Four years after the first satcom on 747-400, here is a snapshot of the technology, the interfaces, and the applications.
History
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:
· 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
Signal-in-space
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)
Chime
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
Cooling
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
Issues
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
ACRONYM LIST
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