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T O P I C R E V I E W
Posted - 11 oct. 2003 : 14:34:47 According to the the Navcen sitenear live report of the 2d Space Operations Squadron (2 SOPS) at Schriever AFB, CO who operates the GPS system there are currently 28 satellites in activity. (10/11/03 at 8.43 EST)
Unlike one might think, GPS satellites are not geostationnary (except the SBAS satellites) as they orbit at about 20,000kms of altitude. This means that depending on your position, at certain times, the "sky" will be unfavorable(poor alignement and elevation of satellites) and it will be difficult for your GPSr to get a "fix".
Posted - 13 oct. 2003 : 18:01:54 Well it doesn't have an extremely intuative user interface, but once you got the clue, it is easy to use.
I found it really useful for planning tests. For example you can easily compare the precision difference depending of the number of sats in view (or depending of DOP values), because you know for every moment how long, how many satellites will be in view for a chosen position. So I imagine it could be useful for some of your surveys too.
It's just important (as I already mentioned) to use a recent ephermis file, because over the period of some days the real sat constellation can remarkably change from the calculated one.
Posted - 13 oct. 2003 : 17:37:07 Thanks, I donwloaded the software some time ago but never installed it. How do you like it?
Posted - 13 oct. 2003 : 17:34:55 There is a free software available from the Trimble website, with which you can "forecast" the theoretical number of satellites in view for a specific position (as well as other parameters, like DOP, etc.)
The concept of GPS was nominally only for 24 satellites, so I think some of the 28 satellites are just for backup, but I'm not sure if the backup satelllites are also in service (marked "healthy") for positioning.
By the way: the constellation of the GPS satellites should be theoretically the same every 23 hours and 56 minutes (but of course changes slighty in practice). The reason for the 4 minute difference to 24h is the celestial time reference used for the GPS satellites orbits.