Atheist Chaplain wrote:
> how can aGPS help you to lock on to a satellite signal any quicker ??
It's inherent to the way GPS works. They constantly transmit
a signal that includes the time according to the satellite's
atomic clock, and also the satellite's postion. The receiver
uses the time taken for the signals to reach it to determine
the distance from the satellite. Using a bit of math, as
long is it knows the distance to 4 or more satellites, and
the position of those satellites, it can triangulate. But
there's a catch - the satellite transmits this data at a
fairly low bitrate, and so it takes 30 seconds for the whole
data packet to be transmitted. In a best-case scenario, it
will take 30 seconds to get the position information from
the satellite, but if the signal is interrupted, corrupted
etc during this 30 seconds, the GPSr doesn't know where the
satellite is, so it can't compute the position accurately
(or at all). This can cause delays in getting a fix of
sometimes several minutes. Additionally, the satellite
position data can be up to 2 hours old, and it takes approx
12 minutes for the entire catalog of satellite data to be
downloaded. Until this is received, the accuracy of the fix
can be poor.
To speed this process up though, and to get a more accurate
fix, some GPSr's allow you to download predicted satellite
positions. Because the GPSr already knows the satellite
position, it only needs the timecode from the satellite, not
the whole packet. This allows a faster, and more accurate fix.
> that's just another rort by the Telco's to extort money from the
gullible, I
> though aGPS was to download things like voice direction and POI's
(things
> that were intentionally stripped out of the Telco supplied GPS software
so
> they could then charge you for it at exorbitant data transfer fee rates)
>
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