Zoom in the Lab

47 - Lunar Radar Control

Nostradamus can see beyond the horizon and beyond the atmosphere as well! The soil, or even the subsoil, of the Moon can be studied… without leaving the Earth! This is indeed what ONERA’s Nostradamus radar can do, if its electromagnetic sight is pointed towards the Moon rather than towards the horizon.

Until now Nostradamus, ONERA’s “sky wave” transhorizon radar, has used the atmosphere’s stratification properties to detect objects well beyond the horizon. For this, Nostradamus’ radar waves are reflected by the ionosphere, at altitudes between 100 and 300km, and make it possible to detect moving targets well beyond the horizon, from a few hundred to several thousands of kilometers away. The applications of Nostradamus concern defense and safety, of course, since the radar is able to detect the movements of aircraft and boats that are very far from our borders. However, Nostradamus is also a powerful research tool for geophysicists, oceanographers and aeronomists, who use it to study the movements of the ionosphere, to chart the state of the sea and sea currents, or to receive seism or tsunami warnings.

With the aim of diversifying the possible targets and practices of this powerful detection tool, new functionalities have been explored since 2009, with the Paris Institute of Earth Physics (IPGP), a regular collaborator with ONERA. From now on, its waves will travel towards our natural satellite, using a novel method: trans-ionospheric propagation, which consists in crossing the ionosphere, this time using the waves of highest frequencies of the HF spectrum.

Why the Moon? There are two reasons: the first, according to Jean-Philippe Molinié, of the Electromagnetism and Radar department: “We are mainly interested in the radar aspect: when a high frequency wave crosses the ionospheric layers, it is slightly deflected. The correction of this bias would improve the location of objects within the ionospheric layers, or even beyond them”. These measurements would complement those of the Graves space surveillance radar, also designed by ONERA, which determines the orbits of satellites flying over France.

The Nostradamus radar antenna network

The second reason for such research, for the researchers of the IPGP, is the study of the lunar surface. “That interests them because at HF, it must be possible to penetrate the lunar surface, under the regolith, to probe the subsoil a little” says the radar specialist. The wave penetration depth is thus estimated, which provides information on the composition of the subsoil, where some still hope to find a good quantity of water. In addition, measurements of the Moon with HF waves would make it possible to obtain knowledge on some lunar surface parameters, such as the dielectric permittivity and conductivity of the surface.

This is what the moon looks like on the Nostradamus radar screen! For the specialists, this trace makes sense (see below)

For the moment, the evaluation of the feasibility of studying the Moon using Nostradamus has started with an energy assessment of the signature of the Moon: it takes into account the distance and the size of the celestial body. The results obtained from the first experiment were satisfactory and the Earth’s natural satellite was indeed detected. However, as Jean-Philippe Molinié says: “We have had to probe deeper, because the Moon does not have a point echo, like an airplane. It is a large sphere, so the radar resolution means it has to be divided into sections”. Another issue to be solved is that of the observation resolution. “Resolutions of a few kilometers are obtained, depending on the wavelength used. In the future, “synthetic” radar imaging methods will take advantage of the displacement of the Moon and be able to obtain a much more precise two dimensional image of the Moon’s surface.”.

We’ve come a long way since the times of the Cold War, when the radar echoes of the Moon were mistaken for a flotilla of Soviet missiles!

Interview and article by Eric Millet

 

More technical supplements

The echo back-scattered by the lunar surface is divided into distance points (for a radial distance resolution of a few kilometers). The distance processing divides the surface of the Moon into iso-distance rings. Likewise, a Doppler treatment would show iso-Doppler half-rings.

Doppler-distance image of the back-scattered echo. A small object would have a specific echo. The length of the echo segment corresponds to the moon's radius (that is to say, 1700 km).

Section according to the distances from back-scattered echo

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