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Wandering Gps


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I put my GPS in my vehicle mount an left it out on the driveway overnight and found out the next morning that my GPS had wandered around during the night. See this image: a4d93d10-7c41-42e0-99bf-a9e334b44f64.jpg

 

Questions:

1. What is the most likely explanation for this track?

2. I repeated this procedure the next night and there was no track. How does it happen one night and not the next?

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You sure the GPS didn't decide to go joy riding? :rolleyes:

 

When a GPS is first turned on, it goes though a process of acquiring the different satellites and downloading the detailed almanac data for each one. As it's doing so, it's "guess" of where it is may wander around as settles in. This is quite normal.

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You sure the GPS didn't decide to go joy riding? :rolleyes:

 

When a GPS is first turned on, it goes though a process of acquiring the different satellites and downloading the detailed almanac data for each one. As it's doing so, it's "guess" of where it is may wander around as settles in. This is quite normal.

Yea, I know. But examining the uploaded track points shows that it was still "wandering" several hours after turning it on and acquiring satellites.

 

I know the unit's OK and that this wandering is normal. Just wanted an explanation why it occurs.

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The available sats are constantly changing. They are up there at 12,000 miles up and take 12 hours to do a complete orbit of the earth. There are times during the day when theres no more than 3 or 4 sats directly overhead. As the sats move in the sky the GPS has to quit using one sat and finding another sat to use. when your hiking past big fat trees your GPS is switching sats like crazy because the sats are blocked randomly by the trees, causing your GPS to wander like crazy. Also if the GPS is stationary, by having it doing AVERAGING, then the GPS knows that it's stationary. If not AVERAGING the GPS may think it's drifting around in a boat, and not parked in a driveway. If there are say 5 satellights all in a perfect line across the sky going East-West, then your position error will be North-South.

Edited by GOT GPS?
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The available sats are constantly changing. They are up there at 12,000 miles up and take 12 hours to do a complete orbit of the earth. There are times during the day when theres no more than 3 or 4 sats directly overhead.  As the sats move in the sky the GPS has to quit using one sat and finding another sat to use. when your hiking past big fat trees your GPS is switching sats like crazy because the sats are blocked randomly by the trees, causing your GPS to wander like crazy. Also if the GPS is stationary, by having it doing AVERAGING, then the GPS knows that it's stationary. If not AVERAGING the GPS may think it's drifting around in a boat, and not parked in a driveway. If there are say 5 satellights all in a perfect line across the sky going East-West, then your position error will be North-South.

Seems like a plausible answer. I was wondering if, as satellites come and go during the night, the GPS was getting "handed off" from sat to sat and during each hand-off there may be introduction of error.

 

Thanks for the help

Edited by CacheMonkeez
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