ESA collaborated with the VTT Technical Research Centre of Finland to test 77 GHz 'frequency-modulated continuous-wave' (FMCW) radar systems. These systems are typically used in vehicles and were assessed for their ability to handle entry, descent, and landing on planetary surfaces.

"These kinds of radars are commonplace in automotive vehicles today; the first one using the E-band millimetre-wave frequencies was introduced by Mercedes Benz before the turn of the century," said ESA microwave engineer Vaclav Valenta, who oversaw the project.

"Most current space-based altimeters and ranging radar systems operate in the pulsed mode - emitting a pulse and then measuring the time it takes for reflected pulse to be received. By contrast, FMCW radars emit a continuous signal that is chirped, that is, swept rapidly in frequency - so the reflected signals can be continuously compared with the transmitted one without any interruption and processed according to build up a coherent picture of multiple targets. This brings several advantages over pulsed radar systems."

The principle of FMCW radar is not new. It was used in the Apollo landing and rendezvous radar and the Huygens probe that landed on Saturn's moon Titan in 2005. However, these older systems operated at lower frequencies than the FMCW system used in this project.

Valenta added, "It's a very simple, straightforward implementation. That is why it is so interesting for us - we know it is cutting-edge technology and we can at the same time benefit from economies of scale because millions of these radar chipsets are being produced, to a high level of reliability."

The tests in Torbacka, Finland, involved a drone-mounted lander radar using automotive radar chipsets. The radar mimicked the descent of ESA's ExoMars Rosalind Franklin rover.

"We're also interested in the use of FMCW radar for orbital rendezvous, but focused on entry, descent and landing because this is especially challenging due to the relatively low output power of these chips, at the level of few milliwatts," commented Henrik Forsten of VTT.

"Therefore, if you want to have a first signal acquisition at an altitude of 6 km - which was the requirement from ExoMars - then we had to boost the signal gain, which is why we added horn antennas to the drone's radar payload. For practical reasons drone tests were carried out at up to 500 m, though the functionality was verified up to 6 km overall."

Valenta explained, "In the end we demonstrated we can achieve the necessary range, velocity, and measurement rates for a radar that is extremely cost-effective, compact and low power. We would like to perform de-risking activities, for instance to confirm the various chipsets can endure space radiation, then the next step would really be to fly a demonstrator mission in space."

The project was supported through ESA's Technology Development Element, which investigates promising new technologies for space.

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