Last Updated on May 23, 2026 by John Berry
The Moon as reflector is complicated. I’ll try to build an explanation here and on other pages.
Our Moon is about 390,000km away so the path loss there and back is large. That path loss varies depending on where the Moon is on its elliptical orbit. Its diameter is about 3,500km.
Radio waves from antennas on the surface of the Earth pointed at the Moon will be reflected or scattered by the Moon’s surface.
Since its diameter is large compared to an isotropic radiator, it has gain over an isotropic reflector or scatterer. That gain makes moon bounce possible.
Scattered reflections
The lunar terrain features deep craters and hills as well as boulders and plains. From scattering theory, the effectiveness of roughness increases with frequency. At VHF, the surface appears relatively smooth. Therefore, reflection is predominantly specular. Conversely, at SHF frequencies and above, the surface is rough and causes scattering. I discuss the path loss including refection and scattering separately.
Consider the advance of a flat wavefront (toward the Moon). This is returned from a plurality of reflectors and scatterers. The energy returned will describe a new wavefront. The signal at each point on this wavefront will have different amplitude and phase. This is caused by different scattering and reflection efficiencies and different path lengths from each point on the lunar surface. Ultimately, the signal received will be the aggregate of all arrivals back on Earth.
The Moon’s movement and the signal return is shown graphically above.
The Moon orbits the Earth and rotates. Combined with the Earth’s orbit and rotation, those movements cancel. As a result, the Moon presents approximately the same face to the Earth always.
The Moon does however have some irregular movement. Relative to someone observing on Earth, it nods (North-South), and it wobbles (East-West). And since it is on an elliptical orbit, it is moving at speed away from Earth and back again with a period of a month.
Considered locally, reflections and scatters are more effective near the centre of the Moon’s face. That’s logical since many of the reflections from the sides (towards the Moon’s outer edges or limbs) will be lost to space. This loss heightens with distance from the centre. And although this appears a steady state, the irregularity of the Moon’s movement means irregular reflections and libration fading.
Reflection coefficient
The effectiveness of a reflecting surface is also characterised by its reflection coefficient. If the energy incident on a reflector is all returned, the reflection coefficient is 1, or 100%.
Whilst the Moon is big (and hence there’s reflection and scattering happening over a large area), much of the energy from a signal is absorbed.
Attempts at measurement of the reflection coefficient put the figure at between 6.5% and 11%. So just a few percent of the energy is returned. But there’s no definitive value, and little understanding about how reflections and scattering vary with frequency.
The Moon as reflector
In summary:
- Waves arriving at the Moon’ surface will be reflected and scattered.
- The Moon has gain with respect to an isotropic reflector or scatterer.
- The signal received back on Earth is the aggregate of all refections and scatters.
- Libration introduces Doppler shift and Doppler distortion (spectral spreading).
- Only the centre of the Moon’s surface is effective in reflection and scattering.
- The Moon’s reflection coefficient is between 6.5% and 11%.
I discuss the path budget and fade margins in other pages.