Loss of signal

[r6man]

Apologies if this is a noob question but I've had a couple of occasions recently when my dash mounted Haicom gps mouse has lost signal completely despite being out on a main road with no large buildings, no overhanging trees etc. Within a few minutes its picked up again and been fine for the rest of the journey. Is this normal?
I generally get 5-7 sats picked up but quite often see it as low as 4. Again is this ok? Its in a vw passat with no heated screen so i expected higher.

Its seems to pick up noticabley fewer in the evening/night time too? Any reason for that or is it just my over active imagination :| !?

[lbendlin]

Aliens are coming. Probably Mars invaders striking back...

Seriously - having 4 to 7 sats doesn't yet mean you have a good fix. They need to be in a good constellation as well. If they are all in the same area of the sky, or if they line up then you won't get a good fix. They need to be as far apart from each other as possible.

So maybe you just had some unlucky constellations. Or - and we've seen that jsut recently - one of the sats has gone crazy again and is distorting the signal. DavidW would know...
_________________
Lutz

Report Map Errors here:
TomTom/TeleAtlas NAVTEQ

[DavidW]

None of the information I'm looking at suggests any system faults on Navstar GPS. I've got no more access to GPS operational information than anyone else here - save that I do keep up with the places where good information and analysis of SPS faults often appears (most of the official monitoring is of PPS, the encrypted military version of GPS).

The GPS SPS specifications for Hazardously Misleading Information, such as happened in the 1 January 2004 fault with PRN23/SVN23, make it clear that this is a very rare occurrence. Usually a GPS satellite realises itself that it's faulty (they have considerable on-board monitoring) and goes off air instead.


Lutz's comments on geometry are right; sometimes a four satellite fix can be a good one, sometimes it can be awful. The DOP (dilution of precision) figures will give a measure of this - though you can't see them in TomTom Navigator or similar programs (unless you were capturing a log - you can read it out of the TomTom GPS log file if you know where to look).

Winfast Navigator shows the HDOP figure, which is the one that gives an assessment of how good or bad the latitude and longitude are. The lower the figure, the better the likely quality of the figures.


It might be that the GPS doesn't have as good a view of the sky as you'd hoped. My Vauxhall Zafira, like most MPVs, has a fairly decent view of the sky through the windscreen, as it has a large, sloping windscreen. However, many car windscreens are not as deep as the Zafira's.

The time of day in the UK (evening as opposed to morning) does not in itself determine the number of GPS satellites visible. The GPS satellites are not in geostationary orbit; they're in a much lower orbit of around 20,000km. This means that they move over the surface of the earth - which means that number of satellites visible at any particular position, and where they are in the sky, changes.


It would probably be worth following the instructions in this article and taking a look at the Signal Level display.

Here's an example:


If you look at my example, you'll see that the view was significantly obscured to the north. Assuming satellites 16 and 2 were on air (which I believe they both are), then I would have expected to pick them up with a clear view of the sky - though sometimes you just can't "see" satellites that are lower to the horizon.

In fact, that particular view was deliberately contrived - it was taken indoors next to a window where I knew the view would be significantly obscured in one direction.

(In case anyone is wondering, the icon over the window title is that of Spb's GPRS monitor, and the configured SIP - that is the pop-up input panel - is Textware's excellent Fitaly).


You'll probably have to try this on several different occasions in your vehicle - that's both parked in different directions and at different times - to see if there is a problem with the visibility of the sky from your GPS. Hopefully if you do this, you can get some idea as to whether your GPS reception is blocked in particular directions with the current position of your receiver.



David

[r6man]

I'll have a play with Winfast Navigator and see if i can find a better location in the car. Thanks v much for all the info.

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