or Connect
New Posts  All Forums:Forum Nav:

GPS in a whiteout - Page 2

post #31 of 34
Wikipedia has a pretty good article that contains an error analysis of civilian GPS error sources:


Newfydog is mixing up short term repeatability with accuracy. Even if his unit did repeat to 2cm in the time it took to finish his beers, he could still have been off in absolute position (accuracy) by up to 40m. I have seen errors that large on my Garmin 60cs (a good civilian GPS unit) especially in canyons or mountainous areas. On the other hand sometimes it is good to 3m if I have a good lock on 4 or more satellites in open terrain.

In addition to the GPS accuracy you have to RSS (square root of the sum of the errors) in the mapping error of whatever maps you are using as others have mentioned. There are other error sources such as poor satelite coverage or multipath errors that would come into play if you are in a canyon or region of other steep terrain. Even if you have occasional accuracies of 3m, on average the error is going to be much larger. Using a GPS to navigate while driving in a whiteout would be a really bad idea. It still however would be valuable for telling you what you are near like an exit or rest area.

By the way, the geolocation accuracy of the Worldview 1 satellite imagery used by Google maps (my company built Worldview 1) is about 5m for those of you who like to overlay satellite maps with GPS tracks. Not all the imagery used by Google is that good though.
post #32 of 34

as said by rcahill, wikipedia has ton's of info on GPS and simply speaking, for driving, civilian GPS is not sufficiently accurate even with relative readings, inertial reading, waas, etc especially when mobile. 

besides the errors, i doubt the other drivers would appreciate one plowing head on into them if you happen to be on the road.   

Aircraft has multiple intercommunicating systems including radio landing link, beacons, etc.  today have runways overlayed upon very lovely heads up displays to aid the pilot in landing.   Heathrow is one of the first airports to enable low to zero visibility instrument landings. 

land surveying GPS using reference reads can be accurate to 2cm or less but that's relative to known markers and time to interpolate multiple readings. 

DARPA had their Urban challenge, a 2.8mi course where teams had autonomous vehicles compete.  4 hrs was a good time.  granted, they navigated in a city but the previous desert challenge, 150mi was done in 7hrs ... maybe 20mph is good in white out but i suspect the routes accuracy was less critical and the equipment used a bit more expensive.


but, another 10 yrs ....

post #33 of 34
Originally Posted by rcahill View Post

Newfydog is mixing up short term repeatability with accuracy. Even if his unit did repeat to 2cm in the time it took to finish his beers, he could still have been off in absolute position (accuracy) by up to 40m


And I'll bet you the rest of the six pack it wasn't.   If I really wanted to dig out the deltas and sigmas I'd show that I had a max error of 2 cm and a median error of less than one.  (in the short term as you said)  Try running a track log for a few hours at full magnification without moving the unit.  They used to drift all over, but now are pretty darn tight.

But point well taken, my study was less than scientifically rigorous, I just wanted to show that  one can't just claim a 15 metre  standard error.  I've used those things for years, going back to the first gulf war, when they turned off "the dithering", and I've seen them exceed expectations more often than not.  Accuracy depends on where the satellites are, how long you let it refine the location etc.


Edited by newfydog - 11/20/09 at 1:23pm
post #34 of 34
Check-out John Deere's "auto steer system" the accuracy is there, but at a cost - and it ain't GPS alone. AND it doesn't apply to the position of the hiway you are traveling on.
Hope you don't try it while I'm on the same road.
New Posts  All Forums:Forum Nav:
  Return Home
  Back to Forum: General Skiing Discussion