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To: 49thMIMOMander who wrote (13924)7/18/2001 7:54:37 AM
From: Clarksterh  Read Replies (3) of 34857
 
llmarinen - From your statements I am reasonably sure that you aren't familiar with how SnapTrack works. The first clue to how it works is in the name. In an ordinary GPS system there are the following problems:

1) The receiver has to find all of the satellite signals each of which is coded differently. This is a long process due to the fact that the GPS signal is so low in power. SnapTrack, true to their name, avoids this problem by having the cell sites tell the receiver all of the info from the satellites in view except the actual timing inherent in the GPS signals. This makes lock-up very fast.

2) The GPS system inevitably has errors. The satellites are not broadcasting the right ephemeris or the right time or .... . This is the primary cause of location errors up to several 10s of meters. SnapTrack fixes this by having the cell basestation use its knowledge of its own location to figure out the local error in the various satellite signals. It then broadcasts this information to the phones so that they correct the satellite signals. This is called differential GPS and allows it to be much more accurate - it has been a long time since I worked this but my memory says the accuracy is about 10 meters.

3) Obviously this system allows significantly greater privacy since the actual determination of position is done by the handset and only when needed. In contrast, triangulation methods require the basestation network to be continuously figuring out the location of the handset.

4) Due to the fact that the basestations are sending out all of the GPS satellite information except the actual timing inherent in the GPS signal itself, the handset needs only the timing. This is repetative (like the ticking of a clock) and thus easy to track even in a building. Thus a SnapTrack system will lock up anywhere a cell phone works unless the basestation with which it is communicating is underground.

Clark
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