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.
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