GPS (the military version, the PLGR) is one of the items I give classes on here at the School house. Here are a few items that GPS companies don't tell allot of people about.
1) The NAVSTAR sats transmint on 2 bands the first the P-codes can be recieved by everyone, the second the Y-codes can only recieved by systems that have cryto-fills. The P-codes are less accurate than the Y-code, with a military GPS recieving P and Y codes you can get accuracy down to +/- 10 meters on good days.
2) The military at anytime can decide to go to "selective availability" When this happens civilian receivers will be accurate to +/- 1000 to 2000 meters. Those units that recieve Y-codes will not be effective.
3)GPS transmission is LOS, Line of Sight, if your receivers loses LOS of the required number of sats you start loosing accuracy. Normally 3 sats are required for location, 4 for altitude, but many of the newer recievors do parallel tracking, so the more sats you can track the more accurate your position will be. Many types of terrain can block the GPS signals, some examples are a thick canopy of trees, canyon walls, buildings, etc. So in many tyoes of terrain you are SOL.
4. GPS direction should not really be relied upon, it has been found that direction option from GPS receivers can be up to 200 milaradians out (1 milaradion is equal to 1/17.77777778 of a degree). So you should also carry a compass to use as your method to determine direction.
5. Most GPS are battery hogs, constantly tracking satellites will drain your battteries fast, the other option is operating in the "Quick Fix" get your new pos and shut it off.
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God truly fights on the side with the best artillery