Hardware Degradation

[+Tidalflame]

Okay, something weird is going on. My geocaching partner and I have been on a DNF streak recently - lately, we haven't even been able to tell whether or not we're in the right spot because the GPS is leading us on wild goose chases! The only thing consistent about the GPS is now that it consistently leads us to the wrong spot. It still works for getting to the general area, but once we get within twenty metres or so, it just cannot "decide" where the spot is supposed to be. The GPS used to work great, so it's all very odd.

 

Now, I imagine it's pretty common to get caught in a rut once in a while, but we're pretty sure our poor performance is due to something being wrong with the GPS. Most of the "evidence" supports this - the GPS is consistently leading us in circles. We actually managed to break our DNF streak today with a single find, but it was mostly due to luck and a very obvious hide. When I found the cache, the GPS was bouncing between 15-25 metres away. We had been going in large circles before the find. After finding the cache, I continued to follow the GPS as a test and it led me in almost the opposite direction it should have. It seems it's been doing this for a while now.

 

I can imagine a couple possible reasons for this. A couple weeks back, we did a long hike in the rain to plant a cache, and it was really raining - to the point that the GPS got wet on the inside. Supposed to be water resistant, but apparently the seal doesn't work to well on this one anymore (it's a Garmin eTrex, and it's quite old now). This was about the last time it seemed to be working properly. Of course, it could just be that I've accidently banged it against a rock one time too many.

 

There's one problem I have with this theory, though. I'm a programmer, not an electronics guru, but I do know a few things about electronics, and in my experience high tech devices have binary states of operation - which is to say that either it works perfectly or not at all. I can't understand how the GPS would suddenly become "less accurate." It certainly seems to be the case, but I don't know if this is even possible. Can someone provide insight?

 

Of course, if the GPS isn't broken, this is all still a bit of a mystery. The only other explanation would be that we suddenly became much worse at finding caches, which doesn't make much sense either

Edited July 15, 2005 by Tidalflame

[Stefanoodle]

I don't know whether what I say will really answer your question, though you seem to have pointed out something valid: generally, the unit either works, or it doesn't, as you mentioned about the binary states.

 

Quite a while back I had one day where all my values were off by about 150 metres, which is a huge error. I also thought it might be the unit, and that it might need calibration. Two electrical engineering colleagues (who have in the past been involved with building some form of GPS units) I consulted, told me what you have said: there is no calibration; either it works, or it will give you nothing. Since I'm a mechanical engineer, I have less insight into the electronics and programming, so I will take that at face value.

 

What I have discovered, and it's been mentioned in various places in the forums, satellite geometry is probably most influential on the position you get. If I have only three satellites, then my error is much greater than when I have five or six or more. Also, the closer I get to a cache, the more variability I get, so sometimes the position does seem to take me in circles. That's why I use a compass, and triangulate from a bit further out. After all, I have an antique Magellan Trailblazer XL from the last century!!

 

So, some days I have spot-on results, and other days I also don't get great results. Check for number of satellites you are picking up, whether there is poor satellite geometry (e.g. they all are in a line, etc.) or just poor reception because you are under some trees. And then the final mystery on my unit: if the batteries are running down, I also get worse results. I would like to ascribe this to the number-crunching going on in the unit, and if the power is dropping, perhaps the algorithms are not being run as quickly, hence the slower response and jumpy results. Then again, if there is a voltage control in the unit, perhaps my reasoning is faulty. Could be a chance thing.

 

Happy caching

[+IVxIV]

Yeah I don't think a GPS device would be prone to "degrade" if used & handled properly. But as you say, if water has entered the unit, perhaps in particular around the internal antenna structure, then yeah I could see it causing an negative influence on satellite signal performance. I agree to, that most of the time an electronic device will either "work or not work", but water entry (+ perhaps an accumulation of other issues) might have rendered your beloved etrex a spot in your junk drawer, and time to go shopping for it's replacement

 

How about trying to go find a cache you had already found? Just so you can see how the thing behaves in an old familiar setting? It would be intolerable to Geocache with a GPSr that I didn't have confidence in.

[+Sputnik 57]

This is a long shot, but when people report that the GPSr is 'suddenly' 30+ meters off on a consistent basis, I always suspect that the datum has accidentally gotten changed. Be sure that your datum is still WGS84. If it somehow got switched to NAD27, you'll consistently be 100 to 200 feet off.

 

If that doesn't work, you may want to try a hard reset, then re-"bake" the unit for 30-40 minutes in an area with a clear view of the sky to let it reload its almanac.

[+Crystal Sound]

I'm probabally stating the obvious, but.... You left out a number of details, so I"m going to just throw some questions out there in response...

 

Are you in the woods? This time of the year, foilage is quite dense, and affects satellite reception. Last night, while visiting one of my own caches, my unit had me over a 100' area (underneath dense tree cover). When I placed this cache, it was about 2 months ago, and much less cover, and i was only bouncing maybe 30'.

 

How strong of a signal are you gettting? Are you noticing that you arent getting as good of a signal from the satellites as you used to?

 

The comment about antennna is pretty good. Yes, the GPS unit is MOSTLY digital and "programming" but there is an RF analog "front end" - particularly an amplifier between the antenna and the chips. RF amps can sometimes get a little funny, so it's possible there is an issue to your unit.

 

You said it got wet on the inside? Did you mean inside the battery compartment, or inside the unit itself? If it was just the battery compartment, you are probably okay. If it's inside the case itself - your best bet is to have Garmin repair it.

 

If none of the above are suspicious, I'd be willing to bet your receiver is fine...

 

How about map datum? Is your unit set to WGS84? Depending on where you are, the differences between NAD27, NAD83, and WGS84 (and/or any others) can explain some inaccuracies. This could make or break you, once you are at the cache location.

 

Does your unit have a battery or power save function? That will make a unit bounce more than the "normal" mode.

 

Do you have WAAS, is it on or off?

 

It's possible any of the above settings (if you have such) could have gotten changed somehow - it might be worth checking them.

 

Just my $.02 - before taxes, tariffs, and other surcharges.

 

Edited for spellung and some minor corrections.

Edited July 15, 2005 by Crystal Sound

[+Byron & Anne]

Any radio receiver has to have a bit of analog circuitry. You said you had gotten water inside. If the was any residue left on the circuit board near the antana input it could effect the analog part of the device. If the power was on when you got water inside it could have damaged some of the analog compontents. Another possibility is a stuck bit. A digital device could have the bit that doesn't change state and still not have a catastrophic failure.

 

Therefore you might want to send the unit in to have it check and/or repaired.

 

Byron

[+Mapper Mark]

What day was that happening? We've been having some really strong solar storms for about a week. That alone will cause weird things to happen to your GPSr. When I use my Leica survey grade GPS, I check the solar weather websites. If there is alot of solar activity we pack it in. Lightning storms within a 100 miles can cause accuracy problems too. Try this site out: http://www.n3kl.org/sun/status.html or my favorite, http://www.sec.noaa.gov/today.html I guess if it were me I wouldn't be setting out any caches during these periods and finding them could be an exercise in futility. By the way, the manufacturers say that GPS is all weather, and they are as long as they are waterproof. Other than as the previously stated lightning problems overpowering the signal and causing signal degradation, a little rain isn't going to hurt anything. So if you are having problems with lock and location, check out the solar activity, it may be the problem. I check it everyday before I use my survey GPS. Saves me time and fustration.

 

Mapper Mark

Edited July 15, 2005 by Mapper Mark

[+Tidalflame]

I did leave several things out, so I'll elaborate:

 

How about trying to go find a cache you had already found? Just so you can see how the thing behaves in an old familiar setting? It would be intolerable to Geocache with a GPSr that I didn't have confidence in.

 

We actually tried this. We went to a cache we had found previously in a wide-open area on top of a hill to check the accuracy. At first, it put us at least 15 metres (~50 feet) away from the proper location, but it did slooooooowly bring us closer. It bounced back and forth a fair bit, though.

 

This is a long shot, but when people report that the GPSr is 'suddenly' 30+ meters off on a consistent basis, I always suspect that the datum has accidentally gotten changed. Be sure that your datum is still WGS84. If it somehow got switched to NAD27, you'll consistently be 100 to 200 feet off.

 

If that doesn't work, you may want to try a hard reset, then re-"bake" the unit for 30-40 minutes in an area with a clear view of the sky to let it reload its almanac.

 

The datum is still on WGS 84, but I may want to try a hard reset at some point. How would I do that?

 

Are you in the woods? This time of the year, foilage is quite dense, and affects satellite reception. Last night, while visiting one of my own caches, my unit had me over a 100' area (underneath dense tree cover). When I placed this cache, it was about 2 months ago, and much less cover, and i was only bouncing maybe 30'.

 

Yes and no. I've been having trouble with several caches. Some of them have been in the woods in deep foliage, but one of the ones I did on Thursday was out in an open field and the GPS was leading us in circles. I did manage to find that one, but it was mostly a matter of luck - the GPS still said I was still 25 metres away when I found it.

 

How strong of a signal are you gettting? Are you noticing that you arent getting as good of a signal from the satellites as you used to?

 

I haven't noticed any change in signal strength. Sometimes it's not great, but it does get down to 6 or 7 metres sometimes. However, even when it's supposedly that accurate, we're having no luck.

 

You said it got wet on the inside? Did you mean inside the battery compartment, or inside the unit itself? If it was just the battery compartment, you are probably okay. If it's inside the case itself - your best bet is to have Garmin repair it.

 

I'm pretty sure all of it got wet inside. There was actually moisture collecting under the plastic covering the LCD. Frankly, I'm surprised it still worked at all.

 

I just checked, and it is indeed on "Normal" mode. The unit isn't WAAS capable.

 

Oh, by the way, does anyone have an idea of what repair might cost (including shipping to/from Northern BC)? I just want a rough estimate - I have a feeling it might end up costing me less to buy a new GPS.

 

What day was that happening?

 

We've been having trouble from roughly the 7th to the 14th (14th was the last day we went caching.)

 

Hm. I'm still suspicious. Either way, I probably won't be caching for a while because I'll be very busy with an exchange program, so I guess I'll see how it's working once I'm able to start again (probably a couple weeks)... if it still seems to be buggered, then I think I'll have to start looking at sending it in or getting a new one.

[marty621]

My thoughts:

 

Lots of wet tree cover can cause the loss of accuracy.

 

I would check the units accuracy by letting it sit on a benchmark of known accuracy for about 15 minutes in plain view of the sky.

 

Regarding moisture, I would carefully let it bake out on the dashboard of my car for several days. But be careful not to let the LCD display get too hot as it might get ruined. I would not leave it there unattended.

[+Sputnik 57]

If that doesn't work, you may want to try a hard reset, then re-"bake" the unit for 30-40 minutes in an area with a clear view of the sky to let it reload its almanac.

 

The datum is still on WGS 84, but I may want to try a hard reset at some point. How would I do that?

Hold the "up" and "enter" keys and then press the "on/off" button to reset the entire machine. You will receive a warning prompt. You can avoid this message using the sequence "Page" plus "Enter" plus "Up" and then press the "on/off" button. Hold the keys down for 5 seconds to perform the reset.

 

The etrex also has "secret" modes for test purposes. Hold the "up" and "page" keys and then press the "on/off" button to enter this screen. The ROM test may show a false failure on this screen since a factory tester is not attached. You can run the key tests by pressing each of the keys. You can use the page key to cycle through all of the display tests. The screen shows the status of the power on diagnostics plus internal battery voltage and external if present. A clock display shows seconds, the revision level of the software is shown, and a thermometer reading shows the internal temperature in degrees Celsius.