Swivelling map when coming to a halt

[aterlecki]

I'm wondering if my Go has a fault because many times, as I am coming to a halt, the map which is usually oriented correctly and showing my direction of travel suddenly starts to swivel around. I can actually have it so when stopped the map is pointing at 90 degrees from where I am going and it only corrects itself once I start driving off again. This can be very confusing at certain junctions when suddenly I can get a bit disoriented.

I know when the Go is first turned on it might be pointing in the wrong direction (although why they couldn't fit a small compass to the unit to overcome this I don't know) but it seems strange that it swivels after I've been moving and just come to a halt, say at a traffic light or junction.

Is this map swivel characteristic a "feature" of the Go unit or is my unit faulty? What does yours do?

[manicguitarist]

I have seen this happen when GPS reception is poor and the unit tries to guess which way you are facing.

[nej]

[Sniff]

I can't say that I've ever noticed that problem myself.

How would a small compass help though?

[BanziBarn]

Mine does that too. I'd put it down to poor reception.

[carl_w]

Every position fix that comes out of the GPS is a little bit out. If you plot a distribution of these you'll get a kind of 3D normal distribution, with the actual position at the "peak".

So if on one poll you get a position that's say 3 ft East of your actual position, and on the next poll you get a position that's 3 ft West of your actual position, Tomtom will think you're travelling East to West and rotate the map accordingly. Obviously the less precision (i.e. less satellites) you have, the greater this effect is likely to be.,

[Juey]

I'm with aterlecki on this one

Once I've stopped the arrow points 90 degrees right.

It's as if the receiver points east- west in the unit rather than north- south (when screen facing you and pointing ahead is north) It's only when it's moving that it recognises you're pinting the other way.

It always does it, even when 5 bars are lit for reception.

Maybe it's the camber on the road, my driveway also cambers heavily to the left (unit down on left hand side)

Not a problem I'm worried about though

[DavidW]

The precision is dependent not only on the number of satellites in the fix, but on the geometry of the satellites - as well as various other factors, such as ionospheric disturbances.



David

[Dave]

Personally I would disable ASN

[Frazzle]

Hi folks,

Just a quick comment on this one ...

GPS systems plot positions
Changes in the plotted position provide velocity and beearing data that the TTG uses to show your direction of TRAVEL

If the TTG is stationary there is theoretically no change of position so the direction of travel is not known. TTG will then randomly hunt until you begin moving agian.

Be aware ... the GPS antenna in the TTG is a Ceramic patch with a hemispherical reception plot. IT IS NOT DIRECTIONAL. The GPS does not use it's patch antenna as a "radio direction finder".

Hope this helps.

Oh and the "loss of signal velocity and direction prediction " system, if it works, should not effect direction plotting as the GPS signal is present and you are NOT moving. It uses hardware acceleromters to detect changes in velocity. No movement = no change.

[wildcard]

As others have said GPS systems plot position - it calculates direction (and speed ) by comparing positions at very short time intervals . Also commercial GPS has a margin of error of say 3 meters radius or so . You can imagine at low speed the fuzziness ( circle of error ) of one position overlaps the fuzziness of the next giving the unit an number of different possibilities within split seconds , hence it swivels around . At higher speeds the margins of error cease to overlap and the readout would become more accurate - particularly if the software applied some weighting or smoothing to the results . For example if within 5 seconds , it calculated 5 results , 4 approximately the same , the 5th markedly different , it could discard it and combine the remaining 4 to make a running weighted average.
Do we know for sure a Go contains accelerometers ? - I would be seriously impressed if it did - at a guess I would have thought the ASN solution is purely a software based predictive tool based on what was happening moments before the signal was lost and as such would be fooled by sharp changes of direction - an underground junction would be the ideal test .

Geoff

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