Rod Speed wrote:
> Atheist Chaplain <abused@[EMAIL PROTECTED]
> wrote:
>> "Doug Jewell" <ask@[EMAIL PROTECTED]
> wrote in message
>> news:48185535$0$9716$5a62ac22@[EMAIL PROTECTED]
>>> 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 pisses me orf.
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