The purpose of full doppler correction is to keep your signal in the exact same spot on the receiver in the satellite. It has a finite passband and if you don't correct your uplink, you move within that finite passband. Theoretically a single linear transponder can support many conversations but it will not support the maximum unless everyone stays put at the satellite receiver. Those who don't doppler correct slide within the passband and run into those who are staying put. Of course if you adjust your uplink so you stay put at the satellite, you have to adjust your receiver to hear yourself because of the doppler shift in the other direction. Both uplink and downlink require doppler correction in this case.
If you don't have a rig that can be computer controlled or if you are operating portable and can't readily incorporate a laptop into your setup for whatever reason, then by all means manually adjust the highest frequency since that is the one that changes three times faster than the lower frequency in a v/u satellite. You simply do the best you can. However, if you have a home rig that can be computer controlled, then by all means use a computer to adjust your doppler. Stay in one spot in the satellite's passband. You can make faster contacts since you only have to pick up the mic to talk or use the key to send cw. If you are working dx and the window is short, being on frequency and staying put will help put that distant grid in the log. The program does all the work but you must be sure that your keps are current AND your computer clock is accurate to the second. On a high pass when the satellite is overhead, the doppler changes very fast and even a few seconds of clock inaccuracy will have you not hearing yourself in a normal voice. At lower elevations the doppler is not as severe so a few seconds won't make much difference. Use the HO-68 beacon to test your doppler correction. Set your radio for CW with a 500 Hz or better bandwidth. Pick a high pass where the bird will get up over 45 degrees. Tune the beacon in at your AOS and then don't touch the dial. If the CW note stays constant over the entire pass, your system is operating perfectly. Make sure your program settings are such that there are frequent frequency updates. I have mine set at 1 second no matter what. Some older radios will not accept frequency changes while you are transmitting. In that case, make sure you only key down for short periods of time so the radio frequency can be updated. Of course you must adjust your uplink calibration on SatPC32 so that you hear yourself on the downlink. Hearing yourself in a normal voice will be an indication that you have your calibration set properly. Most of the birds stay on frequency, but I've noticed that AO-7 Mode B will be off by 1700Hz when it changes from Mode A to Mode B. Normally we would not observe this phenomenon but since lately we have had this mode change while over North America we are able to experience it. Within a few minutes, the satellite is down to a 1200 Hz offset. I'm not sure what happens on the next pass because it is out of my footprint. The next morning, however, the bird is back to my normal setting. This means the frequency drifts about 1700 Hz back down to it's steady state value. The other birds may have some slight drifting, probably due to spacecraft temperature but it is minimal, less than a few hundred Hz. So when you are trying to make a contact on the first AO-7 pass in Mode B, be prepared to readjust your uplink calibration so you can hear yourself. It may be as much as 1700Hz at the start. In my opinion, everyone that can control their radio for doppler should do so. Doppler correction gives us the maximum usage out of our linear transponders. Why go through the agony of fiddling with your radio when your computer can make your life so much easier. You will never have to apologize for not being on frequency. If I wasn't able to have doppler control on my radio, I wouldn't spend much time on the linear birds, unless of course there was a rare grid to be worked. <grin> 73, John K8YSE _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
