Although the follow-on Skynet 3 system never materialized, the Skynet 4 series of spacecraft was successfully deployed during 1988-1990. Built under the direction of British Aerospace and derived from its earlier OTS and CS satellites, Skynet 4 satellites are 3-axis stabilized with an initial on-orbit mass of slightly less than 800 kg. The spacecraft bus dimensions are 1.4 m by 1.9 m by 2.1 m with a 1.2 kW solar array span of 16 m. The communications payload includes three X-band transponders and two UHF transponders.
The SKYNET 4 satellite is 3-axis stabilised, and is based upon the successful ECS design, with a Service Module and a Payload Module. British Aerospace are the prime contractor, and the payload is built by Marconi Space Systems. The design may be engineered for launch into geostationary orbit either by Ariane or by the Shuttle with boost into transfer orbit by payload assist module PAM D-2. In-orbit control is based on a momentum-wheel bias system coupled with sun and earth sensors. Lightweight carbon fibre materials are used for many structural parts. The power supply is regulated in sunlight, and unregulated in eclipse.
The SHF amplifier technology is based on GaAs FETS, with a receive noise temperature of 1000 K, and 40 W T1TAs for the output stages. Four SHF channels are provided, with BWs ranging from 60 to 135 MHz, and single conversion is employed with a translation frequency of 725 MHz. The uplink antenna is an Earth-cover TEll mode corrugated horn in all cases, except where a spot beam is selected (to give *higher gain).
There are a variety of transmit antenna options, with separate higher gain offset-fed reflector antennas serving to concentrate the transmit power over smaller regions, to provide increased capacity with small terminals. In addition to Earth cover, the Wide beam antenna serves interests in the North Atlantic region, the Narrow beam serves the European area, and the Spot beam provides a high EIRP over central Europe, (specifically to serve small Manpack and other tactical army terminals).
A Beacon transmission is provided at SHF, from a solid-state transmitter. It feeds an Earth cover antenna, and may be used by terminals for acquisition and 0 tracking. The SHF payload incorporates several filter units: these include bandpass filters to prevent intermodulation products from multi-carrier signals in one channel falling into adjacent channels, bandstop filters to reject interfering signals at the beacon frequency, bandstop filters to reject transmitted noise and intermodulation products at the receiver frequency, and low pass filters to reject TWTA harmonics.
Two UHF channels are available, each with 25 kHz bandwidth, and operating within the band 305 - 315 MHz (uplink), 250 - 260 MHz (downlink). A common helix antenna is used for both transmit and receive, with a multiplexer unit separating the two. This antenna is relatively large (2.4 m length), and is deployed once the satellite is on station, yet it provides only Earth coverage. The UHF transponder is all solid-state, with each channel delivering 40 W of RF power, and an EIRP of 26 dBW. Receive G/T is -18 dB/K.
An experimental EHF receiver is a feature on SKYNET 4. Operating in the 43 to 45 GHz band, this is an advanced R&D package, originally funded by RSRE as a step towards the future exploitation of the EHF bands.
A self-contained spread-spectrum on-board receiver provides communication facilities at SHF with protection against jamming. A Nulling Antenna facility is also provided for use with the SHF payload. This complex payload may be reconfigured by telecommand, for example to change the SHF antennas. Reliability is a major requirement, and there is considerable redundancy and associated redundancy switching, to allow for substitution of failed units. There are extensive associated telemetry and telecommand systems, supported by dedicated computer facilities in the ground segment.
Because SKYNET 4 aims to provide facilities for all three UK armed services, it is necessarily complex: if a greater number of satellites were provided, each dedicated to a particular user community (as is more common in the US), each might be somewhat simpler. In particular, the close proximity of a number of antennas on the satellite give potential problems of electromagnetic compatibility (EMC), and represents a major design challenge.
The first phase of the SKYNET 4 programme calls for three satellites, the first to be placed at 1°W, the second at 60E, and a spare over the Indian Ocean. The Skynet 4 constellation consists of three spacecraft: Skynet 4A (launched 1 January 1990) located near 326 degrees E, Skynet 4B (launched 11 December 1988) located near 53 degrees E, and Skynet 4C (launched 30 August 1990) located near 1 degrees W. Skynets 4B and 4C were launched by Ariane, whereas Skynet 4A was launched by the U.S. Titan 3. With design lifetimes of only seven years, two more Skynet spacecraft are scheduled for launch later in this decade: 4D and 4E in 1997-1998. A variant of the Skynet 4 spacecraft has also been flown under the NATO 4 series. A pan-European military communications system, perhaps as Skynet 5, is under consideration for launch soon after the turn of the century (References 417-419).
The follow on generation Skynet 4, with six satellites providing global secure communications. Designed to operate in geostationary orbit for 15 years and still providing coverage today. Skynet 4 is the longest serving satellite constellation at 29 years old and continues to deliver vital communications.
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