Space-based laser measured from Earth

Thanks to the extraordinary cooperation between the space mission Aeolus and the Pierre Auger Collaboration, it was possible, for the first time, to measure a laser beam emitted from space by an Earth-based observatory. The laser transmitter was part of the first spaceborne Doppler wind lidar ALADIN (Atmospheric LAser Doppler INstrument), which was launched in August 2018 on board the European Space Agency's (ESA) Aeolus satellite.

As the ALADIN laser emitted in the same spectral region at 355 nanometers, the fluorescence detector of the Pierre Auger Observatory was able to detect the laser beam when Aeolus flew over the measurement area (figure 1).

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Figure 1: Artist's impression of the Aeolus satellite passing over a fluorescence detector site of the Pierre Auger Observatory. The ultraviolet laser beam of the space-based lidar instrument is scattered by molecules, clouds and aerosols as it passes through the atmosphere. The scattered light is detected similar to the fluorescent light induced by secondary particles of air showers. (© S. Saffi, O. Lux, CC BY-ND-NC 3.0)

Similar to the fluorescent light emitted by air showers, the laser was detected as a linear track in the cameras of the fluorescence telescopes. This served as the basis for the geometric reconstruction of the laser beam on its path from space to the ground. The results of the first ground measurements in 2019 revealed an error in the calculation of the satellite's ground track in the Aeolus processor, which was subsequently corrected (figure 2a). In addition, precise modelling of the scattering of molecules and aerosols as well as the optical losses along the path through the atmosphere made it possible to determine the laser energy at the exit of the instrument in space (Fig. 2b). This was a spectacular way of independently quantifying the performance of the lidar instrument and narrowing down the root cause of a steady signal decrease  detected in the ALADIN receiver in the period from 2019 to 2021 to the emission path.

The combination of cutting-edge technology in space and on ground marks an important milestone both for active satellite remote sensing and for the physics of cosmic rays. The synergy between two very different research areas for space and earth observation was surprising for both communities and the method was established for the first time through the collaboration. This unprecedented "ground-truth" measurement by a ground observatory is hence also available for future space lidar missions such as EarthCARE with a planned launch in May 2024 and Aeolus-2 in the 2030s. In addition to the independent verification of space-based lasers, there is the potential for cross-calibrations between different observatories for the detection of cosmic rays.

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Figure 2: (a) Comparison of the ground track of the ALADIN laser beam that was calculated with different versions of the Aeolus processor (version 7.11.2 – orange line and version 7.12 – purple line) with the ground track measured during the transition of the beam through the Pierre Auger Observatory area (blue dots). The crosses represent the sites of the four fluorescence detectors (FD). (b) Reconstructed energies for three Aeolus overpasses in 2019, 2020, and 2021. The average energy per overpass is marked by the dashed line. The energy is given in mJ by the top axis and in PeV (1015 eV) by the bottom axis. The figures were reproduced from the publication in Optica. (CC-BY-ND-NC 3.0)

Abridged version of the article published at https://www.dlr.de/pa/en/desktopdefault.aspx/tabid-2342/6725_read-90430/

Related paper:

Ground observations of a space laser for the assessment of its in-orbit performance
The Pierre Auger Collaboration, Optica 11 (2024) 263-272

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