[OSM-talk] GPS receiver orientation

Christopher Woods christof at infinitus.co.uk
Sat Jul 26 14:04:40 BST 2008

> On Jul 26, 2008, at 13:37 , Franc Carter wrote:
> >
> > My entirely annecdotal experience has been that my TomTom 910 takes 
> > longer to get a fix when I am moving than stationary. I have an 
> > external aerial, so the movement should be the main determinent
> The same for me with my Hamlet GPS receiver. I've seen that 
> if I'm moving it could be unable to get a fix indefinitely 
> (tested up to 30 minutes), while stopping and turning it off 
> and then on usually works less than a couple of minutes.

Is that not how GPS works? Give it a nice stable basis to get its initial
fix on, so it can accurately detect and compare the positions of all the
sats it can see - and then it bases any readings from movement using
differential comparison with the baseline from the initial reading... At
least, that's how I've always considered my GPS receivers to operate (using
less power and processor cycles to boot). Maybe there's an occasional
re-poll for an absolute lock and reference, but surely if the receiver was
recalculating its exact position every second, it'd be out of juice within a
few minutes?

I've always noticed my bluetooth GPS receivers (SiRF chipsets, formerly a
Qstarz BT-Q880 and currently a Navman B10) take much longer to establish a
lock when I'm already driving in the car. But then, I suppose it's obvious
that it'd take longer while a vehicle or person is in motion (and not in a
linear direction either). Or am I barking up the wrong tree?

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