The ALADIN instrument (Atmospheric LAser Doppler INstrument) is a direct detection Doppler Wind Lidar operating in the ultraviolet and consists of three major elements:
- a laser transmitter
- a combined Mie and Rayleigh backscattering receiver assembly
- a Cassegrain telescope with a 1.5 metres diameter
The laser transmitter architecture is based on a pulsed diode-pumped Nd:YAG laser, frequency-tripled to provide 80 mJ pulses of ultraviolet light at 355 nm. This frequency was chosen because of the increased Rayleigh scattering in the ultraviolet region of the spectrum, and because it is eye-safe at distances greater than several hundred metres.
The laser is a complex system of laser sources and amplifiers, which are all packaged closely together. There are two small lasers to fix the frequency of the emitted pulses, a laser oscillator to generate pulses, two amplifier stages that boost the energy of the light pulses to the required value and a frequency conversion crystals stage to produce the correct wavelength.
ALADIN has a very large telescope, which is used to collect the backscattered light from the atmosphere and then directs it to the receiver. Although large, it is made of lightweight ceramic material so it weighs only 55 kg.
The processing of the backscatter signals produce line-of-sight wind-component profiles above thick clouds or down to the surface in clear air along the satellite track, every 10 (particle and cloud backscatter winds) to 87 (clear air backscatter winds) kilometres. Aeolus is also providing information about the vertical distribution of atmospheric cloud and particle (aerosol) layers. The data are provided to users in near-real-time (within three hours of sensing), and the wind product will be used in operational weather forecasts by centres across the world.
Highly sensitive photo-detectors then transform the light signals into electronic signals. The wind profiles are accumulated over at least 20 individual measurements before being downlinked to Earth for further averaging.
The Mie receiver consists of a Fizeau interferometer with a resolution of 100 MHz (equivalent to 18 m/s). The received backscatter signal produces a linear fringe whose position is directly linked to the wind velocity; the wind speed is determined by the fringe centroid position to better than a tenth of the resolution (1.8 m/s).
The Rayleigh receiver employs a dual-filter Fabry–Pérot interferometer with a 2 GHz resolution and 5 GHz spacing. It analyses the wings of the Rayleigh spectrum with a CCD; the etalon is split into two zones, which are imaged separately on the detector.
How Aeolus measures
ALADIN measures the atmospheric wind by emitting short powerful pulses of ultraviolet light at a frequency of 50 Hz though its telescope down into the Earth's atmosphere. Between each pulse, the telescope collects the light that is backscattered from air molecules, aerosols and hydrometeors within the instrument field of view along its line-of-sight. The light is then directed to two receivers measuring the frequency of the backscattered light as a function of time. These recordings are then compared with the frequency of the emitted light pulses during the on-ground data processing. The movement of the air molecules and particles along the satellite view causes the frequency of the backscattered light to change with a tiny fraction due to the so-called Doppler effect. The Doppler effect occurs when an electromagnetic wave is emitted or reflected by a moving object. When the object is moving towards a target, the waves get compressed and hence result in a higher wave frequency. When it moves away, the waves get elongated and hence result in a lower wave frequency. This can for example be observed when an ambulance, emitting sound waves, is passing by.
Because ALADIN is performing time-gated differential frequency measurements, it is hence possible to determine the atmospheric wind at various altitudes above the surface from the its measurements. Aeolus measures the average (mostly east-west) wind component in 24 vertical layers for each channel, resulting in wind profiles every 250 m to 2 km from the surface up to about 30 km (lower stratosphere).
Aeolus Atmospheric Sampling – Range Bin Settings
The current operational WIND Range Bin Settings (RBS) are defined based on latitude bands and small boxes (plot on the left); in each zone the atmosphere is sampled at different altitudes, as shown in the right plot.