Originally Posted by BackPacker
IFMU, but how does the GPS calculate the velocities in the first place? That has to be done using position differences, not?
BackPacker,
You got me on a technicality. But here's the thing - velocities are calculated from position differences with respect to the satellites, rather than the solution of where on earth you are.
Originally Posted by BackPacker
The only other way that I can see would theoretically work is to work out the velocities by Doppler shift but with satellites literally all over the place I don't think that would work.
Beyond doppler shift, one of the neat technologies they came up with was to figure out the GPS solution in terms of the number of cycles of the carrier waves between the satellite and the receiver. And, they do this for both the L1 and the L2 frequencies, even though they can't use the military frequency to get the 'real' position information. Those clever devils. Novatel was one of the innovators of this technology back in the mid 90's.
Originally Posted by RansS9
Interesting.
Is there anyway you could explain that without me having to have Graduate Physics/Maths.
GPS is all about using timing to figure out range from a satellite, then figuring out where the solutions considering all the satellites converge, or at least the closest guess. Like 421C says. Now, if you ignore the solution of where on earth you are, and use the GPS timing to figure out your rate of closure WRT the satellites, you can come up with a velocity figure even without great knowledge of where you are. That's my simplified explanation.
The position solution of where we are on earth tends to jump around because of errors either from the atmosphere, or selective availablity (like the bad old days). If you calculate your velocities directly from your time & distance to the satellites you take some of this error out of the calculation. GPS velocity tends to be a better measurement than absolute position.
-- IFMU