The high power, large aperture Mesosphere-Stratosphere-Troposphere (MST) Radar established at National Atmospheric Research Laboratory (NARL), Gadanki nearly two and half decades ago was designed to study the middle and upper atmospheric dynamics. This radar has now been successfully employed to detect the moon echoes, providing opportunity of probing planetary bodies, a new dimension of future research activities from NARL. The detection of moon echoes also provides an opportunity to characterise the large phased array antenna system including the calibration of the radar.

Since the moon is about 3,68,000 km away from the earth, the RF pulse has to travel for ~2.5 seconds to return to the radar from the moon’s surface. Since the primary design of the radar was not meant to study the moon, experiment was conducted cleverly in range-ambiguous mode in such a way that the range folded echoes from the moon fall in the height region where apparent source of atmospheric echoes were absent. The radar beam was positioned at 18° East, 12° East, 6° East, Zenith, and 6° West sequentially so as to receive moon echoes while it was apparently transiting from East to West. Due to the finite beam width (2.8°) of the antenna pattern and the apparent angular dimension of the moon is about 0.5°, the transit time of the moon across the antenna beam is estimated to be about 15 minutes. Observations were made for 26 minutes at each beam position in order to detect echoes beyond the half power beam width of the antenna radiation pattern.

The experiment was successful and an example of moon echoes observed in different directions, in the form of range-time variations of Signal-to-Noise Ratio (SNR), is shown in the figure. In this Figure, the true range ≈ observed range + 3,67,740 km. Salient results obtained are:

the moon echoes are found to have SNR as high as 20 dB

echo power changes as the radar pulse travels from the sub-terrestrial point towards the limbs consistent with the angular dependence of echo power

Doppler spectral property changes from narrow single peak spectrum to multi-peak spectrum as the radar pulse travels from the sub-terrestrial point towards the limbs

multi-peak spectra are found to be originating from multiple discrete targets on the moon

The first results from the moon echo experiments conducted using the MST radar are shown to be in agreement with those reported earlier at similar frequencies. The strong signals with narrow spectral features have been attributed to radar returns at normal incidence from places close to the sub-terrestrial point on the moon while the weak echoes with relatively broad spectral features have been attributed to discrete multiple scattering centers along the limb, which have also been confirmed by higher-order spectral analysis.

Radar techniques have been extensively used for studying the moon ever since the radio signals reflected from the moon were detected using ground-based radar. Both earth-bound and satellite-based radar techniques have since been employed for probing the moon’s surface topography, impact craters morphology, and regolith. Most of the radar observations for studying the moon, however, have been made at centimeter wavelengths except for a few which were done from the earth in the meter-decameter wavelength. Radar experiments at meter-decameter wavelengths provided important information on the wavelength dependence of radar cross section and angular dependence of backscattering, which were used for characterising lunar surface properties, regolith and underneath rocks.

It is planned to employ the delay-Doppler technique on the MST radar for mapping the moon’s surface, which would be a reality in the near future once the active array MST radar project is upgraded for incoherent scatter radar applications. Such observations then can be compared with those of Mini-SAR onboard Chandrayaan-1 not only to understand the moon surface but also to study the Very High Frequency (VHF) echoes from the moon and the ionospheric effects on radio experiments.

Details on the echo characteristics and its implications can be found in a research paper published recently in Current Science.

Link to MST Radar

SNR of moon echoes as a function of ambiguous-range and time observed in different beams. Echoes marked as A, B, C, D, and E correspond to observations made in 18° East, 12° East, 6° East, Zenith, and 6° West beams, respectively