Gps Off By 20-30 Feet

[suthrncop66]

Thanks for the reply, I'll have to keep track of the accuracy and try some out of tree coverage..

Thanks

Robert

[Kerry.]

Edit: As far as WAAS goes, I believe that any satellite from 33 and above is a WAAS satellite. Maybe someone who knows for sure can confirm that.

Right now from a GPS perpective the maximum number of GPS satellites that the system allows is 32 relative to the PRN numbers (GPS sats are PRN 1 thru 32). Above this different manufacturers call things a little differently.

 

What garmin call 35 (and shows as 35) should really be PRN 122, similar with Garmin's 47, which is PRN 134.

 

Sometime in the not all that distant future some receivers will require to be able to determine satellites from several (possibly 3 separate systems), which could total all up around 80 odd satellites.

 

But right now with a pure GPS receiver (Garmin at least) anything over 32 is a WAAS or EGNOS or MTSAT geostationary satellite.

 

Cheers, Kerry.

[suthrncop66]

I just went and found my second cache, GPS off by 99 ft. so for the two I've found it has been off 71 and 99 ft..not even close, this thing is under warranty should I contact Meridian??any suggestions??

[+Tahoe Skier5000]

 

just curious....

First remember that all gpsr units are receving the same signals... (Unless you have one that can pick up military signal/etc etc.)

So, greater cost = better reception isn't really true. Patch atenna work fine, provided they are receving enough good signals (which is truee of all units).

Professional grade units have improved software. Trimble, for example, has several patents on their software with improved processing algorithms. Add improved antennas, multipath filtering, interference shielding, realtime DGPS from a local base station not just from the Coast Guard beacons, better signal masks, etc and you can consistently get submeter accuracy in a handheld unit -- centimeter under good conditions.

 

Of course you are going to pay $$ for it.

Trimble is awesome!

 

GeoXT

 

They even have downloadable screensavers on the GPS... hilarious

[Kerry.]

I just went and found my second cache, GPS off by 99 ft. so for the two I've found it has been off 71 and 99 ft..not even close, this thing is under warranty should I contact Meridian??any suggestions??

Suggestions? (if not already checked and checked again) check your datum and format settings and then check them again (just to make sure )

 

If this consistency exists/continues then it is probably more likely a user/configuration issue.

 

Cheers, Kerry.

[+c0yote & co]

I just went and found my second cache, GPS off by 99 ft.  so for the two I've found it has been off 71 and 99 ft..not even close, this thing is under warranty should I contact Meridian??any suggestions??

How many sat's did you have for the 2nd? Are you slowing down / stopping when near the cache to let the GPSr average? Are you holding it away from your body? When close, I usually set my bag down and my GPSr down on top of it and let it sit for a few minutes - usually does the trick. If I set it down to where it thinks the cache is, it also gives me my 20-30 foot radius to search.

[Major Catastrophe]

I was interested in the part of this thread that talked about day vs night accuracy. Part of the information that is included in the data from the satellites, is estimated correction factors for atmospheric factors. Do our civilian consumer-grade GPSrs use this information?

 

Also, FWIW, I'm well satisfied with the accuracy of my SporTrak. It is nearly always accurate within 0.01 mile, and sometimes as good as 0.002 mile.

[dana_mix]

Yes, you can do better. Take a number of readings at a spot and take the median of the two coordinates separately. I like to use UTM, and write the numbers down. Take maybe 10 readings and you have a good chance of better than 10ft accuracy. I've found this out by comparison with a National Geodetic Survey station disk.

[dantonac]

I just went and found my second cache, GPS off by 99 ft. so for the two I've found it has been off 71 and 99 ft..not even close, this thing is under warranty should I contact Meridian??any suggestions??

 

Maybe, but I doubt it.

 

First make sure your GPS is set to use WGS84. Second, make sure your coordinates are entered correctly. Third, if your datum is WGS84, do NOT enter the UTM coords that geocaching.com displays (along with WGS84) as there is no mathmatical formula that converts between the two that is 100% accurate in all locations.

 

Your accuracy isn't out of bounds, but it isn't desirable if you are in a clear area, you should be seeing 20-40' accuracy in most cases.

 

However you indicated that for your first find you had 3 satellites and were under tree cover. This can easily explain the problem. You need 4 satelites and to have the sky relatively open for reasonable accuracy.

 

You asked about how to tell if your magellan is locked on a WAAS sat. Go to the sat screen and look for red W's. Those are how Meridian's display WAAS sats. If the red W (assuming there is one in view) has a red square around it, then you are getting the signal. If it's a red W with no red square around it, you are not.

 

To test the accuracy of your GPSR you should ***NOT*** use a geocache as you are not guaraunteed the listed coords are accurate at all. What you should do is lookup reference points (I forget the web site that lists them, but basically there are precisely surveyed markers in various places often municipal buildings) in your area or go into a clearing, let your unit average for several minutes and then save the waypoint. Turn your unit off, then back on, let it average for a few more minutes and then see how far it says you are from the saved waypoint. That is the accuraccy of your unit(at that time and in that area anyway). It helps to do this both with a WAAS sat in view and without so you can see the real world difference in your part of the world.

 

I have the Meridian color which is essentially the same unit you have and without WAAS I have about 30 ft accuracy. Sometimes it is 3ft accuracy, sometimes 30ft. No way to know in advance of the hunt which it is going to be though which is why you need to test to see the typical range of accuracy.

 

If you do this and consistently find you are off by 100ft with your unit I would sent it back, but I rather doubt the innaccuracy is being caused by a defective unit.

[Kerry.]

....as there is no mathmatical formula that converts between the two that is 100% accurate in all locations.

 

From Lat/Long to UTM in the same datum is purely a mathematical function and has nothing to do with locations. We are not talking about changes between datums but changes between coordinates within the SAME datum.

 

Why is this point apparently getting mixed up with the process between datum's? this is not the case.

 

No way to know in advance of the hunt which it is going to be though which is why you need to test to see the typical range of accuracy.

 

Not precisely what the actual accuracy might be but certainly one can tell in advance what is best time to achieve the best possible results/accuracy for a given point on a given day at a given time.

 

....but I rather doubt the innaccuracy is being caused by a defective unit.

 

Certainly agree with this one, defective unit? very unlikely, User issue? most likely.

 

Cheers, Kerry.

[+dekster]

There are two WAAS satellites. One on each coast of the US: Sat # 35 is off the East coast and #47 is the West coast one.

 

Edited for broken arm spelling....

Edited June 19, 2004 by dekster

[+dekster]

Followup Comment/Question: I live on the Texas Gulf Coast, kinda midway between the WAAS satellites. I can only acqure one in the evenings when propagation is good (but not reliably). Does anyone have any information or links to information about the 'footprints' of the WAAS satellites?

[Kerry.]

It's not so much the satellite footprints but the relationship of your location to the correction data broadcast.

 

POR is well and truely visible almost right accross Australia but doesn't to a dam thing as far as accuracy goes, actually degrades it.

 

LINK to POR (PRN134) and LINK to AOR-W (PRN 122) which interactively will show elevation from any location.

 

Elevations much below 10 to 12 degrees not really the best.

 

Cheers, Kerry.

[+EraSeek]

Followup Comment/Question: I live on the Texas Gulf Coast, kinda midway between the WAAS satellites. I can only acqure one in the evenings when propagation is good (but not reliably). Does anyone have any information or links to information about the 'footprints' of the WAAS satellites?

Yes I do and will post shortly. But just a quick note. WAAS is most helpfull during the day when there is the most disturbance in the ionosphere. As you get to evening, and then into night its benefit of corrections deminishes, because there is much less correction that needs to be made.

[+EraSeek]

Here is a REAL TIME service snapshot link:

http://www.nstb.tc.faa.gov/vpl.html

[+EraSeek]

Here are the sat postions and small graphic of footprint:

[+EraSeek]

Here is a link with lots of info and a larger footprint about half way down the page:

http://www.gpsinformation.org/dale/dgps.htm#waas

[+EraSeek]

Here is a moving GIF of what waas is up against, and active ionosphere through the day:

http://gpsinformation.net/exe/iono-day.gif