Three Years Operations of the ALADIN Instrument on Aeolus

Kurzfassung

The European Space Agency’s (ESA) Aeolus satellite was launched on 22 August 2018 from Centre Spatial Guyanais in Kourou, French Guyana. The first atmospheric returns and first wind measurements were retrieved within the first two weeks of operations after launch. Aeolus has continually provided wind measurements since this time with only limited outages in availability. Aeolus successfully met it’s nominal mission lifetime at the end of November 2021, and it has been agreed to extend the mission operations until the end of 2022. It was discovered that the atmospheric return signals from the ALADIN instrument were lower than expected before launch by a factor of x2 to x3. Extensive investigations were performed which led to the conclusion that the most likely cause for the loss in signal was an over-illumination of the 88μm diameter instrument field stop. In addition to this initial loss, it was found that the output energy of the first flight laser transmitter (FM-A) was decreasing. An investigation showed that this decrease was due to a continual misalignment of the laser master oscillator. The decrease in the FM-A UV energy and the resulting degradation of the wind measurement random error led to the decision to change to the second flight laser (FM-B) in June 2019. The FM-B laser has performed well in the subsequent 2.5yrs of operations retaining UV output energies above 60mJ and has recently provided UV output energies around 90mJ, corresponding to the levels that were reached in the on-ground thermal vacuum performance tests. Despite the good performance of the FM-B transmitter, losses in the atmospheric return signal were evident. These losses were eventually traced to the emit path of the instrument (i.e. those optics after the laser transmitter up to the telescope output), largely due to independent measurements of the ALADIN emit energy by the Pierre Auger Observatory in Argentina, which indicated some 50% reduction in the emit energy between 2019 and 2022. This has led to recent interest to revert back to the FM-A transmitter (correcting for the misalignment) or to lowering the Aeolus altitude from the current 320km to 255km, in order to improve the signal returns and extend the mission duration. Several issues with systematic bias in the wind measurements were also discovered. Pixels with slightly increased dark current levels in the memory zone of the Accumulation Charge-Coupled Devices (ACCDs), so called “hot pixels”, were found, with a new hot pixel arising approximately every 2-3 weeks. A novel pseudo-dark current correction method was employed in order to correct for these hot pixels. An important sub-orbital bias caused by differing background illuminations of the ALADIN telescope due to albedo variations was also discovered. This was mitigated by employing a correction which utilised the ALADIN telescope temperatures. The Aeolus data has been extensively analysed by a number of meteorological centres and found to have a positive impact on NWP forecasts, particularly in the tropics and polar regions. These very positive results, along with the successful in-orbit demonstration of the measurement concept and associated technologies utilised on Aeolus, resulted in a statement of interest from EUMETSAT in a future, operational DWL mission in the 2030 to mid-2040’s timeframe. This paper will summarise the performance of Aeolus’ ALADIN instrument in its 3+ years of operations, detailing the issues that have been described briefly above, as well as the mitigation actions taken, and drawing lessons learned for a future operational Doppler wind lidar mission.