Re: [amsat-bb] doppler shift

[Franklin Antonio <antonio@xxxxxxxxxxxx>]

At 03:05 PM 9/30/98 , Mark Flanagan wrote: 
> Am I correct in stating that the doppler shift
> of a receive frequency starts higher than the actual frequency as it is
> approaching then continues lower as it going away? 

Yes.  Just like cars going around a racetrack.  The vroooom
sound is higher in pitch when the car is coming toward you, and lower as
it is going away.  


>  Does the frequency appear to stabilize
> when the distance from the satellite to the qth becomes constant, even if
> it is briefly?

We have to be careful with use of words.  You said "when the
distance from satellite to the qth becomes constant, even if it is
briefly".  I think I know what you meant, but to be sure, I'll
state it in my own words.  Doppler shift is caused by rate of change
of distance.  If the distance is not changing, then the doppler
shift is zero.  This happens when a satellite (or race car) reaches
the point of closest approach.  Of course a low-earth-orbit
satellite doesn't stop at the point of closest approach.  It zips
thru really quickly.  But at that one point, the doppler shift is
zero for an instant.

You used the words "appear to stabilize", and that raises
another issue.  When you listen, you notice the rate of change of
frequency.  At the point of closest approach, the doppler shift goes
thru zero, but the rate of change of doppler shift is really high at that
time.  So I wouldn't describe this point as a point where anything
"appears to stabilize".  In fact, the time around closest
approach is the time when doppler shift is changing most rapidly. 
As with the race car, the received frequency appears stable when the
satellite is far away (either coming or going), but as it zips past you,
things change rapidly.  

The formula for the frequency shift is df = - f * dr/dt / c  where f
is the transmitted frequency, and c is the speed of light, and dr/dt is
the rate of change of distance vs time.  The only thing to remember
from this formula is that things are simply proportional. 
Proportional to f, and proportional to dr/dt.

Hope that was helpful.