Doppler shift and satellite radio: why the frequency moves and how to follow it

The ambulance siren, at 7.5 km per second

You already know the Doppler effect: a siren sounds higher as it comes toward you and drops as it passes. Radio waves do the same. A satellite in low orbit moves at about 7.5 km per second, so its signal arrives a little higher in frequency while it approaches and a little lower while it moves away.

The shift is proportional to the frequency and to how fast the distance between you and the satellite is changing (the range rate). In numbers: shift = frequency × range rate ÷ speed of light.

How big is it on each band?

For a satellite at about 400–800 km, the largest shifts near the horizon are roughly:

• 137 MHz weather satellites: up to about ±3 kHz.
• 145 MHz (2 m band): up to about ±3.5 kHz.
• 435–438 MHz (70 cm band): up to about ±10 kHz.
• 2.4 GHz (13 cm): up to about ±55 kHz.
• Geostationary satellites (such as QO-100) barely move relative to you, so their shift is tiny.

When it changes fastest

At the start of a pass the shift is at its highest positive value and changes slowly. It crosses zero at the moment of closest approach, which is also the highest point of the pass. On a pass straight overhead, the frequency swings fastest right at that moment: about 200 hertz per second on 70 cm. On a low pass far off to the side it changes gently all the way through.

So the middle of a high pass is where you need to tune most actively, and the edges are where you can relax.

Tuning FM satellites

FM signals are wide (about 12 kHz), so you only need to stay roughly on frequency. A common habit for satellites that listen on 2 m and transmit on 70 cm is to leave the 2 m uplink alone and step the 70 cm downlink in 5 kHz steps: +10 kHz at the rise, +5, the published frequency at the top of the pass, then −5 and −10 kHz as it sets. Save these as memory channels before you go out.

The rule behind it: correct the higher band, because that is where the shift is biggest.

Tuning linear transponders (SSB and CW)

Linear satellites relay a whole slice of spectrum, and SSB or CW signals are narrow, so every few hundred hertz matters. The usual method is the "one true frequency" rule: keep your own signal at a fixed spot in the satellite's passband and adjust your radio so it stays there. Many transponders are inverting: when you tune your uplink up, your downlink goes down, which takes a few passes to get used to.

Full-duplex radios with computer control make this almost automatic. Software reads the satellite's orbit, works out the shift every second and retunes both uplink and downlink. Our radar shows the corrected uplink and downlink for each transmitter during the pass, and can drive a radio's frequency over CAT.

A quick check you can do tonight

Open the radar for any satellite with a beacon, pick a high pass, and watch the corrected frequency count down through the pass. Then tune an SDR to the published frequency and watch its waterfall: the line drifts across exactly as the numbers predict. Seeing physics line up with a calculation, live, is one of the small joys of this hobby.