The Lunar Radar Sounder (LRS) Onboard the KAGUYA (SELENE) Spacecraft

Abstract

The Lunar Radar Sounder (LRS) onboard the KAGUYA (SELENE) spacecraft has successfully performed radar sounder observations of the lunar subsurface structures and passive observations of natural radio and plasma waves from the lunar orbit. After the transfer of the spacecraft into the final lunar orbit and antenna deployment, the operation of LRS started on hours worth of radar sounder data and 8961 hours worth of natural radio and plasma wave data have been obtained. It was revealed through radar sounder observations that there are distinct reflectors at a depth of several hundred meters in the nearside maria, which are inferred to be buried regolith layers covered by a basalt layer with a thickness of several hundred meters. Radar sounder data were obtained not only in the nearside maria but also in other regions such as the farside highland region and polar region. LRS also performed passive observations of natural plasma waves associated with interaction processes between the solar wind plasma and the moon, and the natural waves from the Earth, the sun, and Jupiter. Natural 146 T. Ono et al. radio waves such as auroral kilometric radiation (AKR) with interference patterns caused by the lunar surface reflections, and Jovian hectometric (HOM) emissions were detected. Intense electrostatic plasma waves around 20 kHz were almost always observed at local electron plasma frequency in the solar wind, and the electron density profile, including the lunar wake boundary, was derived along the spacecraft trajectory. Broadband noises below several kHz were frequently observed in the dayside and wake boundary of the moon and it was found that a portion of them consist of bipolar pulses. The datasets obtained by LRS will make contributions for studies on the lunar geology and physical processes of natural radio and plasma wave generation and propagation.

Radio occultation measurement of the electron density near the lunar surface using a subsatellite on the SELENE mission

“…Measurements of the lunar electromagnetic environment by other instruments onboard SELENE are mostly limited to altitudes higher than ∼50 km, and thus electron clouds confined below 30 km altitude cannot be studied. An exception would be the natural plasma wave receiver (NPW) and waveform capture (WFC) instruments, which are subsystems of the lunar radar sounder (LRS) [ Ono et al , 2010]. The instruments detected auroral kilometric radiation propagating from the Earth and reflected by the lunar surface, and using these data the electron density near the lunar surface below the spacecraft can be constrained [ Goto et al , 2011].…”

High Frequency Analyzer (HFA) of Plasma Wave Experiment (PWE) onboard the Arase spacecraft

“…In order to achieve the requirements mentioned above, the HFA was designed based on the heritage of radio and plasma wave receivers installed on Japanese spacecraft such as Jikiken (EXOS-B) (Oya et al 1981), Ohzora (EXOS-C) (Oya et al 1985), Akebono (EXOS-D) (Oya et al 1990), Nozomi (Planet-B) (Ono et al 1998), and Kaguya (SELENE; Selenological and Engineering Explorer) (Ono et al 2008(Ono et al , 2010. To determine some specifications such as frequency resolution and dynamic range, we considered the past analyses of datasets obtained by Akebono/PWS (Plasma Waves and Sounder Experiment).…”

GPR observation of the Moon from orbit: Kaguya Lunar Radar Sounder

“…ALSE was an HF (5 MHz) radar system which was operated from an orbit at 120 km above the lunar surface. The report that ALSE detected deep subsurface echoes [2] motivated planetary scientists to send orbiting HF radars to Mars [3], [4] and to the Moon [5] in the first decade of the twenty first century (Table I). These HF radars proved that an orbiting HF radar is a powerful tool to survey subsurface structure of planets up to a few km depth [ Kaguya was a lunar exploration project of Japan Aerospace Exploration Agency (JAXA) which had been formerly started as the SELenological and ENgineering Explorer (SELENE) project in 1997.…”