS with its worldwide coverage at high levels of accuracy has given a tremendous impetus to the navigation capabilities of modern aircraft. However, system integrity concerns have meant that the certification authorities have stopped short of relying solely on GPS.

Bearing in mind physical and radio propagation factors, and the relative traffic densities for various phases of flight, a number of requirements are specified for the use of GPS. Advisory circular AC 90-94 specifies the considerations that apply for the use of GPS as a sole or supplementary method of navigation.

These considerations apply for the following phases of flight:

  1. Oceanic en route. Operation over long oceanic routes means that the aircraft will be denied the availability of most of the line-of-sight radio navigation aids such as NDB, VOR, TACAN, etc. LORAN-C may be available in some circumstances. The aircraft will need to depend upon an approved primary long-range method of navigation. For most modern transport aircraft that means equipping the aircraft with a dual- or triple-channel INS or ADIRS. Supplementary means such as GPS may be used to update the primary method of navigation. Aircraft using GPS under instrument flight rules (IFR) must be equipped with another approved long-range navigation system: GPS is not certified as a primary and sole means of navigation. Certain categories of GPS equipment may be used as one of the approved long-range navigation means where two systems are required. The availability of a functioning receiver autonomous integrity monitor (RAIM) capability is also important owing to the impact that this has upon GPS integrity. Providing RAIM is available, the flight crew need not actively monitor the alternative long-range navigation system.
  2. Domestic en route. Once overland, most of the conventional navigation aids may be available, unless the aircraft is transiting a wilderness area such as Siberia. For the most part, NDB, VOR, TACAN and LORAN-C will be operational and available to supplement GPS. These ground-based systems do not have to be used to monitor GPS unless RAIM failure occurs. Within the United States, Alaska, Hawaii and surrounding coastal waters, IFR operation may be met with independent NDB, VOR, TACAN or LORAN-C equipment. This may not necessarily be the case outside the US NAS.
  3. Terminal. GPS IFR operations for the terminal phases of flight should be conducted as for normal RNAV operations using the standard procedures:
    • Standard instrument departures (SIDs);
    • Standard terminal arrival routes (STARs);
    • Standard instrument approach procedure (SIAP).

    The normal ground-based equipment appropriate to the phase of flight must be available, out, as before, it does not need to be used to monitor GPS unless RAIM fails.

  4. Approach. In the United States an approach overlay programme has been introduced by the FAA to facilitate the introduction of instrument approaches using GPS. The key features of the GPS overlay programme are described below.

8.5.2 GPS Overlay Programme

The GPS overlay programme allows pilots to use GPS equipment to fly existing VOR, VOR/DME, NDB, NDB/DME, TACAN and RNAV non-precision instrument approach procedures. This facility only applies in US airspace and was introduced in February 1994. The approach aid appropriate to the type of approach being flown must be available for use, but need not be monitored provided RAIM is available. In April 1994, ‘phase III’ approaches introduced the first GPS specific approaches with GPS specifically included in the title. For these approaches the traditional avionics need not be available – either ground-based or airborne equipment – provided RAIM is available. For aircraft fitted with GPS without a RAIM capability these navigation aids must be available.

8.5.3 Categories of GPS Receiver

The different types of GPS are mandated in technical standing order (TSO) C-129a. This categorises the different GPS receivers by three major classes:

  1. Class A. This equipment incorporates a GPS receiver and the navigation capability to support it.
  2. Class B. This consists of GPS equipment providing data to an integrated navigation system such as a flight management system or an integrated multisensor navigation system.
  3. Class C. This includes equipment comprising GPS sensors which provide data to an autopilot or flight director in order to reduced flight technical errors.

This classification therefore categorises the GPS equipment types according to function. TSO C-129a also specifies which class of equipment may be used for the typical flight phases described above. It also specifies whether the RAIM function is to be provided by the GPS or the integrated system.

8.6 Flight Management System

It is clear from the foregoing description of the aircraft navigation functions that navigation is a complex task and becoming more so all the while. FMS functionality has increased rapidly over the last decade, and many more enhancements are in prospect as the future features required by FANS are added. A typical FMS will embrace dual computers and dual Multifunction control and display units (MCDUs) as shown in Figure 8.8. In military bomber and transport aircraft the system implementation is likely to be in the form portrayed in this figure. For military fighter aircraft the functions will be similar but embedded in the avionics system navigation computers and mission computers as appropriate.

Figure 8.8 is key to depicting the integration of the navigation functions described above. System sensor inputs, usually in dual-redundant form for reasons of availability and integrity, are shown on the left. These are:

  • Dual INS/IRS;
  • Dual navigation sensors: VOR/DME, DME/DME, etc;
  • Dual GNSS sensors – usually GPS;
  • Dual air data sensors;
  • Dual inputs from on-board sensors relating to fuel on-board and time.

Figure 8.8 Typical flight management system (FMS).

These inputs are used by the FMS to perform the necessary navigation calculations and provide information to the flight crew via a range of display units:

  • Electronic flight instrument system (EFIS);
  • Communications control system;
  • Interface with the autopilot/flight director system to provide the flight crew with flight direction or automatic flight control in a number of predefined modes.

The FMS–crew interface is shown in Figure 8.9. The key interface with the flight crew is via the following displays:

  1. Captain’s and first officer’s navigation displays (ND), part of the electronic flight instrument system (EFIS). The navigation displays may show information in a variety of different ways.
  2. Control and display units 1 and 2, part of the FMS. The CDUs both display information and act as a means for the flight crew manually to enter data.

The FMS computers perform all the necessary computations and show the appropriate navigation parameters on the appropriate display. The navigation displays show the navigation and steering information necessary to fly the intended route. These are colour displays and can operate in a number of different formats, depending upon the phase of flight.

Figure 8.9 FMS control and display interface.

8.6.1 FMS CDU

The FMS CDU is the key flight crew interface with the navigation system, allowing the flight crew to enter data as well as having vital navigation information displayed. A typical FMS CDU is shown in Figure 8.10. The CDU has a small screen on which alpha-numeric information is displayed, in contrast to the pictorial information displayed on the EFIS navigation displays. This screen is a cathode ray tube (CRT) monochrome display in early systems; later systems use colour active matrix liquid crystal display (AMLCD) (see Chapter 11). The tactile keyboard has alpha-numeric keys in order to allow manual entry of navigation data, perhaps inserting final alterations to the flight plan, as well as various function keys by which specific navigation modes may be selected. The line keys at the sides of the display are soft keys that allow the flight crew to enter a menu-driven system of subdisplays to access more detailed information. On many aircraft the CDU is used to portray maintenance status and to execute test procedures using the soft keys and the menu-driven feature. Finally, there are various annunciator lights and a lighting control system.

Examples of the data displayed on the CDU are indicated in Figure 8.11. The CDU displays the following parameters using a menu-driven approach:

  1. An ETA waypoint window. This shows the estimated time of arrival (ETA) at the waypoint, in this case waypoint 15.
  2. Early/late timing information. This represents the earliest and latest times the aircraft can reach the waypoint given its performance characteristics.
  3. Information on the runway – an ILS approach to runway 27.
  4. Wind information for the approach – wind bearing 290.
  5. Information on the navigation aids being used: VOR, DME and ILS/LOC.
  6. An ANP/RNP window. This compares the actual navigation performance (ANP) of the system against the required navigation performance (RNP) for the flight phase and navigation guidance being flown. In this case the ANP is 0.15 nm against a RNP of 0.3 nm and the system is operating well within limits.

Figure 8.10 Typical FMS control and display unit.

Figure 8.11 Typical FMS CDU display data.

Figure 8.12 Top-level FMS functions.

8.6.2 FMS Functions

The functions of the FMS at a top level are shown in Figure 8.12. This diagram gives an overview of the functions performed by the FMS computers. These may be summarised as follows:

  1. Navigation computations and display data. All the necessary navigation computations are undertaken to derive the navigation or guidance information according to the phase of flight and the sensors utilised. This information is displayed on the EFIS navigation display or the FMS CDU. Flight director and steering commands are sent to the autopilot for the flight director with the pilot in the loop or for the engagement automatic flight control modes.
  2. Navigation sensors. INS, GPS, VOR, ILS, ADF, TACAN and other navigation aids provide dual-sensor information to be used for various navigation modes.
  3. Air data. The ADCs or ADIRS provides the FMS with high-grade corrected air data parameters and attitude information for use in the navigation computations.
  4. Fuel state. The fuel quantity measurement system and the engine-mounted fuel flow-meters provide information on the aircraft fuel quantity and engine fuel flow. The calculation of fuel use and total fuel consumption is used to derive aircraft and engine performance during the flight. When used together with a full aircraft performance model, optimum flight guidance may be derived which minimises the fuel consumed.
  5. Sensor fusion and Kalman filter. The sensor information is fused and validated against other sources to determine the validity and degree of fidelity of the data. By using a sophisticated Kalman filter, the computer is able to determine the accuracy and integrity of the navigation sensor and navigation computations and determine the actual navigation performance (ANP) of the system in real time.
  6. Communications management. The system passes information to the communication control system regarding the communication and navigation aid channel selections that have been initiated by the FMS in accordance with the requirements of the flight plan.
  7. Navigation database. The navigation base contains a wide range of data that are relevant to the flight legs and routes the aircraft may expect to use. This database will include the normal flight plan information for standard routes that the aircraft will fly together with normal diversions. It will be regularly updated and maintained. A comprehensive list of these items includes:
    • Airways;
    • Airports – approach and departure information, airport and runway lighting, obstructions, limitations, airport layout, gates, etc;
    • Runways including approach data, approach aids, category of approach (Cat I or Cat II/III) and decision altitudes;
    • Routes, clearance altitudes, SIDS, STARS and other defined navigation data;
    • Procedures including notification of short-term airspace restrictions or special requirements;
    • Flight plans with standard diversions.
    • Wind data – forecast winds and actual winds derived throughout flight.
  8. Aircraft performance model. The inclusion of a full performance model adds to the systems ability to compute four-dimensional (x, y, z, time) flight profiles and at the same time make optimum use of the aircraft energy to optimise fuel use.

The FMS provides the essential integration of all of these functions to ensure that the overall function of controlling the navigation of the aircraft is attained. As may be imagined, this does not merely include steering information to direct the aircraft from waypoint to waypoint. The FMS also controls the tuning of all the appropriate aircraft receivers to navigation beacons and communications frequencies via the communications control units and many other functions besides. The flight plan that resides within the FMS memory will be programmed for the entire route profile, for all eventualities, including emergencies. More advanced capabilities include three-dimensional navigation and the ability to adjust the aircraft speed to reach a waypoint within a very small time window (typically ±6 s). The various levels of performance and sophistication are summarised in Table 8.1. Military aircraft such as the Boeing multirole maritime aircraft (MMA) will be fitted with an FMS developed to provide all these features for a civil operator. The FMS will incorporate additional specific modes of operation to facilitate performance of the mission, e.g. flying mission attack profiles at low level.

The FMS capabilities will be examined in a little more detail.

8.6.3 LNAV

Lateral navigation or LNAV relates to the ability of the aircraft to navigate in two dimensions, in other words, the lateral plane. LNAV was the first navigation feature to be implemented and involved navigating aircraft to their intended destination without any other considerations. LNAV comprises two major implementations:

  • Airway navigation;
  • Area navigation or RNAV.

Table 8.1 Summary of FMS capabilities

Function Capability
LNAV The ability to navigate laterally in two dimensions
VNAV The ability to navigate laterally in two dimensions plus the ability to navigate in the vertical plane. When combined with LNAV, this provides three-dimensional navigation
Four-dimensional navigation The ability to navigate in three-dimensions plus the addition of time constraints for the satisfaction of time of arrival at a waypoint
Full performance based navigation The capability of four-dimensional navigation together with the addition of an aircraft specific performance model. By using cost indexing techniques, full account may be made of the aircraft performance in real time during flight, allowing optimum use of fuel and aircraft energy to achieve the necessary flight path
Future air navigation system (FANS) or global air transport management (GATM) The combination of the full performance model together with all the advantages that FANS or GATM will confer, eventually enabling the concept of ‘free flight’

8.6.4 Airway Navigation

Airway navigation is defined by a predetermined set of airways which are based primarily on VOR stations, although some use NDB stations. In the United States these airways are further categorised depending upon the height of the airway:

  1. Airways based on VOR from 120 ft above the surface to 18 000 ft above mean sea level (MSL) carry the V prefix and are called victor airways.
  2. Airways using VOR from 18 000 ft MSL to 45 000 ft MSL are referred to as jet routes.

Each VOR used in the route system is called either a terminal VOR or a low- or high-altitude en-route VOR. Terminal VORs are used in the terminal area to support approach and departure procedures and are usable up to ∼25 nm; they are not to be used for en-route navigation. Low-altitude en-route VORs have service volumes out to a range of 40 nm and are used up to 18 000 ft on victor airways. High-altitude VORs support navigation on jet routes and their service volume may extend to a range of ∼200 nm from the ground station. Clearly, the range of the VOR beacons is limited by line-of-sight propagation considerations, as has already been described.

Airway width is determined by the navigation system performance and depends upon error in the ground station equipment, airborne receiver and display system.

8.6.5 Area Navigation

Many aircraft possess an area navigation (RNAV) capability. The on-board navigation together with the FMS can navigate along a flight path containing a series of waypoints that are not defined by the airways. Navigation in these situations is not confined to VOR beacons but may use a combination of VOR, DME, LORAN-C, GPS and/or INS. Random routes have the advantage that they may be more direct than the airway system and also that they tend geographically to disperse the aircraft away from the airway route structure. Advisory circular AC 90-45A defines the regulations that apply to aircraft flying two-dimensional RNAV in IFR conditions in the US national airspace system. Advisory circular AC 90-45A is the original guidance on the use of RNAV within the United States, while advisory circular AC 20-130A is a more recent publication on navigation or FMS systems using multiple sensors – including GPS – and is probably more relevant to the sophisticated FMS in use today.

8.6.6 VNAV

Following on from the LNAV and RNAV capabilities, vertical navigation (VNAV) procedures were developed to provide three-dimensional guidance. Present VNAV systems use barometric altitude, as it will be recalled that the GPS satellite geometry does not generally provide accurate information in the vertical direction. Whereas DGPS systems such as WAAS will address and overcome this issue, these systems will not be available for some time. Advisory circular AC 90-97 provides guidance for the use of VNAV guidance in association with RNAV instrument approaches with a VNAV decision altitude (DA). One disadvantage of using barometric means to provide the VNAV guidance function is the non-standard nature of the atmosphere. Therefore, VNAV approaches embrace a temperature limit below which the use of VNAV decision height is not permitted. If the temperature on a particular day falls below this limit, then the flight crew must instead respect the published LNAV minimum decision altitude (MDA).

8.6.7 Four-Dimensional Navigation

The combination of LNAV and VNAV provides a three-dimensional navigation capability. However, in a busy air traffic management situation the element of time is equally important. A typical modern FMS will have the capability to calculate the ETA to a specific waypoint and ensure that the aircraft passes through that point in space within ±6 s of the desired time. Furthermore, calculations can be made in response to an air traffic control enquiry as to when the aircraft can reach an upcoming waypoint. By using information regarding the aircraft performance envelope, the FMS can perform calculations that determine the earliest and the latest possible time within which the aircraft can reach the waypoint. The ability to determine this time window can be of great use in helping the air traffic controller to maintain steady traffic flow during periods of high air traffic density.

8.6.8 Full Performance Based Navigation

If the FMS contains a full performance model provided by the aircraft manufacturer, then even more detailed calculations may be performed. By using the aircraft velocity and other dynamic parameters, it is possible to compute the performance of the aircraft over very small time increments. By using this technique, and provided that the sensor data are sufficiently accurate, the future dynamic behaviour of the aircraft may be accurately predicted. Using this feature, and knowing the four-dimensional trajectory and gate speeds that are detailed in the flight plan, the aircraft can calculate the optimum trajectory to meet all these requirements while conserving energy and momentum and ensuring minimum fuel burn. When this capability is combined with the increasing flexibility that FANS will provide, further economies will be possible. Today, most FMS systems are being developed with these emerging requirements in mind such that future implementation will depend upon system software changes and upgrades rather than aircraft equipment or architecture modifications.

8.6.9 FMS Procedures

Although the foregoing explanations have concentrated on performance enhancements, the assistance that the FMS provides the flight crew in terms of procedural displays cannot be forgotten. Typical examples include:

  • Standard instrument departure (SID);
  • En-route procedures;
  • Standard terminal arrival requirements (STAR);
  • ILS approach.

Examples of these procedures are given in the companion volume (Moir and Seabridge, 2003).

8.6.10 Traffic Collision and Avoidance System (TCAS)

The TCAS combines the use of the ATC mode S transponder with additional computing and displays to provide warning of the proximity of other aircraft within the air traffic control system. The operation of ATC mode S and the TCAS is described in detail in Chapter 7.

8.6.11 GPWS and EGPWS

While the TCAS is designed to prevent air-to-air collisions, the ground proximity warning system (GPWS) is intended to prevent unintentional flight into the ground. Controlled flight into terrain (CFIT) is the cause of many accidents. The term describes conditions where the crew are in control of the aircraft, but, owing to a misplaced sense of situational awareness, they are unaware that they are about to crash into the terrain. The GPWS takes data from various sources and generates a series of audio warnings when a hazardous situation is developing.

The terrain awareness and warning system (TAWS) embraces the overall concept of providing the flight crew with prediction of a potential controlled flight into terrain. The new term is a generic one since the ground proximity warning system (GPWS) and enhanced GPWS became associated mainly with the Allied Signal (now Honeywell) implementation. The latest manifestation is designed to provide the crew with an improved situational awareness compared with previous systems. The FAA is presently in the process of specifying that turbine-equipped aircraft with six seats or more will be required to be equipped with a TAWS by 2003. Advisory circular AC 25-23 addresses the airworthiness requirements associated with the TAWS.

The GPWS/TAWS uses radar altimeter information together with other information relating to the aircraft flight path. Warnings are generated when the following scenarios are unfolding:

  • Flight below the specified descent angle during an instrument approach;
  • Excessive bank angle at low altitude;
  • Excessive descent rate;
  • Insufficient terrain clearance;
  • Inadvertent descent after take-off;
  • Excessive closure rate to terrain – the aircraft is descending too quickly or approaching higher terrain.

Inputs are taken from a variety of aircraft sensors and compared with a number of algorithms that define the safe envelope within which the aircraft is flying. When key aircraft dynamic parameters deviate from the values defined by the appropriate guidance algorithms, appropriate warnings are generated.

The installation of GPWS equipment for all airliners flying in US airspace was mandated by the FAA in 1974, since when the number of CFIT accidents has dramatically decreased.

More recently, enhanced versions have become available. The EGPWS offers a much greater situational awareness to the flight crew as more quantitative information is provided, together with earlier warning of the situation arising. It uses a worldwide terrain database which is compared with the present position and altitude of the aircraft. Within the terrain database the earth’s surface is divided into a grid matrix with a specific altitude assigned to each square within the grid representing the terrain at that point.

The aircraft intended flight path and manoeuvre envelope for the prevailing flight conditions are compared with the terrain matrix and the result is graded according to the proximity of the terrain, as shown in Figure 8.13:

Figure 8.13 Principle of operation of the EGPWS (TAWS).

Terrain responses are graded as follows:

  • No display for terrain more than 2000 ft below the aircraft;
  • Light-green dot pattern for terrain between 1000 and 2000 ft below the aircraft;
  • Medium-green dot pattern for terrain between 500 and 1000 ft below the aircraft;
  • Medium-yellow dot pattern for terrain between 1000 ft above and 500 ft below the aircraft;
  • Heavy-yellow display for terrain between 1000 and 2000 ft above the aircraft;
  • Heavy-red display for terrain more than 2000 ft above the aircraft.

This type of portrayal using coloured imagery is very similar to that for the weather radar and is usually shown on the navigation display. It is far more informative than the audio warnings, given by earlier versions of GPWS. The EGPWS also gives audio warnings, but much earlier than those given by the GPWS. The earlier warnings, together with the quantitative colour display, give the flight crew a much better overall situational awareness in respect of terrain and more time to react positively to their predicament than did previous systems.

8.7 Navigation Aids

As aviation began to expand in the 1930s, the first radio navigation systems were developed. Initially, these were installed at the new growing US airports, and it is interesting to note that the last of these early systems was decommissioned as recently as 1979.

One of the most prominent was the ‘radio range’ system developed in Italy by Bellini and Tosi, which was conceived as early as 1907. The operation of the Bellini–Tosi system relied upon the transmission of morse characters A (dot-dash) and N (dash-dot) in four evenly spaced orthogonal directions. When flying the correct course, the A and N characters combined to produce a humming noise which the pilot could detect in his earphones. Deviation from the desired course would result in either the A or N characters becoming most dominant, signifying the need for corrective action by turning left or right as appropriate.

Following WWII, the International Civil Aviation Organisation (ICAO) produced international standards that led to the definition of the very high-frequency omnirange (VOR) system which is in widespread use today and is described below.

The use of radio navigation aids is important to military aircraft as much of their operation involves sharing the airspace with civil users. Therefore, military aircraft, especially those adopted from a civil aircraft platform, will utilise a similar suite on navigation aids. Typical aids are:

  1. Navigation aids:
    • Automatic direction finding (ADF);
    • VHF omnirange (VOR);
    • Distance-measuring equipment (DME);
    • Tactical air navigation (TACAN);
    • Long range navigation (LORAN).
  2. Landing aids:
    • Instrument landing system (ILS);
    • Microwave landing system (MLS).

8.7.1 Automatic Direction Finding

Automatic direction finding (ADF) involves the use of a loop direction finding technique to establish the bearing to a radiating source. This might be to a VHF beacon or a non-distance beacon (NDB) operating in the 200–1600 kHz band. Non-directional beacons, in particular, are the most prolific and widely spread beacons in use today. The aircraft ADF system comprises integral sense and loop antennas which establish the bearing of the NBD station to which the ADF receiver is tuned. The bearing is shown on the radio magnetic indicator (RMI) in the analogue cockpit of a ‘classic’ aircraft or more likely on the electronic flight instrument system (EFIS), as appropriate. ADF is used by surveillance aircraft such as MPA on an air sea rescue mission to home on to a personal locator beacon used by downed airmen or installed in life rafts.

8.7.2 Very High-frequency Omnirange (VOR)

The VOR system was accepted as standard by the United States in 1946 and later adopted by the International Civil Aviation Organisation (ICAO) as an international standard. The system provides a widely used set of radio beacons operating in the VHF frequency band over the range 108–117.95 MHz with a 100 kHz spacing. Each beacon emits a morse code modulated tone which may be provided to the flight crew for the purposes of beacon identification.

The ground station radiates a cardioid pattern which rotates at 30 r/min generating a 30 Hz modulation at the aircraft receiver. The ground station also radiates an omnidirectional signal which is frequency modulated with a 30 Hz reference tone. The phase difference between the two tones varies directly with the bearing of the aircraft. At the high frequencies at which VHF operates there are no sky wave effects and the system performance is relatively consistent. VOR has the disadvantage that it can be severely disrupted by adverse weather – particularly by electrical storms – and as such it cannot be used as a primary means of navigation for a civil aircraft.

Overland in the North American continent and Europe, VOR beacons are widely situated to provide an overall coverage of beacons. Usually these are arranged to coincide with major airway waypoints and intersections in conjunction with DME stations – see below – such that the aircraft may navigate for the entire flight using the extensive route/beacon structure. By virtue of the transmissions within the VHF band, these beacons are subject to the line-of-sight and terrain-masking limitations of VHF communications. Advisory circular AC 00-31A lays out a method for complying with the airworthiness rules for VOR/DME/TACAN.

8.7.3 Distance-measuring Equipment (DME)

Distance-measuring equipment (DME) is a method of pulse ranging used in the 960–1215 MHz band to determine the distance of the aircraft from a designated ground station. The aircraft equipment interrogates a ground-based beacon and, upon the receipt of retransmitted pulses, unique to the on-board equipment, is able to determine the range to the DME beacon (Figure 8.14). DME beacons are able to service requests from a large number of aircraft simultaneously but are generally understood to have the capacity to handle ∼200 aircraft at once. Specified DME accuracy is ±3% or ±0.5 nm, whichever is the greater (advisory circular AC 00-31A).

Figure 8.14 DME principle of operation.

DME and TACAN beacons are paired with ILS/VOR beacons throughout the airway route structure in accordance with the table set out in Appendix 3 of advisory circular AC 00-31A. This is organised such that aircraft can navigate the airways by having a combination of VOR bearing and DME distance to the next beacon in the airway route structure. A more recent development – scanning DME – allows the airborne equipment rapidly to scan a number of DME beacons, thereby achieving greater accuracy by taking the best estimate of a number of distance readings. This combination of VOR/DME navigation aids has served the aviation community well in the United States and Europe for many years, but it does depend upon establishing and maintaining a beacon structure across the land mass or continent being covered. New developments in third-world countries are more likely to skip this approach in favour of a global positioning system (GPS), as described later in the chapter.

8.7.4 TACAN

Tactical air navigation (TACAN) is military omnibearing and distance-measuring equipment with similar techniques for distance measurement as DME. The bearing information is accomplished by amplitude modulation achieved within the beacon which imposes 15 and 135 Hz modulated patterns and transmits this data together with 15 and 135 Hz reference pulses. The airborne equipment is therefore able to measure distance using DME interrogation techniques while using the modulated data to establish bearing.

TACAN beacons operate in the frequency band 960–1215 MHz as opposed to the 108–118 MHz used by DME. This means that the beacons are smaller, making them suitable for shipborne and mobile tactical use. Some airborne equipment have the ability to offset to a point remote from the beacon which facilitates recovery to an airfield when the TACAN beacon is not co-located. TACAN is reportedly accurate to within ±1% in azimuth and ±0.1 nm in range, so it offers accuracy improvements over VOR/DME.

TACAN also has the ability to allow aircraft to home on to another aircraft, a feature that is used in air-to-air refuelling to enable aircraft to home on to the donor tanker.

8.7.5 VORTAC

As most military aircraft are equipped with TACAN, some countries provide VORTAC beacons which combine VOR and TACAN beacons. This allows interoperability of military and civil air traffic. Military operators use the TACAN beacon while civil operators use the VOR bearing and TACAN (DME) distance-measuring facilities. This is especially helpful for large military aircraft, such as transport or surveillance aircraft, since they are able to use civil air lanes and operational procedures during training or on transit between theatres of operations.

8.7.6 Hyberbolic Navigation Systems – LORAN-C

Hyperbolic navigation systems – of which long range navigation (LORAN) is the most noteworthy example – operate upon hyperbolic lines of position rather than circles or radial lines. Figure 8.15 illustrates the principle of operation of a hyperbolic system in a very elementary manner. This shows hyperbolic solid lines which represent points that are equidistant from the two stations. These points will have the same time difference between the arrival of signals from the blue-master and blue-slave stations (the term secondary station is probably a better and more accurate description). This in itself will not yield position, but, if a second pair of stations is used – angled approximately 45° to the first – shown as dashed lines, then position can be obtained. The relative positioning of the lines in this dual-chain example shows that three outcomes are possible:

  1. At point A the lines cross at almost 90°, and this represents the most accurate fix.
  2. At point B the lines cross at a much more acute angle and the result is a larger error ellipse.
  3. At point C there are two possible solutions and an ambiguity exists that can only be resolved by using a further station.

Figure 8.15 Principle of operation of a hyperbolic navigation system.

LORAN-C is the hyperbolic navigation system in use today and was conceived in principle around the beginning of WWII. Worldwide coverage existed in 1996 and new facilities were being planned in the late 1990s. LORAN operates in the frequency band 90–110 kHZ as a pulsed system which enables the ground wave to be separated from the sky wave, the ground wave being preferred. A LORAN chain will comprise at least three stations, one being nominated as the master. The time difference of arrival between the master and slaves allows position to be determined. Each of the stations in a chain transmits unique identifiers which allow the chain to be identified. A typical example of a LORAN-C chain is shown in Figure 8.16 which shows the north-eastern US chain.

Within the defined area of coverage of the chain, LORAN-C will provide a user with a predictable absolute accuracy of 0.25 nm. A typical chain will have over 1000 nm operating range coverage. LORAN-C is also capable of relaying GPS positional error within the transmissions. LORAN-C is expected to remain in commission until at least 2008. Advisory circular AC 20-121A provides information to assist with the certification of LORAN-C navigation systems for use within the United States and Alaska.

Figure 8.16 Typical LORAN-C chain – north-eastern United States.

8.7.7 Instrument Landing System

The instrument landing system (ILS) is an approach and landing aid that has been in widespread use since the 1960s and 1970s. The main elements of the ILS include:

  1. A localiser antenna centred on the runway to provide lateral guidance. A total of 40 operating channels are available within the 108–112 MHz band. The localiser provides left and right lobe signals which are modulated by different frequencies (90 and 150 Hz) such that one signal or the other will dominate when the aircraft is off the runway centre-line. The beams are arranged such that the 90 Hz modulated signal will predominate when the aircraft is to the left, while the 150 Hz signal will be strongest to the right. The difference in signal is used to drive a cross-pointer deviation needle such that the pilot is instructed to ‘fly right’ when the 90 Hz signal is strongest and ‘fly left’ when the 150 Hz signal dominates. When the aircraft is on the centre-line, the cross-pointer deviation needle is positioned in the central position. This deviation signal is proportional to azimuth out to ±5° of the centre-line.
  2. A glideslope antenna located beside the runway threshold to provide lateral guidance. Forty operating channels are available within the frequency band 329–335 MHz. As for the localiser, two beams are located such that the null position is aligned with the desired glideslope, usually set at a nominal 3°. In the case of the glideslope, the 150 Hz modulated signal predominates below the glideslope and the 90 Hz signal is stronger above. When the signals are balanced, the aircraft is correctly positioned on the glideslope and the glideslope deviation needle is positioned in a central position. As for the localiser needle, pilots are provided with ‘fly up’ or fly down’ guidance to help them to acquire and maintain the glideslope (see Figure 8.17 for the general arrangement of the ILS). Figure 8.18 illustrates how guidance information is portrayed for the pilot according to the aircraft position relative to the desired approach path. On older aircraft this would be shown on the compass display, while on modern aircraft with digital cockpits this information is displayed on the primary flight display (PFD). The ILS localiser, glideslope and DME channels are paired such that only the localiser channel needs to be tuned for all three channels to be correctly aligned.
  3. Marker beacons are located at various points down the approach path to give the pilot information as to what stage on the approach has been reached. These are the outer, middle and inner markers. Location of the marker beacons are:
    • Outer marker approximately 4–7 nm from the runway threshold;
    • Middle marker ∼3000 ft from touchdown;
    • Inner marker ∼1000 ft from touchdown.

    The high approach speeds of most modern aircraft render the inner marker almost superfluous and it is seldom used.

  4. The marker beacons are all fan beams radiating on 75 MHz and provide different morse code modulation tones which can be heard through the pilot’s headset. The layout of the marker beacons with respect to the runway is as shown in Figure 8.19. The beam pattern is ±40° along track and ±85° across track. The overall audio effect of the marker beacons is to convey an increasing sense of urgency to the pilot as the aircraft nears the runway threshhold.

Figure 8.17 ILS glideslope and localiser.

Figure 8.18 ILS guidance display.

A significant disadvantage of the ILS system is its susceptibility to beam distortion and multipath effects. This distortion can be caused by local terrain effects, large man-made structures or even taxiing aircraft which can cause unacceptable beam distortion, with the glideslope being the most sensitive. At times on busy airfields and during periods of limited visibility, this may preclude the movement of aircraft in sensitive areas, which in turn can lead to a reduction in airfield capacity. More recently, interference by high-power local FM radio stations has presented an additional problem, although this has been overcome by including improved discrimination circuits in the aircraft ILS receiver.

Figure 8.19 ILS approach markers.

8.7.8 Microwave Landing System (MLS)

The microwave landing system (MLS) is an approach aid that was conceived to redress some of the shortcomings of the ILS. The specification of a time-reference scanning beam MLS was developed through the late 1970s/early 1980s, and a transition to the MLS was envisaged to begin in 1998. However, with the emergence of satellite systems such as the GPS there was also a realisation that both the ILS and MLS could be rendered obsolete when such systems reach maturity. In the event, the US civil community is embarking upon higher-accuracy developments of the basic GPS system: the wide-area augmentation system (WAAS) and local-area augmentation system (LAAS) have already been outlined. In Europe, the United Kingdom, the Netherlands and Denmark have embarked upon a modest programme of MLS installations at major airports.

The MLS operates in the frequency band 5031.0–5190.7 MHz and offers some 200 channels of operation. It has a wider field of view than the ILS, covering ±40° in azimuth and up to 20° in elevation, with 15° useful range coverage. Coverage is out to 20 nm for a normal approach and up to 7 nm for back azimuth/go-around. The co-location of a DME beacon permits three-dimensional positioning with regard to the runway, and the combination of higher data rates means that curved-arc approaches may be made, as opposed to the straightforward linear approach offered by the ILS. This offers advantages when operating into airfields with confined approach geometry and tactical approaches favoured by the military. For safe operation during go-around, precision DME (P-DME) is required for a precise back azimuth signal.

Figure 8.20 Microwave landing system coverage.

A groundbased MLS installation comprises azimuth and elevation ground stations, each of which transmits angle and data functions which are frequency shift key (FSK) modulated and which are scanned within the volume of coverage already described. The MLS scanning function is characterised by narrow beam widths of around 1–2° scanning at high slew rates. Scanning rates are extremely high at 20 000 deg/s which provides data rates that are around 10 times greater than is necessary to control the aircraft. These high data rates are very useful in being able to reject spurious and unwanted effects due to multiple reflections, etc.

Typical coverage in azimuth and elevation for an MLS installation is shown in Figure 8.20.

8.8 Inertial Navigation

8.8.1 Principles of Operation

The availability of inertial navigation systems (INS) to the military aviation community during the early 1960s added another dimension to the navigation equation. Now flight crew were able to navigate by autonomous means using an on-board INS with inertial sensors. By aligning the platform to earth-referenced coordinates and present position during initialisation, it was now possible to fly for long distances without relying upon TACAN or VOR/DME beacons overland or hyperbolic navigation systems elsewhere. Waypoints could be specified in terms of latitude and longitude as arbitrary points on the globe, more suited to the aircraft’s intended flight path rather than a specific geographic feature or point in a radio beacon network (Figure 8.13). This offered an enormous increase in operational capability as mission requirements could be defined and implemented using an indigenous on-board sensor with no obvious means of external detection except when the need arose to update the system.

Figure 8.21 Principles of inertial navigation.

The principles of inertial navigation depend upon the arrangement of inertial sensors such as gyroscopes and accelerometers in a predetermined orthogonal axis set. The gyroscopes may be used to define attitude or body position and rates.

The output from the accelerometer sensor set is integrated to provide velocities, and then integrated again to provide distance travelled (Figure 8.21). First in the military field and then in the commercial market place, inertial navigation systems (INS) became a preferred method for achieving long-range navigation such that by the 1960s the technology was well established.

The specifications in force at this time also offered an INS solution to North Atlantic crossings as well as the dual-Doppler solution previously described. The inertial solution required serviceable dual INS and associated computers to be able to undertake the crossing. There were also limitations on the latitudes at which the ground alignment could be performed – 76° north or south – as attaining satisfactory alignment becomes progressively more difficult the nearer to the poles the INS becomes.

For civil operators, accuracy requirements set forth requirements for operating an INS as a sole means of navigation for a significant portion of the flight. These requirements were described earlier in the chapter.

8.8.2 Stand-alone Inertial Navigation System

For reasons of both availability and accuracy, systems were developed with dual and triple INS installations. A typical triple INS installation of the type used by modern wide-body commercial transport aircraft is presented in Figure 8.22, showing three INS units integrated with the other major systems units. This type of system would be representative of an INS installation of a large aircraft before the availability of satellite sensors in the 1990s. By this time the gimballed IN platform would have been replaced by a more reliable strapdown system similar to the Litton LTN-92 system.

Figure 8.22 Typical triple INS system.

This integrated system comprised the following units:

  1. Dual sensors:
    • VOR for bearing information;
    • DME for range information;
    • Air data computer (ADC) for air data;
    • Provision for a dual GPS interface.
  2. Controls and displays:
    • Control and display unit (CDU);
    • Electronic flight instrument system/flight director (EFIS/FD);
    • Mode selector unit.
  3. Other major systems receiving INS data for stabilisation or computation:
    • Weather or mission radar;
    • Flight management system (FMS);
    • Autopilot.

The weight of this system, which comprised three LTN-92 platforms with back-up battery power supplies, two CDUs and two mode selector units, was in the region of 234 lb.

Figure 8.23 Sensor fusion of air data and inertial sensors.

By integrating the air data information with inertially derived flight information, the best features of barometric and inertial systems can be combined. Figure 8.23 illustrates the principle of sensor fusion where the short-term accuracy of inertial sensors is blended or fused with the long-term accuracy of air data or barometric sensors.

Means of taking external fixes were evolved so that longer-term inaccuracies could be corrected by updating the INS position during long flights. Some fighter aircraft systems such as Tornado also added a Doppler radar such that Doppler-derived data could be included in the navigation process. The availability of on-board digital computers enabled statistical Kalman filtering techniques to be used to calculate the best estimated position using all the sensors available.

The fundamental problem with the INS is the long-term and progressive accrual of navigation error as the flight proceeds, and, irrespective of the quality or type of the gyroscopes used, this fundamental problem remains.

8.8.3 Air Data and Inertial Reference Systems (ADIRS)

The system illustrated in Figure 8.22 utilises stand-alone ADCs; however, the use of ADCs in many new large aircraft systems was superceded by the introduction of air data modules (ADMs) in the late 1980s. The new integrated air data and inertial reference system (ADIRS) developed in the early 1980s combined the computation for air data and inertial parameters in one multichannel unit. As large civil plaftforms are increasingly adapted for use in the transport, air refuelling, anti-surface warfare (ASuW) or surveillance roles, this ‘commercial’ implementation is finding use in military applications.

Taking the B777 as an example, the primary unit is an air data and inertial reference unit (ADIRU) which provides the main source of air data and inertial information. This unit is supported by an attitude and heading reference system (AHARS) which on the B777 is called the secondary attitude air data reference unit (SAARU). This provides secondary attitude and air data information should the primary source, the ADIRU, become totally unusable.

Figure 8.24 B777 ADIRU.

The B777 ADIRU is shown in Figure 8.24. There are six laser rate gyros (LRGs) and six accelerometers included in the unit. It can be seen that both sets of sensors are arranged in a hexad skew-redundant set in relation to an orthogonal axis set. This means that, by resolving the output of each of the six sensors in the direction of the axis set, each sensor is able to measure an element of the relevant inertial parameter – body rate or acceleration – in each axis. This provides a redundant multichannel sensor set with the prospect of achieving higher levels of accuracy by scaling and combining sensor outputs. Additionally, the output of erroneous sensors may be detected and ‘voted out’ by the remaining good sensors. This multiple-sensor arrangement greatly increases the availability of the ADIRU as the performance of the unit will degrade gracefully following the failure of one or more sensors. The ADIRU may still be used with an acceptable level of degradation until a replacement unit is available or the aircraft returns to base and only has to be replaced following the second failure of a like sensor (e.g. second rate or accelerometer sensor). By contrast, the failure of a sensor in an earlier three-sensor, orthogonally oriented set would lead to a sudden loss of the INS.

Coupled with the dual-hexagonal sensor arrangement, there are four independent lanes of computing within the ADIRU, each of which computes a wide range of navigation parameters:

North velocity Corrected computed airspeed
East velocity Corrected Mach number
Ground speed Corrected total pressure
Latitude Corrected static pressure
Longitude CG longitudinal acceleration
Wind speed CG lateral acceleration
Wind direction CG normal acceleration
True heading Flight path acceleration
Magnetic heading Vertical speed
True track angle Roll attitude
Magnetic track angle Pitch attitude
Drift angle Track angle rate
Flight path angle Corrected angle of attack (AoA)
Inertial altitude Roll attitude rate
Computed airspeed Pitch attitude rate
Mach number Heading rate
Altitude rate Body yaw rate
Altitude Body pitch rate
Total air temperature (TAT) Body roll rate
Static air temperature Body longitudinal acceleration
True airspeed Body lateral acceleration
Static pressure (corrected) Body normal acceleration
Impact pressure

Finally, the ADIRU interfaces with the remainder of the aircraft systems by means of triple flight control ARINC 629 digital data buses: left, centre and right. The unit is provided with electrical power from a number of independent sources.

Further information on typical ADIRS implementations is given in the companion volume (Moir and Seabridge, 2003).

8.8.4 Inertial Platform Implementations

There are two methods by which the IN function may be achieved. These are as follows:

  1. Gyrostabilised platform. In the gyrostabilised platform the sensing elements – gyroscopes and accelerometers – are placed on a platform that is itself stabilised to maintain a fixed position in space. This requires fine servomotors and mechanisms to maintain this stabilisation, and consequently gyrostabilised platforms are expensive to manufacture and tend to be unreliable. All of the earlier platforms were implemented in this fashion and many are still in service today.
  2. Strapdown or analytical platform. The advent of digital computation and its application to avionics applications enabled the introduction of the strapdown or analytical platform. In this implementation the sensors are strapped directly on to the body of the vehicle and the necessary axis transformations to convert from the vehicle to space axes are performed numerically using digital computers. Originally this technique was used for military applications; today, virtually all IN platforms work in this way. The benefits of strapdown platforms are that they are easier and cheaper to manufacture and are more reliable as they contain none of the servomotors and mechanisms that are a feature of the gyrostabilised platform. Consequently, they are more reliable by a factor of around 3.

Figure 8.25 Inertial platform implementation.

Both the gyrostabilised platform and the strapdown platform need to undergo a series of axis transformations before they may be used for navigation on the surface of the earth (Figure 8.25). These axis transformations are as follows:

  • Space axes to the Greenwich meridian;
  • Greenwich meridian axes to true north;
  • True north axes to great circle;
  • Great circle axes to vehicle body (strapdown only).

These transformations are fully described in the following pages.

The initial space axes are shown in Figure 8.26. This initial reference shows a generic axis set: X(I), Y(I), Z(I) as a set of axes determined in space by the platform. For ease of reference it may be assumed that the inertial axis Z(I) coincides with the earth axes Z(E) at the outset.

8.8.5 Space Axes to the Greenwich Meridian

Given that Z(I) and Z(E) already coincide, the completion of the earth axis transformation is achieved by rotating the X(I) axis to coincide with the Greenwich meridian by rotating by Ωt to X(E). The bold axes now represent the earth reference set X(E), Y (E), Z(E) (Figure 8.27).

8.8.6 Earth Axes to Geographic Axes

As it is unlikely that the navigation task will begin precisely aligned with the earth axes, account needs to be taken of where the platform is on the surface of the earth when powered up. This process is also known as platform alignment and will be described separately shortly. The alignment process rotates the X axes from X(E) to X(N) by the angle of longitude. The process also aligns Y(G) such that it points north by rotating by the angle of latitude Φ. The outcome is that Z(G) is in line with the local earth vertical (Figure 8.28).

Figure 8.26 Initial space reference axis set X(I), Y(I), Z(I).

Figure 8.27 Space axes to the Greenwich meridian X(E), Y(E), Z(E).

Figure 8.28 Greenwich meridian axes to geographic axes X(G), Y(G), Z(G).

8.8.7 Geographic to Great Circle (Navigation)

By aligning the platform at the wander angle α, the platform may be used to navigate a great circle route which represents the shortest possible path between two points on the surface of the globe (Figure 8.29).

Figure 8.29 True north axes to great circle route (navigation) X(N), Y(N), Z(N).

8.8.8 Great Circle/Navigation Axes to Body Axes (Strapdown)

In the case of the analytical/strapdown platform, the navigation axes X(N), Y(N), Z(N) have to be realigned to the vehicle body axes X(B), Y(B), Z(B) respectively. This is achieved by executing the following rotations in turn:

  • Rotating in the yaw axes by Ψ;
  • Rotating in the pitch axis by θ;
  • Rotating in the roll axis by Φ;

Refer to Figure 8.30.

8.8.9 Platform Alignment

The process of platform alignment follows the processes defined in Figure 8.31. The process is split into the following phases:

  • Entry of the present position latitude and longitude of the platform into the INS.
  • Platform levelling – both course and fine phases.
  • Gyrocompass alignment.

During these processes the sensors are used to sense misalignment between the platform and the desired platform datum. The platform is inched towards these datums over a period of several minutes and, when the datums are reached, the platform is considered to be aligned and in a suitable state to be used for navigation. The platform attitudes for alignment correspond to the navigation axes X(G), Y (G), Z(G) described in Figure 8.31 above.

Figure 8.30 Great circle/navigation to body axes (strapdown) X(B), Y (B), Z(B).

Figure 8.31 Process of platform alignment. Platform Levelling

In the levelling phase the platform sensors and servomotors drive the platform to ensure that the Z direction corresponds to the local vertical at the point on the earth’s surface where the platform is located. Now one of the three platform axes is correctly aligned (Figure 8.32). Gyrocompass Alignment

Gyroscopic compassing uses a similar technique and commences once the fine alignment process is under way. In this case the Y axis is driven to align with true north. In an orthogonal axis set, when the Z axis corresponds to the local earth vertical and the Y axis corresponds to true north, the X axis corresponds to east and the platform is aligned and ready to perform navigation tasks.

Figure 8.32 Aligned platform axes.

Figure 8.33 Historical development of inertial platforms.

The accuracy of navigation depends to some degree upon the accuracy of the alignment process, so, in general, but also within reason, the longer the alignment, the better is the accuracy. Accurate alignments are difficult above ±70° north or south as greater inaccuracies are experienced.

8.8.10 Historical Perspective – Use of Inertial Platforms

The historical development of IN platforms is shown in Figure 8.33. Gimballed technology came into prominence in the 1960s, followed by strapdown in the early 1980s. In both cases the military avionics community was the first to exploit the technology. In the late 1980s and early 1990s the civil community developed the integratedADIRS concept which rapidly became adopted as the primary means of navigation. Meanwhile, the US military in particular were developing the satellite-based global positioning system (GPS). More recently, IN/GPS coupling has been adopted which enables the fusion of IN and GPS sensors in a similar fashion to baro-IN fusion. Both loosely coupled and tightly coupled implementations are commonly used.

Key attributes of the stabilised and strapdown platforms are as follows:

Gimballed platform Gyro dynamic range ∼105
Accelerometer dynamic range ∼106
Calculations undertaken ∼20–30 Hz
Strapdown technology Laser ring gyro (LRG) dynamic range ∼107
Digital computing technology
Calculations undertaken ∼2000 Hz

Table 8.2 Typical INS performance and physical characteristics

Parameter Value
Navigation accuracy 0.8 nm/h
Velocity accuracy 2.5 ft/s rms
Pitch/roll accuracy 0.05° rms
Azimuth accuracy 0.05° rms
Alignment 3–8 min
Volume 4–8 ATR/MCU
Weight 20–30 lb
Power 30–150 W
Acceleration capability 30 g
Angular rate capability 400 deg/s
MTBF (fighter environment) 3500 h
MTBF (civil environment) 10 000 h

The future trend for IN/GPS products is to use cheaper, lower-performance IN sensors, smaller packages and with increasing integration.

The performance and physical characteristics of a typical LRG strapdown performance of 1996 vintage are summarised in Table 8.2.

8.9 Global Navigation Satellite Systems

8.9.1 Introduction to GNSS

Global navigation techniques came into being from the 1960s through to the 1990s when satellites became commonly available. The use of global navigation satellite systems (GNSS), to use the generic name, offers a cheap and accurate navigational means to anyone who possesses a suitable receiver. Although the former Soviet Union developed a system called GLONASS, it is the US ground positioning system (GPS) that is the most widely used. The European Community (EC) is developing a similar system called Gallileo which should enter service in the 2008–2010 timescale. A comparison of the three systems is given in Table 8.3.

GPS is a US satellite-based radio navigational, positioning and time transfer system operated by the Department of Defense (DoD) specifically for military users. The system provides highly accurate position and velocity information and precise time on a continuous global basis to an unlimited number of properly equipped users. The system is unaffected by weather and provides a worldwide common grid reference system based on the earth-fixed coordinate system. For its earth model, GPS uses the world geodetic system of 1984 (WGS-84) datum.

The Department of Defense declared initial operational capability (IOC) of the US GPS on 8 December 1993. The Federal Aviation Administration (FAA) has granted approval for US civil operators to use properly certified GPS equipment as a primary means of navigation in oceanic and certain remote areas. GPS equipment may also be used as a supplementary means of instrument flight rules (IFR) navigation for domestic en-route, terminal operations and certain instrument approaches.

Table 8.3 Comparison of global navigation satellite systems

Soviet Union launched 1982 24 satellites (only 10 in orbit in 2000)
Three planes
Inclination 64.8°
Height 19 130 km
United States – early 1990s 24 satellites (29)
Six planes
Inclination 55°
Height 20 180 km
Europe – 2008–2010 30 satellites (27 + 3)
Three planes
Inclination 55°
Height 23 616 km

8.9.2 Principles of Operation

The principles of satellite navigation using GPS are illustrated in Figure 8.34. GPS comprises three major components as characterised in the figure:

  1. The control segment embraces the infrastructure of ground control stations, monitor stations and ground-based satellite dishes that exercise control over the system.
  2. The space segment includes the satellite constellation, presently around 25 satellites, that forms the basis of the network.
  3. The user segment includes all the users: ships, trucks, automobiles, aircraft and hand-held sets. In fact, anyone in possession of a GPS receiver is part of the user segment.

Figure 8.34 Principles of GPS satellite navigation.

The baseline satellite constellation downlinks data in two bands: L1 on 1575.42 MHz and L2 on 1227.60 MHz. A GPS modernisation programme recently announced will provide a second civil signal in the L2 band for satellites launching in 2003 onwards. In addition, a third civil signal, L5, will be provided on 1176.45 MHz on satellites to be launched in 2005 and beyond. Finally, extra signals for military users (Lm) will be included in the L1 and L2 bands for satellites launched in 2005 and beyond.

GPS operation is based on the concept of ranging and triangulation from a group or constellation of satellites in space which act as precise reference points. A GPS receiver measures distance from a satellite using the travel time of a radio signal. Each satellite transmits a specific code, called course/acquisition (CA), which contains information on the position of the satellite, the GPS system time and the health and accuracy of the transmitted data. Knowing the speed at which the signal travelled (approximately 186 000 miles/s) and the exact broadcast time, the distance travelled by the signal can be computed from the arrival time.

The GPS constellation of 24 satellites is designed so that a minimum of five are always observable by a user anywhere on earth. The receiver uses data from a minimum of four satellites above the mask angle (the lowest angle above the horizon at which it can use a satellite).

GPS receivers match the CA code of each satellite with an identical copy of the code contained in the receiver database. By shifting its copy of the satellite code in a matching process, and by comparing this shift with its internal clock, the receiver can calculate how long it took the signal to travel from the satellite to the receiver. The value derived from this method of computing distance is called a pseudorange because it is not a direct measurement of distance but a measurement derived from time. Pseudorange is subject to several error sources; for example, ionospheric and tropospheric delays and multipath. In addition to knowing the distance to a satellite, a receiver needs to know the exact position of the satellite in space; this is known as its ephemeris. Each satellite transmits information about its exact orbital location. The GPS receiver uses this information to establish precisely the position of the satellite. Using the calculated pseudorange and position information supplied by the satellite, the GPS receiver mathematically determines its position by triangulation. The GPS receiver needs at least four satellites to yield a three-dimensional position (latitude, longitude and altitude) and time solution. The GPS receiver computes navigational values such as distance and bearing to a waypoint, ground speed, etc., by using the known latitude/longitude of the aircraft and referencing these to a database built into the receiver.

8.9.3 Integrity Features

The GPS receiver verifies the integrity (usability) of the signals received from the GPS constellation through a process called receiver autonomous integrity monitoring (RAIM) to determine if a satellite is providing corrupted information. At least one satellite, in addition to those required for navigation, must be in view for the receiver to perform the RAIM function. Therefore, performance of the RAIM function needs a minimum of five satellites in view, or four satellites and a barometric altimeter (baro-aiding) to detect an integrity anomaly. For receivers capable of doing so, RAIM needs six satellites in view (or five satellites with baro-aiding) to isolate the corrupt satellite signal and remove it from the navigation solution.

RAIM messages vary somewhat between receivers; however, generally there are two types. One type indicates that there are insufficient satellites available to provide RAIM integrity monitoring. Another type indicates that the RAIM integrity monitor has detected a potential error that exceeds the limit for the current phase of flight. Without the RAIM capability, the pilot has no assurance of the accuracy of the GPS position. Areas exist where RAIM warnings apply and which can be predicted – especially at higher latitudes – and this represents one of the major shortcomings of GPS and the reason it cannot be used as a sole means of navigation.

8.9.4 GPS Satellite Geometry

The geometry of the GPS satellites favours accurate lateral fixes. However, because a number of the visible satellites may be low in the sky, determination of vertical position is less accurate. Baro-aiding is a method of augmenting the GPS integrity solution by using a non-satellite input source to refine the vertical (height) position estimate. GPS-derived altitude should not be relied upon to determine aircraft altitude since the vertical error can be quite large. To ensure that baro-aiding is available, the current altimeter setting must be entered into the receiver as described in the operating manual.

GPS offers two levels of service: the standard positioning service (SPS) and the precise positioning service (PPS). The SPS provides, to all users, horizontal positioning accuracy of 100m or less with a probability of 95% 300m with a probability of 99.99%. The PPS is more accurate than the SPS; however, this is intended to have a selective availability function limiting access to authorized US and allied military, federal government and civil users who can satisfy specific US requirements. At the moment, the selective availability feature is disabled, making the PPS capability available to all users pending the availability of differential GPS (DGPS) solutions to improve the SPS accuracy. This step has been taken pending the development of differential or augmented GPS systems which will provide high accuracy to civil users while preserving the accuracy and security that military users demand.

The basic accuracy without selective availability is about ±100m as opposed to ±1m when the full system is available. Developments are under way in the United States to improve the accuracy available to civil users. These are:

  • The wide-area augmentation system (WAAS) to improve accuracy en route;
  • The local-area augmentation system (LAAS) to improve terminal guidance.

8.10 Global Air Transport Management (GATM)

The rapidly increasing commercial air traffic density is leading to a pressing need to improve the air transport management (ATM) system by all available means and move on from the techniques and technologies that have served the industry for the last 40 years. This evolution will embrace the use of new technologies mixed with existing capabilities to offer improved air traffic management. The aims and objectives of ATM and a full description of the future air navigation system (FANS) may be found in Chapter 12 of the sister publication ‘Civil Avionics Systems’ (Moir and Seabridge, 2003). GATM is the military version of FANS and has to be compatible in all respects to enable the interoperability of civil and military aircraft within controlled airspace. This section provides a brief overview of some of the key features.

To this end, the air traffic control authorities, airline industry, regulatory authorities and airframe and equipment manufacturers are working to create the future air navigation system (FANS) to develop the necessary equipment and procedures. In order to be able to use controlled airspace on equal terms with commercial users, military platforms will need to embody GATM objectives.

The areas where improvements may be made relate to communications, navigation and surveillance, commonly referred to as CNS. The key attributes of these improvements may be briefly summarised as follows:

  • Communication. The use of data links to increase data flow and permit the delivery of complex air traffic control clearances.
  • Navigation. The use of GPS in conjunction with other navigational means to improve accuracy and allow closer spacing of aircraft.
  • Surveillance. The use of data links to signal aircraft position and intent to the ground and other users.

These headings form a useful framework to examine the GATM improvements already made and those planned for the future.

8.10.1 Communications

The main elements of improvement in communications are:

  • Air-to-ground VHF data link for domestic communications;
  • Air-to-ground SATCOM communications for oceanic communications;
  • High-Frequency data link (HFDL);
  • 8.33. kHz VHF voice communications. Air-to-Ground VHF Data Link

The emergence of data links as means of communications versus conventional voice communications has developed recently in the commercial community; they have long been used for military purposes.

Voice links have been used in the past for communications between the air traffic control system or ATM and the airline operational centre (AOC). The use of data links, controlled and monitored by the FMS or other suitable method on-board the aircraft, facilitates improved communication with the AOC and ATM systems. These data links may be implemented using one or more of the following:

  • VHF communications;
  • Mode S transponder;
  • Satellite links.

Data link communications are being designed to provide more efficient communications for ATC and flight information services (FIS). Although these systems essentially replace voice communication, there will be a provision for voice back-up in the medium term.

Flight plan data, including aircraft position and intent in the form of future waypoints, arrival times, selected procedures, aircraft trajectory, destination airport and alternatives, will all be transferred to the ground systems for air traffic management. The data sent to the ground ATM system will aid the process of predicting a positional vector for each aircraft at a specific time. This information will aid the task of the ground controllers for validation or reclearance of the flight plan of an aircraft. Furthermore, use of the required time of arrival (RTA) feature will enable the air traffic controllers to reschedule aircraft profiles in order that conflicts do not arise.

For ATC flight service and surveillance, VHF data link (VDL) communications will be increasingly used for domestic communications. VHF communications are line-of-sight limited, as has already been explained. A number of options exist:

  1. VDL mode 1. Compatible with existing ACARS transmitting at 2.4 kbps. This mode suffers from the disadvantage that it is character oriented.
  2. VDL mode 2. Data only transmitted at 31.5 kpbs. As well as having a higher bandwidth, this protocol is bit rather than character oriented, making it 50–70% more efficient than the ACARS protocol. VDL mode 2 is able to support controller to pilot data link communications (CPDLC).
  3. VDL mode 3. Simultaneous data and analogue voice communications using time division multiple-access (TDMA) techniques.
  4. VDL mode 4. Used with the 1090 MHz signal of ATC mode S.

It is expected that the introduction of data link technology will benefit all users owing to a more efficient and less ambiguous nature of the messages passed. Significant improvements in dispatch delays and fuel savings are expected as these technologies reach maturity. Air-to-Ground SATCOM Communications

SATCOM is a well proven data link that, as has already explained, is limited at very high latitudes in excess of about 82°. The SATCOM system is supported by the INMARSAT constellation already described earlier in the chapter. HF Data Link

Modern technology enables HF data link transmissions to be more robust than HF voice and therefore less susceptible to the effects of the sunspot cycle. HF data link provides primary coverage out to 2700 nm and secondary coverage beyond that should propagation conditions be favourable. There is extensive cover by ground stations located in the northern hemisphere such that HF data link is a viable alternative to SATCOM for north polar transitions. Refer to the communications and Navaids description earlier. 8.33 kHz VHF Voice Communications

Conventional VHF voice channels are spaced at intervals of 25 kHz throughout the spectrum. A denser communications environment has resulted in the introduction of digital radios that permit spacing at 8.33 kHz, allowing three channels to be fitted in the spectrum where only one could be used previously. With effect from 7 October 1999, these radios have already been mandated in Europe for operation above 20 000 ft and will follow in the United States within a number of years; one of the difficulties in predicting the timescale is the vast number of radios that have to be replaced/retrofitted. Protected ILS

Within Europe some ILS installations suffer interference from high-power FM local radio stations. Modifications have been mandated that introduce receiver changes to protect the ILS systems from this interference.

8.10.2 Navigation

A number of navigational improvements are envisaged:

  1. Introduction of required navigation performance (RNP) and actual navigation performance (ANP) criteria. This defines absolute navigational performance requirements for various flight conditions and compares this with the actual performance the aircraft system is capable of providing.
  2. Reduced vertical separation minima (RVSM).
  3. Differential GPS (DGPS) enhancements:
    • WAAS – described earlier;
    • LAAS – described earlier.
  4. Protected ILS.
  5. Introduction of the microwave landing system (MLS) in Europe. Area Navigation (RNAV)

Area navigation (RNAV) systems allow the aircraft to operate within any desired course within the coverage of station-referenced signals (VOR, DME) or within the limits of a self-contained system capability (IRS, GPS) or a combination of these. RNAV systems have a horizontal two-dimensional capability using one or more of the on-board navigational sensors to determine a flight path determined by navigation aids or waypoints referenced to latitude and longitude. In addition, the RNAV system provides guidance cues or tracking of the flight path. Many modern RNAV systems include a three-dimensional capability to define a vertical flight path based upon altimetry, and some include a full aircraft and engine performance model.

The performance of pre-RNAV systems has historically been defined according to the following criteria:

  • Along-track error;
  • Across-track error;
  • Flight technical error (FTE).

The total navigation error is the root sum square (RSS) of these elements for a given navigation means or phase of flight.

The availability of the navigation capability is defined at 99.999%, and the integrity requirement for misleading navigation information is set at 99.9999%. RNP RNAV and Actual Navigation Performance

The actual navigation performance (ANP) of the aircraft navigation system is represented by a circle that defines the accuracy of the aircraft navigation system for 95% of the time. The value of the ANP is derived by taking the value of all the navigation sensors and statistically weighing them against the other sensors. After a period of time a degree of confidence is established in which are the most accurate sensors and therefore the ANP value is established. The 95% probability circle is that which is compared with RNP to decide whether the navigation system performance is good enough for the route segment being flown. The ANP and RNP values are displayed on the FMS CDU such that the flight crew can readily check on the navigation system status. Should the ANP exceed the RNP value for a given route sector for any reason – for example owing to a critical navigation sensor failing – the crew are alerted to the fact that the system is not maintaining the accuracy necessary. This will result in the aircraft reverting to some lower-capability navigational means. In an approach guidance mode it may necessitate the crew executing a go-around and reinitiating the approach using a less accurate guidance means. Required Navigation Performance (RNP)

The RNP defines the lateral track limits within which the ANP circle should be constrained for various phases of flight. The general requirements are as follows:

  1. For oceanic crossings the RNP is ±12 nm, also referred to as RNP-12.
  2. For en-route navigation the RNP is ±2 nm (RNP-2).
  3. For terminal operations the RNP is ±1 nm (RNP-1).
  4. For approach operations the RNP is ±0.3 (RNP-0.3).

Other specific RNP requirements may apply in certain geographical areas, e.g. RNP-4 and RNP-10 (Figure 8.35).

It is clear that this represents a more definitive way of specifying aircraft navigational performance, versus the type of leg being flown, than has previously been the case. Other more specific criteria exist: RNP-5 (also known as BRNAV or area navigation) has already been introduced in parts of the European airspace with the prospect that RNP-1 (also known as PRNAV or precision navigation) will be introduced in a few years. There are precision approaches in being – notably those in Juneau, Alaska – where RNP-0.15 is required for new precision approaches developed for mountainous terrain. RNAV Standards within Europe

Two RNAV standards are being developed in Europe:

  1. Basic RNAV (BRNAV). BRNAV was introduced in 1988 and is equivalent to RNP-5 for RNAV operations. Navigation may be accomplished by using the following means:
    • DME/DME;
    • VOR/DME with a 62 nm VOR range limit;
    • INS with radio updating or limited to 2 h since the last on-ground position update;
    • LORAN-C with limitation.
    • GPS with limitation.

    Until 2005, primary sources of navigation will be DME/DME, VOR/DME and GPS. Advisory circular AC 90-96 on the approval of US operators and aircraft to operate under instrument flight rules (IFR) in European airspace designated for basic area navigation (BRNAV), 20 March 1998, approves the operation of US aircraft in European airspace under the application of existing advisory circulars.

  2. Precision RNAV (PRNAV). PRNAV is intended to be introduced at some time in the future but not before 2005. PRNAV will invoke the use of navigation under RNP-1 accuracy requirements or better.

Figure 8.35 ANP versus RNP requirements. RVSM

One of the other ways of increasing traffic density is the introduction of the reduced separation vertical minima (RVSM) criteria. For many years aircraft have operated with a 2000 ft vertical separation at flight levels between FL290 and FL410. As traffic density has increased, this has proved to be a disadvantage for the busiest sections of airspace. Examination of the basic accuracy of altimetry indicated that there were no inherent technical reasons why this separation should not be reduced. Accordingly, RVSM was introduced to increase the available number of flight levels in this band and effectively permit greater traffic density. The principle is to introduce additional usable flight levels such that the flight level separation is 1000 ft throughout the band, as shown in Figure 8.36.

Originally a trial was mounted in 1997 to test the viability of the concept on specific flight levels – FL 340 and FL360 as shown in the figure. RVSM is now implemented throughout most of Europe from FL290 to FL410, introducing six new flight levels compared with before. All the specified flight levels on the North Atlantic were implemented in 2001. Other regions in the globe will have RVSM selectively implemented to increase air traffic density according to Figure 8.36 and Table 8.4.

Figure 8.36 RVSM – insertion of new flight levels. RVSM Implementation

At the time of writing, the plans for the worldwide implementation of RVSM are as shown in Table 8.4, and many have been implemented to plan. The Federal Aviation Authority (FAA) RVSM website lists the most recent schedule and level of implementation.

RVSM operation requires the aircraft to possess two independent means of measuring altitude and an autopilot with an accurate height hold capability. The operators of RVSM-equipped aircraft are not taken on trust: independent height monitoring stations survey aircraft passing overhead, measuring actual height compared with flight plan details, thereby assuring the performance of each aircraft and operator. RVSM implementation therefore embraces a watchdog function that ensures that all users are conforming to the RVSM accuracy and performance provisions. Differential GPS Enhancements

DGPS enhancements are being developed for en-route and precision landings in the United States. The GPS enhancements – WAAS and LAAS implementations – in the United States have already been described earlier in the chapter and their introduction should lead to the following accuracies being achieved as a matter of course:

  1. WAAS is anticipated to yield an accuracy of ∼7 m which will be sufficient for Cat I approaches.
  2. LAAS is expected to provide enhanced accuracies of ∼1 m which will be sufficient for precision approaches Cat II and Cat III.

Table 8.4 RVSM implementation schedule – worldwide

RVSM status – Americas and Europe
North Atlantic March 1997 FL330-370
October 1998 FL310-390
24 January 2002 FL290-410
West Atlantic route system (WATRS) 1 November 2001 FL310-390
24 January 2002 FL290-410
Europe tactical (UK, Ireland, Germany, Austria) April 2001 FL290-410
Europe-wide 24 January 2002 FL290-410
South Atlantic 24 January 2002 FL290-410
Canada North domestic April 2002 FL290-410
Canada South domestic Coordinate with US domestic
Domestic US – phase 1a 1 December 2004 FL350-390
Domestic US – phase 2 Late 2005–2006 FL 290-390 (FL410)
Caribbean/South America RVSM group established
RVSM status – Asia/Pacific
Pacific February 2000 FL290-390
Tactical use FL400-410
Australia November 2001 FL290-410
Western Pacific/South China Sea 21 February 2002 Consult publications
Middle East November 2003 TBD
Asia-Europe/South of Himalayas November 2003 TBD

a DRVSM plan to be finalised not later than January 2002 on the basis of ATC simulation results and user inputs.

The introduction of DGPS technology is also envisaged for Europe and the Far East. In Europe there are two programmes in the planning stage that will enhance satellite navigation. The European Space Agency (ESA), the European Commission (EC) and the European organisation for the Safety of Air Navigation (Eurocontrol) are working together on the development of a global positioning and navigation satellite system (GNSS) plan. The GNSS programme is being carried out in two phases:

  1. GNSS-1. This involves the development of the European geostationary navigation overlay system (EGNOS) which will augment the US GPS and Russian GLONASS systems.
  2. GNSS-2. This involves the development of a second-generation satellite navigation system including the deployment of Europe’s own satellite system – Galileo. At the time of writing the EU nations were wrangling about budget increases needed to fund the programme, so delay appears to be inevitable.

8.10.3 Surveillance

Surveillance enhancements include the following:

  • TCAS II;
  • ATC mode S;
  • Automatic dependent surveillance A (ADS-A);
  • Automatic dependent surveillance B (ADS-B).

The operation of TCAS and ATC mode S has already been described in Chapter 7, but their use in a FANS/GATM context will be briefly examined here.

When operating together with a mode S transponder and a stand-alone display or EFIS presentation, TCAS is able to monitor other aircraft in the vicinity by means of airborne interrogation and assessment of collision risk. TCAS II provides vertical avoidance manoeuvre advice by the use of RAs. TCAS II will soon be made mandatory for civil airliners – aircraft with a weight exceeding 15 000 kg or 30 or more seats – operating in Europe. This will be extended to aircraft exceeding 5700 kg or more than 10 seats, probably by 2005.


Moir, I. and Seabridge, A. (2003) Civil Avionics Systems. Professional Engineering Publicating/American Institute of Aeronautics and Astronautics.

Advisory circular AC 121-13, Self-contained navigation systems (long range), 14 October 1969.

Advisory circular AC 120-33, Operational approval for airborne long-range navigation systems for flight within the North Atlantic minimum navigation performance specifications airspace, 24 June 1977.

Advisory circular AC 25-4, Inertial navigation systems (INS), 18 February 1966.

Advisory circular AC 90-94, Guidelines for using global positioning system equipment for IFR en-route and terminal operations and for non-precision approaches in the US national airspace system, 14 December 1994.

Technical standing order (TSO) C-129a, Airborne supplementary navigation equipment using global positioning system (GPS), 20 February 1996.

Advisory circular AC 90-45A, Approval of area navigation systems for use in the US national airspace system, 21 February 1975.

Advisory circular AC 20-130A, Airworthiness approval of navigation or flight management systems integrating multiple sensors, 14 June 1995.

Advisory circular AC 90-97, Use of barometric vertical navigation (VNAV) for instrument approach operations using decision altitude, 19 October 2000.

Advisory circular AC 25-23, Airworthiness criteria for the installation approval of a terrain awareness and warning system (TAWS) for Part 25 airplanes, 22 May 2000.

Advisory circular AC 00-31A, National aviation standard for the very high frequency omnidirectional radio range (VOR)/distance measuring euipment (DME)/tactical air navigation (TACAN) systems, 20 September 1982.

Advisory circular AC 20-121A, Airworthiness approval of LORAN-C navigation systems for use in the US national airspace systems (NAS) and Alaska, 24 August 1988.

Advisory circular AC 00-31A, National aviation standard for the very high frequency omnidirectional radio range(VOR)/distance measuring equipment (DME)/tactical air navigation systems, 20 September 1982.

Advisory circular AC 90-96, Approval of US operators and aircraft to operate under instrument flight rules (IFR) in European airspace, March 1998.

Federal Aviation Authority (FAA) RVSM website: www.faa.gov.ats/ato/rvsm1.htm