Okay, The Good Luck Stopped

[+Thot]

On our first few hunts, after walking around some the GPS homed right in on the target. This time it pointed all over the place. I noticed it was doing flaky things in the car. We’d be moving down a straight road and the target would suddenly jump from a modest distance off to the left side of the road to the about the same distance off to the right. As we finally neared the target it pointed to four very different locations as far apart as 40'. ( I tried to narrow them down each of these points by coming from different directions.)

 

The description rated it one star (easy) to find, but given the large amount of dense shrubbery in the area encompassed by these points I decided it was futile to try to find a 1” x 2” microcache wrapped in camo tape.

 

How would others handle this situation?

Edited June 30, 2004 by thot

[+TotemLake]

It'll help to know what you have but the first thing that comes to mind is to cycle the power and see if that fixes it.

 

You might have a firmware corruption which can happen for oddball reasons and a firmware update would be in order.

 

If you post what brand and model you have, you might receive some more targeted info about that particular unit.

[+Thot]

If you post what brand and model you have, you might receive some more targeted info about that particular unit.

It's a Magellan GPS 315.

 

I should have mentioned, there was pretty heavy cloud cover, but the location was in the open with lots of open sky area.

Edited June 30, 2004 by thot

[+TotemLake]

Somebody who knows this unit will soon step up to the plate. My guess is you probably just needed to cycle the power off and then on again. That tends to happen with my MeriPlat on a blue moon and that's how I fix it.

[+Thot]

So, you think the behavior I described suggests the unit was really malfunctioning? I don’t have enough experience with them to know. The thing is spec’d with an error of 50’ (I asked about this in another thread. But, up to now it seemed to be getting within 2-3 feet after repeated trials. I had been confused by the difference between specified accuracy and previous performance. This latest performance is more consistent with the spec. I thought maybe they are flakey inside the car, and maybe heavy cloud cover screws them up.

Edited June 30, 2004 by Thot

[+EraSeek]

Cloud cover has nothing to do with it. I don't know Magellan systems, and not quite sure what you are experiencing. However, if this thing jumps like crazy when you are traveling at speed, it could be that you have the speed filter setting messed up. In your menu see if you can find it. It should be set to auto. If you have it set too far off, your movements over run the screen. Also, if you are going at speed and have the unit zoomed in too much you will also have this problem.

This is just an outside chance guess. Maybe you can describe the situation a bit more detailed. It is also possible you are just losing the sats coverage in your car and it makes jumps whenever it locks on.

[+TotemLake]

So, you think the behavior I described suggests the unit was really malfunctioning? I don’t have enough experience with them to know. The thing is spec’d with an error of 50’ (I asked about this in another thread. But, up to now it seemed to be getting within 2-3 feet after repeated trials. I had been confused by the difference between specified accuracy and previous performance. This latest performance is more consistent with the spec. I thought maybe they are flakey inside the car, and maybe heavy cloud cover screws them up.

The stated accuracy of the unit is the minimum you will see. EraSeek made a good point about not getting a good lock on the satellites in the car. The roof will block the GPSr(eiciever) from attaining the signal unless you have it close to a window or windshield. I don't know about the older units, but my Meridian doesn't have a Speed setting. There is an Auto setting for laying down a track with more trackpoints , particularly when you are going around corners, but I've never seen this be affected by speed, only by coverage.

 

Depending on the type of coverage you have, that is the constellation of the satellites has to be wide and not narrow, and more birds is better than less, your estimated position error can range greatly. Bear in mind the EPE is nothing more than a sophisticated formulated guess of where you are about 95% of the time. You could be outside of that estimate 5% of the time. How do you tell? Unless you're really good at triangulating yourself on a map, or standing over a benchmark you will never really know for sure. You just got lucky with a good constellation at the time of your searches.

[+AtoZ]

Okay Trouble shooting 101.

Did the unit work correct the day before. If yes then the problem is as you stated: I should have mentioned, there was pretty heavy cloud cover, but the location was in the open with lots of open sky area.

Heavy Cloud cover WILL affect a GPSr reception. Moister WILL attnuate the signal the heaver the cloud cover the more attenuation and the flaker the signal. We have a GIS department here at work and use GPSr to track our accets and I talked to the head guy and he confirms the above. My resent exp is that YES cloud cover will affect the GPSr. Usually I have been getting a 5 fot accuracy the other day under HEAVY cloud cover it was 35 to 40 feet.

But I have had a GPSr that was flaky,under clear open skys, and according to Garmin it was faulty so I took it back and exchanged it for my current GPSr.

Suggestion is wait for a sunny CLEAR sky and see if the problem exist still but to discound the HEAVY cloud cover is not correct.

cheers

[+EraSeek]

Okay Trouble shooting 101.

Did the unit work correct the day before. If yes then the problem is as you stated: I should have mentioned, there was pretty heavy cloud cover, but the location was in the open with lots of open sky area.

Heavy Cloud cover WILL affect a GPSr reception. Moister WILL attnuate the signal the heaver the cloud cover the more attenuation and the flaker the signal. We have a GIS department here at work and use GPSr to track our accets and I talked to the head guy and he confirms the above. My resent exp is that YES cloud cover will affect the GPSr. Usually I have been getting a 5 fot accuracy the other day under HEAVY cloud cover it was 35 to 40 feet.

But I have had a GPSr that was flaky,under clear open skys, and according to Garmin it was faulty so I took it back and exchanged it for my current GPSr.

Suggestion is wait for a sunny CLEAR sky and see if the problem exist still but to discound the HEAVY cloud cover is not correct.

cheers

Sorry, simply NOT true. And I quote once again:

 

"Rain, Snow, Clouds and GPS Reception

1) Does RAIN or SNOW or CLOUDS affect the reception of my GPS receiver?

Answer: No. Not so as the user can tell without instrumentation.

 

2) I notice that when it rains and I turn on my windshield wipers my GPS has trouble locking. Doesn't this mean that rain affects my GPS reception?

Answer: No. What is affecting your GPS signal reception is the fact that the wipers running back and forth across your windshield is intermittently blocking reception and making it difficult for your GPS to get a complete navigation data string. Turning off your wipers until the signal locks will speed the initial lock.

 

3) Sometimes my GPS gets a thick coating of water on it and it does not receive as well. Does this not prove that rain can cause signal degradation?

Answer: No. A coating of water, even a fairly thin one is NOT the same as raindrops. The GPS signal frequency of about 1575mhz was chosen expressly because it is a "window" in the weather as far as signal propagation is concerned.

 

4) This all sounds good, but do you have a reference that supports these statements?

Answer: Yes.

See: Ref: http://www.navcen.uscg.gov/pubs/gps/sigspec/default.htm (Courtesy of Sam Wormley)

 

Cloud, Rain, Snow, weather in general does NOT attenuate the GPS signals enough to effect accuracy. As can be seen below, the total atmospheric loss (from all causes including rain, clouds, snow, fog, etc.) is but 2db. This is small compared with other variables.

 

L1 and L2 Navigation satellite Signal Power Budget

 

Parameter L1 P-Code L1 C/A-Code L2 P-Code

-----------------------------------------------------------------------------------------------------

User minimum received power -163.0 dBw -160.0 dBw -166.0 dBw

Users linear antenna gain 3.0 dB 3.0 dB 3.0 dB

Free-space propagation loss 184.4 dB 184.4 dB 182.3 dB

Total atmospheric loss 2.0 dB 2.0 dB 2.0 dB

Polarization mismatch loss 3.4 dB 3.4 dB 4.4 dB

Required satellite EIRP +23.8 dBw +26.8 dBw +19.7 dBw

Satellite Antenna gain at 14.3° 13.5 dB 13.4 dB 11.5 dB

worst case Block II off-axis angle

Required minimum satellite antenna +10.3 dBw +13.4 dBw +8.2 dBw

Input power 10.72W 21.88W 6.61W

 

5) Ok then, can I use my GPS underwater?

Answer: No. Just a few millimeters of "solid" water will severely attenuate the GPS signal.

 

Joe Mehaffey "

 

 

Clouds, rain, snow, will not be a problem. Satellite configuration, reception, metalized windshields, will.

[+Thot]

EraSeek made a good point about not getting a good lock on the satellites in the car. The roof will block the GPSr(eiciever) from attaining the signal unless you have it close to a window or windshield.

 

After I saw the jumping around I began holding it forward -- over the dashboard near the windshield. It didn't seem to help a lot. I've had so little experience with this thing my observations aren't probably worth much but usually I've been able to hold it in front of me at a normal distance and it seemed to work fine until I got very near a target. Then it seems to respond too slow -- have a lag.

 

I don't know about the older units, but my Meridian doesn't have a Speed setting. There is an Auto setting for laying down a track with more trackpoints , particularly when you are going around corners, but I've never seen this be affected by speed, only by coverage.

 

So far I haven't been able to find a speed setting.

 

Depending on the type of coverage you have, that is the constellation of the satellites has to be wide and not narrow, and more birds is better than less, your estimated position error can range greatly. Bear in mind the EPE is nothing more than a sophisticated formulated guess of where you are about 95% of the time. You could be outside of that estimate 5% of the time. How do you tell? Unless you're really good at triangulating yourself on a map, or standing over a benchmark you will never really know for sure. You just got lucky with a good constellation at the time of your searches.

 

I don't know what factors control the number of satellites one gets. I haven't been paying close attention to this but the few times I've checked I was getting 7 or 8 and the looked pretty solid (tall bars).

 

If this problem happens again I'll check the count.

 

Assuming one has a lot of sky, what effects this most -- location on the earth or time of day or what?

[+EraSeek]

Biggest factor with an unobstructed view of the sky (no tree cover, canyons, buildings, metalized windshields) is the position of the satellites. There are occasions where there may be fewer in view OR they are all kinda lined up instead of spread out for good triangulation. Also an active ionosphere can throw accuracy off because it causes signal delay, and timing is important. Another factor is multipathing, which means signals are bounce of of objects (like buildings, trees)before you recieve them. This often causes your postion to move about or bounce around (a now here, now there type thing).

[+AtoZ]

Okay, I guess I was miss informed, UGH experts. Ol well they only thing I know is tha since it has been very over cast and cloudy here is my accuracy has gone from 5 feet in the local school yard where I had several satelites marked as diff. To 30 to 40 feet accuracy in the same location and no satelites marked as diff. This is at approximately the same time of day, when im bored and want to go out side.

 

I know when I was using a Garmin Ledgend that I had it in a field and using the compass/point screen that the compass was vary erradic swinging all over the palce and reporting the cache as 150 feet in on direction and then 540 feet in another direction with in a matter of seconds. This I think was a faulty GPSr. I know with my Garman GPSMAP76 it is very stable even when I stop. the needle doesnt move, I knowit only points correclty when your moving but the Legend went haywire when I stopped.

 

I guess I would say see what happens on a nother day and differant conditions. Basically GPS technology isn't highl percise nor accurate. Atleast at the consumer lever. If it were we would all stop when our GPSr said "Arrived at Destination" and we would be standing on the cache every time.

 

Thanks Earseek for the URL.

cheers

[+Niss Feiner]

I too have the Magellan 315 and i too had the same problem. I was walking through the woods and had my waypoint jump from side to side, also i had my coordinates move, i was looking at the screen that displayed the LAT LONG and it was moving when i stood still! So far i enjoy this unit, and it works pretty well but sometimes i get the mentioned problem. Try changing the batteries? I dont know if the weather changed much but when i changed the batts. it worked better.

 

Niss

[+Thot]

I too have the Magellan 315 and i too had the same problem. <snip> Try changing the batteries? I dont know if the weather changed much but when i changed the batts. it worked better.

 

Niss

Thanks for the reply. I always start a new hunt with freshly charged NiMH batteries. In this case the unit had not been on for more than 5-10 minutes when the jumping from left to right began.

 

I haven't had a reason to use the unit again, but I'll probably try that cache again next week. If it happens again I'll do as someone suggested and turn it off and back on. I should have though of that -- "Dammit Jim, it's just a computer."

Edited July 1, 2004 by Thot

[dana_mix]

Just wiped out my post, until I check something.

Edited July 1, 2004 by dana_mix

[+XVampire]

I have had MUCH better accuracy when 3d mode is enabled. If it is off a cache can jump 200 feet! Just my 2 cents.

[+Thot]

I have had MUCH better accuracy when 3d mode is enabled. If it is off a cache can jump 200 feet! Just my 2 cents.

I don't think my unit has a "3D" mode.

[+XVampire]

If its a magellan 315 it sould have 2d and 3d mode in the settings somewhere.

[Pipanella]

I have to agree with changing the batteries. I have an eTrex yellow and that's happened to me a couple of times - it had me going in circles! - and a change of batteries solved it.

[+Niss Feiner]

I went out looking for the Gables Cache + Jakes treasure, in Barrie Ontario, and I had my waypoints just jumping all over the place.....it would be 200m ahead of me then 200m behind, then 200m to my right.....all over the place! When i get in close it stops but in the mean time im chasing a ghost!

 

Niss

Edited July 2, 2004 by Niss Feiner