On 22 August 2018 the European Space Agency (ESA) satellite Aeolus was launched into space. Named after the Greek god of the Winds, the satellite carries the instrument ALADIN (atmospheric laser Doppler lidar instrument) as its single payload. It is the first European lidar and the world's first ever Doppler wind lidar in space. Consisting of a pulsed ultraviolet laser, a telescope (1.50 m diameter) and a highly sensitive optical spectrometer, ALADIN measures one component of the wind vector along the laser line-of-sight from ground up to altitudes of 30 kilometers with a resolution of 0.5 km to 2 km and an accuracy of about 2 m/s to 5 m/s. The emitted laser pulses are scattered in the atmosphere from air molecules, aerosols and cloud particles that move with the wind, so that a small portion returns to the satellite. The wind speed can be derived from the frequency difference between emitted and backscattered pulses which is caused by the Doppler effect the, while the duration of the pulses contains the altitude information. The wind data supplied by the satellite are directly incorporated into weather forecasting models, where they close large data gaps in the global observation system, especially over the oceans and in the tropics.
Schematic of the ALADIN airborne demonstrator (A2D). (Graphics: ©DLR)
Already several years before the satellite launch, an airborne prototype of the direct-detection Doppler wind lidar, the ALADIN airborne demonstrator (A2D), was developed at DLR in collaboration with the space industry EADS-Astrium (now: Airbus Defence and Space). The A2D provides high data coverage under various atmospheric conditions with regard to cloudiness and aerosol content thanks to the special design of the receiver, which - like the satellite instrument - has two complementary channels. While the Doppler frequency shift from the molecular backscatter signals is determined in the Rayleigh channel consisting of two Fabry-Pérot interferometers, the analysis of the particulate backscatter from clouds and aerosols is performed with the Mie channel based on a Fizeau interferometer. Due to its representative design, the A2D served to demonstrate the technical and scientific functionality of the satellite instrument prior to the mission. For this purpose, the A2D was deployed in several field campaigns aboard the DLR research aircraft Falcon to accurately measure, for example, the North Atlantic jet stream. This allowed to validate the measurement principle, to optimize the operation procedures and to improve the wind retrieval algorithms already before the start of the satellite.
Since the launch of Aeolus in August 2018, already three flight campaigns have been conducted to validate the satellite instrument in different phases of the mission. Besides the A2D, also the 2-µm Doppler wind lidar was used aboard the DLR Falcon. While the first two campaigns were conducted from Oberpfaffenhofen in November 2018 and May 2019, the base for the third validation campaign was located in Keflavík, Iceland. A total of 20 satellite underflights were carried out during the three campaigns, with wind measurements being taken over a distance of more than 15,000 kilometres along the satellite measurement track. Using this extensive data set under various conditions with respect to season, geographical region as well as ground albedo, cloud types and atmospheric dynamics, the quality of the Aeolus data products was assessed in detail and processor updates were proposed. The validation campaigns thus make a significant contribution to the refinement of the data processors and to improving the quality of the satellite data. Another airborne campaign is planned for July 2020 on the Cape Verdean Islands in which not only wind data but also aerosol parameters, which Aeolus provides as a by-product, will be validated.
ALADIN airborne demonstrator (A2D)
60 mJ (Feb. 2020)
30 MHz (FWHM)
50 MHz (FWHM)
Direct-detection with double-edge and fringe imaging technique
Sequential Fabry-Pérot interferometers for molecular backscatter (Rayleigh channel) and Fizeau interferometer for particulate backscatter (Mie channel)
250 m to 2000 m
depending on range gate settings
300 m to 1200 m
depending on range gate settings
Key instrument parameters of ALADIN on Aeolus and the ALADIN airborne demonstrator