Presentation Information
[PPS01-P05]Evaluation of the potential for passive radio sounding of icy moons based on radar equation and ray tracing studies
*Ayuto Kawakami1, Yasumasa Kasaba1, Atsushi Kumamoto1, Fuminori Tsuchiya1 (1.Planetary Plasma and Atmospheric Research Center, Tohoku University)
Keywords:
Passive Radar,icy moons,Jovian radio emission,Radar equation,Ray tracing
Passive radar uses the reflections of radio waves emitted by natural sources, unlike active radar in which a spacecraft transmits the signal and detects its reflection from a target. By using natural radio sources such as Earth's auroral kilometric radiation (AKR; a few 10s to 100 kHz), Jupiter's auroral radio emission (hundreds of kHz to tens of MHz), and solar radio bursts (hundreds of kHz to tens of MHz), this approach contributes to smaller, lighter, and lower power instruments. It also enables the use of long wavelength waves that are difficult to transmit from the spacecraft, so less attenuation potentially allows deeper subsurface sounding. In addition, because the transmitting and receiving locations differ, radio waves can be observed at a variety of reflection angles, making it possible to estimate permittivity from the reflection angle dependence of reflectivity. Our RPWI aboard the JUICE mission plans passive radar observations to investigate the surface and shallow subsurface of icy moons during flybys and on the orbit around Ganymede. The use cases for surface and subsurface sounding in future ice giant missions and icy moon lander missions are also investigated.
In order to establish the feasibility of passive radar observations by JUICE RPWI, we are estimating the strength of reflected Jovian auroral radio waves from the moon's surface at various distances and reflection angles. We use two methods, the radar equation and the ray tracing.
The radar equation for active radio sounding for a point source is well established. In passive radar using a plane wave from a distant radio source, the transmitted wave does not undergo spherical spreading from a point source, so the incident intensity does not depend on observation altitude. The reflected intensity would be the same for a perfectly plane,but in reality it decreases with the altitude due to surface scattering and the curvature of the target body. Therefore, the standard radar equation must be modified [Schroeder et al. 2016], and we are currently working on this evaluation.
To validate the radar equation results, we are also evaluating the ray tracing. The assumed observation frequency is 1 to a few MHz, which is sufficiently higher than the plasma frequency of the Ganymede ionosphere. So unlike our occultation studies [e.g. Yasuda et al., 2025], refraction in the ionosphere is not included. At orbital altitudes of 100, 200, 500, and 1000 km, we evaluate the reflected radio intensity as a function of sub Jovian angle (related to reflection angle) for three cases; a perfect reflection, rocky (ε=8), and an icy (ε=3) surface. We evaluate TE and TM modes separately to assess polarization effects. These results are compared with expected Jovian radio intensities and the sensitivity of JUICE RPWI to assess the feasibility of passive radar observations. For comparison with the radar equation, we plan to include the effects of surface scattering as well.
We will also compare these results with the lunar surface reflection of Earth's auroral radio emission (AKR) measured by the lunar orbiter Kaguya LRS and verify the validity. JUICE received AKR during its lunar flyby in August 2024 at an altitude above 1000 km, and we will also evaluate whether lunar surface reflections could have been received along that trajectory.
In order to establish the feasibility of passive radar observations by JUICE RPWI, we are estimating the strength of reflected Jovian auroral radio waves from the moon's surface at various distances and reflection angles. We use two methods, the radar equation and the ray tracing.
The radar equation for active radio sounding for a point source is well established. In passive radar using a plane wave from a distant radio source, the transmitted wave does not undergo spherical spreading from a point source, so the incident intensity does not depend on observation altitude. The reflected intensity would be the same for a perfectly plane,but in reality it decreases with the altitude due to surface scattering and the curvature of the target body. Therefore, the standard radar equation must be modified [Schroeder et al. 2016], and we are currently working on this evaluation.
To validate the radar equation results, we are also evaluating the ray tracing. The assumed observation frequency is 1 to a few MHz, which is sufficiently higher than the plasma frequency of the Ganymede ionosphere. So unlike our occultation studies [e.g. Yasuda et al., 2025], refraction in the ionosphere is not included. At orbital altitudes of 100, 200, 500, and 1000 km, we evaluate the reflected radio intensity as a function of sub Jovian angle (related to reflection angle) for three cases; a perfect reflection, rocky (ε=8), and an icy (ε=3) surface. We evaluate TE and TM modes separately to assess polarization effects. These results are compared with expected Jovian radio intensities and the sensitivity of JUICE RPWI to assess the feasibility of passive radar observations. For comparison with the radar equation, we plan to include the effects of surface scattering as well.
We will also compare these results with the lunar surface reflection of Earth's auroral radio emission (AKR) measured by the lunar orbiter Kaguya LRS and verify the validity. JUICE received AKR during its lunar flyby in August 2024 at an altitude above 1000 km, and we will also evaluate whether lunar surface reflections could have been received along that trajectory.