GPS Accuracy - More questions
John Bond
Posts: 369
Hi Guys
Paul Paker said in the previous post on GPS accuracy...
Here is a table of the accuracies availible from the different systems:
My question:
If you know accurately the starting position of your moving GPS and you have one or more static GPS stations, is it possible to record position to an accuracy of better than 500mm (20 inches)? Is it possible to record direction (heading) of the moving GPS·to better than 0 degrees 15 Minutes.
If not, what other options are there?
I want to be able to repeatably and reliably get to a location from a known fixed point.
Kind regards on this grey and blustery day on the Dark Continent
John Bond
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Paul Paker said in the previous post on GPS accuracy...
Here is a table of the accuracies availible from the different systems:
GPS 10m DGPS 1-5m (depending on how close you are to a beacon) WAAS 3m LAAS 1.5m
My question:
If you know accurately the starting position of your moving GPS and you have one or more static GPS stations, is it possible to record position to an accuracy of better than 500mm (20 inches)? Is it possible to record direction (heading) of the moving GPS·to better than 0 degrees 15 Minutes.
If not, what other options are there?
I want to be able to repeatably and reliably get to a location from a known fixed point.
Kind regards on this grey and blustery day on the Dark Continent
John Bond
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Comments
consistently. Most modern GPS equipment can only calculate a position to
within 5-meters. Note that this error is strictly from the calculations internal
to the GPS and "not-so-accurate" clock *BUT* does NOT include errors in the
actual received signals from the GPS satellites (e.g. ionospheric diffraction
amongst other error-inducing phenomenon).
A number of technical papers have discussed averaging of positions taken from
a commercial GPS and obtaining accuracy of less than 1-meter. Alas, this wouldn't
be useful for a moving vehicle.
As a side note, when I was working on one of the first GPS receivers at Texas
Instruments back in the early 1980's we were able to use two of our GPS receivers
to survey (with differential techniques) distances on the order of 1-kilometer
(I believe) with sub-millimeter accuracy. BUT, that was with a GPS that would be
considered military-grade in that we could listen and decrypt the L2 channel coming
down from the GPS satellites. No GPS reciever can do that without a key from the
US DoD.
Static GPS stations don't help you unless you've got their positions properly
surveyed to the accuracy you desire (*IF* surveyors could get you that accuracy).
Multiple "static" stations really won't help much except allow one to over-calculate
a possible position and each would have to sit atop a properly surveyed positions
as well.
The GPRMC sentence spews forth the current heading to 1/10-degree or 6-minutes
precision. Since this is calculated (by the GPS itself) from the current position and
(probably the last position) it would be dependent upon the accuracy of those points.
Through out any exercise using GPS it would be wise to monitor the HDOP (Horizontal
Dilution of Position) value from the GPGGA sentence out of the GPS. This value gives
you an idea of how the current group of GPS satellites that you're receiving and their
relative positions to each other in the sky are affecting your GPS's calculated accuracy.
It would depend on what sort of range your moving GPS will be moving within.
Radio triangulation may be your best bet but it (probably) won't get you 1/2-meter accuracy.
If there is a system that will get your 1/2-meter accuracy I suspect it will be
quite expensive.
Hope this helps!
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-Rusty-
--
Rusty Haddock = KD4WLZ = rusty@fe2o3.lonestar.org
**Out yonder in the Van Alstyne (TX) Metropolitan Area**
Microsoft is to software what McDonalds is to gourmet cooking
Post Edited (Fe2o3Fish) : 11/26/2006 7:30:50 AM GMT
I've been using them in a project and you can see what side of the road you are on!
James
Rusty, I would love to get the Key but some of the people round here wear turbans (well they don't actually wear them except when they go to Mosque Friday lunchtime) so I don't think US DoD would give it to me.
I am 1000 yards from a survey beacon though I don't know how accurate that beacon is, probably not more that 500mm. It's position was updated in the 1980's.
Radio triangulation is beyond my level of electronic expertise.
Another idea bites the dust.
Kind Regards from the Dark Continent.
John Bond
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without expensive equipment is just beyond capabilities of today's commercial
products. Having a portable cesium or rubidium atomic clock helps tremendously
but having equipment that can use the clock... even more so.
I have an acquaintance in West Texas that used GPS and differential techniques
to locate huge "bundles" of cotton that needed to be picked up for delivery. These
"bundles" of cotton were right next to each other in the field and a driver was to
pick up only certain blocks (the ones he had purchased) by the lat/lon from the field
owner. So, 1-3 meters accuracy is fairly easy to achieve and make use of.
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-Rusty-
--
Rusty Haddock = KD4WLZ = rusty@fe2o3.lonestar.org
**Out yonder in the Van Alstyne (TX) Metropolitan Area**
Microsoft is to software what McDonalds is to gourmet cooking
of the errors.
Notice that all those high accuracy claims are not live but for GIS data collection.
Such accuracies come from GPS data that is post-processed. Basically, you run
around, collect your data points, then download the data to a PC and have it grind
on the collected data along with satellite data to regurgitate the solutions. Not real
useful if you want that accuracy out in the field. Quite useful for surverying and
trying to re-locate that spot you explored 2-months ago where you might want to drill
for undersea oil.
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-Rusty-
--
Rusty Haddock = KD4WLZ = rusty@fe2o3.lonestar.org
**Out yonder in the Van Alstyne (TX) Metropolitan Area**
Microsoft is to software what McDonalds is to gourmet cooking