why gps cordinates needed to setup linkstar modem

The LinkStar return link system uses Time Division Multiple Access (TDMA) for transmissions from remote sites to the teleport hub.

Remotes transmit occasional brief packet bursts, for example when you click the mouse.  If you send a long file you are allocated a series of time slots and then send a series of packet bursts.  The exact timing of all these bursts, as they arrive at the satellite, is critical.  Your transmitter shares the same transmit frequency as other sites, so it is important that your transmit bursts arrive at the satellite at the correct time, so they do not overlap in time with bursts from the other sites.  Each remote terminal is told by the hub when to transmit and this timing is based on the time slot position in the TDMA frame when the burst is intended to arrive at the satellite and also the path length distance from your site to the satellite. The path length distance is based on your lat/long and the current position of the satellite.

A knowledge of the site lat/long helps the initial acquisition of the site at commissioning.  Once an acquisition burst is successfully received at the hub (via a wide, empty time slot), the hub will do a very accurate ranging test. If you move a site your bursts will drift sideways (in time) and will interfere with someone else.  Tell the hub so they can do a ranging update and check your polarisation alignment.

The diagrammatic image below illustrates a highly simplified and very slow TDMA time frame:

Real TDMA frames are normally shorter, with many more bursts per frame from all the different sites. The guard time between bursts allows for small errors path length.

Read more: Explanation of TDMA

Best regards, Eric.