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Garmin 670 - Unimpressed
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philpugh
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Joined: Dec 28, 2005
Posts: 2003
Location: Antrobus, Cheshire
PostPosted: Mon Nov 19, 2007 3:31 pm    Post subject: Reply with quote
Having driven around for a couple of days with max zoom on my 660 running 4.20 and no route calculated - the 'lag' was a couple of seconds out at any time. There weren't too many POIs in view as I live out in the sticks and don't tend to go into towns unless 'forced' Laughing One second is about the best you will get due to GPS update rates. Having read the rest of the posts I would suggest you try to get the unit swapped as your experience is at odds with other's here.
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philpugh
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Joined: Dec 28, 2005
Posts: 2003
Location: Antrobus, Cheshire
PostPosted: Mon Nov 19, 2007 4:54 pm    Post subject: Reply with quote
MaFt wrote:
philpugh wrote:
... it updates about every second...


i was under the impression that GS signals were sent every second anyway so wouldn't expect any higher update frequency than this otherwise the unit would be guessing where you were between signals?

MaFt


MaFt - not sure how the one second interval came about but that is certainly what I believe it to be. It could be due to the fact that as part of the receiving process you get an atomic clock accurate clock on the GPS and a very precise 1Hz signal for timing purposes. A couple of my earlier GPS receivers gave me access to this signal. I guess they just decided to use this as the timing for data output.

The sats transmit navigation information continuously at 50 bits/sec and they piggyback ephemeris and almanac data onto this. Almanac and ephemeris span multiple navigational messages as the volume of data is larger. This is why a cold fix takes time - you need to get lots of info at a slow rate. It takes about 12 mins to get a complete almanac.

Not the place to go into it here but the navigation information is a psuedo-random code and the GPS receiver calculates a time delay for each satellite (and hence a distance to satellite) using the almanac and ephemeris to calculate an absolute position on or near the surface of the earth. So it should be possible for updates more regularly than 1 second - and there are commercial 5Hz units available - but they are expensive as they probably require multiple receivers and processors.
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