17:15 〜 19:15
[PCG20-P03] The Next Generation small-body Sample Return mission: Bi-static radar for comet nucleus sounding – Antenna and radar transponder
キーワード：バイスタティックレーダ、次世代小天体サンプルリターンミッション、彗星核レーダ内部探査
Investigations for a new mission of sample return from inactive comet as 289P/Blanpain are performed in the Next Generation small-body Sample Return Working Group (NGSR-WG). In addition to the sample return and in-situ analysis of the subsurface material of the comet nucleus, observations of the internal structures of the comet nucleus by radar and seismometer are also planned in NGSR mission. By using radar, we can know whether the comet nucleus is rubble pile formed through catastrophic disruptions and re-accumulation, or pebble pile formed through the accumulation of dust and pebbles.
In order to achieve the purpose, a new bi-static radar system is proposed in NGSR mission. The radar system consists of Radar-A (radar transceiver) installed on Deep Space Orbital Transfer Vehicle (DS-OTV) which stays at a distance of 10 km from the comet nucleus, and Radar-B (radar transponder, which receives radar pulses from Radar-A and immediately transmits them back) installed on the Lander which can get closer to the comet nucleus in sample operation, and also kept on the orbit with a distance of about 0.5 km from the comet nucleus in radar tomography operation. NGSR Radar-A and B are chirp radar operated in a frequency range from 140 to 200 MHz. The range resolution is therefore 2.5 m in vacuum and ~2 m in the media with a permittivity of 2.
For transmission in wide frequency range, we plan to apply a bowtie antenna with a length of 0.7 m and a width of 0.2 m, which can be stored on the panel of the spacecrafts. The characteristics of the bowtie antenna were evaluated using 1/8 scale model in December 2024. The return loss was measured to be 20 dB in 150-190 MHz, and 10 dB in 140-210 MHz. The radiation pattern was almost isotropic. The performance of the antenna is generally degraded if the antenna is located too close to the ground plane. In the measurement using 1/8 scale antenna, the height of the antenna was decreased from (0.6/8) m to (0.3/8) m and found that the decrease of the receiving level was limited within 3 dB. The unexpected results need to be verified by numerical analyses using method of moments (MoM). In addition to the baseline plan using the rigid bowtie antenna, we also investigate another backup plan using aluminized Kapton etched antennas implemented on the solar cell sheet of the Lander.
Development of Radar-B bread-board model (BBM) was also performed from September 2024 to February 2025. The Radar-B BBM consists of directional coupler and power amplifier with attenuator. The signal generated by direct digital synthesizer (DDS) are fed to the Radar-B BBM through EO/OE converters and optical fiber to simulate propagation delay from DSOTV to Lander. The waveform of the signal sent back from the Radar-B BBM was sampled using digital oscilloscope. We could confirm that the total gain of power amplifier of Radar-B have to be less than the isolation of the directional coupler. In order to avoid the oscillation of the power amplifier, its gain have to be less than 34 dB, less than the isolation of the directional coupler. If we can use radar transponder as Radar-B BBM which relays radar pulse with no delay using analog circuits, the mass, and power consumption could be reduced up to 0.7 kg and 0.7 W, and synchronization of the clock between Radars-A and B would not be required.
In order to achieve the purpose, a new bi-static radar system is proposed in NGSR mission. The radar system consists of Radar-A (radar transceiver) installed on Deep Space Orbital Transfer Vehicle (DS-OTV) which stays at a distance of 10 km from the comet nucleus, and Radar-B (radar transponder, which receives radar pulses from Radar-A and immediately transmits them back) installed on the Lander which can get closer to the comet nucleus in sample operation, and also kept on the orbit with a distance of about 0.5 km from the comet nucleus in radar tomography operation. NGSR Radar-A and B are chirp radar operated in a frequency range from 140 to 200 MHz. The range resolution is therefore 2.5 m in vacuum and ~2 m in the media with a permittivity of 2.
For transmission in wide frequency range, we plan to apply a bowtie antenna with a length of 0.7 m and a width of 0.2 m, which can be stored on the panel of the spacecrafts. The characteristics of the bowtie antenna were evaluated using 1/8 scale model in December 2024. The return loss was measured to be 20 dB in 150-190 MHz, and 10 dB in 140-210 MHz. The radiation pattern was almost isotropic. The performance of the antenna is generally degraded if the antenna is located too close to the ground plane. In the measurement using 1/8 scale antenna, the height of the antenna was decreased from (0.6/8) m to (0.3/8) m and found that the decrease of the receiving level was limited within 3 dB. The unexpected results need to be verified by numerical analyses using method of moments (MoM). In addition to the baseline plan using the rigid bowtie antenna, we also investigate another backup plan using aluminized Kapton etched antennas implemented on the solar cell sheet of the Lander.
Development of Radar-B bread-board model (BBM) was also performed from September 2024 to February 2025. The Radar-B BBM consists of directional coupler and power amplifier with attenuator. The signal generated by direct digital synthesizer (DDS) are fed to the Radar-B BBM through EO/OE converters and optical fiber to simulate propagation delay from DSOTV to Lander. The waveform of the signal sent back from the Radar-B BBM was sampled using digital oscilloscope. We could confirm that the total gain of power amplifier of Radar-B have to be less than the isolation of the directional coupler. In order to avoid the oscillation of the power amplifier, its gain have to be less than 34 dB, less than the isolation of the directional coupler. If we can use radar transponder as Radar-B BBM which relays radar pulse with no delay using analog circuits, the mass, and power consumption could be reduced up to 0.7 kg and 0.7 W, and synchronization of the clock between Radars-A and B would not be required.