7 Communications and Identification – Military Avionics Systems

Communications and Identification

7.1 Definition of CNI

All military aircraft need certain computing sensing and computing resources apart from the mission sensors and weapons to enable them to complete their mission. These are:

  1. Communications. The ability to be able to communicate by either voice or data link means with cooperative forces, be it wingmen in the same flight of aircraft, airborne command centre or troops on the ground.
  2. Navigation. The military platform needs to be able to navigate with sufficient accuracy to a target, rendezvous point, waypoint, or initial point as dictated by the mission requirements.
  3. Identification. The rules of engagement for a given theatre of operation will necessitate the classification and identification of a target before permission to engage is given.

The American military refer to this collection of resources as communications, navigation, identification (CNI).

Some of the CNI sensors such as air data, radar altimeters and inertial systems are autonomous to the platform, in other words the platform needs no other input from third-party sources. Others such as communications, radio navigation beacons and global navigation satellite systems (GNSSs) require the participation of other organisations to respond or the provision of a network of aids or a constellation of satellites to provide the navigational framework. Military platforms use a combination of all these sensors with the additional rider that, for certain covert stages of a mission, no emissions are made by the platform as radio silence – more correctly known as emission control (EMCON) – procedures are enforced.

Figure 7.1 CNI RF spectrum.

7.1.1 RF Spectrum

The RF spectrum associated with the CNI functions is shown in Figure 7.1. The CNI spectrum covers a range of different equipment spanning almost five decades from 100 kHz to 4 GHz and comprising a range of functions as described below. Communications and identification are addressed within this chapter while navigation is discussed in Chapter 8. For ease of reference, the equipment is listed in ascending order of operational frequency:

  1. Communications:
    • High-frequency (HF) communications;
    • Very high-frequency (VHF) communications;
    • Ultrahigh-frequency (UHF) communications;
    • Satellite communications (SATCOM);
    • Data links.
  2. Identification:
    • Air traffic control (ATC) mode S;
    • Traffic collision and avoidance system (TCAS);
    • Identification friend or foe (IFF).

With one or two exceptions, this equipment is all freely available for use by the civil community as well as by the military platform. All operational frequencies are published on aeronautical charts to ensure safe and successful integration and interoperability of all traffic within the wider airspace. There are a few exceptions, namely:

  1. Civil traffic does not usually use the UHF communications band. Military users may also use UHF SATCOM which is not widely available.
  2. Civil traffic would not ordinarily be equipped with TACAN.
  3. Certain GPS codes offering more accurate navigation capabilities may be denied to the civil user community.
  4. IFF is compatible with ATC modes S but not available to civil users.

7.1.2 Communications Control Systems

The control of the aircraft suite of communications systems, including internal communications, has become an increasingly complex task. This task has expanded as aircraft speeds and traffic density have increased and the breadth of communication types have expanded. The communications control function is increasingly being absorbed into the flight management function as the management of communication type, frequency selection and intended aircraft flight path become more interwoven. Now the flight management system can automatically select and tune the communications and navigation aids required for a particular flight leg, reducing crew workload and allowing the crew to concentrate more on managing the on-board systems.

7.2 RF Propagation

The number of antennas required on-board an aircraft to handle all the sensors, communications and navigation aids is considerable. The CNI aspects of RF systems integration on a fighter aircraft have already been described in Chapter 2.

Civil aircraft adopted for military applications also have a comprehensive CNI antenna complement. This is compounded by the fact that many of the key pieces of equipment may be replicated in duplicate or triplicate form. This is especially true of VHF, HF, VOR and DME equipments. Figure 7.2 shows typical antenna locations on a Boeing 777 aircraft; this is indicative of the installation on most civil aircraft operating today, particularly those operating transoceanic routes. Owing to their operating characteristics and transmission properties, many of these antennas have their own installation criteria. SATCOM antennas communicating with satellites will have the antennas mounted on the top of the aircraft so as to have the best coverage of the sky. ILS antennas associated with the approach and landing phase will be located on the forward, lower side of the fuselage. Others may require continuous coverage while the aircraft is manoeuvring and may have antennas located on both upper and lower parts of the aircraft; multiple installations are commonplace. In addition to these antennas, military aircraft will have additional communications fitted commensurate with their military role.

Figure 7.2 Typical aircraft CNI antenna (Boeing 777 example).

In aviation, communications between the aircraft and the ground (air traffic/local approach/ground handling) have historically been by means of voice communication. More recently, data link communications have been introduced owing to their higher data rates and in some cases superior operating characteristics. As will be seen, data links are becoming widely used in the HF, VHF and UHF bands for basic communications but also to provide some of the advanced reporting features required by FANS. In the military community, data links have a particular significance in relation to target reporting and the sharing of tactical and targeting information, as will be described in the section on network-centric operations. The most common methods of signal modulation are:

  1. Amplitude modulation (AM). This type of modulation concentrates the information being carried by the transmission in relatively narrow sidebands. AM communications are susceptible to noise and jamming.
  2. Frequency modulation (FM). FM modulation is more sophisticated and spreads the transmission across a wider frequency spectrum than AM, thereby reducing the vulnerability of the signal to interference and jamming. This technique is generically known as spread spectrum modulation and can be used in a number of ways using differing modulation techniques. The spread spectrum is a useful adjunct to low probability of intercept (LPI) systems where the intention is to make the task of an adversary detecting the signals more difficult.

Figure 7.3 HF communications signal propagation.

There are many extremely sophisticated methods of signal modulation. In the military environment these are used to maximise the performance of the signal under adverse operating conditions while minimising the probability of intercept.

7.2.1 High Frequency

High frequency (HF) covers the communications band between 3 and 30 MHz and is a very common communications means for land, sea and air. The utilised band is HF SSB/AM over the frequency range 2.000–29.999 MHz using a 1 kHz (0.001 MHz) channel spacing. The primary advantage of HF communications is that this system offers communication beyond the line of sight. This method does, however, suffer from idiosyncrasies with regard to the means of signal propagation.

Figure 7.3 shows that there are two main means of propagation, known as the sky wave and the ground wave.

The sky wave method of propagation relies upon single- or multiple-path bounces between the earth and the ionosphere until the signal reaches its intended location. The behaviour of the ionosphere is itself greatly affected by radiation falling upon the earth, notably solar radiation. Times of high sunspot activity are known adversely to affect the ability of the ionosphere as a reflector. It may also be affected by the time of day and other atmospheric conditions. The sky wave as a means of propagation may therefore be severely degraded by a variety of conditions, occasionally to the point of being unusable.

The ground wave method of propagation relies upon the ability of the wave to follow the curvature of the earth until it reaches its intended destination. As for the sky wave, the ground wave may on occasions be adversely affected by atmospheric conditions. Therefore, on occasion, HF voice communications may be corrupted and prove unreliable, although HF data links are more resistant to these propagation upsets, as described below.

HF communications are one of the main methods of communicating over long ranges between air and ground during oceanic and wilderness crossings when there is no line of sight between the aircraft and ground communications stations. For reasons of availability, most long-range civil aircraft are equipped with two HF sets with an increasing tendency also to use HF data link if polar operations are contemplated.

HF data link (HFDL) offers an improvement over HF voice communications owing to the bit encoding inherent in a data link message format which permits the use of error-correcting codes. Furthermore, the use of more advanced modulation and frequency management techniques allows the data link to perform in propagation conditions where HF voice would be unusable or incomprehensible. An HFDL service is provided by ARINC using a number of ground stations. These ground stations provide coverage out to ∼2700 nm and on occasion provide coverage beyond that. Presently, HFDL ground stations are operating at the following locations (Figure 7.4):

  1. Santa Cruz, Bolivia.
  2. Reykjavik, Iceland.
  3. Shannon, Ireland.
  4. Auckland, New Zealand.
  5. Krasnoyarsk, Russia.
  6. Johannesburg, South Africa.
  7. Hat Yai, Thailand.
  8. Barrow, Alaska, USA.
  9. Molokai, Hawaii, USA.
  10. Riverhead, New York, USA.
  11. San Francisco, California, USA.
  12. Bahrain.
  13. Gran Canaria, Canary Islands.

7.2.2 Very High Frequency

Voice communication at very high frequency (VHF) is probably the most heavily used method of communication used by civil aircraft, although ultrahigh frequency (UHF) is generally preferred for military use. The VHF band for aeronautical applications operates in the frequency range 118.000–135.975 MHz with a channel spacing in past decades of 25 kHz (0.025 MHz). In recent years, to overcome frequency congestion, and taking advantage of digital radio technology, channel spacing has been reduced to 8.33 kHz (0.00833 MHz) which permits 3 times more radio channels in the available spectrum. Some parts of the world are already operating on the tighter channel spacing – this will be discussed in more detail later in the chapter in the section on global air transport management (GATM).

Figure 7.4 HF data link ground stations.

Figure 7.5 VHF signal propagation.

The VHF band also experiences limitations in the method of propagation. Except in exceptional circumstances VHF signals will only propagate over line of sight. That is, the signal will only be detected by the receiver when it has line of sight or can ‘see’ the transmitter. VHF transmissions possess neither of the qualities of HF transmission and accordingly neither sky wave nor ground wave properties apply. This line-of-sight property is affected by the relative heights of the radio tower and aircraft. This characteristic applies to all radio transmissions greater than ∼100 MHz, although the precise onset is determined by the meteorological conditions prevailing at the time (Figure 7.5).

The formula that determines the line-of-sight range for VHF transmissions and above is as follows:

where R is the range (nautical miles), Ht is the height of the transmission tower (ft) and Ha is the height of the aircraft (ft).

Therefore, for an aircraft flying at 35 000 ft, transmissions will generally be received by a 100 ft high radio tower if the aircraft is within a range of around 235 nautical miles.

Additionally, VHF and higher-frequency transmissions may be masked by terrain, by a range of mountains, for example. These line-of-sight limitations also apply to equipment operating in higher-frequency bands and mean that VHF communications and other equipment operating in the VHF band or above – such as the navigation aids VOR and DME – may not be used except over large land masses, and then only when there is adequate transmitter coverage. Most long-range aircraft have three pieces of VHF equipment, with one usually being assigned to ARINC communications and reporting system ACARS transmissions, although not necessarily dedicated to that purpose. The requirements for certifying the function of airborne VHF equipment are given in Advisory Circular AC 20-67B (1986), while RTCA DO-186 (1984) specifies the necessary minimum operational performance standards (MOPS). There are advanced techniques that may be used in sophisticated military equipment that mitigate against these fundamental limitations. Such systems are said to possess an over-the-horizon (OTH) capability.

A number of VHF data links (VHFDL) may be used, and these are discussed in more detail later in the chapter. ACARS is a specific variant of VHF communications operating on 131.55 MHz that utilises a data link rather than voice transmission. As will be seen during the discussion on future air navigation systems, data link rather than voice transmission will increasingly be used for air-to-ground, air-to-ground and air-to-air communications as higher data rates may be used while at the same reducing flight crew workload. ACARS is dedicated to downlinking operational data to the airline operational control centre. The initial leg is by using VHF communications to an appropriate ground receiver, thereafter the data may be routed via land-lines or microwave links to the airline operations centre. At this point it will be allowed access to the internal airline storage and management systems: operational, flight crew, maintenance, etc.

All aircraft and air traffic control centres maintain a listening watch on the international distress frequency: 121.5 MHz. In addition, military controllers maintain a listening watch on 243.0 MHz in the UHF band. This is because the UHF receiver could detect the harmonics of a civil VHF distress transmission and relay the appropriate details in an emergency (second harmonic of 121.5 MHz × 2 = 243.0 MHz; these are the international distress frequencies for VHF and UHF bands respectively).

7.2.3 Satellite Communications

Satellite communications provide a more reliable method of communications using the International Maritime Satellite Organisation (INMARSAT) satellite constellation which was originally developed for maritime use. Now satellite communications, abbreviated to SATCOM, form a useful component of aerospace communications. In addition there are dedicated and secure military satellite systems not addressed in this book for obvious reasons. In this publication, SATCOM is described as it uses similar principles of operation and is also used in conjuction with the global air transport management (GATM) described later.

The principles of operation of SATCOM are shown in Figure 7.6. The aircraft communicates via the INMARSAT constellation and remote ground earth station by means of C-band uplinks and downlinks to/from the ground stations and L-band links to/from the aircraft. In this way, communications are routed from the aircraft via the satellite to the ground station and on to the destination. Conversely, communications to the aircraft are routed in the reverse fashion. Therefore, provided the aircraft is within the area of coverage or footprint of a satellite, communication may be established.

The airborne SATCOM terminal transmits on frequencies in the range 1626.5–1660.5 MHz and receives messages on frequencies in the range 1530.0–1559.0 MHz. Upon power-up, the radio-frequency unit (RFU) scans a stored set of frequencies and locates the transmission of the appropriate satellite. The aircraft logs on to the ground earth station network so that any ground stations are able to locate the aircraft. Once logged on to the system, communications between the aircraft and any user may begin. The satellite to ground C-band uplink and downlink are invisible to the aircraft, as is the remainder of the earth support network.

The coverage offered by the INMARSAT constellation was a total of four satellites in 2001. Further satellites are planned to be launched in the near future. The INMARSAT satellites are placed in earth geostationary orbit above the equator in the locations shown in Figure 7.7.

Figure 7.6 SATCOM principles of operation.

Figure 7.7 INMARSAT satellite coverage – 2001.

Table 7.1 SATCOM configurations

Configuration Capabilities
Aero-H/H+ High gain. Aero-H offers a high-gain solution to provide a global capability and is used by long-range aircraft. Aero-H+ was an attempt to lower cost by using fewer satellite resources. Provides cockpit data, cockpit voice and passenger voice services
Aero-I Intermediate gain. Aero-I offers similar services to Aero-H/H+ for medium- and short-range aircraft. Aero-I uses the spot beam service
Aero-C Version that allows passengers to send and receive digital messages from a PC
Aero-M Single-channel SATCOM capability for general aviation users
  1. Two satellites are positioned over the Atlantic: AOR-W at 54° west and AOR-E at 15.5° west.
  2. One satellite is positioned over the Indian Ocean: IOR at 64° east.
  3. One satellite is positioned over the Pacific Ocean: POR at 178° east.

Blanket coverage is offered over the entire footprint of each of these satellites. In addition there is a spot beam mode that provides cover over most of the land mass residing under each satellite. This spot beam coverage is available to provide cover to lower-capability systems that do not require blanket oceanic coverage.

The geostationary nature of the satellites does impose some limitations. Owing to low grazing angles, coverage begins to degrade beyond 80° north and 80° south and fades completely beyond about 82°. Therefore, no coverage exists in the extreme polar regions, a fact assuming more prominence as airlines seek to expand northern polar routes. A second limitation may be posed by the performance of the on-board aircraft system in terms of antenna installation, and this is discussed shortly. Nevertheless, SATCOM is proving to be a very useful addition to the airborne communications suite and promises to be an important component as future air navigation system (FANS) procedures are developed.

A number of different systems are offered by SATCOM as described in Table 7.1. A SATCOM system typically comprises the following units:

  • Satellite data unit (SDU);
  • Radio-frequency unit (RFU);
  • Amplifiers, diplexers/splitters;
  • Low-gain antenna;
  • High-gain antenna.

7.3 Transponders

There are a number of different interrogators and transponders used on military aircraft. These are as follows:

  1. Distance measurement equipment (DME) is used as a navigation aid for both civil and military aircraft (see Chapter 8).
  2. Tactical air navigation (TACAN) is used as a navigation aid for military aircraft solely (see Chapter 8).
  3. ATC mode S is used both on military and civil aircraft, usually in association with the traffic collision avoidance system (TCAS). ATC mode S and TCAS are described below.
  4. Automatic dependent surveillance – address mode (ADS/A) is used to support FANS and GATM developments during oceanic crossings using HF communications.
  5. Automatic dependent surveillance – broadcast mode (ADS/B) is used to support FANS and GATM developments over land using VHF communications.
  6. Identification friend or foe (IFF) is used by the military specifically for identification of threat aircraft. IFF is compatible with ATC mode S and works on the same frequencies of 1090 MHz (TX or interrogator) and 1090 MHz (RX or transponder) but carries additional identification codes specifically for military purposes.

DME and TACAN are described under the Communications and Navaids section while ATC mode S is described under the GATM section of Chapter 8.

7.3.1 Air Traffic Control (ATC) Transponder – Mode S

As a means to aid the identification of individual aircraft and to facilitate the safe passage of aircraft through controlled airspace, the ATC transponder allows ground surveillance radars to interrogate aircraft and decode data which enables correlation of a radar track with a specific aircraft. The principle of transponder operation is shown in Figure 7.8. A ground-based primary surveillance radar (PSR) will transmit radar energy and will be able to detect an aircraft by means of the reflected radar energy – termed the aircraft return. This will enable the aircraft return to be displayed on an ATC console at a range and bearing commensurate with the aircraft position. Coincident with the primary radar operation, a secondary surveillance radar (SSR) will transmit a series of interrogation pulses that are received by the on-board aircraft transponder. The transponder aircraft replies with a different series of pulses which give information relating to the aircraft, normally aircraft identifier and altitude. If the PSR and SSR are synchronised, usually by being co-boresighted, then both the presented radar returns and the aircraft transponder information may be presented together on the ATC console. Therefore, the controller will have aircraft identification (e.g. BA 123) and altitude presented alongside the aircraft radar return, thereby greatly improving the controller’s situational awareness.

Figure 7.8 Principle of transponder operation.

The system is also known as identification friend or foe (IFF)/secondary surveillance radar (SSR), and this nomenclature is in common use in the military field. On-board the aircraft, the main elements are as listed below:

  1. ATC transponder controller unit for setting modes and response codes.
  2. A dedicated ATC transponder unit.
  3. An ATC antenna unit with an optional second antenna. It is usual to utilise both upper and lower mounted antennas to prevent blanking effects as the aircraft manoeuvres.

The SSR interrogates the aircraft by means of a transmission on the dedicated frequency of 1030 MHz which contains the interrogation pulse sequence. The aircraft transponder replies on a dedicated frequency of 1090 MHz with a response that contains the reply pulse sequence with additional information suitably encoded in the pulse stream.

In its present form the ATC transponder allows aircraft identification – usually the airline call-sign – to be transmitted when using mode A. When Mode C is selected, the aircraft will respond with its identifier together with altitude information.

More recently, an additional mode – mode S or mode select – has been introduced with the intention of expanding this capability. In ATC mode S the SSR uses more sophisticated monopulse techniques that enable the aircraft azimuth bearing to be determined more quickly. Upon determining the address and location of the aircraft, it is entered into a roll call file. This, together with details of all the other aircraft detected within the interrogator’s sphere of operation, forms a complete tally of all the aircraft in the vicinity. Each mode S reply contains a discrete 24 bit address identifier. This unique address, together with the fact that the interrogator knows where to expect the aircraft from its roll call file, enables a large number of aircraft to operate in a busy air traffic control environment (see section 7.3.2 for details of the traffic collision avoidance system).

ATC mode S has other features that enable it to provide the following:

  • Air-to-air as well as air-to-ground communication;
  • The ability of aircraft autonomously to determine the precise whereabouts of other aircraft in their vicinity.

Mode S is an improved conventional secondary radar operating at the same frequencies (1030/1090 MHz). Its ‘selectivity’ is based on unambiguous identification of each aircraft by unique 24 bit addresses. This acts as its technical telecommunications address, but does not replace the mode A code. There are also plans for recovery of the A and C codes via mode S.

Apart from this precise characterisation of the aircraft, mode S protects the data it transmits owing to the inclusion of several parity bits which means that up to 12 erroneous bits may be tolerated by the application of error detection and correction algorithms. For transmission, these parity bits are superimposed on those of the mode S address.

Finally, mode scan may be used to exchange longer, more varied data streams, which can even be completely unplanned. To do this, mode S transmissions between the station and the transponder use highly sophisticated 56 or 112 bit formats called frames. They fall into three main categories: 56 bit surveillance formats, 112 bit communication formats with a 56 bit data field, which are in fact ‘extended’ surveillance formats (uplink COMM-As and downlink COMM-Bs), and 112 bit communication formats with an 80 bit data field (uplink COMM-Ds and downlink COMM-Ds). This feature will be of use in facilitating the transmission and interchange of flight plans dynamically revised in flight which is one of the longer-term aims of FANS.

Mode S also has the capability of providing a range of data formats, from level 1 to level 4. These are categorised as follows:

  1. Level 1. This is defined as the minimum capability mode S transponder. It has the capability of reply to mode S interrogations but has no data link capability. All the messages provided by level 1 are short (56 bit) messages.
  2. Level 2. These transponders support all the features of the level 1 transponder with the addition of standard length data link word formats. This can entail the use of longer messages (112 bit). Some of the messages are used for TCAS air-to-air communication while others are utilised for air-to-ground and ground-to-air communication as part of the enhanced surveillance data access protocol system (DAPS) requirements.
  3. Level 3. The level 3 transponders embrace the same functionality as level 2 with the additional ability to receive extended length messages (ELM) which comprise 16 segments of information, each containing a 112 bit message.
  4. Level 4. Level 4 has the full functionality of level 3 with the capability of transmitting ELM messages of up to 16 segments of 112 bit word messages.

Originally it was envisaged that ATC mode S would be the primary contender to provide the CNS/ATM functionality by providing large block transfers of information. More recently it has been realised that VDL mode 4 might better serve this need, and levels 3 and 4 are no longer required.

When used together with TCAS, ATC mode S provides an important feature for FANS, that of automatic dependent surveillance – A (ADS-A). This capability will assist the safe passage of aircraft when operating in a direct routing mode.

7.3.2 Traffic Collision and Avoidance System

The traffic collision and avoidance system (TCAS) was developed in prototype form during the 1960s and 1970s to provide a surveillance and collision avoidance system to help aircraft avoid collisions. It was certified by the FAA in the 1980s and has been in widespread use in the United States in its initial form. The TCAS is based upon a beacon interrogator and operates in a similar fashion to the ground-based SSR already described. The system comprises two elements: a surveillance system and a collision avoidance system. The TCAS detects the range bearing and altitude of aircraft in the near proximity for display to the pilots.

The TCAS transmits a mode C interrogation search pattern for mode A and C transponder equipped aircraft and receives replies from all such equipped aircraft. In addition, the TCAS transmits one mode S interrogation for each mode S transponder equipped aircraft, receiving individual responses from each one. It will be recalled that mode A relates to range and bearing, while mode C relates to range, bearing and altitude and mode S to range, bearing and altitude with a unique mode S reply. The aircraft TCAS equipment comprises a radio transmitter and receiver, directional antennas, computer and flight deck display. Whenever another aircraft receives an interrogation it transmits a reply and the TCAS computer is able to determine the range depending upon the time taken to receive the reply. The directional antennas enable the bearing of the responding aircraft to be measured. The TCAS can track up to 30 aircraft but only display 25, the highest-priority targets being the ones that are displayed.

The TCAS is unable to detect aircraft that are not carrying an appropriately operating transponder or that have unserviceable equipment. A transponder is mandated if an aircraft flies above 10 000 ft or within 30 miles of major airports; consequently, all commercial aircraft and the great majority of corporate and general aviation aircraft are fitted with the equipment.

The TCAS exists in two forms: TCAS I and TCAS II. TCAS I indicates the range and bearing of aircraft within a selected range; usually 15–40 nm forward, 5–15 nm aft and 10–20 nm on each side. The system also warns of aircraft within ±8700 ft of the aircraft’s own altitude.

The collision avoidance system element predicts the time to, and separation at, the intruder’s closest point of approach. These calculations are undertaken using range, closure rate, altitude and vertical speed. Should the TCAS ascertain that certain safety boundaries will be violated, it will issue a traffic advisory (TA) to alert the crew that closing traffic is in the vicinity via the display of certain coloured symbols. Upon receiving a TA, the flight crew must visually identify the intruding aircraft and may alter their altitude by up to 300 ft. ATA will normally be advised between 20 and 48 s before the point of closest approach with a simple audio warning in the flight crew’s headsets: ‘TRAFFIC, TRAFFIC’. TCAS I does not offer any deconfliction solutions but does provide the crew with vital data in order that they may determine the best course of action.

TCAS II offers a more comprehensive capability with the provision of resolution advisories (RAs). TCAS II determines the relative motion of the two aircraft and determines an appropriate course of action. The system issues an RA via mode S, advising the pilots to execute the necessary manoeuvre to avoid the other aircraft. An RA will usually be issued when the point of closest approach is within 15 and 35 s, and the deconfliction symbology is displayed coincident with the appropriate warning.

A total of ten audio warnings may be issued. Examples are:


Finally, when the situation is resolved: ‘CLEAR OF CONFLICT’.

Figure 7.9 TCAS architecture showing related equipment and displays.

TCAS II clearly requires a high level of integration between the active equipment. Figure 7.9 shows the interrelationship between:

  • TCAS transmitter/receiver;
  • ATC mode S transponders;
  • VSI display showing vertical guidance for TAs and RAs;
  • Optional horizontal situational indicator for RAs that could be the navigation display;
  • Audio system and annunciators;
  • Antennas for ATC mode S and TCAS.

This is indicative of the level of integration required between ATC mode S transponders, TCAS, displays and annunciators. It should be noted that there are a variety of display options and the system shown does not represent the only TCAS option.

More recently, further changes have been introduced to TCAS II – known as TCAS II change 7. This introduces software changes and updated algorithms that alter some of the TCAS operating parameters. Specifically, change 7 includes the following features:

  • Elimination of nuisance warnings;
  • Improved RA performance in a multiaircraft environment;
  • Modification of vertical thresholds to align with reduced vertical separation minima (RVSM) – see the section on global air transport management (GATM) and the civil equivalent future air navigation system (FANS);
  • Modification of RA display symbology and aural annunciations.

The change 7 modifications became mandatory in Europe for aircraft with 30 seats or more from 31 March 2001 and for aircraft with more than 19 seats from 1 January 2000. The rest of the world will be following a different but broadly similar timescale for implementation. Change 7 is not mandated in the United States but it is expected that most aircraft will be equipped to that standard in any case. Further information can be found on AC-12955A, RTCA DO-181 DO-185 certification and performance requirements for TCAS II and mode S.

7.3.3 Automatic Dependent Surveillance – Address Mode (ADS-A)

ADS-A will be used to transmit the aircraft four-dimensional position and flight plan intent based upon GPS position during oceanic crossings. The communications media will be SATCOM or HF data link (HFDL). ADS-A requires the aircraft to be fitted with an FMS and CDU and with some means of displaying message alerts and annunciation.

7.3.4 Automatic Dependent Surveillance – Broadcast Mode (ADS-B)

ADS-B will be used to transmit four-dimensional position and flight plan intent based upon GPS position using line-of-sight VHF communications. Either mode S or digital VHF radio will be used to transmit the data. ADS-A requires a cockpit display of traffic information.

7.3.5 Identification Friend or Foe (IFF)

There are two ways in which IFF equipment may be used:

  • Providing 360° coverage in order to be able to respond to interrogation and receive transponder returns from friendly aircraft in any direction. In this respect the operation is very similar to the airborne operation of ATC mode S when used in association with the TCAS.
  • Used in association with a primary radar sensor in order to be able specifically to identify targets appearing within the radar scan. This operates in the same way as a ground surveillance radar interrogating aircraft in the vicinity of an airfield.

An example of the first type is the advanced IFF (AIFF) AN/APX-113(V) used on-board the F-16 aircraft but which is typical of equipment of this type (Figure 7.10). IFF equipment is sometimes referred to in a generic sense as IFF mark XII which relates the generic family of present IFF equipment. Equipment such as the AN/APX-113(V) has the following capabilities:

Figure 7.10 IFF set AN/APX-113(V). (BAE SYSTEMS)

Figure 7.11 Co-boresighting of IFF interrogator with radar.

  • Multiple antenna configurations – electronic or mechanical scan;
  • MIL-STD-1553 interface to connect to the rest of the avionics system units;
  • Ability to provide encryption capability;
  • Provision of growth to accommodate future functional modes.

The use of the interrogator co-located and co-boresighted with the main radar creates a problem as illustrated in Figure 7.11. The primary radar will be operating at a higher frequency than the 1090/1030 MHz that the interrogator uses. An airborne early warning radar will be operating at ∼3 GHz, while an airborne intercept (AI) radar will be operating higher still at ∼10 GHz. For a given radar antenna size the beamwidth is inversely proportional to frequency, so, the higher the frequency, the more narrow is the beamwidth. The IFF beamwidth will therefore encompass the main beam and several sidelobes of the radar beam and may therefore be receiving returns from targets that are not of fundamental interest to the radar. This effect will be more pronounced for AI as opposed to AWACS radar.

7.4 Data Links

The use of voice was the original means of using RF communications. However, the use of speech has severe limitations; it is slow in terms of conveying information and prone to misunderstanding, whereas high bandwidth data links can delivery more information, if necessary incorporating error correction or encryption. In the avionics sense, typical data link users are portrayed in Figure 7.12.

Primary users include:

  • Strategic airborne sensor platforms such as E-4, E-6, E-8, Global Hawk and satellites;
  • Tactical airborne sensors and shooters – F-15, F-16, F-18, Harrier, Tornado, Eurofighter Typhoon and tactical UAVs/UCAVs among others;
  • Shipborne sensors;
  • Land forces.

Figure 7.12 Typical data link users.

Many of the data links are limited to line-of-sight operation owing to the transmission characteristics of the RF frequencies being employed. However, the use of communications satellites to perform a relay function permits transmission of data over the horizon (OTH), thereby enabling intra- and intertheatre communications.

Typical data packages that may be delivered by data links include:

  • Present position reporting;
  • Surveillance;
  • Aircraft survival, EW and intelligence information;
  • Information management;
  • Mission management;
  • Status.

The primary data links used for communications between airborne platforms and space and surface platforms are as follows:

  1. Link 16. This is the most commonly used avionics data link and is usually manifested in avionics systems as the joint tactical information distribution system (JTIDS). The JTIDS is also compatible with the US Navy data link satellite tactical data information link J (S-TADIL J). Link 16 operates in the UHF frequency band in the same frequency range as identification friend or foe (IFF), distance measurement equipment (DME) and TACAN, as described below.
  2. Link 11. Certain strategic aircraft assets such as E-4 and Nimrod MRA4 associated with joint maritime operations also have the capability of operating with link 11 – a data link commonly used by naval forces.

Figure 7.13 JTIDS frequency band.

7.4.1 JTIDS Operation

The characteristics of the JTIDS frequency band and how this is shared with other equipment is shown in Figure 7.13. The JTIDS characteristics are as follows:

  1. Data are transmitted in the UHF band between 969 and 1206 MHz.
  2. Frequency-hopping techniques are employed to provide ECM jam-resistant properties.
  3. A total of 51 channels are provided at 3 MHz spacing.

JTIDS transmissions are constrained to avoid interfering with the IFF frequencies at 1030 MHz (TX) and 1090 MHz (RX), and JTIDS is not employed within ±20 MHz of these frequencies. The problem of mutual equipment interference is one that has to be frequently faced on highly integrated military avionic platforms.

Integration with the host aircraft avionics system usually takes the form shown in Figure 7.14. The JTIDS terminal and associated antenna are shown on the right of the diagram. The equipment shown in this particular case is the URC-138 terminal, a typical example of which is shown in the inset. Such a terminal – compatible with tanker/transport, fighter and helicopter environments will have the following capabilities/characteristics:

Data rate 28.8–238 kbps
Weight 40 lbs
Dimensions 12.5 in deep
7.5 in high
10 in wide
(equivalent to 8ATR)
Power 750 W

Figure 7.14 JTIDS integration with the avionics system.

The JTIDS terminal typically interfaces with the host aircraft mission system computer via MIL-STD-1553B data buses. The host mission systems computer is connected in turn to the aircraft platform sensors embracing radar, electrooptics, ESM/EW and other operational sensors depending upon the host platform sensor fit.

Such a system will in most cases include secure voice capabilities and the ability to transmit encrypted data. Clearly, in a real battlefield scenario there is a need to share classified information between some but certainly not all of the participants. With the data that many participants will utilise in a communications systems used in a military environment, therefore, there needs to be the capability of separating secure/classified data from the data that are ‘in the clear’ or open to all participants.

7.4.2 Other Data Links

Apart from JTIDS which specifically operates in the UHF band, other transmission techniques may be used to communicate between military platforms. These are:

  • HF data links;
  • Local cooperative data links.

SATCOM and HF data links or HFDL are already used extensively by the maritime and civil aviation communities. The same transmission capabilities are open to the military community, except that in many cases data protection/encryption may be required depending upon the sensitivity of the message content. Therefore, many military communications systems are designed to include an encryption/decryption device at the front end – between the processor and transmit/receive elements. By using suitable encryption ‘keys’, the necessary levels of encryption may be achieved depending upon the sensitivity of the message content.

Aircraft such as the F-22 Raptor use a local cooperative data link to aid in the data sharing and coordination of a group of aircraft embarked upon a shared mission. As outlined in Chapter 2, on the F-22 it was the intention to utilise two phased array related cooperative data links. These are:

  • A common high-band data link (CHBDL) or in-flight data link (IFDL) operating at around 10 GHz and utilising three antenna locations to pass data between adjacent aircraft;
  • A cooperative engagement capability (CEC) using similar antenna configurations.

It is not clear whether either or both of these facilities have been included in the final aircraft configuration.

7.5 Network-centric Operations

Network-centric operations are becoming the latest ‘force multiplier’ element of modern airborne warfare in the same way as air-to-air refuelling and the availability of digital signal processing have been in the past. The use of high-bandwidth digital communications, together with sophisticated signal processing capabilities and the high bandwidth of internal platform interconnective buses and highways, have enabled the data interchange between a variety of sensor and weapons platforms to ascend to much higher levels. The command and control (C2) structure, sensor and weapon delivery platforms have become integrated at a level that would previously have been unimaginable. This connectivity, allied with the capability of ultrahigh-resolution sensors, enables target and threat data to be shared at all levels of the force structure with unprecedented speed and fidelity.

The nature of network-centric operations may be appreciated by reference to Figure 7.15. This figure illustrates three tiers of interconnected centres or nodes, each of which are interconnected at the three respective levels and which are also interconnected between levels or layers by specific network nodes. In the figure, interconnecting nodes between layers are shown in black while supporting nodes within a layer are portrayed in white.

The network comprises three layers, which in descending order of importance/authority are as follows:

Figure 7.15 Nature of network-centric operations.

  1. Weapons control layer. This embraces a limited number of participants – 20 or fewer – all operating at the strategic level, all interconnected within the layer and also connected to subordinate layers to implement force control. It is at this level that the rules of engagement (RoE) for a particular task execution will be decided and implemented. Airborne platforms in this category may include platforms such as AWACS or similar airborne assets.
  2. Force control layer. This layer exercises control over the force structure, implementing the RoE and engaging targets on the basis of sensor and tactical information exchange. This joint data network will be typified by link 11 and link 16 users exchanging data at the tactical level and deciding the priorities according to different target types depending upon geographical and time currency of intelligence and target data. This layer may include up to 500 users including strategic and tactical force assets. Aircraft platforms may include maritime reconnaissance and fighter aircraft.
  3. Force coordination layer. This layer embraces a joint planning network invoking force coordination at the local or theatre level and exercising force coordination to achieve maximum force effect or to avoid ‘blue-on-blue’ fratricide engagements. In the aviation context this may include fighter aircraft, attack helicopters, UAVs and forward air controllers (FAC). This network may extend across 1000 users or contributors.

It is noteworthy that the nature of the information changes as it migrates from the lowest to the highest level within this hierarchy according to the simple tabulation below:

Layer Information timecales Information accuracy
Weapons control Subseconds High accuracy
Force control Seconds ^
Force coordination Minutes ^
Low accuracy

Figure 7.16 Sensor/shooter information grid.

The doctrines associated with network-centric operations have many proponents, especially in the United States. One of the most celebrated and vocal proponents is USN Vice Admiral Arthur K. Cebrowski together with J.J. Garstka (1998).

Key elements to the operation of a network-centric operation relate to the information grid interrelationship between the ‘sensors’ and ‘shooters’ involving the command and control element (Figure 7.16). This depicts the information flow and command links that exist between the detection of a target on the left to the engagement of the target on the right. It embraces the overlapping nature of information and engagement grids that determine the process by which information is processed between sensor, command and control and shooter to ensure that the necessary information is provided to the command function in order that a target may be correctly assessed, command and control may be exercised and battle damage assessment may be accomplished.

The high-bandwidth communications available for intelligence and target data interchange between these functional entities mean that radar video or electrooptic images may be exchanged in near real time. Therefore, the decision time to identify, categorise and authorise target engagement has reportedly decreased from hours (1991 Gulf War) to tens of minutes (Afghanistan War), with the aim of reducing this to a matter of minutes in future conflicts.

In the avionics environment this information exchange is achieved by using data links using a series of transmission means as described elsewhere in this chapter. On-board the airborne platform the availability of high-bandwidth fibre-optic or fibre-channel communications as described in Chapter 2, Technology and Architectures.


Advisory circular AC 20-67B, Airborne VHF communications installations, 16 January 1986.

Advisory circular AC 20-131A, Air worthiness approval of traffic alert and collision avoidance systems (TCAS II) and mode S transponders, 29 March 1993.

Advisory circular 129-55A, Air carrier operational approval and use of TCAS II, 27 August 1993.

Cebrowski, A.K. and Garstka, J.J. (1998) Network-centric warfare: its origin and future. Proceedings of Naval Institute, January.

RTCA DO-181, Minimum operational performance standards for air traffic control radar beacon system/mode select (ATCRBS/mode S) airborne equipment.

RTCA DO-185, Minimum operational performance standards for traffic alert and collision avoidance systems (TCAS) airborne equipment.

RTCA DO-186, Minimum operational performance standards (MOPS) for radio communications equipment operating with the radio frequency range 117.975 to 137.000 MHz, dated 20 January 1984.