Garmin eTrex snesitivity issues

[Guest redd]

I've had my basic yellow eTrex for a few weeks now, and it seems like just this week my sensitivity and accuracy has gone downhill. At first, I wondered if the DoD was tinkering with SA due to recent events, but a friend with another eTrex, as well as some other Groundspeak forum users have reported no such problems.

 

I can't recall dropping the unit, or giving it any excessive shocks lately.

 

I seem to get good locks while walking around in my back yard, but I can drive for miles in my pickup-truck without getting a single steady lock.

 

I never had this problem before.

 

Has anyone else ever noticed this symptom? I'd hate to have to figure out how to return it if there's a problem.

 

Thanks...

 

Scott

redd@interbug.com

http://interbug.com/pigeon

[Guest mcb]

Assuming that there is nothing wronge with you unit there are a couple of reason for such behavior. Most likely it is satellite geometry. Remember the satellite orbit the earth every 12 hours and thus the position of the satellite is alway changing. Sometime they just get them selves in a bad arangment for your location. If you combine this with some well placed trees or building you can pretty much stop the unit from getting a lock.

 

You said that is get a lock in the backyard but in the truck it won't lock on. In you truck the roof can block some of the satellites. This is usually not a problem if there are alot over head but if the geometry of the satellites is such that the roof is block the majority of the satellites then this could also give you problems. A GPS will alway take longer to aquire a lock if it is moving when attempting to lock on. Most of the time this is not much longer but if the satellite geometry is bad and you are moving you can have problems.

 

So maybe you where just a victume of bad satellite position. If however your unit continues this even when you are sure there are lots of satellite in good position then you may have a defective unit.

 

Hope that helped

mcb

[Guest redd]

ccuracy and usability has returned to normal. There are two things that happened that may, or may not, help to explain the problem.

 

First, I had some rechargable NiMH cells loaded. Perhaps the lower working voltage threw off the unit? I've since loaded some alkalines. I personally don't think this would be the case. If it's got enough voltage to operate, then it's got enough voltage to operate, regardless of the type of cells loaded.

 

Secondly, the unit had been inactive for several days. Perhaps the geometry of the constellation had changed to the point that it was difficult for the unit to get a good lock.

 

Once I let the unit stay on in my home (which I do get a fair signal through my roof), it seemed to lock on normally. I took the eTrex for a drive, and also used it on a cache hunt the next day. Also this morning I had the unit on while driving and everything seemed to have returned back to normal.

 

I'm guessing that when I have had the unit off for some time, I need to allow it enough time to readjust to the new constellation configuration before expecting normal accuracy.

 

That being said, don't the units have some sort of internal almanac to predict the locations of the satellites?

 

Thanks...

 

Scott

redd@interbug.com

http://interbug.com/pigeon

[Guest mcb]

I don't think the rechargeable should have effected the GPS at all. I run my GPSMAP 76 and eTrex on NiMH with out a problem. Your second reason is a very likely the cause. I have also notice that after sitting on the shelf several days it is a little flaky for the fist little bit of use. The GPS does have an almanac but this is an approximation only to determine if the satellite will be visible or not. The unit must download the ephemeral data each time the GPS passes over head to get data accurate enough to navigate by. Sometimes they do actually allow satellite to drift into different orbits and if you leave a GPS off for a long time the satellite may not be just where the GPS thinks it should be. This can take time for the unit to figure out what is going on. I try to make sure I my GPSs on and let it lock on at least once a week. Not necessary but it helps avoid problems like you had.

 

Later

mcb