One of the first questions new satellite operators ask is whether they can program their radio once and be done with it. The short answer is no — and understanding why will make you a much more effective operator on the birds.
What Is Doppler Shift?
Doppler shift is the change in perceived frequency caused by the relative motion between a transmitter and a receiver. You experience it every time an ambulance passes — the pitch of the siren drops as it moves away from you. The same physics applies to satellites.
As an amateur satellite approaches your location, it's moving toward you at several kilometres per second. That motion compresses the radio waves, making the received frequency slightly higher than what the satellite is actually transmitting. As it passes overhead and begins to recede, the waves stretch out and the frequency drops. At closest approach (TCA — Time of Closest Approach), the shift is minimal.
For FM satellites like SO-50 or AO-123, the total Doppler swing from AOS to LOS can be as much as ±3–5 kHz on the downlink. On SSB satellites like FO-29 or RS-44 the effect is even more noticeable — a few hundred Hz of Doppler error is enough to make SSB audio sound distorted and difficult to copy.
Why the Frequencies Are Different Every Pass
Even if you memorized the Doppler values from last Tuesday's SO-50 pass, those numbers won't be accurate for today's pass. Several factors change the Doppler profile each time:
- Maximum elevation — a high overhead pass has a much larger Doppler swing than a low pass near the horizon. The satellite's velocity component toward or away from you is greatest during steep passes.
- Approach geometry — whether the satellite rises to your north, south, east, or west changes how the Doppler shift evolves throughout the pass.
- Your location — operators even a few hundred kilometres apart will experience noticeably different Doppler curves for the same pass.
This is why tools like Ham Sat Tracker calculate Doppler-corrected frequencies specifically for your location and each individual pass, showing you the expected uplink and downlink at AOS, TCA, and LOS. Those values are computed fresh every time using the satellite's current orbital elements.
How Operators Handle Doppler in the Field
In practice, the approach varies depending on the satellite type and how seriously you want to chase contacts.
For FM satellites, many operators simply program the published nominal frequencies and tune by ear during the pass. On a low to medium elevation pass the total shift is manageable — you can nudge the VFO up a couple of kHz at AOS and back down toward LOS while transmitting on the programmed memory. Some operators program three memory channels for a given satellite covering AOS, TCA, and LOS frequencies and step between them during the pass.
For SSB and linear transponder satellites like FO-29, RS-44, and AO-73, Doppler correction is essential. These satellites use an inverting transponder — your uplink and downlink shift in opposite directions, which adds complexity. The standard technique is to tune your downlink to a clear frequency and then adjust your uplink to compensate, keeping the downlink stable as the pass progresses. Pre-calculating a few waypoint frequencies before the pass and stepping through them on a memory bank makes this much more manageable.
Using Ham Sat Tracker's Doppler Values
The pass predictions in Ham Sat Tracker show corrected uplink and downlink frequencies at three points in each pass: AOS, TCA, and LOS. These are calculated for your specific location and that specific pass window, so they're as accurate as the underlying orbital elements allow.
A practical workflow before a pass: note the AOS uplink and downlink, program them into two memory channels, glance at the LOS values to know which direction you'll be tuning, and adjust as the satellite moves. After a few passes you'll develop a feel for how quickly the shift progresses and how much correction a low pass versus a high pass typically requires.
The short answer to the original question: program your nominal frequencies as a starting point, but plan to tune during every pass. The birds are fast — a typical LEO satellite pass is only 8 to 12 minutes — and a little preparation goes a long way toward pulling a clean contact out of the noise.
73 de VE3AKK