Drone test of planetary landing radar

This drone took to the sky over Finland to assess the suitability of one particular terrestrial technology for space: the radar systems found in many of today’s cars, responsible for automated cruise control and other safe driving functions.

ESA worked with the VTT Technical Research Centre of Finland to test the suitability of automotive 77 GHz ‘frequency-modulated continuous-wave’ (FMCW) radar for entry, descent and landing on a planetary surface and for in-orbit rendezvous scenarios.

“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,” explains ESA microwave engineer Vaclav Valenta, overseeing 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 is not new, FMCW radar at lower frequencies acquired space heritage long time ago – already the Apollo landing and rendezvous radar relied on the FMCW principle, likewise, the Huygens probe that landed on the surface of the Saturn’s moon Titan back in 2005 employed FMCW radar. However, those radars operated at much lower frequencies than the FMCW system deployed in this project.

Vaclav adds: “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 test campaign in Torbacka, Finland, assessed the performance of a drone-mounted lander radar using automotive radar chipsets. They were tasked with mimicking the planned 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,” comments Henrik Forstén 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.”

Vaclav explains: “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, investigating promising new technologies for space.