Doppler Effects on SSTV Pictures

Satellite communication experiences the doppler effects just like the sound of a speeding train whistle. The end effect will be that the received frequency will either rise or fall. The effect is less noticeable on the high satillites, but the Low Earth Satellites (LEOs) may see a shift in frequency as much as 50 HZ a second.

The top picture of myself is the original image transmitted Scottie 1 up to the LEO amateur satellite FO29 on 145 Mhz.

The middle picture shows the effect of doppler on the received frequency of 435 Mhz. In this case, I did not correct the receive frequency until about eye level. This received Scottie 1 picture had drifted down in audio frequency and the resultant color grey scales had all become darker.

At the eye level point I adusted either the receiving or the transmitting frequency to compensate for the down doppler shift so that the received audio frequencies were once again correct. Then a few seconds later, the audio frequencies had again drifted dark, so when the scan line hit the microphone area, I once again adjusted either the receiving or the transmitting frequency so that the audio frequencies were again correct.

Obviously, this will not be a quality picture, when the doppler is adjusted in coarse steps. AO10 doppler is much slower, and the receive frequency drifts high.

 

The correct way to compensate for doppler is to smoothly tune the receiver or transmitter frequency as shown in the FO29 received picture at the right. In this case, the receive or transmit frequency was adjusted 10Hz at a time, while watching the 1200Hz sync pulse on the ChromaPix spectral display.

Although the transmit and receiving computer timings were perfectly matched, notice the slight left slant at the bottom of the received picture, another doppler problem.

An operationlly easier method would be to correct for doppler automatically by having the satellite tracking program not only display the satellite location and move the tracking antennas, but also modify the VHF/UHF transceiver's receive or transmit frequency. Another possibility would be to use an external computerized circuit which monitors the 1000Hz VIS signal and the 1200HZ Sync signal and sends a CAT signal to the VHF/UHF transceiver to keep things tuned to the right frequency.