Why is my GPSr so far off?

[+SilentYellowDogTeam]

I have a Garmin Venture that is very well taken care of. While caching this weekend, I found myself wandering in large circles (100-200 feet) looking for the caches. Most of the time my GPSr read that the spot of the cache was at least 100 feet away when standing on it.

 

Second, while seaching for the cache, I would put my GPSr in my pocket for a minute. When I take it out (and wait a minute for it to get a good signal) the cache waypoint has 'moved' a few hundred feet when I was nearly standing on it before.

 

Third, when returning back to my car, I followed the track on the GPSr and when walking down the *exact* same trail, it says I was walking 80-100 feet parallel.

 

I should say that I normally operate in Battery Saver mode with WAAS off. Not sure that really makes a difference. I have tried it with WAAS on, but it didn't seem to improve.

 

So, it my GPSr just a little wacky and needs to be reset (which I don't know how to do), or is this just one of the limits of GPS accuracy?

 

Karl

Silent Yellow Dog Team

[+EtrexRose]

Was there tree cover? I think taking it out of battery saver mode does effect it-in batter saver mode, it stops recieving satelites off and on like when on the road on long straight high ways and if you are making a track with batt saver on, its not going to be AS accurate. Hope this helps

[+SilentYellowDogTeam]

quote:

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Originally posted by Mr. Rose:

Was there tree cover? I think taking it out of battery saver mode does effect it-in batter saver mode, it stops recieving satelites off and on like when on the road on long straight high ways and if you are making a track with batt saver on, its not going to be AS accurate. Hope this helps

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No tree cover. Very clear view of sky in almost all directions.

 

I didn't think Battery Saver completely stopped receiving from the satelites, but the manual is not specific.

 

[This message was edited by Silent Yellow Dog Team on December 30, 2002 at 09:51 AM.]

[+Renegade Knight]

These can account for being off.

 

1) you are using the wrong datum (WGS84 is what you want for geocaching) If you never messed with your GPS's datum you probably are using the right one.

 

2) Tree cover (which you said isn't an issue).

3) Being near a rock face and/or bridge or building that reflects the satalite signals.

4) The hider's did a typo on the cache when they hid it.

5) Dense cloud cover.

6) You are holding your GPS wrong and blocking the reception. This isn't too likley unless you hold it upside down or from the top, or cover the top with your hand for some odd reason. You could have it too close to your body...

7) A bad satalite constelation not giving your GPS much to work with.

 

That's about all I can think of. All of these can affect your GPS signal and cause it to be off. Or can have your GPS be dead on but the coordinats were off.

 

Wherever you go there you are.

[+cat-n-the-hat]

Battery saver mode will affect your accuracy in a situation with marginal signal. Because it only polls the sat. every 5 seconds. This can make a bigf difference if you are getting poor reception to begin with.

[+SilentYellowDogTeam]

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Originally posted by Renegade Knight:

These can account for being off.

 

1) you are using the wrong datum (WGS84 is what you want for geocaching).

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Nope, datum is correct.

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3) Being near a rock face and/or bridge or building that reflects the satalite signals.

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There are mountains around, but 10+ miles away. I was having proglems standing in an open wash (dry river bed).

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4) The hider's did a typo on the cache when they hid it.

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I ruled this out because it is pretty much off on _every_ cache. Not to mention there is that 'track' issue.

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5) Dense cloud cover.

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Perfectly cloudless day. I'm in AZ

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6) You are holding your GPS wrong and blocking the reception. This isn't too likley unless you hold it upside down or from the top, or cover the top with your hand for some odd reason. You could have it too close to your body...

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Are you saying I need to go on a diet? Just kidding. I always walk around with the screen pointing straight up.

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7) A bad satalite constelation not giving your GPS much to work with.

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I pretty typically get a full strength signal from 6-8 satelites.

 

This is why I am so puzzled. I'm in what should be the ideal GPS reception environment, clear sky, little to no trees, wide open spaces.

 

Is there a way to do a reset on the Garmin Venture?

[+Stunod]

quote:

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7) A bad satalite constelation not giving your GPS much to work with.

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I pretty typically get a full strength signal from 6-8 satelites.

 

You can still have strong signal strength, but if the satalites are in a poor constelation (position in the sky) your GPSr will not be able to calculate your position well.

 

"Just because I don't care doesn't mean I don't understand."

[+Counter Fit Cache]

Are your signal bars locked, i.e. do they have a "D" in them?

 

Are you locking onto WAAS?

 

ChuckyBrown

[Stefanoodle]

I understand your feeling of puzzlement; I have on occasion had similar experiences. I have a Magellan Trailblazer XL, which is pretty ancient. On one weekend I found two caches, but mainly because I had a good idea where to look (and they were virtual caches), but my readings were between 90 metres and 150 metres off. One of the caches was actually a calibration point erected by the Department for Surveying and Mapping.

 

I spoke to two friends (electrical engineers) who have at one time or another been involved with building GPS units commercially. Some of the issues are:

 

* buildings or mountains do NOT reflect the satellite signal (RF signal) because it's much too weak (- 130 dB) to reflect.

* poor satellite geometry, i.e. when they are all in a straight line, certainly will cause poor position calculations

* GPSr don't need to be calibrated if they are not reading correctly; if the calibration is out, they will not pick up satellites AT ALL.

* perhaps SA had been turned on temporarily (in light of the possible Iraq war), but there have been no internet postings to this effect, and the problem was only occasional.

 

This didn't really solve my problem either, but it did exclude some reasons I had thought of. What I have done in the meantime is keep a log of the GPSr's positioning as follows:

 

* choose a place where you can get repeated records (I sit in my garden in the same place)

* take say 20 readings, one every minute or 30 seconds.

* take a note of the satellite geometry and how many satellites you are locked onto

* plot the readings (I use Excel and plot UTM readings)

* repeat over several days or weeks

 

There is also some Shareware called VisualGPS which plots real time readings or logged data specifically for this purpose. I have looked at it, but have not tried it yet.

 

What I have discovered about the accuracy of my unit:

* satellite geometry has a huge effect

* the number of satellites also makes a difference, i.e. only three satellites has more drift than say 6.

 

I'm sure we all have days when our readings are not good, or drift quite a bit.

[+Searching_ut]

quote:

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* buildings or mountains do NOT reflect the satellite signal (RF signal) because it's much too weak (- 130 dB) to reflect.

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Signals arriving through indirect means such as reflections are refered to as Multipath. Signal strength and polarization were selected to minimize the occurance of Multipath problems. It does however happen, quite often in certain conditions, especially with consumer grade equipment. With a Quad helix antenna, theoretically it takes two bounces, wheras with a patch you can sometimes suffer the problem with a single bounce if the signal arrives at a steep angle to the antenna. Tell your engineer friends to get out into some steep canyons etc and play with the toys instead of reading the books. They'll find that on occassion, the receiver does indeed lock onto a satellite the terrain is blocking and that the receiver couldn't possibly see. The position solution will also of course show significant error.

 

For more info on error sources check out these sites to start with:

 

http://www.edu-observatory.org/gps/gps.html

 

http://www.edu-observatory.org/gps/check_accuracy.html

[+scoutingitout]

I also had the same problem with my Garmin 12. And I found that I had go into the setup menu under navigation, and the position format needed to be changed to (hddd'mm.mmm') instead of (hddd'mm'ss.s) This brought my GPS a lot closer to the caches.