Doppler Shift in Amateur Radio: Satellites, EME and Meteor Scatter

Anyone who has heard an ambulance siren pass knows the Doppler shift: the pitch is higher while the vehicle approaches and lower once it has passed. Radio waves do the same. A transmitter that moves toward you is received at a slightly higher frequency than the one it sends, and a transmitter that moves away is received lower. On HF with ground stations the effect is invisible, but on a satellite pass it moves the signal by many kilohertz, on moonbounce it moves it by hundreds of hertz or more and on the microwave bands it can be larger than the whole signal bandwidth. This guide explains the formula, gives realistic sizes for the amateur bands, shows how the effect is corrected in practice and tells you when you can safely ignore it.

What Doppler Shift Is

The Wikipedia article on the Doppler effect defines it as the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. When the source moves toward the observer, each cycle is emitted from a position closer to the observer than the previous one, so the time between cycles is shorter and the frequency higher. When the source moves away, the opposite happens. The article adds that for waves in a vacuum, as with radio waves, only the relative velocity between the observer and the source needs to be considered. For sound, the speed of the medium matters too, but for radio there is no medium, which makes the radio case simpler.

It also tells us that only the part of the velocity along the line of sight counts. The article gives the radial velocity as the speed multiplied by the cosine of the angle between the direction of motion and the line of sight. A satellite that passes straight overhead has no radial velocity at the closest point, and the shift passes through zero at that moment, while a satellite that is low on the horizon and moving toward or away from you has the largest radial velocity.

The Formula for Radio

For speeds much smaller than the speed of light, which is always the case for amateur radio, the shift is simple:

Δf = f × v / c

where f is the frequency, v is the radial velocity between the two stations (positive when approaching) and c is the speed of light, about 299,792 km per second. Because the shift is proportional to the frequency, the same motion produces a larger shift at a higher frequency. A satellite moving at 7 km/s directly toward you would raise a 145.9 MHz signal by about 3.4 kHz and a 435.8 MHz signal by about 10.2 kHz. These are upper bounds. In a real pass the radial velocity is lower, because the satellite is not moving directly at you except at the very beginning and the very end of the pass, and the maximum shift is somewhat lower than the numbers above.

The shift is symmetrical for a two-way path, such as a signal that goes to a satellite and comes back, or to the Moon and back, so the total shift on a round trip is roughly double the one-way value. That is why moonbounce at higher frequencies needs correction even though the Moon moves slowly relative to the Earth, as we explain below.

Satellites: The Largest Everyday Effect

If you work amateur satellites in low Earth orbit, you meet Doppler shift at every pass. At the start of the pass, as the satellite rises over the horizon, it moves toward you and the downlink is received high. The shift falls steadily and passes through zero at the closest approach, then the signal drops below the nominal frequency as the satellite moves away. For an FM satellite on 70 cm, the shift runs from about plus 10 kHz to about minus 10 kHz in the worst case, which is much more than the margin of an FM receiver, so you have to retune during the pass or use a radio that does it for you. The signal moves in the opposite direction on the uplink: to arrive at the satellite on the right frequency, you must transmit slightly lower at the start of a pass and higher at the end, when the satellite is moving away.

Software removes the work. Programs such as the one in our guide to Gpredict for satellite tracking compute the shift from the orbital data of the satellite and your location, and control a radio through CAT so that the uplink and downlink frequencies follow the satellite. If you do it by hand, the usual method is to tune the receiver in steps during the pass, correcting in a few steps of a few hundred hertz or kilohertz, and to keep your uplink within the passband of the satellite. For a start with satellites, our guide to getting started with amateur radio satellites explains the practice, and the Wikipedia overview of amateur radio satellites lists the types.

The type of the satellite decides how you must handle the effect. For an FM repeater satellite, small shifts cause bad reception at the beginning and end of a pass, and you may need to correct only by a few steps. For linear transponders, where many signals share a wide passband and the uplink and downlink must move together, you need an accurate correction in both directions. For a geostationary satellite such as the one in our QO-100 guide, the satellite is fixed in the sky, and the Doppler shift is negligible, so the issue is the stability of your own equipment.

EME: A Small Shift on a Big Path

The Moon is far away and slow, but the Earth rotates under it, and a station on the surface moves at up to about 465 metres per second at the equator. That motion is a radial velocity relative to the Moon that changes during the day and is zero when the Moon is due south, and it is largest at moonrise and moonset. The two-way shift is about twice the one-way value. A rough estimate of the maximum from the rotation alone is about 450 Hz at 144 MHz, 1.3 kHz at 432 MHz and 4 kHz at 1296 MHz. These are rough upper bounds, and the real shift depends on your latitude and the position of the Moon, but they show the right order of magnitude: on the 2 m band the shift is a few hundred hertz, on the microwave bands it is several kilohertz.

EME signals are narrow and very weak, and a shift of a few hundred hertz moves them out of the decoder’s search range, so EME software tracks it. Our EME beginner’s guide describes how the modes and the programs handle it, with the station and the other end both being corrected. On the microwave bands, the correction is a necessity and not an extra.

Meteor Scatter, Aircraft and HF

A meteor trail is an ionised column that moves in the upper atmosphere, and the signals reflected from it have a Doppler shift that is usually small compared to the bandwidth of the fast digital modes used on 6 m and 2 m. Fast modes such as MSK144, described in the meteor scatter guide, tolerate it, though the effect grows with the frequency, and the software handles the small shifts without any action from you. Reflections from aircraft produce a larger shift because the aircraft moves quickly, and an aircraft-scatter signal appears as a smeared tone that moves rapidly in frequency.

On HF between two fixed stations the Doppler shift is not due to the motion of the stations but to the movement of the ionosphere, and it is normally a fraction of a hertz to a few hertz. This is the reason why weak-signal HF modes are designed to tolerate small drifts. The effect increases at times of rapid change in the ionosphere, and it can be seen as a slow smearing of a narrow signal. For a mobile station, such as a car with an FM radio on VHF, the shift is tens of hertz and has no practical consequence.

Doing the Calculation by Hand

1. Find the radial velocity. For a satellite, your tracking program shows the range rate in kilometres per second. A positive value for decreasing range means approaching.
2. Apply the formula. Multiply the frequency by the range rate and divide by 299,792. For 145.9 MHz and a range rate of 5 km/s toward you, the shift is 145.9 million times 5 over 299,792, about 2.4 kHz up.
3. Correct the downlink. Tune the receiver to the nominal frequency plus the shift when the satellite approaches, and minus the shift when it recedes.
4. Correct the uplink in the opposite sense. Transmit below the nominal frequency when the satellite approaches and above it when it recedes, so that the signal arrives at the satellite on the right frequency.
5. Re-check regularly. The shift changes fastest around the closest approach, so update more often during that part of the pass.

Practical Tips for Operating With Doppler

• Check your clock and your frequency accuracy. Doppler correction assumes that your own radio is on frequency. A transceiver that is 1 ppm off adds 435 Hz of error at 435 MHz on top of the shift you are trying to correct.
• Use the right radio mode. FM satellites tolerate small errors, while SSB and CW on linear transponders need corrections accurate to a few tens of hertz to be readable.
• Listen first. Tune to the beacon or the downlink before you transmit and set your uplink after you have found your own signal on the downlink.
• Move in small, regular steps. If you retune by hand, use the same step size and make changes at the same interval, so that the other operators can follow you.
• Trust your tracking software and update the orbital data regularly, because old data gives wrong predictions of the shift and of the times of the pass.

The Bottom Line

Doppler shift changes the received frequency by the transmitted frequency times the radial velocity divided by the speed of light, Δf = f × v / c. For amateur satellites this means shifts of up to roughly 3 kHz at 145 MHz and 10 kHz at 435 MHz, which you correct by tuning during the pass or with tracking software. For EME the shift from the Earth’s rotation is hundreds of hertz on 2 m and kilohertz on the microwave bands, and the software tracks it. On meteor scatter and HF the effects are small and are tolerated by the modes. Learn the formula, use a tracking program and remember that the uplink moves in the opposite sense to the downlink, and the effect becomes a routine part of the operation.

Frequently Asked Questions

What is Doppler shift in ham radio?

It is the change in the received frequency that results from the relative motion between transmitter and receiver. Approaching stations are received higher and receding stations lower.

How large is the Doppler shift on a satellite pass?

Up to roughly 3 kHz at 145 MHz and about 10 kHz at 435 MHz for low Earth orbit, passing through zero at the closest approach.

What is the formula for Doppler shift?

Δf = f × v / c, where v is the radial velocity between the stations and c is the speed of light. It is valid for the small speeds of satellites, aircraft and vehicles.

Do I need to correct for Doppler on HF?

Normally no. Shifts from the ionosphere are a fraction of a hertz to a few hertz, which weak-signal HF modes tolerate.

Why must I correct the uplink as well as the downlink?

The satellite also receives your signal with a shift. To arrive at the right frequency, your uplink must be shifted in the opposite direction to the downlink.

Does EME need Doppler correction?

Yes, especially on higher bands. The rotation of the Earth gives a two-way shift of hundreds of hertz on 2 m and several kilohertz on microwave bands, and EME software tracks it.