Influence of antenna radiation pattern on deep echo detection of KAGUYA Lunar Radar Sounder

Abstract

Deep echo detection of ALSE Half a century ago, in 1972, the last mission of Apollo project, Apollo 17, carried an onboard science mission Apollo Lunar Sounding Experiment (ALSE) to the Moon. ALSE was an HF/VHF radar system of which the center frequency of the lowest frequency observation mode was 5 MHz. One of the notable results of ALSE was detection of deep subsurface echoes in Mare Serenitatis and Mare Crisium, which were interpreted as reflection echoes from deep subsurface media boundaries. The depth of these echoes in Mare Serenitatis was estimated as some 1.6 km with an assumption that the relative permittivity of the subsurface medium was 8.7, and 1.4 km in Mare Crisium. KAGUYA LRS 35 years later, another HF radar, Lunar Radar Sounder (LRS), went to the Moon onboard KAGUYA space craft. LRS worked with the same center frequency as that of ALSE with a bandwidth twice as wide as that of ALSE. Transmission power of LRS was 800 W whereas that of ALSE had been 130 W. Orbit altitude of KAGUYA was 100 +/-20 km whereas that of ALSE was 110 km. Indeed, LRS was a twin to ALSE but with much improved radar engineering specifications. However, none of such deep echo that ALSE reported was confirmed by LRS observation. Enhancement of deep range echoes While we studied SAR processed LRS data looking for deep signals, we found enhancement of echo intensity at an apparent depth of 7.5 km in Mare Crisium (Fig. 1), which is a depth of 2.54 km under the assumption of relative permittivity 8.7. It appears as if it were the bottom of Crisium basin. This enhancement of subsurface echoes might be interpreted as such subsurface echoes that were reflected/scattered by a very rough subsurface boundary located at the apparent depth of 7.5 km. But, strangely, similar echo enhancement was also found in data from other maria where any  subsurface echoes had not been detected at all due to the shielding effect of TiO2 rich surface. Nature of the echo enhancement We further studied this echo enhancement, and finally found that the ranges at which echo intensity enhances depend on the incidence angle of the radar pulse on the lunar surface. Fig. 2. compares those data which were obtained from three orbits: one was over Mare Crisium while two others were closely located orbits over Oceanus Procellarum where the surface is covered by highly TiO2 rich material thus that no subsurface echo has been reported. The incidence angle of the enhancement peak was found commonly at ~24 degrees. This result implies that the enhancement is not a phenomenon which occurs due to subsurface reflection/scattering but, rather, to the characteristics of LRS, most likely to be its antenna radiation pattern. Conclusion The echo enhancement phenomenon could lead to a misunderstanding that the deep echoes are of reflection/scattered echoes from a rough boundary interface at a large depth. Such misunderstanding can be avoided by comparing data from nearby orbit of different altitudes. If the depth of the echo enhancement of concern is independent of the orbit altitude, the echoes are very likely to be actual deep echoes. FiguresFig. 1. A 50-pulse mean radargram of Mare Crisium along the longitude of 58.85 degrees. Enhancement of echo intensity is clearly seen at around a range of 7.5 km.Fig. 2. Comparison of 3000-pulse-mean echo intensity after detrend processing (Fig. 3). Echo intensity is plotted as a function of incidence angle onto the lunar surface. Data from three orbits are shown.Fig. 3 Detrend processing. Decreasing trend of echo intensity as function of incidence angle was defined by single pulse A-scope data, and was subtracted from 50-pulse mean data to produce those data which are plotted in Fig. 2.