A United Kingdom military communications satellite, SkyNet 5A, was launched on an Ariane-5ECA (V175)/535 rocket on March 11, 2007 from French Guyana. Skynet 5A carries an international designator 2007-007A/SSC#30793. Skynet 5A will operate in geostationary orbit at 1 degree west in the Clarke Belt.The Skynet-5A (and 5B) satellites were designed by the European company Astrium SAS in its Stevenage and Portsmouth production centres in the UK, and integrated in Toulouse, for Astrium Ltd/Paradigm Secure Communications based in Stevenage, under the terms of a turnkey contract with the British Ministry of Defence. Paradigm will supply secure high-speed voice and data telecommunications facilities to the British Armed Forces, NATO and other countries. Skynet-A payloads were manufactured at the Astrium site in Porthmouth (UK). Final assembly and testing were carried out in the Toulouse plant (France). Skynet-5A and 5B will replace the current Skynet-4 satellites, also built by EADS Astrium for Paradigm Secure Communications, owner and operator of the spacecraft.
Skynet-5A is the first of the series 5 satellites to be launched by Arianespace, following the series 4 spacecraft. It is also the 26th military payload to fly on the European launcher.
Skynet 4B was launched in December 1988 on flight 27 on an Ariane 44LP, with Astra 1A,
Skynet 4C was launched in August 1990 on flight 38 on an Ariane 44LP, with Eutelsat II F1,
Skynet 4E was launched in February 1999 on flight 116 on an Ariane 44L, with Arabsat 3A,
Skynet-4F was launched in February 2001 on flight 139 on an Ariane 44L, with Sicral.
Skynet -5A forms part of a 3-satellite (Skynet-5A, 5B and 5C) communications system.
This system provides links with terminals on the ground, at sea and on board aircraft, with protected transmission for voice messages and high-speed digital data transmissions. When initially placed in service, the system will operate under the control of the US Air Force Satellite Control Facility (AFSCF) at the Sunnyvale station in the USA. Control will then be transferred to the British RAF ground segment at Oakhanger (with redundancy provided by the Sunnyvale facilities for a given period).
To facilitate manufacture, transport and assembly, the satellite structure has two independent platforms round a central tube on which the payload equipment is installed. The lower, Service Module (SM) carries the greater part of the Attitude Determination and Control System (ADCS), and the power units. This module is substantially derived from the Eurostar 3000 generic platform. The upper, Communication Module (CM) carries most of the communication equipment.
A Liquid Propellant apogee Engine (LAE) is used for orbit circularisation. Four identical tanks (two MMH and two NTO) are secured symmetrically round the central tube and supply the LAE mounted on the central cone, and the seven modules housing the main, redundant nozzles. The helium pressurisation tank is installed inside the central tube. The Service Module equipment is mounted on the North and South panels for the thermal dissipation units. The Chemical Propulsion System (CPS) is mounted on the bottom of the SM.
The North and South panels of the Communication Module (-Y, +Y) carry the travelling wave tubes (TWT) and other high thermal dissipation equipment, and like the Service Module panels also provide a heat-sink function. The batteries, which also require efficient temperature control, are mounted on the side walls of the SM.
The Communication Module (CM) carries two helicoidal UHF antennas. These will be deployed under remote control when the satellite is in geostationary orbit. The central part of the CM also carries an S band telemetry and telecommand (TTC) deployable mast antenna.
Electric power supply for the satellite is delivered by two solar panels, folded during the launch (on ± Y) and deployed after 3-axis stabilisation of the satellite (the outer panel of each wing is deployed one hour after separation from the launcher). Energy is supplied by a Li-lon cell battery in transfer orbit and during eclipses.
In-orbit pointing of the satellite is controlled by the ADCS, a simple and reliable system. Low-thrust (10N) thrusters are used in the pulse or continuous mode for housekeeping and attitude control operations.
