Tuesday, February 18, 2014

Inmarsat Satellite Communications (1994)

Four years after the first satcom on 747-400,  here is a snapshot of the technology, the interfaces, and the applications.


Airborne satellite communications have been envisioned for 30 years

Mechanically steerable parabolic dishes proved to be impractical

Provisions were provided on first 747 (RA001)

Limited number of installations, none on commercial airplanes

In 1987 Inmarsat started actions to offer an aeronautical service

Breakthrough technologies included:

· phased array antenna
· digital signal processing

AEEC started a satellite subcommittee

RTCA started an AMSS special committee

ICAO started the Aeronautical Mobile Communication Panel

Racal developed the Satfone product for corporate applications, including avionics and an antenna in 1989, providing a single channel of voice through a dedicated handset

Rockwell/Collins began to develop and market the SAT-900, a single channel data system.

Boeing initiated a BFE (buyer furnished equipment) program, offering installation of ARINC 741 compliant avionics and antennas.

Customer interest was high, and commitments to install the SAT-900 were made to about nine airlines.

Ball Aerospace developed a reliable, certificable low gain antenna (LGA)

After several delays, the first commercial satellite data communications system was certificated on a United Air Lines 747-400 in August of 1990, used for ACARS data communications only.

The first commercial satellite voice communications system was type certificated on a Malaysia 747-400 in June, 1993

By this time Honeywell and Racal teamed together to provide a two voice, one data channel system

Boeing led industry activities to enable the FAA to allow voice to be used for air traffic control communications

Other Boeing led first time functionality include:

·      cabin packet data communications (SIA 747-400, Feb 1995)

·      dual (fail/operational) (EAD 777, June 1996)

Basic Functionality

·      flight deck voice

·      flight deck data

·      passenger voice

·      passenger data

·      packet

·      pc modem

·      fax


L band is used to communicate between the airplane and the orbiting satellite

        Rx                       GPS                         Tx

1545  -  1555              1575             1646.5  -  1656.5     Mhz

C band is used to communicate between the orbiting satellite and the ground

Tx 3.700 - 4.200 Ghz

Rx 5.925 - 7.075 Ghz

Channel spacing dependent on data rates

600/1200 bps - 2.5 khz     10.5 kbps - 10 khz    21 kbps - 17.5 khz

Satellite Transponder

The orbiting satellite is simply a transponder, up-converting the transmissions from the airplane and down-converting the transmissions from the ground station (a bent pipe)

Additional functionality, such as orbit maintenance, is coordinated from a prime and backup ground station.

Four satellites provide global coverage

·      Atlantic Ocean Region - West (AOR-W)

·      Atlantic Ocean Region - East (AOR-E)

·      Indian Ocean Region (IOR)

·      Pacific Ocean Region (POR)

Satellite Ground Earth Stations (GES)

Multiple ground stations provide support to each satellite

Each ground station supports packet data and telephony services

Typically, each ground station aligns themselves with a consortium to provide global service

·      Satellite Aircom, Skyphone, Skyways Alliance

·      GLOBALink, SITA (ACARS data networking providers)

Each ground station is fitted with channels units to support transmission and reception

The number of channel units is determined by commercial practices and is not legislated

ABB Nera builds the majority of GESs

Toshiba has built two stations

Each ground station provides a connection to the international public switched telephone network

Each ground station provides a connection to an ACARS packet data network

Each ground station may offer other value added services

·      fax

·      pc data (circuit mode)

·      secure circuit mode data (voice)

·      X.25 packet data (shopping)

·      Broadcast data (news)

The ability to bill for services rendered is a big concern to a GES operator.

Packet data channels

Each station can support multiple packet data channels

Airborne reception limited to one packet channel (P) at any instant of time

Airborne transmission limited to one packet channel (R or T) at any instant of time

Data rates can vary from 600 bps up to 10.5 kbps

All packet channels use one-half rate forward error correction

Effective data rate at most one-half the transmitted rate (300 bps - 5.25 kbps)

Air to ground transmission use shared channels (multiple users of each channel), making the effective data rates much lower

 P channel

Broadcast from a single ground station

Multiple P-channels in use

Packet data transmitted from the ground supporting signaling and other functions

R channel

Packet data transmitted from the airplane

Supports signaling and other limited data transmissions

Slotted Aloha, random access shared channel

Multiple R-channels in use

T channel

Packet data transmitted from the airplane

Time division multiple access (TDMA)

Reservation request made on R channel, granted on P channel

Multiple T channels in use

Circuit mode channel 

A C channel is a pair of frequencies dedicated to a particular "call" between the airplane and the ground.  The ground station assigns the frequency pair.

British Telecom Research Labs (BTRL) 9.6 kbps voice coding algorithm is used

·      21 kbps channel is used

·      19.2 kbps used for voice data (1/2 rate FEC)

·      1.8 kbps used for channel management and signaling

Transmit power is adjusted continuously to maintain a specific bit error rate (BER)

Voice communications rely on a 10-3 BER

Also can support 4.8 kbps fax, 2.4 kbps pc modem data (V.22bis)

Special high performance circuit mode data communications can rely on a 10-5 BER (which takes 2 dB more power)

Used for secure voice, slow scan video, and higher data rate communications

A single installation can support up to five C-channels if enough antenna and transmit gain is available

dual installation can support up to 10 C-channels

High gain phased array antennas

Doubling of data rates requires twice the power (3 dB)

Satellite transmit power is limited

Airplane antenna must provide gain for data rates greater than 1200 bps

Inmarsat requires a 12 dBi antenna for circuit mode applications

Four suppliers provide electrically steerable phased array antennas

·      Conformal side mounted (requires external high power relay switching)

·      Top mounted "multiple array"

·      Top mounted single array

·      Side mounted high gain phased array antenna installation

Beam Steering Unit (BSU) 

Pre-programmed to associate beam selection for any elevation/azimuth

Side mounted antenna BSU provides power division and phase shifting

Top mounted antenna BSU provides beam selection, power, and some other signals

Low Noise Amplifier/Diplexer 

Antenna is used for simultaneous transmit and receive

Diplexer provides necessary band pass filtering to separate the transmit frequency from receive frequency

Must be installed within one foot of the antenna feed connector

Diplexer also filters out spurious noise on transmit path to minimize intermodulation products

Diplexer designed to minimize intermodulation products on GPS L1 frequency (1575 Mhz)

Diplexer does not provide adequate filtering of spurious noise to protect GLONASS reception

Low noise amplifier is used to condition the receive signal for transmission to remotely located RF processing

allows lengthy, light weight coax

High Power Amplifier (HPA)

Provides transmit power

Class C amplifier used for single channel applications (data only)

Class A amplifier used for multi-channel applications

Collins provides 60 watts

Honeywell/Racal provides 40 watts

Thermal considerations are paramount, active cooling a necessity

Minimal losses allowed from the HPA output through the Diplexer and into the antenna

keep the HPA close to the antenna and use low-loss coaxial cable

2.5 dB allowance, but the diplexer and the coax connectors take about .8 dB

Radio Frequency Unit (RFU)

Rockwell Collins uses the RFU in a "traditional" manner to up-convert intermediate frequency transmissions to radio frequency and to down-convert received radio frequency signals to an intermediate frequency

Honeywell/Racal uses the RFU as an expansion unit which provides connectivity to the cabin telecommunication and three additional voice channels. 

Satellite Data Unit (SDU) 

The main processing center

Coordinates all of the SatCom components

Handles all interfaces with external systems

·      MCDU (multipurpose control and display unit)

·      CMC (central maintenance computer)

·      IRU (inertial reference unit)

·      ACARS (airplane communication addressing and reporting system)

·      ICAO (International Civil Aviation Organization) 24 bit code (the address of the airplane)

·      CTU (cabin telecommunications unit, Collins only)

·      Cockpit audio interface

·      EICAS (Engine Indication and Crew Alerting System) messages

·      Cockpit chime

·      Cockpit call lights

·      Log-On Process

The airborne satellite terminal (Aircraft Earth Station, AES) must logon to a GES first

The AES uses a pre-programmed preference table to select candidate GESs

The AES tunes to each candidate GES and evaluates P-channel reception

The AES may consider satellite elevation angle and antenna gain also

Based on all available factors, the AES attempts to logon to the best GES, requesting a particular class of service

The GES will process the request and provide confirmation of logon and class of service to be provided

If the AES does not receive logon confirm, the AES will move onto the next most favorable GES

Once logged on, the AES will remain with that GES until conditions warrant a change, at which point the process begins over with determining the most favorable GES

flying out of the GES coverage

Available resource changes (loss of high gain capability or data capability)

Flight Crew Interface - MCDU 

MCDU provides the flight crew the ability to manage the SatCom system

voice channel status, logon GES, AES identification

automatic logon

MCDU is required in order for the flight crew to initiate or terminate a call

Flight crew can accept a ground to air call without MCDU actions

Ground party can hang up to terminate the call

airborne resources may be tied up typically for 70 seconds, occasionally indefinitely

Flight crew can share voice channels with the passengers

Flight crew can camp-on (wait for the passenger to hang up) or pre-empt the passenger via MCDU commands

Flight crew establish a priority for the call (Emergency, Operational High, Operational Low)

Audio Control Panel (ACP)

Flight crew select SatCom voice channels for transmission and/or for monitoring via ACP action

Call lights alert the flight crew to SatCom channels in use.  Stays on for the duration of the call

Ground to air call is answered when flight crew selects SatCom MIC

if MIC already selected, call is answered when Push to Talk (PTT) is selected

SatCom calls are full duplex

Receive and transmit frequencies are different, allowing simultaneous operation

Flight crew audio system requires PTT for outgoing audio to be transmitted

Flight crew can select speakers to monitor SatCom

Flight crew may use hand held microphone, or oxygen mask microphone if needed

SatCom flight crew audio transmissions are recorded on the cockpit voice recorder (CVR)

Some call events are also recorded on the digital flight data recorder (DFDR)


Aural alerts are necessary to provide timely crew awareness

777 chime is set whenever the call light is lit

chime will go off for ground to air AND air to ground calls (a feature)

All other airplanes, chime is controlled directly by SDU

Chime will go off only for ground to air calls

May be programmed to go off when a camped on ground to air call is processed

Chime also may be used to alert the flight crew when an air to ground call attempt fails

Communications alerting messages

.SATCOM Displayed on EICAS

Used to gain flight crew attention for communications purposes

not related to failures

Flight crew action is to refer to the SatCom MCDU menu for further information

Advisory alerting messages 

Displayed on EICAS to alert the flight crew to the status of functionality

SATCOM                     loss of total SatCom system

SATCOM DATA          loss of SatCom data function only

SATCOM VOICE         loss of SatCom voice function only

SATVOICE LOST        temporary loss of SatCom Voice due to logoff condition/no failure

SATVOICE AVAIL       SatCom voice functionality has been restored

Status messages 

Used to determine the dispatchability of the airplane

SatCom functionality may play a role in determining what type of route may be flown and how much fuel must be carried

Messages are used to identify the failed system, not the affected function

Maintenance messages are used to provide additional details of the failur

SATCOM SYS              Total failure of the SatCom system

SATCOM LGA             Failure of the low gain antenna, or associated components

SATCOM HGA             Failure of the high gain antenna, or associated components

ACARS data 

SatCom can be used to convey ACARS data

Data 2 protocol

considered a reliable link service (RLS)

SatCom will re-transmit the message until it is successfully acknowledged

SatCom provides a link available indication to the ACARS airborne router

·      777 - DCMF

·      others - ACARS Management Unit

Cabin Telecommunications Unit

Principal interface to the cabin telephone system to support circuit mode applications

CEPT-E1 data bus

2.048 Mhz data rate

potential interference source for HF radio reception

special connectors must be used to contain radiated interference

supports 32 (64 kbps) channels

64 kbps allows high quality voice digitization

SatCom must convert to 9.6 kbps BTRL voice coding algorithm

Can support 4.8 kbps group III fax and 2.4 kbps V.22bis pc modem communications

Cabin Packet Communications

X.25 packet data connection between appropriate applications

Typically source is cabin file server

Special physically isolated connection

Priority of data is lowest of all communications

Cost effective means to convey low volume data for administrative or passenger use

·      catalog shopping

·      credit card validation

·      reservation requests

·      on-board inventory management

Protocol is named Data 3


SatCom equipment is installed in the passenger cabin (overhead areas)

Toxic smoke is a concern

Cooling system required to evacuate smoke from the cabin

must use a draw through cooling system

draws contaminants through the equipment too (cigarette smoke)

Use of dual, non-essential lav/galley vent system

Provide backup fans to account for loss of the primary cooling system

vents into the overhead

failure of the cooling system in combination with a SatCom failure producing toxic fumes is not considered likely


Quality of service (voice quality, speed of connection, maintenance of connection, performance)

Reliability of installation (RF cables, intermodulation products)

GPS, GLONASS, TFTS (terrestrial flight telephone system) coupling

Reliability of function/limitations

Timely connections with ATC

knowledge of airplane and air traffic controller phone numbers

A new C-channel (8.4 kbps) and voice coding algorithm (4.8 kbps) is emerging

Inmarsat III satellites are entering service and provide spot beams and more powerful global beams

AERO-I equipment is in development to allow cheaper airborne equipment, dependent on spot beams

New satellite operators are emerging, some offering constellations of satellites in low Earth orbit (LEO), which can utilize much simpler equipment

Reference Documents

AMSS SARPS                International Civil Aviation Council (ICAO) Aeronautical
                                        Communications Panel (AMCP) Aeronautical Mobile Satellite System
                                        (AMSS) Standards and Recommended Practices (SARPS)

ARINC 741                 Defines the form, fit and function of the airborne equipment

RTCA DO-210            Defines Minimum Operational Performance Standards

RTCA DO-222, 231  Provides Guidelines in the use of satellite voice

RTCA DO-215            Defines Emd to End Performance Standards

Inmarsat SDM           Defines the necessary requirements for airborne and ground station equipment.  The basis for use of an Inmarsat satellite


AES                       Aircraft Earth Station

GEO                      Geosynchronous Earth Orbit

GES                      Ground Earth Station

GPS                       Global Positioning System

HGA                      High Gain Antenna

HPA                       High Power Amplifier

LEO                       Low Earth Orbit

LGA                       Low Gain Antenna

LNA/Diplexer        Low Noise Amplifier/Diplexer

RFU                        Radio Frequency Unit

SARPS                   Standards and Recommended Practices

SATCOM               Satellite Communications

SDM                      System Definition Manual

SDU                      Satellite Data Unit

TFTS                    Terrestrial Flight Telephone System