Atmospheric Background Radiation Measured by the ALADIN Instrument of ESA's Aeolus Mission

Kurzfassung

After each laser shot and the acquisition of the 24 atmospheric range gates, the acquisition of the atmospheric background radiation was started with a sufficiently long time delay after the ground return at zero altitude. The background radiation measurements were accumulated on-chip the ACCD with a longer duration as the backscattered laser signals due to their, in general, smaller signal strength. For instance, the longest background integration times was 3750 μs, while the shortest integration time for the atmospheric range gates was 2.1 μs. This background radiation was measured with a bandwidth of about 1 nm centred at the laser wavelength of 355 nm, and has been assumed as a constant offset over frequency for the Mie and Rayleigh spectrometer (MSP and RSP) measurements. It needed to be measured and corrected during the processing because it acted as a constant signal contribution to the acquired backscattered laser signals. Additional offsets to the signals, which needed to be corrected for, arisen within the detection chain from the electronic offset voltages, which was applied before digitization (detection chain offset (DCO)). Different approaches to determine DCO have been tested and implemented. Furthermore, the background integration time was tested and changed several times during the mission lifetime in order to find the optimal setting. A normalization with respect to the integration time can be applied to eliminate its impact on the background signals. Moreover, large spiky signals were observed in the background data on different ACCD pixels. They could be caused e.g. by cosmic rays. A spike detection and correction scheme has been developed and applied to the background signals in order to reject them. In total, a harmonised and quality controlled atmospheric background radiation data set has been obtained by Aeolus in the UV at 355 nm of nearly 5 years from September 2018 – July 2023. In our contribution, some aspects of the DCO correction and the spike detection and correction will be presented and discussed first. The main part of the presentation comprises the discussion of the daily and annual behaviour of the background radiation in terms of time and geolocation. It is demonstrated that the background signals were determined by the changing sun irradiation with respect to Aeolus along the track, by the changing Earth albedo with respect to location and time, and by the Earth orbit around the sun and the inclination of the Earth’s rotational axis with respect to the ecliptic. It is furthermore shown that the ratio of the background radiation, measured in the Rayleigh and Mie channel, shows a similar behaviour. Though the atmospheric background radiation, measured by Aeolus over nearly 5 years, was primary aimed to correct the backscattered laser signals and has not been a nominal data product of the mission, it is of a certain value. For instance, it can be used to compare with measurements of other EO instruments in the near UV, for further investigations, or in radiation simulations after a radiometric calibration.