At 11:01 16/04/2002 +0100, you wrote:
Not sure if the Doppler shift due to orbital wobble will affect the
modulation
in the same way it does carrier frequency. Are there any physicists out
there
who can confirm this ?
Andy G4JNT
Dear Andy, LF Group,
I'm not a physicist, but it seems to me that however the line sync is
modulated onto the satellite signal, a change in the path length to and
from the satellite will change the path delay, and so the arrival time of
the sync pulse, by 3.3ns per metre. At 15.625kHz, this will cause a change
in relative phase of 2pi * 3.3ns/64us, which is 3.3*10e-4 radians per metre
of path length. So if the path length changes by 40m/s as Stewart states,
the rate of phase change will be 0.013rad/s, or 2.1 millihertz. Multiplied
up to 136k, this would be about 0.02Hz as Stewart calculates.
How fast is the ionosphere supposed to be moving around? my OCXO reference
seems to be stable within a few parts in 10^9 over a short period of time
on a good day, and this amounts to a fraction of a metre per second
equivalent. So if the effective height of the ionosphere changes by a few
10s of km in a few hours, the frequency/phase change should be detectable
without locked oscillators.
Cheers, Jim Moritz
73 de M0BMU
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