Not very accurate GPS - help!

[+The Happy Harpers]

We have a Magellan SporTrak Pro that we got as a gift a couple of years ago. We love geocaching, but are getting frustrated by searching all over the place because we are not getting very accurate readings. Just today we tried three caches, found two and gave up on the third. The two we found showed that we were still +/- 100' away! Is this normal? It's fall and most of the leaves have dropped, and for one of the caches that showed 100' away we were in an open field! What should we expect, accuracy-wise? How can we increase our accuracy, and if we decide to buy a new GPS, which one should we choose?

 

Thanks for your help!

 

Frustrated, but still having fun!

[itchytweed]

We have a Magellan SporTrak Pro that we got as a gift a couple of years ago. We love geocaching, but are getting frustrated by searching all over the place because we are not getting very accurate readings. Just today we tried three caches, found two and gave up on the third. The two we found showed that we were still +/- 100' away! Is this normal? It's fall and most of the leaves have dropped, and for one of the caches that showed 100' away we were in an open field! What should we expect, accuracy-wise? How can we increase our accuracy, and if we decide to buy a new GPS, which one should we choose?

 

Thanks for your help!

 

Frustrated, but still having fun!

 

Bear with me on this...this may get long.

 

The key to accuracy is timing. In a perfect world, everything would line up just right and you would be able to measure right down to a gnat's whisker. We don't live in a perfect world...darn.

 

If the Magellan has been in a box gathering dust bunnies on the back of the bedroom closet for a long time, it may take some time to get everything back into shape. Electronic equipment is like our bodies, it gets out of tune just sitting around. It takes time for everything to stabilize.

 

Now assuming that you have been using this unit for some time and it is still not behaving, there could be several reasons: multipath, buggy software, poor satellite constellations, no WAAS reception, weak batteries. I could go into a couple of other items but that is something that manufacturers do not like to hear.

 

Ideally, multipath is drastically minimized by using circular polarized signals. You can receive a left-handed signal on a right-handed antenna. Same idea as putting in a left-hand threaded screw into a right-hand threaded hole. The handedness of a signal changes upon bouncing off of a surface. This is a great thing about circular polarized signals.

 

If your software is old, I would see if Magellan has an updated firmware package for your unit. This may help out improving the accuracy. Some GPS units have almanacs hard-coded (IMHO a BIG mistake). New software updates the almanacs.

 

Poor constellations can give you grief. If all you are receiving are satellites that are close together, your position resolution falls off as opposed to receiving satellites that are far apart.

 

There has been a lot of talk about WAAS reception with the new birds being put up. The WAAS system helps compensates for our dynamic atmosphere constantly moving around and messing up the signal timings. There are precisely monumented receiving stations around that listen to the GPS signals. The system compares what it receives vs. where it is and distributes corrections via the WAAS satellites. These corrections allow my GPSr to get +/- 7 ft accuracy most of the time outside. If your GPSr does not have WAAS reception turned on, turning it on will help out with the positional accuracy.

 

Weak batteries would not be the situation. I am sure that your unit would be screaming to be fed when necessary.

 

One thing about patch antennas. They have to be parallel to the ground to work the best. Any tilting and their performance degrades.

 

Hope this helps and now back to making dinner....

[ossumguywill]

Do a unit restart or a hard reset. Not sure how.

[+kohldad]

Another thing that will help is allow the unit to "catch up" after a drive. Usually, I get to the aproximate location and then set the unit down while I start looking. After a couple of minutes, I'll come back and move it in the direction indicated only to set it down again. I repeat the pickup and move process every few minutes til it seems like it has stabilized.

 

A second trick to do is triangulation. Stand one place, give the unit a minute to stabilizer, make a mental note of where the spot is indicated. Move abou twice the cache distance and repeat the process. Again make a mental note of the spot. Repeat one last time. After the third spotting, hopefully all three spots are about in the same location. That should give you a good base point to start looking from. I'll generally look 50'. So far, this would put the cache within my search area with all except one.

[+The Happy Harpers]

Wow! Thanks for all of your help! I'll try the ideas listed. Heading out again soon to try it out!

 

Thanks! You all are awesome!!

 

Sue

[+YuccaPatrol]

 

If the Magellan has been in a box gathering dust bunnies on the back of the bedroom closet for a long time, it may take some time to get everything back into shape. Electronic equipment is like our bodies, it gets out of tune just sitting around. It takes time for everything to stabilize.

 

 

?????? Although having a current almanac loaded is very important to GPS accuracy, I'm not so sure about the idea that it takes time for electronics equipment to get back into shape.

 

I have an old calculator that had been stored in a box for 5 years. It didn't have any problems getting back into shape and added and subtracted correctly as soon as it was turned on.

[+Night Stalker]

My SporTrak Pro was at least as accurate as my new 60CX. I had to learn how to use it though. I found that as I approaced a cache I had to really slow down or I would overshoot. When it showed close to zero I would set set it down and start looking. After about 5 minutes if I hadn't found the cache I would pick the GPS back up and work toward the new zero point. Usually the 2nd zero point was very close to the cache.

[itchytweed]

 

If the Magellan has been in a box gathering dust bunnies on the back of the bedroom closet for a long time, it may take some time to get everything back into shape. Electronic equipment is like our bodies, it gets out of tune just sitting around. It takes time for everything to stabilize.

 

 

?????? Although having a current almanac loaded is very important to GPS accuracy, I'm not so sure about the idea that it takes time for electronics equipment to get back into shape.

 

I have an old calculator that had been stored in a box for 5 years. It didn't have any problems getting back into shape and added and subtracted correctly as soon as it was turned on.

 

A calculator and a GPSr are two completely different animals. The calculator is a device that does not rely on a precision clock to function and get the correct answers. The faster the clock, the faster the answers appear and vice versa. Clock speed does not affect the answer.

 

The clock in a GPSr has to be stable in order to display its location accurately. If the crystal clock inside the GPSr is drifting up and down, your position will be unstable. Mind you, the atmosphere has a big role in making the numbers unstable but there is no need for more sources of instability.

 

I use my GPSr for "amateur surveying" of benchmarks and when I am going to do a benchmark run, I keep it on for 24 hours preceding so that the unit is as stable as it can be. Using the GPS V and its built-in antenna and a 45 minute run, I can get post-processed results of +/- 0.5 meter.

[GermanSailor]

Hi,

 

some people here seem to have a let's say, interesting opinion how GPS works.

 

1. Cross check your GPS with itself. Without moving make 10 Waypoints and repeat the same process the next day. (With a different Satellite constellation.) Calculate the average, or use a program like Easy GPS to look at the Waypoints. If everything is normal, both points should be pretty close. (30 to 50 feet)

2. Check the readings of your GPS with another GPS, again, both units should be pretty close.

3. Check your GPS with a place where you know the coordinates. (Google Earth is not always accurate!) Use a benchmark or a Topo / Trigonometric Point.

4. Do those check preferably where you have no buildings or foliage, so you get a non-interfered signal and more important as many sats as possible.

 

Some guys wrote about letting it warm up and so. Fact is, that some units reacquire the almanac after a reset, which takes some time since it take about 12 minutes to transfer the almanac.

 

All GPS units have a internal clock which will be synchronized by the satellites, since without an absolutely precise time GPS can't work. (It measures the time a signal travels from the sat to your receiver and calculates the distance) That's actually one of the reasons 4 sat are required to get a position and not just three. If you want to know how complicated getting the "proper" time really is, google "time-dilation" and prepare to dig really into physics.

 

Anyways: It's probably not a bad idea to reset you GPS and let it sit in clear view of the sky for half an hour or so to make sure it gets the new almanac. I haven't seen a unit with a hard coded almanac, so I would say it is worth a try.

 

I absolutely doubt that bad batteries negatively affect the performance and accuracy of your GPS. It either works or not!

 

Actually if the GPS is constantly 100' off, I would be a fair amount of good Bavarian beer that it is just a mix of Datums. Like you have WGS 84 coords, but the GPS is set to NAD 27 (which is aprox. 100' off).

If you don't know how to change the datum, the reset should have set the GPS to back to WGS84, since that is the standard setting for almost all GPS receivers.

 

Some units allow you to manually enter a Position correction factor in order to match your GPS to a specific map or chart. If you have such a factor entered in your GPS, it will of course be off by that factor.

 

Hope I could help a little!

 

GermanSailor

Edited November 18, 2006 by GermanSailor

[+budd-rdc]

My SporTrak Pro was at least as accurate as my new 60CX. I had to learn how to use it though. I found that as I approaced a cache I had to really slow down or I would overshoot. When it showed close to zero I would set set it down and start looking. After about 5 minutes if I hadn't found the cache I would pick the GPS back up and work toward the new zero point. Usually the 2nd zero point was very close to the cache.

This one pretty much hits the nail on the head. I have a Magellan GPSr older than the Sportrak Pro (GPS Companion) and I had to learn how to use it. Believe it or not, electronic devices are designed by human engineers, so it shouldn't be a surprise that each brand and make is a result of different compromises.

 

The SporTrak Pro tends to respond slower than the newer GPSr's, and people commonly describe the behavior as the "Boomerang Effect" or "slingshot effect". Once you get used to it, it's a great unit, very, very accurate.

[+BuxCamper]

Checking the datum is the fastest thing to do, it needs to be WGS84.

[+darylt]

Checking the datum is the fastest thing to do, it needs to be WGS84.

 

Does it need to be WGS84 in the UK too?

[adrianjohn]

Abso-caching-lutely!!

[+darylt]

Abso-caching-lutely!!

 

Thank You

[GermanSailor]

Checking the datum is the fastest thing to do, it needs to be WGS84.

 

Does it need to be WGS84 in the UK too?

 

Yes, it always has to be the same as the one the coords you try to find are. GC.com coordinates are always stated in WGS84 datum.

 

GermanSailor