1975 ONWARDS (continued)

The requirement to provide for a remote area broadcasting service was a key driver in the system design, as the service had to operate into small inexpensive earth stations and be able to service the different time zones across the country. This led to a design featuring four high-gain, shaped spot beams, tailored to match the intended service requirements and population distributions. Each satellite has the capability to provide four transmit spot beams, namely the Western Australia beam (WA), the Central Australia beam (NT, SA), the North-East beam (QLD) and the South-East beam (NSW, VIC, TAS). Additionally there are two national transmit and two national receive beams on each satellite. As Papua New Guinea (PNG) maintained an active interest in receiving services via Aussat during the system development, the system was designed to provide dedicated uplink and downlink beams covering PNG. At a late stage in the satellite construction phase it was decided to modify the third in the series to provide uplink and downlink beams covering the South-west Pacific nations, including PNG, to provide an opportunity for the establishment of pilot domestic communications systems within the island nations. Overall the Aussat satellites have a complex antenna system providing ten independent uplink and downlink beams, with the third satellite equipped with a total of twelve beams.

The communications payload on board each spacecraft has 15 active communications channels, each having a minimum bandwidth of 45 MHz with a channel spacing of 64 MHz. The repeaters are arranged in a frequency re-use scheme employing orthogonal linear polarizations, orientated so that nominal horizontal and vertical polarizations are received over Australia. The diversity of applications identified for an Australian national satellite system, ranging from direct broadcasting to telephony and data links, led to the system being designed with both standard power amplifiers for fixed satellite services and high power amplifiers for broadcasting satellite services. There are eleven active standard power amplifiers (nominally 12 Watts) channels 1 to 6 in Repeater A and 9 to 13 in Repeater B, and four active high power amplifiers (nominally 30 Watts), being channels 7 and 8 in Repeater A and 14 and 15 in Repeater B. To ensure that the high payload reliability targets are met, two spare amplifiers of each type are carried, with ground commandable switching in the event of an amplifier failure.

A key feature of the Aussat payload is the substantial channel/beam connectivity provided, with 31 channel/beam combinations available to ensure adequate flexibility to meet changing market demands. The use of both high power and standard power channels, the complex antenna system and the extensive interconnectivity options have resulted in one of the most complex and advanced domestic communications satellite systems built up to that time.

Australian Academy of Technological Sciences and Engineering