I've been doing another anomaly test. One can really tell that the US army has brought back the build in error. Last time I did the test it moved around very little only 1 to 3 meters in many many minutes. Nowadays it can move about pretty far; and quickly as well. Worse test showed a swing of 20 meters and a max rate of change of 5 meters a second. THAT is pretty drastic. So I have to recall some of my earlier comments on GPS use in speed trails. Two years ago the shift was small enough by far to not influences the tracking too much. Now with the Iraq war it sometimes does; although the same test also showed some stretches of 10-15 minutes in which nothing happened.

So I will go to my back-up plan for GPS tracks. Luckily we in the Dutch F16 class have several GPS units and therefor 1 spare. So what we will do next time is leave 1 GPS tracker in the car in a bag underneath the front window and leave it stationary there. We'll take the others along on the boat. Afterwards we just deduct the swing recorded by the stationary unit from the moving units and cancel out the US army build in error. Actually it is sort of our own differential GPS setup but with a higher accuracy then WAAS or commericial DGPS's. MS-Excel comes in really handy now. Maybe I'll write a short program to do the substraction when I have some spare time left.

So for all you speed freaks out there. Restore your old accuracy by having a stationary tracker during your sailing and do some input file hacking.

The other alternative is indeed to look for smooth average read-outs as you decribe. Calm periods are interchanges with active periods. At least the stationary tests I've done show that. See picture



Coding; block size = 5 seconds = tracking sampling rate.

Blue = 2 km/h or less
light blue = between 2km/h and 5 km/h
red = above 5 km/h


If the record attempt was made in the first 4 minutes than the data would have been undependable. If you did it after 4 minutes than the error speed would be been between 0% and 2% ; so at max 0.4 knots when having a base speed of 20 knots. For a base speed of 30 knots the error would at MAX have been 0.6 knots. With a little luck you would have done the attempt in one of the stationary times and get a 100 % accuracy (0% error) as in those stretches no swing occured.

We can device a ground rule that for all record attempts at stationary GPS tracker must be used so we can see that the swing at the record time was and compensate the measured for the swing. At todays prices this should be easy enough. I think I located another cheap GPS tracker that allows storage of up to 50.000 track points and has a stand-alone battery life of about 8 hours with a wireless blue tooth PC link. This is sufficient for cat speed/course tracking.

Wouter