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WAAS accuracy degradation in heavy cover.
Posted by: Rick
Date: April 01, 2010 10:19AM
Hello,
Has anyone ever tested if WAAS corrections degrade a receiver's accuracy in heavy cover? How could someone test the effects of heavy cover on a GPS receiver? It was mentioned a while back that WAAS would degrade accuracy in heavy cover and should be turned off. Does anyone know if this is the case?
Rick

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Re: WAAS accuracy degradation in heavy cover.
Posted by: Andrew Kalinowski
Date: April 01, 2010 02:04PM
I guess, the simplest test would be to compare readings from 2 identical receivers where one would have WAAS turned off and the other one would have it turned on. I haven't tested it but, I don't see a reason why WAAS would degrade accuracy in heavy cover especially that;
because of the positions of the WAAS satellites, for most users in CONUS, Alaska and Canada, satellites are very low in the sky and WAAS signal is too weak to be received under heavy cover.



Andrew Kalinowski

Re: WAAS accuracy degradation in heavy cover.
Posted by: Rick
Date: April 08, 2010 09:21AM
Thank you for your answer Andrew, I will try that test and let you know.
Rick

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Re: WAAS accuracy degradation in heavy cover.
Posted by: edjcox
Date: April 26, 2010 09:41PM
100 meters: Accuracy of the original GPS system, which was subject to accuracy degradation under the government-imposed Selective Availability (SA) program.


15 meters: Typical GPS position accuracy without SA.

3-5 meters: Typical differential GPS (DGPS) position accuracy.

< 3 meters: Typical WAAS position accuracy


Maybe that will help you..






Re: WAAS accuracy degradation in heavy cover.
Posted by: txrpls
Date: June 24, 2010 07:46AM
WAAS will not degrade accuracy in have cover. Heavy cover degrades accuracy, it has nothing to do with WAAS. The less satellites the receiver reads the more your positional accuracy decreases.

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Re: WAAS accuracy degradation in heavy cover.
Posted by: NM5K
Date: January 16, 2011 03:46PM
I've never seen that to be the case with mine. I've seen the unit lose the WAAS bird
at times, and it will toggle between SA and WAAS mode. It says what mode it's using
on the screen. I've never seen problems in heavy tree cover with mine. Or in general
anyway. I agree with one about the location of the WAAS birds. They are often lower
in the sky than the others, and if you loose one or two, the WAAS bird is often one
of the first to go. That may make it seem like WAAS is causing more error, but I think
it's the loss of the WAAS bird that causes more error, not having a weak but usable
WAAS bird in cover. Myself, I'd rather have the weak WAAS coverage, than no WAAS
coverage.
I often look at the sky map to see where the birds are in the sky. In many cases, the
WAAS bird is fairly low.
For me, I find my machine the most accurate when a lot of birds are almost directly
overhead, and have high WAAS coverage. And this seems to usually be in the middle of
the day for some reason. I notice at night, the bird coverage is often not quite as good,
and accuracy might suffer a tad. I don't know if this is a fluke, or by design when planning
the orbits of the birds.
I have land that is mostly wooded up in OK, and I've done a lot of testing up there.
Tree cover doesn't seem to bother my machine much at all. But mine also usually
works fine inside a car too.. As does my car GPS.
When mine is in real time WAAS mode, it will say WAAS..
When it loses the WAAS bird, or the coverage becomes spotty, it will say "WAAS average"
and show the amount of time that it's been in that mode. That elapsed time shows
how long it's been since a "good" WAAS fix. Often just changing the direction of the unit
and it's internal antenna will toggle it back and forth from WAAS to WAAS average mode.
So if I'm really wanting an accurate fix, I make sure I'm in WAAS mode, and it's stable
and not toggling back and forth.
I can measure off distances with mine, and have tested it at my property measuring
the distance between survey pins. I know the actual distances from the survey maps
of the property. If I let it get good and stable, and have a lot of overhead birds, it can
be right on the money for short periods of time. I'm talking measuring the pins right
down to the exact number of feet. But... it's all constantly "wobbling" around due to
the movement of the various birds. So I have to do a reset pretty often if I want it to
be real accurate. By reset, I mean that I set a new waypoint for the #1 pin, and then
hurry on down to the second one, let it get stable and see what it says.
The road frontage to my place is exactly 330 feet, and it will usually read out right on
the money with a stable unit, and fresh waypoints. But as you stand there, you
can see the fix wobble around a tad as you stand there a while. IE: I'll start out
right on the money, but then see it move north a few feet, wobble around to the
east a bit, duck behind me a while, etc.. It's never perfectly still..
But I trust mine more in WAAS mode, than in WAAS average mode. It's obviously
more accurate. And this usually applies to the elevation reading too..
Most all that place is heavy forest, and I never have trouble walking around the
place with trees constantly over me. I have saved "tracks" that show my property
lines. So I can wander around and know where my lines are even though I don't
have fences up. BTW, that's with an older Magellan Meridian. "yellow version".
Anyway, WAAS is supposed to be good to 3 meters or less. Mine often beats
that, and sometimes is right on the money for short periods of time after a fresh
set. In the overall scheme of things, and some error for various reasons, I
consider it to be accurate to about 5 feet in average use without trying to be
ultra accurate. But it's better than that if I do try to be ultra accurate.. :)

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Re: WAAS accuracy degradation in heavy cover.
Posted by: edjcox
Date: February 02, 2011 02:03AM
How was works
The Wide Area Augmentation System (WAAS) is an air navigation aid developed by the Federal Aviation Administration to augment the Global Positioning System (GPS), with the goal of improving its accuracy, integrity, and availability. Essentially, WAAS is intended to enable aircraft to rely on GPS for all phases of flight, including precision approaches to any airport within its coverage area.[1]

WAAS uses a network of ground-based reference stations, in North America and Hawaii, to measure small variations in the GPS satellites' signals in the western hemisphere. Measurements from the reference stations are routed to master stations, which queue the received Deviation Correction (DC) and send the correction messages to geostationary WAAS satellites in a timely manner (every 5 seconds or better). Those satellites broadcast the correction messages back to Earth, where WAAS-enabled GPS receivers use the corrections while computing their positions to improve accuracy






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