Active-sensing instruments have their own signal source. They transmit light or other signals and analyse the reflected irradiance returning from the ground or atmosphere. Compared with passive instruments that use sources like reflected sunlight as a signal source, this has the advantages that:
For remote sensing of the atmosphere, so-called lidars (Light Detection and Ranging), which employs lasers as the light source, are primarily used. Short laser pulses are transmitted from a satellite and the backscatter of the light is analysed. While ground- and aircraft-based lidar systems have existed for quite a while, lidar measurements from satellite sensors are a relatively new development. IMF participates in two lidar missions: Aeolus to measure global wind fields and MERLIN (Methane Remote Sensing LIDAR Mission) to measure the greenhouse gas methane (CH4). In these missions IMF works closely with DLR's Institute of Atmospheric Physics (IPA), where the instrument concept and the algorithms for scientific analysis are developed.
In the Aeolus mission use is made of the fact that light backscattered from an atmosphere in motion experiences a shift in wavelength known as the Doppler effect. The velocity of a backscattered particle in the direction of the beam of radiation can be calculated from the extent and direction of the shift. Aeolous uses for the purpose a laser with a wavelength of 355 nm. The backscattered light is captured at high resolution by a reflecting telescope with a diameter of 1.5 m. Since the exact time the laser pulse was emitted is known, it is possible to calculate not only the mean wind velocity but also to generate a wind velocity profile. Good knowledge of wind profiles is essential for forecasting weather and predicting extreme weather events. So far, movement in the atmosphere can only be ascertained for a specific location using radiosonde measurements. Aeolus will for the first time provide global wind profile data. The horizontal resolution is about 50 km. Aeolus was launched on August 22, 2018 and is expected to have three years of service life. IMF is responsible for the so-called Level 1 processing, which means for calibrating the detector signals. In principle the calibration task consists of converting electronic detector signals, which are binary units, into physical quantities. This is accomplished by sequential calibration steps in accordance with a calibration equation. IMF is also assisting in the development of the operational Level 2A processor. For ESA's Aeolus project the work is carried out in a consortium comprised of CNRS/GAME, IPSL, KNMI and other partners.
The MERLIN mission is designed to measure methane with high accuracy. After carbon dioxide (CO2), methane makes the second highest contribution to climate warming caused by human activity. It primarily arises from biogenic processes like biomass decomposition, rice cultivation, and livestock farming. Knowing the sources and sinks of methane is important for reliably predicting climate change. MERLIN's capability to measure permafrost regions also during polar night is particularly crucial because with passive instruments useful data for these regions can only be obtained during local summertime. As an active instrument, MERLIN does not have this limitation and can measure day and night throughout the year. In order to determine the CH4 content of the atmosphere, MERLIN send out two laser pulses at intervals of about 250 microseconds: one pulse with a wavelength at which there is strong CH4 absorption, and another with a closely neighbouring wavelength without significant methane absorption. MERLIN measures the backscatter signal from both pulses. Since the two wavelengths are very close the difference in the acquired signals is attributed to the methane concentration along the light path. This makes it possible to precisely determine the CH4 concentration. MERLIN is a German-French project, with France responsible for the platform and Germany for the payload. It is planned for launch in the mid-2020s.
IMF is responsible for implementing the Level 1 processors also for MERLIN and, in collaboration with IPA, for developing the calibration algorithms. The team also carries out the long-term monitoring of the instrument. This involves monitoring a variety of instrument parameters in order to react in time to changes in instrument performance. Besides the DLR institutes, CNES (the French space agency), and other French institutes, the MERLIN consortium also consists of Airbus Defense and Space, responsible for instrument construction, and the SciSys company, responsible for transferring instrument data and for the data user interface.