bob_nestor wrote:
Actually the ephemeris data does tell the GPS unit where the satellites are in the sky and that speeds the processing done to correctly locate your ground position. The ephemeris data "ages" due the the satellites not being in an absolute geosynchronous orbit. It is good for about two weeks. The updated data can be downloaded to the ground unit from the satellites or preloaded. Usually on an initial setup of a GPS unit that doesn't have current data it will take longer to acquire the satellites and compute your ground position. GPS units that have been more recently used will tend to acquire quicker because they have more up to date information. That's why on some units (like military GPS) they pre-load the units before going on a mission. They're loading current ephemeris data and may also load the decode keys to access the higher precision signals not available to non-military users. But you are correct in how the GPS uses time and signal phase differences to compute ground position.
Again, very interesting. I always felt that it helped speed up acquisition, but you guys are of differing opinions, so I'm just trying to learn all I can.
WoodGlue
