5:15 PM - 6:45 PM
[PPS09-P14] Lunar Subsurface Exploration Using Passive Radar based on AKR observations by KAGUYA/LRS
Keywords:The Moon, Subsurface structure, KAGUYA, JUICE
Exploration of the lunar subsurface structure will help elucidate the history of the Moon. In addition, the use of lunar subsurface structures to protect human activities on the Moon from cosmic rays is attracting attention, increasing the importance of lunar subsurface exploration. The method to observe the subsurface structure using echoes of natural radio emissions is called passive radar. In this study, we investigated a method of lunar subsurface exploration by passive radar using auroral kilometer radiation (AKR) observations, which has a frequency of 10 kHz to several 100 kHz and is lower than the frequency of typical active radar (4 to 6 MHz in the case of KAGUYA/lunar radar sounder (LRS)). Thus, the passive radar has the potential to investigate subsurface structures more deeply than active radar. In a previous study, interference fringes due to AKR direct arrival waves (hereafter referred to as W1) and lunar reflection waves (W2) were observed by KAGUYA/LRS. In this study, we simulated the interference fringes by adding subsurface layer reflected waves (W3). We developed a tool to simulate interference fringes at arbitrary incidence angles according to the orbit of KAGUYA.
The simulation results showed that the interference amplitudes due to W2 and W3 (<2dB) were smaller than the amplitudes due to W1 and W2. When the AKR is used as a passive radar wave source, the spectral amplitude of AKR itself is larger than the amplitude of the interference fringes. The main reason for the smaller interference amplitude is small Fresnel reflectivity in the subsurface layer. With the KAGUYA/LRS frequency resolution of 6 kHz, fringe observations which satisfy the Nyquist law is possible only in the region where the angle of incidence of the radio wave to the Moon is larger than 79.2 degree. Due to these reasons, it is difficult to obtain information on the subsurface layer reflected waves from the KAGUYA/LRS observations. From the interference fringe simulation, it was evaluated that a frequency resolution of 200 Hz was required to conduct the fringe observation of the Moon. We attempted to estimate the bulk dielectric constant of the lunar surface from the interference fringe amplitudes of W1 and W2 observed by KAGUYA/LRS. While the permittivity of the lunar surface has been evaluated to be 4-10 from previous studies, the permittivity obtained in this study is at most 1.3. The small permittivity is not result of large porosity of the lunar regolith layer but may be result of surface roughness of the Moon which was not considered by the present simulation. Future studies are needed to resolve this issue.
Using the developed interference fringe simulation tool, we evaluated interference fringe spectra at an altitude of 5m from the lunar surface, simulating a radio receiver on a lunar lander. The expected interference fringe amplitude was estimated to be 2 dB in a frequency of 400 kHz. We also evaluated the interference fringe spectra and delay times of the reflected waves (W2 and W3) during the lunar flyby of the JUICE spacecraft scheduled for August 2024. The frequency resolution required to observe the interference fringes at the perihelion is 600 Hz. A length of the waveform required to determine the time difference between W1 and W2/W3 is 7.5ms around the closest approach, and the time resolution required to separate W2 and W3 is 5.8 microseconds.
The simulation results showed that the interference amplitudes due to W2 and W3 (<2dB) were smaller than the amplitudes due to W1 and W2. When the AKR is used as a passive radar wave source, the spectral amplitude of AKR itself is larger than the amplitude of the interference fringes. The main reason for the smaller interference amplitude is small Fresnel reflectivity in the subsurface layer. With the KAGUYA/LRS frequency resolution of 6 kHz, fringe observations which satisfy the Nyquist law is possible only in the region where the angle of incidence of the radio wave to the Moon is larger than 79.2 degree. Due to these reasons, it is difficult to obtain information on the subsurface layer reflected waves from the KAGUYA/LRS observations. From the interference fringe simulation, it was evaluated that a frequency resolution of 200 Hz was required to conduct the fringe observation of the Moon. We attempted to estimate the bulk dielectric constant of the lunar surface from the interference fringe amplitudes of W1 and W2 observed by KAGUYA/LRS. While the permittivity of the lunar surface has been evaluated to be 4-10 from previous studies, the permittivity obtained in this study is at most 1.3. The small permittivity is not result of large porosity of the lunar regolith layer but may be result of surface roughness of the Moon which was not considered by the present simulation. Future studies are needed to resolve this issue.
Using the developed interference fringe simulation tool, we evaluated interference fringe spectra at an altitude of 5m from the lunar surface, simulating a radio receiver on a lunar lander. The expected interference fringe amplitude was estimated to be 2 dB in a frequency of 400 kHz. We also evaluated the interference fringe spectra and delay times of the reflected waves (W2 and W3) during the lunar flyby of the JUICE spacecraft scheduled for August 2024. The frequency resolution required to observe the interference fringes at the perihelion is 600 Hz. A length of the waveform required to determine the time difference between W1 and W2/W3 is 7.5ms around the closest approach, and the time resolution required to separate W2 and W3 is 5.8 microseconds.