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why gps cordinates needed to setup linkstar modem

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Ex Member

Aug 2nd, 2009 at 11:32am  
Why dose the sevice provider need the gps cordinates to setup the modem? Can i move the modem to another site after being locked on? They say i cannot move more than 30 km ist it so?

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Eric Johnston
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Reply #1 - Aug 2nd, 2009 at 12:38pm  
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.
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« Last Edit: Aug 2nd, 2009 at 1:48pm by Eric Johnston »  
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Ex Member

Reply #2 - Aug 2nd, 2009 at 1:01pm  
Hi Cris thanks for the explanation how will i know if the time frame is dynamic, the one sp from south africa said it is not a problem to move the modem they can just send a log file to the modem at the new site. The other sp is in namibia and uses an hub in italy i think. I need to set up a site in far north angola and want to use the namibian sp as it is cheaper but they do nt want to go into angola i think it is because of licences,can you help with sugestions.

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