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Re: Determining max dopler shift for LEO's

Rodney Clayton (KA3BHY) wrote:
> Is there a list or some way to determine the max
> dopler shift I should see for rs-12 rs-15 fo-20 etc?

Some tracking programs do that for you automatically.  But in any
case, if your tracking program will do a pass prediction that includes
the range (distance between your QTH and the satellite), you can
compute it yourself.  (The easiest way would be to import the pass
prediction into a spreadsheet, then you could even plot out the
Doppler shift and/or actual ground station frequencies over the time
course of the pass.

Say you have two lines of the pass prediction listing.  The critical
fields you need are TIME and RANGE.  Let's say they look like the
following (totally fictitious numbers, just want to demonstrate the
simple math involved):

12:01:03   4,123 Km
12:02:18   4,423 Km

Elapsed time = 75 seconds
Range change = 4,423 - 4,123 = 300 Km = 300,000 meters
Range Rate = (Range Change / Elapsed Time)
           = 300,000 / 75 = 4,000 meters per second

Doppler effect = range rate / speed of light in meters per second
               = 4,000 / 299,792,458
               = 0.00001334256
               = 0.001334256%

Doppler Shift = uplink or downlink frequency x Doppler effect

If the frequency of interest was 145 MHz, the Doppler Shift would
be 145,000,000 Hz x 0.00001334256 = 1,934.6712 Hz = 1.934 KHz.

Hope that helps.
John (KB0ZEV)
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