[amsat-bb] Re: Full Doppler Tuning

["Stacey E. Mills" <w4sm@xxxxxxxxxx>]

Jim wrote....


>  Here is what I am doing:
>1. Find the Beacon with the radio
>2. enter the proper LO into NOVA, so that the actual predicted downlink
>frequency matches the rig with doppler correction and 2401.323 entered at
>the satellite. this turns out to be about 2255.999 990 for my UEK-3000.
>3. With frequencies locked at the satellite, I match 2401.323 to the same
>calculated MB value on the Uplink side (either U-band or L1-band). I thus
>move down from the top of the L1 band by the same amount as moving up on the
>S2 band.
>    After doing this shouldn't I get predicted frequencies from NOVA that,
>when i tune my uplink frequency to, I should hear my station on the
>downlink? (assuming I now move off the MB frequency onto a MB+20 value or
>so?)
>What am I missing? some sort of offset correction for inaccuracy?
>Jim
>AC4EA
>ac4ea@arrl.net
>ac4ea@amsat.org

Jim,
As has been mentioned there is enough variation in the beacon freq. due to 
spacecraft temperatures and downconverter drift, that it is virtually 
impossible to implement a fully deterministic solution.

Here's how I do it:
1. Zero beat your signal on the downlink somewhere away from the beacon 
(the software should get you close).
2. Enter your receive frequency into the program in some fashion.  This is 
RxBase.
3. Correct RxBase for doppler to give TxSat (the satellite transmitted freq).
4. Enter your transmit frequency into the program.  This is TxBase.
5. Correct TxBase for doppler to give RxSat.
6. (Satellite offset under current conditions)  SatOff = TxSat - RxSat
7. As doppler changes your program must maintain TxSat and RxSat constant, 
changing TxBase and RxBase as needed.
8. If you manually change frequencies, it must calculate the second 
frequency using the SatOff equation.

You can't assume that SatOff is constant.  Once you've found your "spot," 
SatOff isn't used during full doppler correction, though it is used if you 
hold Tx or Rx constant, or make manual changes.  Remember to keep track of 
the doppler shift sign depending on which way you're calculating.

I've found the above method works quite well for a given session, but if 
the temperature changes, due to clouds, etc. you may have to recalculate 
(apparent) SatOff.

The other problem is that this system assumes the station you're talking to 
is using the same full correction technique.  If they are holding Tx or Rx 
constant, they'll drift relative to you.  My software includes the option 
for full doppler, Tx constant, and Rx constant.



-- 
____________________________________________________________________
Stacey E. Mills, M.D.  WEB:  http://www.people.virginia.edu/~sem2r/
Charlottesville, VA   Office Phone: 434-982-4406  Fax: 434-924-8767
                E-Mail: sem2r@virginia.edu -or- semills@virginia.edu
_____________________________________________________________________ 

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