SatSig topic:

Why exactly GPS position is needed ?

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Aug 4^th, 2009 at 12:45pm

Hello,

Any1 knows why exactly they need my gps coordinates for the .opt file?

What infulence does it have on the system?

Thank!

Joe

Aug 4^th, 2009 at 1:02pm

Your modem needs to know exactly when to transmit a burst, so that the burst does not overlap with anyone elses burst when it arrives at the satellite. The distance from you to the satellite is critical. The distance is calculated from your lat/long position, the size of the earth and the orbit position of the satellite. The signals go at the speed of light, so 300m distance error = 1 microsecond time error. At any moment there may be many bursts on their way up to the satellite and they must all arrive in sequence, with no overlap. Some burst are longer (green) than others (red).

All the remote sites sharing the return link frequency transmit on the same frequency but at different times. Sites transmit short bursts, for a small percentage of the total time - unless they are very busy !

Read answer here: https://www.satsig.net/topics/satsig-topic-num=1249209172.htm

Read here also: TDMA explanation

Best regards, Eric

Aug 4^th, 2009 at 3:10pm

Thank, you very much for the reply.

Aug 6^th, 2009 at 12:50pm

Eric, hit the nail on the head. It is used to calculate Frame Start Delay (FSD).