Safety Investigation Report 2018:1 Factual Information/1.9/1.9.5 Satcom/1
SAFETY INVESTIGATION REPORT MH370 (9M-MRO)
1.9.5 Satellite Communications
1) Satellite Communications System Description
Satellite Communications (SATCOM) is an acronym of, and generic term for, satellite communications. SATCOM operates by using satellites to relay radio signals between the sender and receiver. It can cover far more distance and wider areas than other radios. SATCOM can be used to transmit words, pictures and other forms of information.
The aircraft, 9M-MRO, was equipped with a SATCOM terminal that used the Inmarsat Classic Aero system. The Inmarsat system utilises a constellation of satellites to provide nearly global coverage, the exception being polar areas. The aircraft SATCOM system, also referred to as an Airborne Earth Station (AES) operates on L Band, transmits at 1.6 GHz and receives at 1.5 GHz. For this aircraft, the SATCOM system provided a total of five voice channels and one data channel. The satellite link provides the following functions:
The Earth or Ground Station uses C Band, transmits at 6 GHz and receives at 4 GHz. Inmarsat uses a network of Ground Earth Stations (GES) to communicate with the satellites and connect the SATCOM signal to other terrestrial data networks such as telephone systems, internet, etc.
When the SATCOM AES is first powered on, it sends a log-on request to the GES to initiate service.
There are a number of channels available for messages to be sent between the Satellite and Earth Station. One of the channels is called the ‘common access channel’, which aircraft will constantly listen to when able to do so.
If the GES has not heard from an aircraft for an hour after the last communication, it automatically transmits a ‘log on interrogation’ (“ping”) message on the common access frequency using the aircraft’s unique identifier. If the aircraft receives its ‘unique identifier’, it returns a short message that it is still logged onto the network. Both the initial log-on request and the hourly ping have been termed as a ‘handshake'.
The SATCOM AES consists of the following equipment: Radio frequency unit (RFU), Radio frequency attenuator (RF ATTN), Radio frequency splitter (RFS), Class C high power amplifier (HPA), Class A high power amplifier (HPA), High power relay (HPR), three low noise amplifier/diplexers (LNA/DIPs), Low gain antenna (LGA), two beam steering units (BSUs), two high gain antennas (HGAs), Radio frequency combiner (RFC) and Satellite data unit (SDU).
The SATCOM avionics are located on the E11 rack, which is in the crown area aft of doors 3 left/right. The High Gain antennas are mounted above door 3 left and door 3 right. The Low Gain antenna is mounted on the fuselage centreline. The SATCOM Circuit Breakers (CB) are located in the Main Equipment Center (MEC).
The Satellite Data Unit (SDU) receives 115V AC from the Left Main bus. In flight, this bus can be powered by engine mounted generators or the APU generator. Neither the aircraft battery nor the ram air turbine will power the SATCOM system.
The diagram in Figure 1.9K (below) shows the complete set of SATCOM units, including avionics, High Gain Antenna Subsystem and Low Gain Antenna Subsystem. It also shows interfaces to the aircraft cockpit and cabin systems and functions. The following notes are intended to be read in conjunction with Figure 1.9K (below):
- a) CDU (3) are the three Control Display Units, otherwise known as Multi-function Control Display Units (MCDUs).
- b) CPMU is Cabin Passenger Management Unit, which provides an interface between the Panasonic IFE and the SDU, for any Data-3 SMS/e-mail messages.
- c) AMU is the Audio Management Unit, which feeds cockpit audio to and from the SDU.
- d) CTU is the Cabin Telecommunications Unit, which provides an interface between the in-seat handsets and the SDU, for cabin telephony calls, were that functions available. In the case of 9M-MRO, the in-seat phones can only be used for seat-to-seat calling.
- e) AIMS Cabinet is one of two Airplane Information Management System cabinets, which route numerous information to and from the SDU, including ACARS data, Navigational data, AES ID and Flight ID.
- f) SATCOM Maintenance Switch is not relevant to this document, as no maintenance activity is possible in flight.
Figure 1.9K - SATCOM System
The photo in Figure 1.9L (below) shows the Honeywell/Racal (Honeywell/Thales) MCS-6000 SATCOM Units - RFU (left), SDU (centre) and HPA (right).