European Facility For Airborne Research

European Facility For Airborne Research May 16, 2021, 14:08

Joint Research Activities (EUFAR 2008 - 2013)

Completed Joint Research Activities under EUFAR FP7 contract (2008 - 2013)

JRA1 - Development and evaluation of new and improved hygrometers for airborne research (DENCHAR)

Focused on a fundamental measurement – humidity – which is of central interest across a broad range of atmospheric science, the research activity DENCHAR aimed to facilitate new instrumental developments together with extensive testing, both in the laboratory and in-flight, including routine operations within EUFAR (e.g. investigations on the sampling characteristics of different gas/ice inlets and development of an improved ultra-fast thermometer for near- and in-cloud measurements).

In the first two reporting periods (Oct 2008 – Sept 2011) the four new types of hygrometer using different detection techniques as well as the novel ultra-fast thermometer were developed. Parallel laboratory tests of the individual prototype instruments were made at the humidity calibration facilities at FZJ, Jülich. At the end of RP2 the instruments were tested on their in-flight performance during a research aircraft intercomparison campaign (IFCC-2011) in May 2011. All data from the different hygrometers were compared “blind” to those of the advanced airborne hygrometer (e.g. FISH), and all data were stored “blind” before any comparison were conducted. In general, from these tests the different instruments demonstrated good performance over a wide range of humidity levels covering 3 orders of magnitude between 10-50 ppmv up to 30,000 ppmv.

In RP3 (Oct 2011 – March 2013), the work was focused on the preparatory work for the endurance testing (ET) of the instruments integrated in a small flight package during a routine flight operation of a Learjet aircraft. Therefore a small flight package (SFP) was designed and constructed containing all four hygrometers together with a data acquisition system. The SFP can be flown autonomously on any research aircraft.

For the purpose of the planned endurance testing, the SFP had to be flown unattended (no operator aboard the aircraft) during the routine flight operation of a Learjet aircraft. Although the SFP was ready for the endurance testing, its certification was still pending in RP3, such that this sub-task was delayed. It was evident that at the end of RP3, even if the required certification documents had been obtained to fly the SFP unattended on the Learjet, the JRA1 working group would have not been able to accomplish the full nine months of endurance testing within the timeframe of the EUFAR project. Therefore on this deliverable corrective steps had to be taken for the task “Endurance in-flight testing of compact hygrometer package” which was re-defined to “Calibration and In-Flight Validation”. Therefore in the last reporting period, the SFP was flown during two dedicated research flight campaigns (May and September 2013) as part of the AIRTOSS (AIRcraft Towed Sensor Shuttle) experiments. Aboard the Learjet 35A D-CGFD the SFP was thereby operated side by side with the Lyman FISH Hygrometer together with cloud detecting instruments. All data of the hygrometers (incl. FISH) were stored “blind” before any comparison was made. From the intercomparisons the instruments showed very good and consistent performance over a wide range of humidity levels covering almost three orders of magnitudes between 10-50 ppmv up to 20,000 ppmv water vapour mixing ratios. Flight by flight the DENCHAR-instruments demonstrated significant consistent behaviour in comparison with each other and as well as compared to the FISH-reference instrument. Within their uncertainty range (5-10%), all instruments agreed very well and were traceable within about 10% uncertainty to the DP30 (MBW) frost point hygrometer of the ground-based FISH-calibration bench. The results have been evaluated and compiled in an assessment report.

JRA2 - Quality layers for airborne hyperspectral imagery and data products (HYQUAPRO)

HYQUAPRO sought to develop, implement and test quality indicators and quality layers for airborne hyperspectral imagery, and to develop higher performing water and soil algorithms as demonstrators for end-to-end processing chains with harmonised quality measures.

The concept of Uncertainty Propagation Analysis (UPA) combined with Monte Carlo stochastic simulation has been applied to airborne hyperspectral imagery to explore how uncertainty of input parameters propagates through the processing chain.

Within the consortium of 9 European data providers for airborne hyperspectral imagery (VITO/UZH, DLR, INTA, PML, ISBE, TAU and FUB, with ONERA associated), quality indicators were identified and selected for implementation in the processing chains. Also the data description that accompanies the hyperspectral data (metadata) has been harmonised across all EUFAR providers. The main step was the development of data quality indicators. Therefore generic quality indicators and quality layers for airborne hyperspectral images (based on sensor and scene characteristics) were developed. Sensor characteristics, sensor calibration, data characterisation, sensor performance during acquisition, external conditions during acquisition, quality of auxiliary data used for the processing were translated into generic quality indicators and quality layers. Starting from the generic quality layers, these were adjusted (“personalised”) for the different processing facilities involved since different sensors, software, auxiliary data and processing methods are used at the different processing facilities. After adjustment of the quality layers, the layers were integrated in the respective processing facilities.

A literature review of water quality algorithms, from simple to complex approaches was undertaken and preliminary results were presented at the EWG meeting on “Water Applications” during RP1. PML continued with the algorithm review following advice on variables to consider from the EWG. PML investigated the development of an improved version of an Inherent Optical Properties model for use in inland and coastal waters.

For the integration of the PML higher performing water quality algorithms into an existing PAF, three algorithms were provided by PML to VITO:

      • Algorithm of Gitelson et al. (2007) for CHL-a retrieval
      • SeaWiFS OC4v6 algorithm of O’Reilly et al., 2000 for CHL-a retrieval
      • IOP model of Smyth et al. (2006) for IOP retrieval.

The integration of these algorithms in the PAF was tested with the Lake Balaton 2010 data set (NERC-ARSF AISA Eagle, in-situ and sun photometer data) provided by PML and the University of Stirling.

The validation of water quality retrieval algorithms implemented by the VITO PAF processing chain was implemented at the stages of atmospheric correction and retrieval of water quality indicators, such as chlorophyll-a concentration, water IOPs and TSM concentration.

During RP4 (April 2013 – Sept 2013), the PML inherent optical property model was updated to include optical water types with considerable improvements in retrieval of the absorption (in terms of RMSE). The backscatter was also improved but to a lesser extent in RMSE. The algorithm has since been optimised resulting in a significant improvement in processing time (a factor of 23 faster).

HYQUAPRO developments allow provision of quality indicators/quality layers (and data descriptors/metadata) with the hyperspectral imagery to their users. In addition, validated water quality products (Chl-a, IOP) are now available to the users through the VITO processing facility. Partner GFZ undertook the development of higher performing soil algorithms under the double commitment of using methodologies where automation is possible, and offering multiple algorithms to the users. The focus was on offering both analytical and empirical algorithms for the determination of the following key soil products: clay, iron, carbonate, soil organic carbon, and soil moisture maps.

The HYperspectral SOil MApper (HYSOMA) toolbox was developed and validated with 18 image datasets and allows to produce 11 soil products associated with different methods for soil moisture content, soil organic carbon content, and soil minerals content (iron oxides, clay, carbonates) for every input image file, plus 1 soil quality layer file, and 4 mask files. For the integration of the GFZ higher performing soil algorithms into an existing PAF an automatic version of HYSOMA was developed under the name HYSOMA_AUTO. HYSOMA_AUTO runs without interface under the IDL command prompt and was integrated in the automated DLR processing chain. The validation of the HYSOMA products based on various in-situ validation data sets showed correlations from R2 of 0.52 (clay) up to >0.9 (soil moisture) which both validate the HYSOMA software and provide science validation for the soil algorithms.

The HYSOMA software was adapted to be included in the EUFAR Toolbox. For this, a public release version was developed and the HYSOMA website ( was released on 29 June 2012, to which the EUFAR Toolbox is directly linked. After registration and accepting the license, HYSOMA is freely made available for download for non-commercial purposes through both the N6SP EUFAR Toolbox and the HYSOMA website. Plug-ins for linux/mac/windows can be found on this website. The software is IDL based and distributed for free under the IDL-virtual machine, so that it is easy to use for non-expert users. At the end of the project in 2013, more than 60 users from all over the world had downloaded a plug-in of the HYSOMA software interface, demonstrating the interest of the airborne community for this software.

The PML IOP algorithm and the other water quality algorithms are freely available through the EUFAR toolbox.

For more information, contact the activity leader - Ils Reusen (

Related publications:

Bachmann, M., Makarau, A., Segl, K., Richter, R. (2015): Estimating the influence of spectral and radiometric calibration uncertainties on EnMAP data products - examples for ground reflectance retrieval and vegetation indices. Remote Sensing, 7.

Bachmann, M., Rogge, D., Malec, S., Holzwarth, S., Makarau, A., Richter, R. (2015) Estimating the uncertainty in ground reflectances resulting from radiometric and spectral calibration. EARSeL SIG-IS, 14.-16.04.2015, Luxembourg. 

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.

For more information contact, ALIDS activity leader - Emmanuel Porcheron.

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