Interpreting the GPS Status screen

Zebb · Occasional Visitor Joined: Aug 28, 2008 Posts: 55

Is there an FAQ for this, otherwise (I did a forum search);

14 bars- one per satellite? Max of 14 satellites?
Blue bar equals valid signal? Grey bar = too weak?
Little numbers at the bottom of bars, some in a blue circle? Signal strength? but what actual measurement are they in? mV? Maximum number?
Line across all 14 bars?

N E S W icon, two concentric cirles, closest dots to the centre are overhead, near the edge are near the horizon?

Any clues or any other info.

TIA, Zebb.

philpugh · Posted: Mon Oct 27, 2008 6:31 pm Post subject:

You've got it more or less right.

It's only meant to give you a rough idea of what's going on.

There will be a 'calibration' for the scale of the signal strength - but it will vary from manuf. to manuf. (and probably from model to model).

The usual things to look for are:-

Hollow bars mean it is 'seeing' the satellite but has not yet established its positional data (almanac/ephemeris) - so it is not being used to calculate a fix.

SOLID bars mean that it has all the data required and it is using the info for a calulation of your position.

You will see (at max) 12 satellites plus one or two possible WAAS/EGNOS 'differential fix' satellites. There are more up there than this. 24 in the active constellation of GPS stas plus a number of 'spares' along with several WAAS/EGNOS geo-stationary sats that allow enabled receivers (i.e. most modern ones) to get a slightly more precise fix. These are quite good at equatorial latitudes but become more difficult to receive the further N or S you are. Here in North UK they are virtually useless when you are out in the hills. Even in flat Cheshire (where I live) I don't get a fix with them on most days.

In a perfect world what you would want to see are 12 sats nicely spaced over the display, which is as you say, what it can see in the sky above you. Factors like buildings, hills etc will block out some of the lower ones and have an effect on the accuracy of the fix.

In reasonably open terrain you should be able to 'see' 9 - 10 sats. Anything over 6 well 'dispersed' sats will give you a pretty good fix.

Also remember:- most car sat navs use software to lock the received position onto the road (they assume you can't be off the road).
_________________
Phil

Oldboy · Posted: Mon Oct 27, 2008 7:10 pm Post subject:

Have a look at the Topic here for some more info.
_________________
Richard

TT 910 V7.903: Europe Map v1045
TT Via 135 App 12.075: Europe Map v1120

Zebb · Occasional Visitor Joined: Aug 28, 2008 Posts: 55

Ah, thanks for that info. The highest number in the bars I've seen is 51, (~represents the signal to noise ratio from the other topic). They could have shown all three lat/long readouts at once on that screen, there's enough space.

I wonder how long each satellite stays in view? The numbers in the blue dots must be a code for a particular satellite but I can't find out what the code is online.

Wikipedia doesn't even list it and that's very technical.
http://en.wikipedia.org/wiki/GPS

Pity it doesn't show altitude, I've tried a couple of other programs but I can't seem to get them to work on my One XL.

Regards, Zebb.

mikealder · Posted: Mon Oct 27, 2008 8:24 pm Post subject:

Zebb · Occasional Visitor Joined: Aug 28, 2008 Posts: 55

No I wouldn't use it for flying but on holiday, the names of towns have the height above sea level on them. My body doesn't do so well above 6000 feet these days and I'm curious to see the altitude. Not that I could do anything about it, all around would be of similar altitude.

Very interesting info, it's a long time since I did all this radio theory.
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Hope you enjoy your new toy Mike, it's been a long time coming.

philpugh · Posted: Tue Oct 28, 2008 6:23 pm Post subject:

Zebb · Occasional Visitor Joined: Aug 28, 2008 Posts: 55

philpugh · Posted: Wed Oct 29, 2008 11:07 am Post subject:

See what you mean about the three formats.

+/- 15m would be a dream for altitude accuracy with a GPS - the problem is it's more like +/- 50m a lot of the time and I've seen it over 100m out more than once. My Colorado GPS shows altitude using two methods. GPS derived and barometric pressure derived. Assuming I calibrate the barometric unit correctly before each outing it is far more accurate than the GPS derived value (according to OS mapping).

If you really want to know the altitude buy one of the relatively cheap watches that have a barometric unit in them. If you calibrate it daily (or at least before every outing - usually a 1-2mins job) it will give you more accurate results than GPS.
_________________
Phil

Andy_P · Posted: Thu Oct 30, 2008 8:48 pm Post subject:

mikealder · Posted: Thu Oct 30, 2008 9:00 pm Post subject:

This was something I did wonder about when the 32nd was switched live as the date did occur within a couple of days of the reports of miss position being reported by quite a few users. But its been live quite some time now and miss position on screen with the TomTom devices has been random in terms of area and date for a while so i doubt the two are connected.
Most probable is duff Quick GPS fix data downloaded to the device.

It has been reported some very early GPS based devices cannot use the 32nd bird due to the way the numbers were allocated 00 to 31 or 01 to 32 but this didn't affect the TomTom devices AFAIK - Mike

Zebb · Occasional Visitor Joined: Aug 28, 2008 Posts: 55

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