estimating error in my GPS position.

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

Maybe I don't understand how GPS error is specified, what does the
"30m" actually mean?

The error on my GPS shows as radius of, let say 30 m. I understand it as I
would be somewhere in a circle of 60 m diameter. This errors varies with
signals from satellites. It sometimes shows only R = 3m, but sometimes 120
m.
Jan

Maybe this?

http://en.wikipedia.org/wiki/Kalman_filter

GPS position is not a constant value...that's why differential GPS (and
WAAS) provide corrections based on known positions.

Bob Gardner