GPS Unit Acting Funny

[Xorand]

Did some benchmarking today at lunch and again after work. I was finding the benchmarks primarily based on the descriptions given instead of relying on the GPS. Took pictures of each benchmark (6 found today) along with my GPS unit.

 

The first two, I had only turned on the GPS a couple minutes before, so I can understand why it might not be too accurate. The third one, it was pretty much spot-on for the coordinates and the altitude.

 

After work, I chased down three other benchmarks. The first one, the coordinates were dead-nuts on with the page listing, as well as the altitude (within 5 feet). The second one, just 1/2 mile and 5 minutes up the road, the altitude showed like 48000 feet (true altitude was around 500 feet) and the indicated speed while the unit was stationary on the ground was 12.4 mph.

 

The third benchmark after work, everything was pretty much spot-on, but the altitude was off a bit.

 

I keep hearing rumors that we're receiving degraded signals right now during wartime, then I hear rebuttals indicating that's not the case.

 

I noticed that the track lines that I left while driving to and from the benchmarks were off a couple hundred feet by driving up and back the same exact route. Also, the original waypoint that I assigned to my house while sitting in the driveway with the unit on for quite some time is now about 250-500 feet away from where I end up when driving home.

 

Any comments? It's a Magellan SporTrak Pro, only about a week old. Today's benchmarking was done with a fresh set of AA batteries.

[peter]

Besides the official statements from the DOD that GPS signals have not been affected, there are lots of people who run continuous monitoring stations and they're not reporting any anomalous behavior. I don't do that level of checking, but I do record tracklogs on a daily basis and haven't seen anything out of the ordinary since the war started.

 

As to the behavior you observed, it would be helpful to know what the satellite page was showing at the time. I've sometimes seen GPS receivers give results that are way off, but it's usually with just the minimum number of satellite locks where a multipath signal from one of them could result in a large error or the solution calculated could be ambiguous due to poor geometry.

[+jtice]

I was doing the exact same thing !!!

While bench marking,, i was about 30 feet from tthe benchmark, then all of a sudden, my Garmin etrx Vista tells me to turn 90 degrees right and walk 50 feet !

 

I have also noticed that if i make a track log, then go back on the same trail or road, that it will be off and not track right over tthe same line.

 

AND ,,,!!!,, my home waypoint is also about 200 feet off. Thats what Really bothers me. 10, 20, even 30 feet,, OK , but 200 !

 

To tell you tthe truth, the builtin electronic compass is what REALLY goes wild, it seems to even be 180 degrees off some times, or, flicker all over, and not stay steady in one direction.

 

What in the world is going on!! ??

 

P.S. and I was wondering how the heck our troops could keep getting lost over there when they have GPS units. I know now.

[Kerry.]

One and/or all of the following general possible local anomalies, obstructions, interference, multipath or the other 5%

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+Searching_ut]

Get out and play with your receiver more, you'll get used to it's quirks over time. While the sportrak can be quite accurate, it can also get wild hairs up it's .... sometimes. The problems are sometimes reception related, sometimes software withing the unit itself, sometimes I think elves may be involved.

 

As for any sort of problems due to SA type interference, I haven't seen any indication of it, nor have I seen any data indicating anything strange from the numerous sites that track accuracy continually. Sometimes, they do jamming in local areas, but they pre-announce it first. You can find info about it at the following URL, or most aviation websites for pilots in the form of NOTAM's

 

http://www.navcen.uscg.gov/gps/default.htm

 

If you're curious about atmospheric affects, and other local reception problems, many Universities and buisnesses run sites that continuously monitor errors. One I often use in my area because I like the graph is:

 

http://www.spacenv.com/~rice/BLO/pos/index.html

 

If you look at some of the historical data available on the site, you'll see variation in reception quality is normal.

[shrekTBA]

For those with a Vista, they will go really wacky if you dont calibrate the compass, especially if you change the batteries. They rely on the compass for slow speed nav, so if it is way off, so are you. Also, try to hold the screen as level as possible, that can trick it also, as it seems to catch or bounce alot. I had to learn the hard way on these points, but now I have a very reliable unit since I figured out what it likes.

 

It's not a sport unless there is something dead in the back of the truck when you get home.

[+nincehelser]

I haven't seen any indication that SA has been turned back on.

 

However, last week I ran into a strange situation.

 

For about a minute, my Vista was showing that I was moving backwards very quickly (according to the map view). I was in the car, and the speed backward was a heck of a lot faster than the speed I was acutally moving forward. It got itself about a mile off-track, then corrected itself.

 

I have no idea if it was caused by a software bug in the receiver, or something wacky with sat signals.

 

George

[+jtice]

Yes I've noticed i have to calibrate the compass pretty much every time i turn it on, just to be on the safe side.

 

And,, Ive had that problem with it wonting to all of a sodden show a straight line like 200 yards off some direction when making a track,,, like i walked out to the laft 200 yards than right back to where i was,,,, for the most part, it doesnt seem to effect my distance readings , but it is anoying,,, it mostly happens when im walking, and dont hold the unit out in front of me, sometimes i clip it to the back of my boonie hat. It will keep good reseption,, but goes whacko sometimes.

[Kerry.]

One thing with a carrying a handheld is always try and face the equator as there's basically no satellites over the poles and if your facing the poles then the sats (most of them) are behind you and you become an obstruction.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+tirediron]

Anything around that might have been causing a multi-path effect (spurious signal reflection)? Trees, power-lines, hills, buildings, that sort of thing? This is a problem that is allegedly more serious with the Magellan's line of quad-helix antennae. As well, they prefer to be held horizontally vice vertically. Hope that helps

[Xorand]

quote:

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Originally posted by tirediron:

Anything around that might have been causing a multi-path effect (spurious signal reflection)? Trees, power-lines, hills, buildings, that sort of thing? This is a problem that is allegedly more serious with the Magellan's line of quad-helix antennae. As well, they prefer to be held horizontally vice vertically. Hope that helps

---

 

Good information. I'll remember that. The odd altitude reading was just a minute or so after getting out of my truck to walk over to the BM. In my truck, I mount the GPS in one of those cell phone squeeze holders on the top of my dash. The unit sits in a fairly vertical position there. Also, multipath signals may enter into it at that location due to the proximity of the roof, hood and other metal in the vehicle.

 

BTW, like your IH icon. Big IH fan? I have an uncle that recently retired from Navistar/IH. Back in the day, I toured their plant in Fort Wayne where they were making medium duty trucks in one facility and Scouts in another (that kinda dates me, huh?)

[+GeoForse]

Customer: GPS Unit acting Funny

 

Tech Support: GPS unit told to straighten up and fly right, and to get serious...

 

[+apersson850]

Didn't see this thread when i posted this report.

 

Take a look at that!

 

Anders