Wandering GPS readings -- WHY?

[Guest BigDoggie]

Another question from a semi-newbie...

 

I was out cache-hunting today. The terrain was a bit strange. The area was a fairly thick plant cover, but it was mostly heavy, tall brush rather than huge trees. There were several low buildings.

 

My GPS readings were all over the place. I'd stand in one place for a while (allowing the Magellan to average). It would say BNG 180 DST 45'. Using a compass, I'd go about 45' south. Look at the GPS again, and it would say BNG 010 DST 120' In other words, way back the other way. Then I would narrow it down to DST 10'... walk 10' and it jumps to 75'. No matter where I went, I was sent off in another direction.

 

I frequently looked at the screen that shows the satellites, and it showed at least 6 satellites every time, but not always the SAME satellites every time.

 

The sky was moderately overcast as well.

 

What is going on here? Why did the readings fluctuate so drastically, even though I had locks on multiple satellites? How do you know when you are being led astray? How do you correct/allow for/deal with this sort of thing?

[Guest Kerry]

Maybe not one single problem but combination of several? Sometimes one never knows why. Being an auto average and as the unit appeared to be lost what was it doing while you were walking?

 

If you have an approx location (nearest minute is enough), date, time, time zone and a rough direction/height/distance of the obstructions we can tell what Sats you should have had in view/fix and DOP vales etc. May give some indication or rule it out of the issue.

 

Cheers, Kerry.

[Guest BigDoggie]

I didn't really watch the screen WHILE I was walking... I was watching my compass, the ground, the brush, and the monster mosquitos laden with West Nile Virus.

 

I would stop, give the GPS a minute to average in, then read distance and bearing. Lets say that the GPS showed BNG 90 DST 45. Using the compass, I would walk 15 paces East, which should put me within a few feet. After again waiting a minute, the GPS now says BNG 005 DST 120.

 

Whenever I checked the signal strength screen, it never showed less than 6 satellites. I am not sure that it was always the same six... I didn't really look for that.

 

Can tree cover mess up the GPS in some way that the signal strength is still good, multiple satellites show locked, but the data gets confused?

 

Can an overcast sky mess things up? How?

[Guest Geoffrey]

I went Geocaching today with my new Garmin Etrex Vista, and it had me going all over the place for about 45 minutes. I then remembered to go to the satellite positions page, and moved the GPS around till i good a good signal. I then went to the compass page and took a bering with the electronic compass. Then i walked it, then found the PVC Pipe Cache container.

[Guest brokenwing]

I think you may be expecting too much. Sattelite geometry also plays a big part in EPE calculation. That said, one would think if you had 6 locks, you had a pretty good shot at having had 4 with good geometry.

 

If this happens again, take a look at the EPE. If it's high, this may be the problem. You may have been as close as your reciever could get you under those particular conditions. If the EPE is low and this still happens, the problem may be your reciever.

 

Just a couple of things to think about.

 

brokenwing

 

[This message has been edited by brokenwing (edited 08 August 2001).]

[Guest BigDoggie]

EPE - you learn something every day!

[Guest raptorrider2001]

I am still new at this also, but seems like I had readings like that when I had the gps set on bearing and not course.