Since everyone is insisting that GT-11s have to have their Alpha 500 results calculated using trackpoints, I've been investigating what is involved, and I've discovered something very interesting.

It seems that there is no trackpoint data in the SBN file format. (Spec is at http://www.navmanwirelessoem.com/oem/customer-support/oem-news/product-briefs-and-data-sheets/jupiter-32-xlp-new2/sirf-binary-protocol-reference-manual and the relevant message ID is 41, on page 87)

For each point, there is only one speed measurement, and it is a doppler one.

So I am a bit confused, because obviously Realspeed and GPSResults seem able to work out a "trackpoint" calculation for Alphas without actually having any trackpoint speeds. They must be derived somehow from the other data, perhaps working backwards from the locations?

Any idea how?

I'd love to understand this a bit better.

It's bits 24-27 (Latitude in degrees (+ = North) x 10 ^ 7 ), and bits 28-31(Longitude in degrees (+ = East) x 10^7) isn't it?

Page 3-42 (the one after you said), Message ID 41.

There's a big table that shows which bits do what.


Well done on finding that document btw, when I was thinking of doing something similar I searched high and low but couldn't find it anywhere.

I'm sure you could find the speed between successive trackpoints, and create an array based on that.

Then your fancy trig algorithms would still work, and you'd be doing it on trackpoint data

Careful mate you might open the can of worms that is GPS data software.

There is no way I would rely on these GPS units software for anything requiring scientific type accuracy. Think of those missiles that go slightly off target

However, as the Challenge is not for sheep stations and everyone uses the same software (I'm talking in the actual GPS's here) they are satisfactory for comparative purposes.

"The accuracy of GPS technology is relative, not absolute"

I think Ive even confused myself

What I mean is that your position is determined relative to the location of the satellites.

Your GPS then calculates your position in lat/lon, but then again all measurement is relative and relies on the accuracy of instrument calibration.

Now I'm stuck in a philosophical paradox loop
My brain hurts

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