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Who owns a GPS?
Rod Speed wrote:
> Atheist Chaplain > wrote: >> "Doug Jewell" > wrote in message >> ... >>> 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) >>>> >> while that may be true I have yet to see a time when my Navman GPS >> doesn't see at least 5 satellites and currently my Tom tom phone >> system is tracking 9 sats. Admittedly I'm not in the concrete jungle >> of ****ney but even when I am, the only time I have lost signal was >> when I was in one of the tunnels. to be fair I have never been >> driving so fast in ****ney that the GPS couldn't keep up with my >> position so I fail to see any benefit from aGPS. > > The most obvious difference is seen with the time it takes for > the GPS to decide its got enough info to tell you were to go > next. Thats most noticeable when driving out of a parking area > etc after the GPS has not been able to see any satellites for quite > a while, usually because its been turned off for quite a while. > > And aGPS isnt the only way to reduce that time too. Agreed - and it ****es me orf. |
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