On Wed, Aug 4, 2010 at 10:09 PM, Ray - K9DUR <k9...@rnacs.com> wrote:
> Brian, > > I believe that you will find that the Doppler shift is a proportional to > the > frequency. Therefore, the shift at 432MHZ would be approximately 3 times > the shift at 144MHz. > Yes, it is. But total doppler changes throughout a pass and since most of the transponders are inverting, they move in opposite directions. > A simple thought experiment demonstrates this. Assume you have a 144MHz > frequency source & it is moving away from the receiver fast enough to cause > a 5kHz negative shift. If you decrease the frequency of the source and the > shift remains 5kHz, consider what the result would be if you decrease the > source frequency all the way down to 5kHz -- the resulting received > frequency would be 0Hz. That is obviously not correct. The frequency > shift > is dependent on BOTH the relative speed and the frequency. > That is correct. You get a gold star! The actual formula is: F = ( c/(c-Vs)) Fo Where: F = perceived frequency Fo = actual transmitter frequency c = speed of light ( 300,000,000 m/s) Vs = radial velocity of source (m/s) Since c/(c-Vs) is a simple ratio, the change will be proportional to the source frequency. As long as we are on the subject, the above holds true only for your own signal coming back to you, which is a trivial and [mostly] useless calculation. That is why just knowing the ratio for your own signal isn't good enough. The signal from another station will have a different radial velocity and therefore a different doppler shift. So unless you know your location, the other station's location, the satellite's location, and the satellite's velocity vector, you can't calculate the doppler shift ahead of time. But since both stations can calculate the satellite's position and velocity and they know their own location, they each can correct for doppler on both the uplink and downlink, thus making it possible to have a completely hands-free QSO with the computers on each end tuning both links and pointing the antennas. > 73, Ray, K9DUR > http://k9dur.info > > > > -- Brian Lloyd, WB6RQN/J79BPL 3191 Western Dr. Cameron Park, CA 95682 br...@lloyd.com +1.767.617.1365 (Dominica) +1.931.492.6776 (USA) (+1.931.4.WB6RQN) _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kc.flex-radio.com/ Homepage: http://www.flex-radio.com/
