Understanding Doppler Shift
Doppler across a pass
A satellite's Doppler traces a smooth S-curve over a single pass. At acquisition of signal, when it first rises above the horizon and is heading towards you, the received frequency sits at its highest — the full positive shift. As the satellite climbs the shift shrinks, passing through zero at the point of closest approach — the highest-elevation moment of the pass, where the satellite is moving sideways rather than towards or away from you. It then turns negative and steepens as the satellite sinks back to the horizon. A high, near-overhead pass swings through the whole range fastest and demands the quickest tuning; a low pass that only grazes the horizon shifts more gently.
Why higher bands shift more
The Doppler shift is directly proportional to the carrier frequency, so the same 7.5 km/s of motion barely nudges a VHF signal yet drags a microwave one a long way. That is why the 2 m band is forgiving while operators working 23 cm and above must retune almost constantly. The figures below are approximate peak shifts for a good overhead pass; they are quoted as ± from the published frequency, so the full swing from the start of a pass to the end is about double. Slower-moving, more distant MEO and geostationary satellites shift far less, because their range rate relative to the ground is much lower.
| Amateur band | Frequency | Peak Doppler (LEO) |
|---|---|---|
| 2 m (VHF) | 145 MHz | ±3.5 kHz |
| 70 cm (UHF) | 435 MHz | ±10 kHz |
| 23 cm (L-band) | 1.26 GHz | ±30 kHz |
| 13 cm (S-band) | 2.40 GHz | ±55 kHz |
Correcting Doppler in practice
On wideband FM satellites the receiver's bandwidth swallows most of the shift, so a 2 m downlink often needs no tuning and a 70 cm link only occasional 5 kHz nudges — many operators just store split-frequency memories for the start, middle and end of a pass. SSB and CW on a linear transponder are far less forgiving: a drift of even 100 Hz is audible, so you must retune continuously. The long-standing rule is to tune the higher-frequency band and leave the lower one fixed — for a typical 70 cm-up, 2 m-down transponder that means adjusting the uplink. Many transponders also invert the passband (you transmit lower sideband and hear upper sideband), which makes the uplink and downlink shifts partly cancel. In practice most operators let rig-control software such as Gpredict or SatPC32 apply full correction to both links automatically. The ISS makes a good first target — its 145.800 MHz SSTV downlink shows gentle 2 m Doppler, while its 437.800 MHz FM cross-band repeater shows the larger 70 cm swing.
Doppler as a tool: navigation and tracking
The same effect is as much an instrument as a nuisance. The 1960s TRANSIT system — the first satellite-navigation network — worked purely by Doppler: a receiver watched a satellite's stable carrier rise and fall in frequency as it passed overhead and solved backwards for its own position. Modern GNSS receivers still contend with Doppler, searching a window of roughly ±5 kHz to lock onto each GPS signal and using the measured shift to compute your velocity. Ground stations run the physics in reverse for orbit determination, deriving a spacecraft's range rate from two-way Doppler, while systems such as Argos locate wildlife tags and ocean buoys from the Doppler curve of their transmissions.