Can radio amateurs do something similar?
Learning from planetary radar imaging
Volker Grassmann, DF5AI
Detecting lunar radio echoes is one issue, what about Mercury, Venus and Mars? We may consider moonbounce communication the radio amateur's simple counterpart to scientific radar experiments detecting radio echoes from the planets. However, by using the powerful transmitter of the Goldstone Solar System Radar and by using the Very Large Array as the receiver, the radio astronomer's experimental setup exceeds our capabilities by many orders, of course. Sophisticated instrumentation isn't the only difference though ...
The major difference between planetary radar imaging and amateur radio Earth-Moon-Earth communication results from the data analysis methods. Implementing bigger arrays and more powerful transmitters is the radio amateur's traditional approach to improve the detection of faint lunar radio echoes. With Joe's (K1JT) signal processing methods, EME operation became much smarter in recent years, i.e. we no longer interpret receiver signals by voltages and currents to be processed by electronic circuits but also by bits and bytes supporting mathematical data analysis methods. However, there is one major challenge specific to amateur radio: because we are mainly interested in realtime communication, we cannot rely on noise reduction methods associated with signal integration over long time periods.
As an alternative, we may reduce the speed of the Morse code but the fundamental problem remains (referring to terrestrial radio communication, a prominent example of very very slow speed CW is given by the first 136 kHz transatlantic QSO between VA3LK and G3AQC which took 14 days for completion). While radio amateurs are challenged by the requirements of realtime communication, radio astronomers also find their specific challenge - a challenge unknown in ham radio, i.e. the radio telescope's angular resolution. Astronomers wish to study features of small angular size in the sky but the radio telecope's ability to resolve those small features is limited by the ratio of the radio wavelength and the antenna's geometrical size. To improve the telescope's moderate resolution, radio astronomers use a technique known as radio interferometry in which a number of radio telescopes simultaneously observe the same object in the sky. The angular resolution of an interferometer array is given by the ratio of the radio wavelength and the distance (baseline) between the individual antennas. Since the separation between the antennas may be thousands of kilometers (very long baseline interferometry - VLBI), the angular resolution increases dramatically and may reach values as small as one milli-arcsecond corresponding to the angular size of a person walking on the Moon. In VLBI applications, each radio telecope records the data on a magnetic tape which is finally shipped to the data analysis center. Here, all radio telecope data is analysed by the averaging process of cross-correlation which removes much of the noise that was added to the signal by the electronic receiver circuits. The cross-correlation in particular identifies signal components identical in all data recordings which finally results in radar images such as the ones shown above. The same method was used, by the way, to pinpoint the Huygens space probe in the atmosphere of Titan. Can radio amateurs generate radar images of the lunar surface by an, say Amateur Long Baseline Interferometry Experiment (ALBIE)? This experiment would indeed represent a major effort and it would require an international team of skilled radio amateurs - however, I cannot see fundamental obstacles which couldn't be solved. So, are there any volunteers developing ALBIE into one of the most sophisticated initiatives ever launched in amateur radio?