# Doppler programming question

• Subject: [amsat-bb] Doppler programming question
• From: Phil <phil@xxxxxxxxxxxxxxxx>
• Date: Wed, 16 Jul 2003 14:15:11 +1000
• User-Agent: KMail/1.5

```Hello All,

I don't want to bore the group with a lot of programming code however it is a
satellite related question and perhaps someone will be able to point me in
the right direction.

I'm trying to implement "the one true Doppler tracking rule". Downlink
tracking works perfectly (except for minor rounding errors), it's the uplink
tracking that is giving me trouble.

Briefly, this is the logic behind my code:

1. get the satellite's downlink frequency (this is my rx freq)

subtract the Doppler shift for that freq from the receiver's frequency
save the result as baseRxFreq

This is the actual transmission frequency of the satellite.

2. get the satellite's uplink frequency  (this is my tx freq)

baseTxFreq = freqConstant - baseRxFreq;  // constant is 581800000 for FO-29

This is the actual reception frequency of the satellite (no problem here).

3. every 3 seconds do the following

get the Doppler shift for the baseRxFreq
rxFreq = baseRxFreq + Doppler;
setFreq(satRxVfo, (int)rxFreq); // set the receiver's vfo

get the Doppler shift for the baseTxFreq
txFreq = baseTxFreq - Doppler;
setFreq(satTxVfo, (int)txFreq); // set the transmitter's vfo

Uplink tracking doesn't work, it's the wrong tx frequency.

My uplink frequency logic seems to be wrong and with only two FO-29 passes
each day it's a little tedious to use the trial and error method to find the
fault. So, I hope another set of eyes, or brain, will see the error in my
logic.

--
Regards,
Phil.

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