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I don't get it... why's my eTrex so "off" all of a
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Buzzygirl
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Joined: 06 Apr 2004

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PostPosted: Sun Jun 27, 2004 7:51 pm    Post subject: I don't get it... why's my eTrex so "off" all of a Reply with quote

Hmph. I'm not quite sure if my eTrex has a problem or not, but it sure seems like it.

I went caching tonight with my 2-month-old eTrex to a couple of cache sites. I noticed something strange going on with the eTrex tonight.

I know GPSrs are usually accurate to within 15-30 ft. or so, but tonight, my GPS was all over the place. I had set the waypoint for a cache I wanted to find, headed towards the cache site, and started looking around the area when the eTrex told me I was within 5 feet of the cache.

I hunted around near a couple of trees, under bushes and branches and foilage; nothing. When I went to look up at my eTrex, the distance figure suddenly said I was 65 ft. away from my quarry, instead of five. I went to the screen that checks the accuracy of the lock, and it said that the GPSr was accurate to within 24 feet.

I then stopped navigation to the waypoint, got back into the waypoint and started over again. This time the GPSr pointed me in a totally opposite direction from where it first had the lock. I went to within 5 feet of the lock this time, but there were only a few low plants and nothing to really cover a cache to much extent. Plus, this area was a foot away from a main walking path; not likely to be the hiding spot.

So, I turned off the eTrex, took the batteries out, and let it sit for about 5 minutes. I then proceeded to the entrance of the park to start over from that point. I put the batteries back in, turned it on, and once it got the satellite fix, I checked the accuracy, which said it was within 22 feet.

Then I went to my same waypoint and told it to go to there. I started walking towards my quarry again. This time the eTrex pointed me in the same direction as the beginning; I started walking towards what I thought was a likely hiding spot and practically tore it apart, as well as the surrounding area. Nothing there. Then I looked at the eTrex, and it said that my quarry was now *89* feet away, and in the opposite direction. I walked around and watched the eTrex. It rarely pointed in the same direction more than once and the distances to the purported cache area would change wildly if I moved more than a few feet in any direction.

I thought this was rather strange, mostly because the tree and brush cover in this area wasn't really all that heavy, and I had a clear view of the sky in most areas I searched through.

After repeating the experiment once more, searching the area for the cache and not locating it, I headed home. I decided to do a small test on the way from the park. I told the eTrex to go to my home waypoint. I have used my home waypoint before when going on long walks with the eTrex, so I could determine how far I walked. I originally set the waypoint from my front door. When I got here tonight, the GPS was suddenly off by over 300 feet when I arrived at my front door. It's never been that "off" before.

I checked my eTrex manual for how to reset it, but I didn't see anything in there about how to do it. I believe there is a reset hole on the back of it, but since there aren't any instructions on how to do it in my manual, I felt I should wait until I got some input from someone who may have experienced the same issues with theirs.

I am not sure if I have the most recent version of the eTrex operating system either, but I'm not sure if this would make a difference or not.

Any suggestions on what else to look for when troubleshooting? I'm still really new to this, so there may be tricks and fixes I'm not aware of.

Thanks!

Jackie (Buzzygirl)
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Kitch
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Joined: 18 May 2003

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PostPosted: Sun Jun 27, 2004 8:24 pm    Post subject: Reply with quote

I hate to say it.......

but sounds like a normal night for me!!!!
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Marsha and Silent Bob
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PostPosted: Sun Jun 27, 2004 8:51 pm    Post subject: Reply with quote

I rarely follow the GPS once I am in the general area of a cache. Once it is 30 feet or less put the GPS away and look. You'll likely have less frustration and more luck.
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Buzzygirl
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PostPosted: Mon Jun 28, 2004 8:44 am    Post subject: Reply with quote

Okay, well, I was just wondering if it was normal for a GPSr to be "off" so much on a previously-set waypoint. I thought 300+ feet was a little far off.

Does anyone know if upgrading a GPSr's system software to the latest version sometimes helps with its accuracy?

Jackie
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Moe the Sleaze
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PostPosted: Mon Jun 28, 2004 9:08 am    Post subject: Reply with quote

By any chance is your front door under heavy tree cover? Or in some other location that does not have a clear view of the sky?

For GPS to work well, your receiver needs to be seeing at least four satellites. If it sees only three, it will still be operating in "2D" mode which makes assumptions on your altitude based on where in the world you are. It is not unusual to be off by hundreds of feet when in 2D mode.
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Buzzygirl
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Joined: 06 Apr 2004

Posts: 499

PostPosted: Mon Jun 28, 2004 9:44 am    Post subject: Reply with quote

Moe the Sleaze wrote:
By any chance is your front door under heavy tree cover? Or in some other location that does not have a clear view of the sky?


No, the nearest trees are some distance to the east of my door. I would not have thought it odd for the GPSr to be 50 feet off, but over 300 feet seemed a bit excessive.

I may try and download the most recent version of the system software for my eTrex. Dunno if it'll make a difference, but I guess it won't hurt to have the newest version.

Jackie
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Kitch
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Joined: 18 May 2003

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PostPosted: Mon Jun 28, 2004 9:45 am    Post subject: Reply with quote

Buzzygirl wrote:

Does anyone know if upgrading a GPSr's system software to the latest version sometimes helps with its accuracy?

http://www.garmin.com/support/download.jsp
Considering that your etrex is only month's old....I find it hard to believe that you have an outdated software on it since they haven't updated anything on the software since June 14, 2002.

Also...I'm 99% sure its not going to change your issue.

Quote:
Changes made from version 2.11 to 2.14:

Software modified to support hardware changes. Does not affect user operation.

Changes made from version 2.10 to 2.11:

Prevents excessive number of track points from being recorded.

Changes made from version 2.09 to 2.10:

Added an Advanced Skyview mode on the GPS page that displays the location in the sky of each satellite that the unit is tracking. Along with this skyview is the signal strength for each satellite. To get to this new mode, press the ENTER key while on the GPS page and an option menu will appear. This option menu contains two items: SETUP DISPLAY and ADVANCED SKYVIEW. The SETUP DISPLAY option will take you to the Display Setup page for adjusting contrast and light controls. Selecting the ADVANCED SKYVIEW option will switch the GPS page to display the advanced skyview information.
Allow display of speed and track when velocity is below 1.0 MPH and Selective Availability (SA) is off.
Fixed route leg distance calculation that occurred when the unit did not have a fix.
Fixed Dutch grid conversions and added new Dutch map datum to accompany the Dutch grid.
The map datum list on the unit's Setup page is now displayed alphabetically.

Changes made from version 2.08 to 2.09:

Fixed elevation display on the Compass page so that it does not 'dash out' after 9999.
Improvements in positioning to fix erroneous track logs.
Improved the arrival alarm.
Changed the meter unit symbol from 'mt' to 'm' to be consistent with international standards.

Changes made from version 2.07 to 2.08:

The UTC label is no longer displayed on the time when the UTC offset is zero.
The user selected north reference offset is saved correctly.
The restore defaults button on the units setup page saves the defaults correctly.
Elevation is no longer displayed in kilometers or kilofeet when exceeding 1,000. It now remains in meters or feet.
Shortened the Australian grid abbreviations.

Changes made from version 2.06 to 2.07:

New display option on the Map page for the Goto navigational line. The line can be displayed from your current position (default) or from your original point of navigation as a course line. This option can be changed from the Map page's option menu by selecting 'SHOW BEARING LINE' (default) or 'SHOW COURSE LINE'.

Changes made from version 2.05 to 2.06:

New user configurable north reference offset that can be modified on the units setup page.
New project waypoint feature that allows you to create waypoints from existing waypoints given a distance a bearing.
New hide/show waypoints feature on the Map page that is accessed from the option menu.
Editing the location of a waypoint is now displayed correctly.
Added PVT output support.
Fixed rare memory anomaly caused by uncontrolled shutdown.
Added bearing and distance to the map when veiwing a waypoint from the Review Waypoint page.
The elevation on the Review Waypoint page can now be modified.
Added more time zones. Note: Daylight savings needs to be turned on/off manually for time zones out of the United States.
More zoom levels on the Map page.

Changes made from version 2.04 to 2.05:

Initialized user grid for proper software upgrade to existing units that contain software version 2.03 or less.

Changes made from version 2.03 to 2.04:

Turned on seconds attribute for time display.
Added user grid format and user datum setups.
New data entry pages for number fields and position origin field.
Added new position formats: Dutch grid, Qatar grid, and Finnish KKJ27 grid.
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KC0GRN
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Joined: 22 Feb 2004

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PostPosted: Mon Jun 28, 2004 10:56 am    Post subject: Reply with quote

Soemtimes I'm just thankful my GPSr gets me to the park Wink

But seriously, I've been of by large areas myself in the past. When you get into caching, sometimes you learn to just pocket (or holster?) the GPSr and look in the area it brought you too. Usually you'll see something that stands out as a good location, so start your search there.
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Buzzygirl
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PostPosted: Tue Jun 29, 2004 11:38 am    Post subject: Think I've got it. Reply with quote

Okay, I think I see now why my GPSr experiences wide swings in accuracy.

Since I've had this eTrex, I have to admit that I haven't paid much attention to how many satellites it locks onto during a caching session. Once I have an initial fix, I generally don't go to that initial screen anymore. I pay more attention to the distance/nav screens from then on.

I went out and hid my new "Squished Skeeter" cache last night, and this time, I paid careful attention to the satellite locks as well as the coordinates. I did notice that several times I lost lock on 2 of the 4 satellites for brief periods of time; not enough to get a "weak signals" message, but enough to throw the accuracy off a noticeable amount. Funny thing is, it didn't always lose lock in heavily-treed areas, either.

Anyway, I walked around with the eTrex for awhile and got no better than a 23 ft. accuracy, which I know is quite within the range to be expected.

On the way back home, I also checked the distance to my home waypoint-this time, it was far more accurate than it was the previous night. I got the "arriving destination" message when I was within 60 feet of the door instead of 300 feet.

I guess I am still really new to these receivers and there are likely many factors that effect their accuracy beyond trees and clouds. Does stuff like humidity and temperature affect them as well?

Cache on!

Jackie
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KC0GRN
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PostPosted: Tue Jun 29, 2004 11:52 am    Post subject: Reply with quote

I suppose certain types of radio intereference could affect a signal....

Also sometimes night verse day seems to have different effects.

BTW Buzzygirl, I was out at your Squished Skeeter cache this morning, the coords were pretty close, but I was using WAAS on my GPSr, with an accuracy of 17 feet (I suppose that's theoretical), but I was definately within 25 feet of the cache.
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Marsha and Silent Bob
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Joined: 02 Sep 2003

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PostPosted: Tue Jun 29, 2004 11:55 am    Post subject: Re: Think I've got it. Reply with quote

Buzzygirl wrote:
I guess I am still really new to these receivers and there are likely many factors that effect their accuracy beyond trees and clouds. Does stuff like humidity and temperature affect them as well?

I haven't noticed anything with temp and humidity affecting my GPS but I didn't cache much last summer and I was new when I did... It certainly wasn't affected much by cold temps (except the battery life and the screen updates).

I usually lock onto more than 4 satellites. Four is the absolute minimum for 3D. I wouldn't trust anything with only 4 or less sats myself.
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Buzzygirl
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PostPosted: Tue Jun 29, 2004 12:29 pm    Post subject: Reply with quote

KC0GRN wrote:
I suppose certain types of radio intereference could affect a signal....


Oh, I'll bet it could. Do you know what frequency(ies) the satellites transmit on? Just curious. I usually don't bring my HT with me when I go caching (maybe I should).

Quote:
BTW Buzzygirl, I was out at your Squished Skeeter cache this morning, the coords were pretty close, but I was using WAAS on my GPSr, with an accuracy of 17 feet (I suppose that's theoretical), but I was definately within 25 feet of the cache.


I took eight readings of the cache area before committing to the posted coords... I thought that should be accurate enough.

I'll have to do more reading up on caching lingo. I am not exactly sure what WAAS is Question

Congrats on the FTF, by the way! I added a small hint to my new cache's listing on geocaching.com. Might make it a bit easier to avoid getting hit in the noodle by an errantly-flung disc. Smile

Jackie
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KC0GRN
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PostPosted: Tue Jun 29, 2004 12:48 pm    Post subject: Reply with quote

Quote:
Do you know what frequency(ies) the satellites transmit on?


interesting question. I managed to find this off a Gov site

Code:

GPS SIGNAL CHARACTERISTICS
The satellites transmit on two L-band frequencies: L1 = 1575.42 MHz and L2 = 1227.6 MHz. Three pseudo-random noise (PRN) ranging codes are in use.
The coarse/acquisition (C/A) code has a 1.023 MHz chip rate, a period of 1 millisecond (ms) and is used primarily to acquire the P-code.
The precision (P) code has a 10.23 MHz rate, a period of 7 days and is the principal navigation ranging code.


As for WAAS, it's still experimental, stands for Wide Area Augmentation System. I forget how it works, but I think there's some math equations involved..?

Thanks! it actually took me a bit longer than I thought it would, but there's a lot of area to cover.
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Moe the Sleaze
Geocacher


Joined: 10 Jan 2003

Posts: 1145

PostPosted: Tue Jun 29, 2004 7:57 pm    Post subject: Re: Think I've got it. Reply with quote

Buzzygirl wrote:
On the way back home, I also checked the distance to my home waypoint-this time, it was far more accurate than it was the previous night. I got the "arriving destination" message when I was within 60 feet of the door instead of 300 feet.


Oh, that's much different than saying the waypoint was 300 feet off! The timing of the "arriving destination" message has a lot to do with how fast you are approaching the waypoint. If you slowly walk up, you will get the message much closer to the actual point than you would if you ran, biked, or drove up. The GPSr attempts to warn you at a given time from the point rather than distance. At least that's the way it works with the two units I'm familiar with (Garmin GPSMap 76 and GPSMap 76CS).
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SirPoonga
Geocacher


Joined: 07 Feb 2003

Posts: 144

PostPosted: Tue Jun 29, 2004 10:23 pm    Post subject: Reply with quote

I can tell you what I have heard. I have heard the patch antenna for the etrex isn't as good as the double helix on other garmins. But I also heard patch antennas are suppose to be really good.

Also, near buildings you can be off by a little bit do to reflection and refraction.

Also satellite orientation makes a big difference. I'v enotice on my etrex it can tell me I have 30ft EPE when I am in the bathroom of my house. That doesn't seem right.

The best thing I have found in finding a cache witht he etrex is walk in such a way you make a large 'T' near the cache location. Then look on the map screen in reference to the 'T' breadcrumbs and start looking in that direction.

In hiding just take several readings and average them. You can use a program like USAPhotoMaps to see how close the coords are.

Also remember that the hider could be 30ft off, and you 30ft off, meaning a total of 60ft off is very possible.

I actually did an experiment in my front yard today. I took a rock and placed it in the middle of the lawn. I marked it. I walked about 300ft away and navigated to it with the etrex several times. It tended to put me 20-40ft south and west of the cache.
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