Co-location (satellite)

Co-location is the placing of two or more geostationary communications satellites in orbit, in close proximity so that to reception equipment on the ground they 'appear' to occupy a single orbital position. The technique as applied to a group of TV satellites from a single operator was pioneered by SES with the Astra satellites at 19.2°E.

Communications satellites' orbital positions are normally spaced apart along the geostationary orbit to provide for frequency reuse for both uplink and downlink transmissions. By separating adjacent satellites by a distance greater than the at-orbit beamwidth of the uplink antennas, the same carrier frequencies can be used to uplink to both satellites without interference. Similarly, a sufficient separation so that the beamwidth of the receiving dishes on the ground can distinguish one satellite from its neighbours, allows the same frequency spectrum to be used for adjacent satellite downlinks.

Communications satellites are positioned in orbital 'slots' allocated under international treaty by the ITU and a separation between slots of 2° or 3° of orbital longitude is common.

Co-location of two satellites has long been used on a temporary basis for the transfer of services from a retiring satellite to a replacement satellite, and for pairing of two partially malfunctioning satellites to operate as a single satellite. The planned co-location of two satellites was used by the TDF-1 and TDF-2 and TV-Sat 1 and TV-Sat 2 DBS satellites launched 1987-1990 to provide national DBS broadcasting to France and Germany.

The first co-location of a group of satellites (reaching eight in number) was by SES with the Astra satellites at 19.2°E. Astra 1A was launched in 1988 and followed by Astra 1B (1991), Astra 1C (1993), Astra 1D (1994), Astra 1E (1995), Astra 1F (1996), Astra 1G (1997), Astra 1H (1999), all co-located at the same orbital slot of 19.2°E (although Astra 1D was only co-located with all the other satellites at this position for short periods).

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