Last Updated on July 6, 2025 by John Berry
One EME station points its antennas at the moon and transmits. Another EME station points its antennas at the Moon and receives the weak signal returned. The signal received by the second station is the aggregate of multiple reflections and scatters from the varied terrain on the Moon. The received signal therefore sometimes suffers from libration fading.
The Moon’s orbit is locked to that of Earth, so the same part (or face) of the Moon always presents to observers on Earth. Whilst that’s true, there’s an additional effect. The Moon nods and wobbles. Astronomers are hugely interested in this movement, termed libration, because it allows them to see a bit more of the surface around the edges. That’s of no interest to the radio ham, but the fact that the Moon moves relative to the Earth is.
Libration fading
The the signal returned is not at a steady level. By way of analogy, a car moving in a city, using a receiver tuned to a broadcast FM radio station (or any other for that matter), receives multiple signal reflections from its surroundings. The aggregate signal received follows a Rayleigh frequency distribution with deep fades at a period of half a wavelength. The signal level with position is shown below. Position is that of the car doing the receiving.
This Rayleigh fading signal is described by its median value (value exceeded for 50% of time) and its variance.
Unlike the car, EME stations are fixed on the surface of the Earth. But the reflecting environment – the Moon – is moving relative to those stations and hence the same sort of fading occurs. The Moon’s libration movement is the equivalent of someone walking through a city at about 5km/hr. The received signal on Earth follows a Rayleigh frequency distribution with deep fades at a period of half a wavelength.
Changes in libration
There are times when libration is a minimum, and others when it’s maximum. The corresponding libration fading reduces to give a steadier signal received at the minimums, but at the maximums the deep fades are annoying to EME enthusiasts. The image below of the Moon’s libration shows two periods when the libration is worst, at apogee and perigee, around the points of inflexion. And it shows two periods when it’s least, when the Moon is in its orbit between 370,000km and 400,00km distant.
Nodding is change around the east-west Moon axis. Wobbling is change around the north-south axis. Both nodding and wobbling occur* with slightly different periods of around 27 days. The image below repeats the argument that libration is minimum halfway between perigee and apogee. Since the movement is then least, that’s the best time for radio hams to attempt EME communications. That’s from the perspective of libration fading – there are other effects that don’t necessarily coincide with libration.
Libration and coding
For transmission systems to cope well with fading, they must consider libration. Rayleigh fading is shown above. It has cusps rising above the median, and then deep fades. Amateurs have measured the cusps as giving up to 10 dB of enhancement and up to 23dB degradation over EME paths. It’s a pretty hostile environment in which to transmit.
Classically, communications engineers align the data coding – the way the data is impressed on the carrier signal – with the nature of the signal received. The simplest alignment approach is to increase the median signal to overcome the fades (by reducing the communications distance, for example). Operating targets are quoted as signal exceeded for perhaps 95% of time. That’s enabled by designing for shorter paths. That’s fine when designing commercial systems. But radio hams wouldn’t want to throw up to 23dB of power away.
The approach used by Joe Taylor, K1JT, and others, is to acknowledge the fades, and resend the same information over and over. Riding on multiple cusps, eventually the information will be received. I discuss the nature of coding systems like Q65 in a presentation on data modes.
This is easier at some frequencies than at others (since the fade period is half a wavelength). Designers of transmission systems often demand that settings must be matched to the signal environment, including signal frequency. Q65 has settings for various environments.
Ultimately, the transmission system designers quote thresholds at a frequency and for a given coding system taking account of the nature of the received signal. Sometimes fading can’t be overcome and it’s just a matter of waiting until the Moon’s distance from Earth changes.
*The nodding and wobbling are sometimes more complex. The Moon sometimes lolls around!