ADM-Aeolus

ALADIN instrument mounted on the satellite top face

The core space element of ADM-Aeolus is ALADIN (Atmospheric Laser Doppler Instrument), a direct detection lidar incorporating a fringe-imaging receiver (analysing aerosol and cloud backscatter) and a double-edge receiver (analysing molecular backscatter).

The processing of the backscatter signals will produce line-of-sight wind profiles above thick clouds or down to the surface in clear air along the satellite track, every 200 km. Wind information in thin cloud or at the tops of thick cloud is also attainable. From the data processing, information on other elements like clouds and aerosols can also be extracted. The data will be disseminated to the main weather centres in near-real-time.
 
 

The Doppler Wind Lidar principle

Doppler Wind Lidar instrument principle
 
ALADIN is an active instrument which fires laser pulses towards the atmosphere and measures the Doppler shift of the collected return signal, backscattered at different levels in the atmosphere. The frequency shift results from the relative movement of the scatter elements along the line of sight of the instrument. This movement relates to the mean wind in the observed volume. The measurement volume is determined by the maximum ground integration length of 50 km, the required height resolution and the width of the laser footprint. The measurements are continuously repeated at distances of about 200 km.

Light is scattered either by interaction with aerosol or cloud particles (Mie scattering) or by interaction with air molecules (Rayleigh scattering). The two scattering mechanisms exhibit different spectral properties and different wavelength dependencies such that instruments evaluating only one signal type or both in separate processing chains can be constructed.

For Mie backscattering, the spectrum of the received Doppler shifted light equals the transmitted spectrum slightly broadened by the wind variability within the measurement volume. In case of molecular scattering, the Brownian motion of air molecules significantly broadens the received spectrum to a width equivalent to wind speed such that the spectral width resembles Doppler shifts equivalent to several 100 m/s. The mean Doppler shift resulting from average air motion represents therefore in this case a much smaller fraction of the spectral width than in the case of aerosol scattering. Thus for molecular scattering, a much higher signal is needed for the same velocity measurement performance, but as noted above the received signal level from molecules has different dependencies compared with aerosol scattering.

The return signal strength from aerosols scattering depends on their concentration, which varies largely over different locations, altitudes, and time. Aerosols are most concentrated in the lower 4 km of the troposphere and diminish above the troposphere. A system relying only on aerosol backscattering can therefore not provide measurements at higher altitudes. On the other hand, ground return signals useful for ground-speed calibration and signals from clouds exhibit the same spectral properties as the aerosol signal and can hence be best processed by such a receiver system.

Contrary to the Mie signal, the molecular backscattering signal under clear atmospheric conditions is only weakly dependent on the aerosol content (attenuation) and exhibits only small variation with altitude. This allows more consistent measurements up to altitudes above 20 km. However, Rayleigh receivers suffer from accuracy limitations at low altitudes (< 2 km) due to the aerosol absorption.

 
 

Deployed configuration of the ALADIN instrument

It is due to this complementary behaviour between Mie and Rayleigh return signals that ALADIN has been chosen as the instrument for ADM-Aeolus. By combining two dedicated receivers within a single instrument, ALADIN will be able to make accurate measurements over the entire altitude range.

There are two measurement techniques to measure these effects, namely the coherent heterodyne systems and the direct detection, interferometric systems. There are in fact very profound differences of principle in their respective operation. Coherent heterodyne systems operate by beating the scattered and Doppler shifted radiation with an optical laser oscillator at the surface of a detector. The resultant electrical beat-frequency signal is thus analysed post-detection to produce the Doppler frequency. On the other hand, in the direct detection methods the optical signal field is analysed and dispersed in an interferometric filter prior to detection. Both systems require interferometric precision in the optical manipulation of the signal beam. However, due to the very different physics of these schemes, the performances are very different in principle. For heterodyne systems the key parameter is shown to be the photon degeneracy – that is the number of photo-detections per optical mode (i.e. in a single coherence area and coherence time). In low-backscatter conditions, this requires that the available laser power to be distributed into pulses of the largest possible energy. However, for direct-detection interferometric systems, the accuracy depends only on the total scattered signal and is not dependent on the energy of individual pulses, only on the total laser energy.