>> I thought the 4th satellite was needed to determine the time. Wouldn't >> it take a 5th satellite to also determine the frequency of the local clock?
> Not really. There are two ways to get the postion and time derivatives. One > is to either use two fixes which give you each a (x,y,z,t) tuple, while you > know what your expected delta-t is, you can calculate the "real" delta-t and > get from that your frequency offset. That's the sort of thing I'm looking for, but I don't quite get it yet. I have 4 satellites. If I know f, I can solve for x, y, z, and t. If I don't know f, I'm short an equation. If I get two samples, I have 8 equations and I need to solve for: x0, y0, z0, t0, and f0 x1, y1, z1, t1, and f1 That's 10 unknowns with 8 equations. I get a 9th equation by setting t1 = t0 + 1. I'm still short one equation. Can I do something like assume f0 = f1? That would make sense if the change in frequency is small relative to the noise/error in all the other calculations. > The other way is to use the doppler shifts of the satelites. You know what > position and speed relative to you the satelites have and can from this > calculate what your speed, respektive frequency is. There is a chicken-and-egg problem in there. If I need the local clock frequency to solve for position, I can't use position to solve for frequency. Consider the time-nut case of 1 satellite, known position, and trying to find time. If I know the rough time I can calculate the Doppler. That tells me which FFT bucket to look in. Is the local clock close enough for that even if it's off by a few/10s of PPM? -- These are my opinions, not necessarily my employer's. I hate spam. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
