A compact FMCW Radar as a Proximity Sensor and Subsurface Analyzer for Landers or CubeSats in Planetary or Small Body Missions

Planetary and small-body lander missions, as well as CubeSat-based exploration platforms, require robust proximity sensing capabilities to support descent, landing, and surface operations. This contribution presents a compact, coherent dual-channel FMCW radar designed as a proximity sensor for planetary and small-body missions. The radar features a volume of less than half a CubeSat unit and operates over a wide, mission-configurable frequency range from 10 MHz to 6 GHz, allowing adaptation to antenna accommodation, platform constraints, and planetary protection or regulatory requirements. The integrated power amplifier provides a transmit power of up to 2 W, while the minimum detectable signal reaches −125 dBm. Output power and sensitivity can be further extended using external amplification stages if required.

The radar is fully software-configurable, offering flexible control over RF bandwidth, sweep duration, intermediate-frequency sampling rate, and output power. It supports up to two transmit and two fully independent, phase-coherent receive channels. Depending on the operational duty cycle, average power consumption can be as low as 2.5 W, making the system suitable for resource-constrained CubeSat and lander platforms.

Designed for autonomous operation, the system performs real-time, on-board signal processing to provide deterministic, terrain-relative proximity measurements independent of external navigation or communication infrastructure. In its primary mode, the radar functions as a radar altimeter and descent monitor, delivering continuous estimates of range to the surface and relative vertical velocity. These measurements are well suited for guidance, navigation, and control during terminal descent, landing detection, and post-landing assessment.

In secondary mode, the radar can be used as a surface analyzer and subsurface sounder. Due to its enormous bandwidth and high dynamic range, the radar can be operated as a surface analyzer to map surface permittivity and roughness and, in GPR mode, to characterize the shallow subsurface with high spatial resolution. In the low-frequency range, the instrument is capable of performing deep sounding measurements with high penetration depth to analyze the deep interior of small bodies or planetary subsurface structures.

In addition, a cooperative transponder mode enables two-way FMCW ranging between multiple mission elements, such as a lander and an accompanying CubeSat or orbiter, supporting relative navigation and formation tracking. Operating at low frequencies with link budgets of up to approximately 155 dB, this mode allows the use of simple, non-directional antennas. A low-data-rate communication mode can also be implemented on the same hardware to support beaconing and basic command and housekeeping functions during descent and surface operations.

The presented radar system is intended as mission-agnostic proximity-sensing infrastructure for planetary exploration. Owing to its coherent architecture, it is inherently compatible with advanced processing techniques, including synthetic aperture processing for surface characterization and subsurface sounding, which are identified as promising directions for future work.