Re: estimating error in my GPS position.
- From: RWW <RWW@xxxxxxx>
- Date: Tue, 22 Jan 2008 21:11:55 -0800
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!
This is the age-old classic problem of measurement
accuracy versus system error budget.
The approach to solving this is to characterize the
system error budget in terms of
a) Random component (uncorrelated)
b) Systematic component (correlated)
and the magnitudes of each.
Next, you need to decide on what measurement
accuracy you are trying to achieve. You are looking
for positional accuracy over certain time intervals
on the surface of the ocean. What is the desired
posisional accuracy and the time measurement interval?
It comes down to a statistical excercise to examine the
autocorrelation properties of the measurement signal
versus that of the true position, for the minimum
drift you wish to achieve.
Without some statistical analysis, I would not trust
the raw measurement of position of a drifting Buoy
in any way shape, or form.
The positonal corrections would need to come in the
form of a Kalman Filter, which is excactly what is
used to account for the variations from the systematic
orbital dynamics in the GPS solution. But in this case,
you would be dealing with ocean current dynamics,
which are much less modelable.
.
- References:
- estimating error in my GPS position.
- From: Charlie
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