Tomtom is lost

[PW]

After getting what appeared to be a reliable setup, I had a new problem this morning.
Basicaly Tomtom hadn't a clue where I was. It displayed a position a few miles from where I was and kept jumping location.
The reason for this was probably the fact that I kept losing the fix from some of the Sats. At most I had 4 red bars + another 3 gray bars in the status window, but it was jumping to between 0->4 in an almost random fashion, that is, it was been effected by building etc. In places where I would normally have 6/7/8 fixes I had hardly any. Even when I did have 4 red bars, it still wasn't sure where I was. I always had the scrolling dots so information was being received.
In my 20 min drive to work, it never worked out my location.
The only 2 things different from yesterday (that I can think of) was the weather today was heavy rain/cloud cover and also I am in the process of changing my car stereo and I currently have the electrics hanging from the dash.

Any thoughts? Do people wait until they get a solid fix before driving off?
I have never had a problem just driving off and letting TT catch up.
And what do the grey GPS status bars indicate?

Pete (TT3, GPS 2.09, TT BT GPS, Ipaq 2210)

[omaruk]

I get this very occassionaly. The steps I take are reseat memory card. Turn power off and then on for GPS unit and finally soft rest the PDA. Should cure it with one of the above

[Tim Buxton]

[PW]

Thanks for the information about the grey bars, although I don't get why it would have some information and not be using it.

Regarding the "lost" problem, I did try a soft reset and to power off the GPS. No change. Didn't try reseating the memory card - but I would have thought that if TT has any maps at all on screen, then it should be ok..
Anyone else seen this and what did you do to resolve it? Will see whether it has fixed itself later on today.

[Tim Buxton]

[DavidW]

The normal reason for the grey bar (at least at startup) is that the GPS hasn't yet got an ephemeris for that particular satellite, so it can't use it in a positioning solution. If you're stationary with a good view of the sky, the necessary data should be got within 45 seconds.


At startup, if a GPS has a valid almanac and an idea where it is (it saves its last position), it stands a pretty good chance of knowing which satellites are overhead. If its clock is also correct, it will know exactly which satellites are overhead.

If the GPS can't find the expected satellites overhead, it will start searching. With less than 32 satellites and typically 12 channels, this normally doesn't take too long - once it finds one satellite, it can get the right time, and if its the time that's the problem, it then knows what satellites it should be listening for.

If the almanac is incomplete, damaged or not present, or the GPS has been moved a long way since it was last used, you may have to wait several minutes for the GPS to find at least one satellite, start downloading the almanac as well as the ephemeris of any satellites it can hear, and eventually it will have enough satellites locked and with an ephemeris that it knows where it is.


Though it's not entirely technically correct, the almanac is a bit like a directory of the satellites (I believe a new almanac is issued weekly, but an old almanac will give a GPS a start). The almanac is enough to know which satellites to listen out for if the GPS knows roughly where it is and what time it is.

The ephemeris for a satellite is a more accurate set of orbital parameters needed to use a satellite for positioning - these go out of date every few hours, which is why when you turn your GPS on after a few hours, you get grey bars - you're waiting for the GPS to get ephemeris information for the satellites it has found so it can use them for positioning.


Driving off without a fix can greatly prolong the time it takes to get a fix. Sometimes the geometry of the satellites overhead can be poor, sometimes it's better than average. I typically have no fewer than five satellites in my positioning solution in all but the most built up areas, and I have seen twelve happen very briefly a couple of times!

A 3D position only needs four satellites, but modern GPSes use what's called an overdetermined solution - if they have more than four satellites available, they'll use as many as it's sensible to use.


I suspect what happened is that you had marginal geometry - probably only four usable satellites overhead, and driving off before you got a fix made it too difficult to acquire a fix on the move. I believe the problem with moving before you have a fix is that it's far harder to receive ephemeris information correctly when you're moving - possibly because if the GPS is unlocked, it can't compensate for the doppler effect. However, that sort of question would have to be answered by someone who has more detailed GPS technical knowledge than I do.



David

[PW]

David,
Thanks for the excellent reply which clears up a number of gaps in my knowledge of how GPS works and the possible causes for my GPS being lost. The response was very comprehensive and much more than I expected. I must go and do a bit of research into almanacs and epherm...wotists :-)
To update the story, I turned on the GPS this evening, sat in the car park for about 3 minutes, and all is hunky dorey.
I was a little concerned over the condition of the receiver when I was getting no green flashing leds at all, even though the GPS dots on the iPaqs status screen were implying it was receiving info from the GPS.
I guess I just didn't expect this type of behaviour. Perhaps I should have been after the GPS took ages to get locked when I moved it from the UK to the US.

Pete

[DavidW]

The dots were scrolling because information was correctly getting from the GPS to the Pocket PC - it's just that that information didn't contain a position fix.

The behaviour you describe indicates that the GPS may well have lost its last position, or at least the time - it would have taken more like a minute if it had accurate last position and time details in its memory (all it's got to do for a 3D position is listen for the satellites it believes are overhead then acquire a lock and an ephemeris for at least four, which can usually be done in just under a minute). Do you happen to know if the batteries had gone flat?


Anyway, as you've seen, it will recover if you leave it switched on and stationary for a few minutes.



David

[PW]

I think the batteries were pretty well charged.

[bagmans911]

This may not help but, I have had a couple of incorrect positions displayed from mine, normally a few miles out in the middle of nowhere! I have 'always display your current GPS position' enabled, but still it occasionally disappears. I have to tap on the GPS indicator (No of Sats tracked and bar chart in map mode) for it to jump to the correct position, once done it works a treat.


IPAQ 3970, Fortuna Clip-on BT, TT3, Lexar 32x 256mb SD card.

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