Communications and Navigation Aids 339

VHF Antenna

VHF Transceiver

128.15VHF

Controller

ACARS

Management

Unit

Digital Tuning

Transmitter Keying

AudioACARS

Control Unit

Data

Printer

Flight

Management

System

Figure 9.8 Typical ACARS installation

facility enabled the investigators to recover a modicum of data during the investigation into

the Air France 447 accident before the crash recorders were recovered.

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 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).

9.2.4 SATCOM

Satellite communications provide a more reliable method of communications, originally

using the INternational MARitime SATellite (INMARSAT) organisation satellite constel-

lation which was developed for maritime use. Now satellite communications, abbreviated to

SATCOM, form a useful component of aerospace communications over a range of different

frequency bands and provided by a number of service providers.

The principles of operation of SATCOM are shown in Figure 9.9. The aircraft communicates

via the INMARSAT constellation and remote ground station by means of C-Band uplinks and

340 Civil Avionics Systems

Inmarsat

Satellite

Ground

Earth

Station

AircraftUplink

C-Band

Downlink

C-Band

Uplink

L-Band

Downlink

L-Band

C-Band: 4 - 6 MHz

L-Band: 1530 - 1660 MHz

Figure 9.9 SATCOM principles of operation

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, then

communication may be established.

The airborne SATCOM terminal transmits on frequencies in the range 1626.5 to 1660.5 MHz

and receives messages on frequencies in the range 1530.0 to 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 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/downlink

is 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 from different competitors have been launched more recently. The

INMARSAT satellites are placed in geostationary orbit above the Equator in the locations

shown in Figure 9.10:

• Two satellites are positioned over the Atlantic: AOR-W at 54◦ West and AOR-E at 15.5◦

West.

Communications and Navigation Aids 341

54 W

AOR-W

15.5W

AOR-E

64E

IOR178E

POR

Limitations of SATCOM:

• The geostationary

nature of the INMARSAT

satellite constellation

means that SATCOM is

ineffective at latitudes

greater than 80° North or

80° South, i.e. in the polar

regions

• The aircraft

installation is also a

significant limiting

factor

Figure 9.10 INMARSAT satellite coverage around 2001

• One satellite is positioned over the Indian Ocean: IOR at 64◦ East.

• One satellite is positioned over the Pacific Ocean: POR at 178◦ East.

This represents the coverage provided by the four I-3 satellites launched in the 1996–97

timeframe. In 2005 to 2008, three I-4 satellites were commissioned, which offer a higher

bandwidth Broadcast Global Area Network (BGAN) capability.

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. Due 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 onboard 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 procedures

compatible with future air navigation system (FANS) are developed.

A typical SATCOM system typically comprises the following units:

• satellite data unit (SDU);

• radio frequency unit (RFU);

• amplifiers, duplexers/splitters;

• low gain antenna;

• high gain antenna.

342 Civil Avionics Systems

Satellite Data

Unit

Radio

Frequency

Unit

Beam

Steering

Unit -

Starboard

Beam

Steering

Unit -

Port

<

<

<

<

<

<

<

<

RF

Splitter

A429 (4)

A429 (4)

A429 (2)

A429 (2)

High Gain

Antenna -

Port

High Gain

Antenna -

Starboard

High

Power

Relay

High Power Amp

- Low Gain

Antenna<

Low Gain

Antenna

A429 (2)

High

Power

Amp

High

Gain

Antenna

Figure 9.11 SATCOM system using conformal antennae

A typical SATCOM system as installed on the B777 is shown in Figure 9.11. This example

uses extensive ARINC 429 data buses for control and communication between the major

system elements. This configuration demonstrates the use of two high gain conformal antennae

which are mounted on the upper fuselage at positions approximately ±20◦ respectively from

the vertical. Conformal antennae lie flush with the aircraft skin, offering negligible additional

drag. Alternatively, the system may be configured such that a single top-mounted antenna

may be mounted on the aircraft spine. Both systems have their protagonists and opponents.

Claims and counter-claims are made for which antenna configuration offers the best coverage.

Conformal configurations reportedly suffer from fuselage obscuration dead-ahead and dead-

stern, while the top-mounted rival supposedly suffers from poor coverage at low grazing

angle near the horizon. Whatever the relative merits, both configurations are widely used by

airlines today.

9.2.5 Air Traffic Control (ATC) Transponder

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 enable correlation of a radar track with a specific

aircraft. The principle of transponder operation is shown in Figure 9.12. 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