European Facility For Airborne Research

JRA3 - Airborne Laser Interferometric Drop Sizer (ALIDS)

This joint research activity tackled a challenge in cloud physics by extending to airborne operation a very innovative technique that has recently been developed in the laboratory for drop sizing. Prior to this activity, no instrument existed for the accurate measurement of the drop size distribution in the diameter range from 20 to 200 μm, a range that is essential for studies of the onset of precipitation and this is what JRA3 aimed to develop. All measurement principles already applied to airborne operation suffer from a poor sampling area, hence poor statistical significance in this range.

The new principle, referred to as Interferometric Laser Imaging Droplet Sizer (ILIDS), offers in contrast, a much larger (by a factor of 1000) sampling area. The sizing accuracy of existing airborne instruments is sensitive to contamination of the optics, which occurs frequently in flight. This is not the case for Phase Doppler Anemometer (PDA), and ILIDS which provide absolute measurements of the drop size, using interference detection. Unlike the PDA, however, the sampling area of the ILIDS is large and it is less sensitive to vibrations. So far, application to airborne operation has been limited by technological deficiencies in laser power, ultra-short pulse generation, and camera sensitivity, but recent progress in optics technology now allows this very innovative technique to be transferred to aircraft use.

The first reporting period of EUFAR FP7 (2008-2013) was dedicated to the definition of the initial specifications of the airborne instrument such as the geometry and components of the optical setup, the acquisition and processing system. Successful comparative measurements of droplets size were conducted inside a spray with a Phase Doppler Anemometer and the ILIDS technique using the global image processing algorithm. The second period was essentially devoted to the finalisation of the specifications of the airborne instrument, the final definition of the optical setup which includes the identification, the purchase and the test of all the optical components of the probe. Laboratory tests were also performed with the actual components implemented in the final optical setup. In addition, ILIDS acquisition and data processing was tested in the laboratory at high frequency - up to 100 Hz. The CAD design of the instrument has started and should be finalised shortly. Due to some important evolutions of the ALIDS concept decided at the end of the Year 2, the specifications had to be revisited regarding the acquisition and mass storage systems and also the geometry and the sizing of the probe. Thus, the final specifications proposed an instrument with fully integrated components such as acquisition and mass storage systems, laser including power supply and cooling system.

Accordingly, the sizing of the probe is slightly increased. The final optical setup has been defined taking into account the ILIDS principle, the requirements needed for the high speed data processing, the constraints linked to the probe geometry, the constraints induced by airborne measurements using aircraft. The final optical setup has been elaborated with actual components and tested during droplet size measurements. Previously, a difficult search was necessary to identify a laser with the target specifications. In RP2, the CAD of the probe was in progress. The required specifications of the BAe146 aircraft operated by FAAM (used for the certification) were obtained which allow the implementation of plans for the probe to be completed.

RP3 was essentially devoted to the finalisation of the architecture and the 3D design of the airborne instrument which is now under construction. The final design of the ALIDS probe is inspired by that of the instrument “X-probe” developed by COMAT for Météo France a few years ago. The X-probe was certified on the ATR 42 aircraft operated by SAFIRE. The objective for the ALIDS probe is then to simplify the certification phase having almost the same characteristics (drag coefficient, mass, sizing) of the X-Probe.

This last period, was devoted to the finalisation of the construction of the mechanical structure of the airborne instrument and the integration of the optical components in the probe. Laboratory tests were performed in order to check the functioning of the instrument and to qualify the measurements of size distribution of droplets. The ALIDS instrument was implemented on SAFIRE’s ATR 42 and two flight tests were realised and demonstrated the good functioning of the instrument despite that improvements were identified.