Re: estimating error in my GPS position.

Charlie wrote:
Hello all,


I have a time series of GPS positions, longitude and latitude, from a
drifter at the top of the ocean, and I am trying to estimate the
velocity of surface drift.

What I am doing is taking the difference between my first and last
longitude coordinate and convert them to meters. This gives me my
"delta x", and dividing by the time over which this occurred "delta t"
at the surface, I obtain an estimate of surface velocity U (positive
east)

I do the same for latitude,find the difference in latitudes, convert
to meters then divide by "delta t", to obtain my velocity V (positive
north).


But I need some kind of estimate of the error in these velocities, U
and V. I know the appropriate error formulas for propagating error but
I am unsure what number to use for my estimate in the error of my
longitude and latitude data respectively.

The manual says the GPS is good to within 30m but this is for the
radius around a point (lon,lat). What I require is an estimate of the
error in the longitude, and a separate estimate of the error in the
latitude. Or should I use 30m each for both? or perhaps half that for
each ?

Is there any way to figure this out?


Any help you can provide me on this would be greatly appreciated!

cheers,

- C

The estimated errors in the position are just that, estimates only. So you can only assume some sort of error distribution in both lat and lon, and compute an error based on that. If the errors followed a normal distribution, you could assume the lat and lon errors each contributed about 1/sqrt(2) to the total, ie around 20 m each. Real world GPS positions suffer from all sorts of non-random errors, so doing the usual RMS error analysis is unlikely to help much.

A further suggestion:
Many GPS units will output a velocity measurement which is independent of the position, done by using Doppler shift on the signals themselves. This value is often available to within 0.1 m/sec or better, depending on the receiver, and might be better for measuring short term drift velocity. At least its an independent measurement you could use as a cross-check. Check your specific GPS unit specs. I know for instance that some Garmin GPS engines will give velocity measurements to within around 5 mm/sec resolution, if not accuracy.



--
Regards,

Adrian Jansen adrianjansen at internode dot on dot net
Design Engineer J & K Micro Systems
Microcomputer solutions for industrial control
Note reply address is invalid, convert address above to machine form.
.

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