Jump to content

Gps Accuracy


exgint
Followers 0

Recommended Posts

I've had my Garmin GPSMAp 60c for a few weeks now. When I first got it I was regularly getting accuracy to +/- 3m. About 10 days ago that went down to 6 or 7m at best and now no better than 5m with an occasional 4m now and then.

 

Also WAAS seems to have no effect on the accuracy.

 

I'm wondering if WAAS is at fault, atmospherics, the GPS or me at fault.

 

I live in Bristol, England. Anyone know a way of getting the general status of the GPS accuracy in a region? A government website maybe?

 

Love the 60c in every other way though, and even this is not a real problem, i just got curious.

Link to comment

Exgint,

 

With my handheld I have seen as good as 1 m accuracy, but usually 3-5m. Gets worse with heavy tree cover, etc.

 

Not sure if you knew this but I thought I would bring it up.

 

quoted from the U of Calgary

The accuracy of navigation systems using very low frequency signals depends on accurately knowing the altitude of the ionosphere's lower boundary. During a geomagnetic storm the altitude of the ionosphere's lower boundary of the ionosphere changes rapidly and can introduced errors of several kilometers.

Global Position System (GPS) operates by transmitting radio waves from satellites to the ground, aircraft or other satellites and therefore is sensitive to ionospheric changes due to geomagnetic storms.

 

Another regarding navigation

http://www.sec.noaa.gov/info/Navigation.pdf

 

I'm not sure how much it affects hand held units(I suspect similarly), but when ever we are planning on doing a GPS survey (mm accuracy) we always check the space weather sites to see if there are storms predicted.

 

this site has the article listed above plus links and info about space weather.

http://www.sec.noaa.gov/

Edited by Snootleather
Link to comment
...waas doesn't work worth crap even here in America.

Using an amplified antenna on the roof of my truck, I regularly see 7' EPEs using WAAS.

$40 bucks, humm... and to think all this time i've been making the kids were aluminum foil and walk behind me in a semi-circle with their arms fully extended!

Link to comment
7' what??

EPE = Estimated Position Error.

 

And WAAS works quite nicely throughout the United States. I frequently get accuracies (measured, by the way, not estimated) of better than 3 m.

 

Common courtesy dictates that if a geocacher has a WAAS-capable unit and it can get a lock on the WAAS satellite(s), then it should be used in taking coordinates for hidden caches. As you no doubt know, the error in coordinates for cache hides is much more significant than the errors in the GPS receivers for those hunting the caches.

Edited by fizzymagic
Link to comment

The EPEs on the GPSrs are not accurate. It's a guess. Because the GPSr says 3' it could still be off 20 ft . or it could say EPE 70 ft and be within 3'.

 

The EPE has an EPE if that makes sense.

 

WAAS works more for surveyors. When driving, it's a waste of battery power, esp. with "lock on road" feature turned on. For a geocache I don't use it because no matter how accurate my GPSr comes to the coordinates, it's still only as accurate as the person who put it there.

 

Someone could have just hid the cache, turned on their GPSr and logged the first coordinate it grabs, then leaves. What's 30 feet when finding a cache anyway? Part of the fun is turning off the GPSr when you get there and searching for it! :rolleyes:

Link to comment
Also WAAS seems to have no effect on the accuracy.

 

I'm wondering if WAAS is at fault, atmospherics, the GPS or me at fault.

 

I live in Bristol, England. Anyone know a way of getting the general status of the GPS accuracy in a region? A government website maybe?

The equiv to WAAS in your part of the world is EGNOS and works exactly the same.

EGNOS is still only transmitting a test signal so could have its ups and downs.

 

General status of GPS accuracy can be obtained in hindsight, otherwise one must assume the system accuracy specs of less than 13 metres SIS, 95% of the time world average or worst case scenario 36 metres SIS 95% of the time.

 

Cheers, Kerry.

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
Followers 0
×
×
  • Create New...