Etrex Accuracy

[GeekKitty]

I have recently aquired an etrex for use when geocaching. On the whole I am happy with it, but I have noticed that even when standing still it jumps about *ALOT*. As in I moved as much as 50m at one point while standing still. Other times I have walked towards the cache with the distance counting down 8m, 5m, 4.5m, 3.2m, 1.8m, 12.8m. Instantly it jumps, is this normal? am I doing something wrong? Any ideas?

 

Thanks

 

J

[+Beds Clangers]

Hi Geekitty,

 

Sounds quite normal to me. It is all to do with number and location of satellites that your GPS can "see". I'm sure you will get a much better/detailed answer than that.

 

I use 76CS and as you approach the "cache" it does almost the same thing. I tend to stop about 30m away and let the GPS settle as I was forever walking past the cache locations. At best the accuracy (EPE) is around 6/7m in the open under trees could be anything. Not sure if you have it on an Etrex but I always look at the GPS Satellite Information page which shows the number/location of satellites.

 

If I'm having real problems I quite often turn the GPS off and back on, don't think there is a technical reason why that helps but is just something I do.

 

When you have done one or two caches you will see the likely cache locations as you approach the area. Once in the right area I usually turn the GPS off and just look around for the obvious "out of place" signs.

 

Found a cache last week that had been muggled and re-hidden by another cacher about 100m from original location. Just walking in a search pattern saw the obvious "log pile".

 

Regards

 

Nick

[+The Forester]

The symptoms you describe sound a lot like multipath.

 

This is caused by the radio waves from the satellites bouncing off surfaces such as buildings, roadsigns, rocks etc.

 

Because GPS works by calculating distances from the satellites, the longer raypath of the bounced radio wave means that the receiver 'thinks' that the straightline distance to the satellite is greater than is actually is.

 

The reason for a sudden jump in the apparent position when your receiver is actually stationary is usually a result of a new satellite being used or an old one discarded by the receiver. If that satellite has a duff indicated range then the resultant fix will suddenly hop. Remember that the satellites are not geostationary. They are whizzing along at great speed so the geometry of the satellites being monitired by the GPSr is constantly changing.

 

There isn't a simple cure for the problem. In some situations you can easily see that something like an adjacent building is causeing a reflection and it is sometimes possible to guess which satellite raypath is being reflected. You can block such reflections quite simply with your hand beside the antenna as the radio energy is incapable of travelling through the flesh of your hand. This can sometimes make a fix more stable and more accurate, but sometimes it's a hit or miss technique.

 

Another possible cause can be partial obscurement of line of sight to the satellites by tree cover. We are now coming into the season when foliage becomes dense enough to severely restrict GPS reception, so that's a problem which will get worse in woodland over the next few weeks.

 

Cheers, The Forester

[+wildlifewriter]

Instantly it jumps, is this normal? am I doing something wrong? Any ideas?

I'd agree with the previous posters, that the behaviour you see could be just the 'normal' rough-and-tumble of using a handheld GPS in mixed conditions.

 

However...

 

There have been some reports that the eTrex can start to behave in an unpredictable manner when it's tracklog is full. It's worth checking this: RTM for your version. Find the "Tracks" display and clear the tracklog. If you aren't actually using Tracks, then turning track recording OFF altogether will give a (small) benefit in performance.

 

-Wlw.

[+CuplaKiwis]

Instantly it jumps, is this normal? am I doing something wrong? Any ideas?

I'd agree with the previous posters, that the behaviour you see could be just the 'normal' rough-and-tumble of using a handheld GPS in mixed conditions.

 

However...

 

There have been some reports that the eTrex can start to behave in an unpredictable manner when it's tracklog is full. It's worth checking this: RTM for your version. Find the "Tracks" display and clear the tracklog. If you aren't actually using Tracks, then turning track recording OFF altogether will give a (small) benefit in performance.

 

-Wlw.

That's quite interesting - hadn't heard that before. Do you have a reference for this?

 

I find that my eTrex is pretty stable (at least to with a metre or two) as long as I have decent sat coverage (5-7m accuracy). Best way to find a cache if coverage is dodgy is to triangulate the position from areas with better coverage a few (?10) metres away. Much easier....

[+wildlifewriter]

That's quite interesting - hadn't heard that before. Do you have a reference for this?

Nope - only a couple of anecdotal reports on other discussion boards, where clearing a filled tracklog was reported to cure the "problem".

 

I haven't tested this myself, life being too short.

 

-Wlw.

[+BedlingtonDuo]

Strange this!

 

Our eTrex went absolutely haywire yesterday around 1730. It wasnt just jumping a few metres but sometimes a couple of kilometres. We had to abandon our caching because we thought it was faulty.

 

We were near trees and walls but no more than usual.

[+Simply Paul]

While setting a cache in Herts the other day I fired my etrex up and asked it to list all my nearest caches, to ensure I wasn't placing anything too close to anyone else's. Once it got a lock it told me I was 60 miles from Doune (in Scotland, where I've not been since 2003. It must be 500 miles away) and that was the only cache it listed. In the end I had to turn the unit off and on again to get any sense out of it.

 

Generally the arrow and distance dances around a bit, and walking in straight lines and observing the direction and range seems the best way of getting an 'average' location to hunt. I've also compared two identical extrexi next to each other and they were showing anything up to 80ft apart.

 

SP

[+The Forester]

walking in straight lines and observing the direction and range seems the best way of getting an 'average' location to hunt.

That sounds like a classic indication that the machine is using a Kalman Filter to smooth out the jumps.

 

Such a filter presumes that you are moving along a straight or curved path, with little change in rates of change of rates of acceleration.

 

If you are actually stationary, then the random jumps which are natural can confuse the filter and cause it to start believing that you are moving off at a high rate of acceleration in some direction or other. In other words, it can actually make the jumps worse than they would be without the filter.

 

WlW's described cure suggests to me that perhaps resetting the track log also sets the internal parameters of the Kalman Filter to zero or whatever are the default parameters and therefore quietens down the jittery filter.

 

I don't know anything about the eTrex's actual internal smoothing algorithm, but I do know that the Magellan discontinues using its filter when it detects a groundspeed of less than 0.5mph and then starts a simple arithmetical averaging of the fixes.

 

Cheers, The Forester