Tracklog Gone Berserk

[Bazzer]

Can anyone help with this.

Was out on a fell race at the weekend using my wrist mounted Garmin Foretrex 201, which I have used many times before.

However, when I got home and uploaded the tracklog to my PC, I discovered the the tracklog was all over the place.

You can see from the extract below. My route (give or take a few yards) is the purple line, whereas the tracklog just literally goes berserk.



Does anyone have an explanation for this.

Cheers

Baz
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[AllyCat]

Hi Baz,

If the gps has worked correctly before, in similar (mountainous) conditions, then I can only suggest that there may have been poor (or unfortunate) gps satellite geometry (positions) on that occasion. Here is part of an answer I posted on the gpspassion site in answer to a similar question from "pwel" (near the end of the page).

The GPS system calculates distances (from known reference points) by the "time of flight" of radio signals. It is possible for a mountain to act as a "mirror" to the radio signal, so the time of flight indicates a larger distance to the satellite. Normally, there are more satellites "visible" than the minimum required for a fix (3 for lat/long only, 4 for altitude as well) and the GPS engine can eliminate any "bad" signals. But many of the 9-12 satellites typically above the (sea) horizon may be hidden behind mountains. If there is only a minimum configuration, and the direct signal from a "reflected" satellite is hidden, then the GPS cannot know of the error.

Also, the basic "reference" data (e.g. for calculating the satellite positions) is transmitted relatively infrequently, and in difficult reception conditions it might become slightly corrupted. In this case, there may be some delay before it is corrected (which may explain the period when your track appears to have been "offset" by a relatively constant distance).

Cheers, Alan.
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[philpugh]

Also - did you have any other electronic equipment nearby? I have had similar sort of results when running 2 GPS units side-by-side.

The sorts of positional errors you are seeing (around 1-1.5 Km) would imply very poor sat geometery / poor reception conditions.
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Phil

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