Fraunhofer Institute for High Frequency Physics and Radar Techniques FHRThe Space Observation Tracking and Imaging Radar TIRA, short for Tracking and Imaging Radar, has obtained surprising data on the rotational speed of the malfunctioned environmental satellite ENVISAT. It had been spinning, unable to maneuver on a populated orbit – with decreasing rotational speed, as the scientific community had assumed. However, measurement campaigns with TIRA show that its rotation is accelerating again. The results are particularly relevant for a potential deorbiting mission.
As space debris weighing about eight tons on a heavily trafficked orbit, ENVISAT not only poses a danger to active satellites, but can also contribute significantly to the creation of new debris fragments through collisions. ESA is currently exploring solutions to move decommissioned satellites to a lower orbit, so that they subsequently re-enter the Earth’s atmosphere more quickly and largely burn up. For such a deorbiting mission, the rotational rate must, however, be precisely known.
Contact with ENVISAT broke off in 2012, after its mission duration lasted twice as long as originally planned. Soon after, ESA announced the end of the mission. At the same time, Fraunhofer FHR began analyzing and imaging the inactive satellite with TIRA. The result: Neither the radar data nor optical images of the French Pléiades Earth observation satellite showed external damage, but ENVISAT began to rotate. Further measurements showed that the rotational rate reached its maximum in 2013 and has since declined continuously – presumably due to inductive effects triggered by the Earth’s magnetic field. The scientific community agreed that this trend would continue in the future.
TIRA disproves predictions of rotational speed
Sporadic measurements with TIRA in 2018 showed that the forecast of a steadily decreasing rotational speed did not hold. Measurement campaigns in the following years up to 2021 confirmed this surprising result: the rotation is actually speeding up again. Compared to in 2013, in 2018 the satellite rotated at only half the speed, with a full rotation taking 240 seconds. In the first half of 2021, it took only 210 seconds – a clear difference. The cause of this acceleration is the subject of further investigations.
Target-tracking radar and precise imaging for effective mission support
For ENVISAT measurements, Europe’s unique capabilities of TIRA come into play: When the satellite appears on the horizon and crosses the sky above the Earth, the L-band target-tracking radar tracks it until it disappears from the field of view ten to fifteen minutes later. During the tracking, Ku-band radar data are acquired for imaging. With the collected data, researchers produce high-resolution ISAR images, short for Inverse Synthetic Aperture Radar. This provides mission operators with decision-relevant information that is crucial not only for deorbiting missions but also for overall safety of space infrastructure.