Visualizing Satellite Doppler Shift

Per Wikipedia, Doppler Shift is defined as:

The change in frequency or wavelength of a wave for an observer who is moving relative to the wave source.

In regards to amateur radio satellites, Doppler Shift is experienced as the satellite (wave source) is moving relative to you, the observer (or receiver) of the transmitted wavelength.

So, tonight there was a decent passing of the AO-91 satellite. Although it is not nearly as active after midnight (EST) as it is during the day time, activity can still be heard on it at all hours over the United States. All of my AO-91 QSOs have been logged after midnight, as the satellite is less crowded and easier to work.

 

Tonight’s AO-91 pass was pretty good. Even with a normal outdoor omni directional antenna I have been able to work the satellite well.

 

I decided to create a visual of AO-91’s Doppler Shift by tuning my RTL-SDR dongle in GQRX to AO-91’s advertised downlink frequency of 145.960 MHz.

 

As the satellite rose over my horizon to the North, the receiving frequency was greater than the documented frequency of 145.960 Mhz and as it exited about ten minutes later to the South was less than the documented frequency. The difference in all reality is minimal, and the shifted frequency still falls within the normal filter width of the standard 10 k narrow FM filter.

Adjusting the downlink frequency to receive AO-91 is not really necessary, however AMSAT does recommend adjusting the transmit frequency for best performance. View the table below for reference.

AO-91 Doppler Shift Correction
Memory TX Frequency
(w/ 67 Hz Tone)
RX Frequency
Acquisition of Signal (AOS) 435.240 MHz 145.960 MHz
Approaching 435.245 MHz 145.960 MHz
Time of Closest Approach (TCA) 435.250 MHz 145.960 MHz
Departing 435.255 MHz 145.960 MHz
Loss of Signal (LOS) 435.260 MHz 145.960 MHz

 

If you want to learn more about the Doppler Effect / Doppler Shift, Google (or DuckDuckGo, for those who value a bit of privacy) is going to be a great source to get you started. I do not know enough to write about it at any great lengths, I only wanted to create and share with you the visual above.