AIRTOSS – AIRcraft TOwed Sensor Shuttle

The idea of a Tandem Platform

One of the unique features of the Learjet 35A D-CGFD is its capability to carry external loads of up to 450 kg under each wing. GFD, the operator of the aircraft, uses this feature for a variety of special missions for military training, e.g. to carry jammers for training of electronic warfare or to tow targets for ground troops.

The possibility to tow targets with up to 8 km long steel cables and the long lasting experience of GFD in the operation of towed targets (since 1966 without any problems even when flying difficult flight manoeuvres) was the trigger to think about possible utilisations for atmospheric research.

First ideas of a "tandem platform" or "triple platform" were iterated between researchers, GFD, and enviscope since the modification of the Learjet 35A D-CGFD for atmospheric research in 1997/1998.

Finally, a scientific project to study the influence of cloud particle size, -shape and -chemical composition on radiative transport was initiated. Headed by Dr. Stephan Borrmann, Professor at the Institute for Physics of the Atmosphere and one of the directors of the Max Planck-Institute for Chemistry, Mainz a group of researchers planned to instrument a towed target with a Forward Scattering Spectrometer Probe (FSSP), up- and downward looking radiation sensors, and a LAser Backscatter Sonde (LABS) in order to study processes within cirrus clouds.

Towing these instruments by a fully equipped Learjet 35A research aircraft with appropriate instrumentation on board and by changing the horizontal and vertical distance between the towed instrumented probe(s) and the research aircraft simultaneous measurements at two (three) different altitudes should be possible. This would enable e.g. the in-situ characterisation of physical and chemical properties of particles and the determination of radiative fluxes at different altitudes. Such simultaneous observations of the radiation balance and particle properties could be used to study the influence of cloud particle size, -shape, and chemical composition on the radiation balance using radiative transport models.

Due to safety regulations such measurements never could be achieved using two or more instrumented aircraft, either the horizontal and/or vertical separation would to be too big (two or more aircraft flying according to IFR rules) or to small (two or more a/c flying wing by wing).


airtoss_dosk6_2.jpgIn a first study enviscope developed a concept which uses as carrier a towed target type Do-SK6. This towed target consists of three sections with an inner diameter of approx. 18 cm, has a total length of approx. 266 cm and an empty weight of 19 kg. Manufactured in high quantities by company EADS Friedrichshafen (former Dornier GmbH) and certified to be flown with a max. weight of up to 70 kg the Do-SK6 body offers a cost-effective approach to integrate instrumentation for atmospheric research of up to 30 kg (assuming that basic infrastructure will weigh between 20 and 25 kg).
To make these technique to tow targets available for atmospheric science, the existing types of targets shall be modified and equipped with:

  • a modular mounting frame suitable for a variety of instruments,
  • a power supply for the scientific instrumentation,
  • a flexible data acquisition system open for different users,
  • a telemetric system to transfer data and commands between aircraft and AIRTOSS,
  • avionic and meteorological basic instrumentation,
  • hard points to mount inlet systems and sensors at the fuselage.

In the following picture the mechanical concept of the AIRcraft TOwed Sensor Shuttle (AIRTOSS) is given:
airtoss_dosk6_sw1.jpgAn instrument rack was fixed to the middle section of the towed target in a way that the front and aft section of the housing could be removed easily in order to have good access to the instruments. The front part was designed to accommodate a particle spectrometer, while the middle and aft section enable the mounting of the basic infrastructure like batteries, data acquisition, standard avionic and meteorological instrumentation and the adaptation of instrument modules according to the scientific goals of the missions.

airtoss_fem_1.jpg The certification of AIRTOSS was done in close co-operation with EADS Friedrichshafen. Enviscope performed the stress and load analysis and delivered all relevant information to the EADS Design Bureau.

Proof of concept

First measurements with the tandem platform to test the "proof of the concept" took place in September 2007 during a measurement campaign in northern Germany. A week-long campaign was funded by the German Science Foundation within the frame of the SFB 641 "Tropospheric Ice Phase". In total 5 measuring flights were performed in a restricted area over the North Sea.
The instrumentation was choosen according to the objectives for the AIRTOSS project during the first phase of the SFB 641:












AIRTOSS was instrumented with the

  • Cloud Imaging Probe (CIP)
  • stat. pressure and temperature sensors

and sensors to characterize the attitude of the sensor shuttle

  • iMAR Inertial Navigation System
  • GPS
  • telemetry and data acquisition unit

while the Learjet was instrumented with

  • Forward Scattering Spectrometer Probe (FSSP-300)
  • Fast In-Situ Stratospheric Hygrometer (FISH)
  • VIS and NIR spectrometers for down- and upwelling radiance and irradiance measurements
  • Standard Learjet Instrumentation
The following slides give some impressions of the AIRTOSS instrumentation and the challenging flights…

The first flights with AIRTOSS turned out to be a great success!

The team of scientists from Mainz was happy about first data sets measured by the Tandem Platform AIRTOSS-Learjet35A, and GFD and enviscope were happy that the technical concept prooved well and all flights could be performed safely.
Thanks to all groups involved, and especially to Andreas Fleck from EADS for his support.

The data evaluation of the first flights is still in process, first results have been presented at the 15th International Conference on Clouds and Precipitation (ICCP 2008), Cancun, Mexico, 7-11 July 2008.


AIRTOSS Campaign 2013

In order to enhance the obtained experience from the 2007 campaign, a second twin campaign was scheduled for spring and late summer 2013 in form of a test- and main-campaign. Both Learjet and the towed probe AIRTOSS should be equipped with atmospheric instrumentation for the investigation of multiphase inhomogeneous cirrus clouds. For this purpose, a) a new set of sensors had to be adapted to the shuttle and b) the airworthiness certification had to be expanded for flights above 25.000 feet.

AIRTOSS was instrumented with the following set of sensors provided by the involved scientific institutes, Institute for Physics of the Atmosphere of University Mainz (IPA Mainz), the Max Planck-Institute for Chemistry Mainz (MPI-C), and the Institute for Meteorology Leipzig (LIM):

3D Model of the AIRcraft TOwed Sensor Shuttle 3D Model of the AIRcraft TOwed Sensor Shuttle with Cloud Combination Probe in the leading bow and Spectral Modular Airborne Radiation measurement system (SMART-AIRTOSS) installed at the rear fuselage. A Rosemount intake is installed between the upper tail fins.

Interior components of the SMART-AIRTOSS platformInterior components of the SMART-AIRTOSS platform consisting of CCP, SMART, temperature and humidity measurement unit (ICH-TB), GPS/INS unit, power distribution and data storage system.

A real challenge of this set-up was given by the asymmetric geometry of the CCP installed in the leading bow of the towed shuttle. To ensure a safe flight stability of the towed system the momentum caused by this asymmetrical geometry had to be compensated by aerodynamical changes of the shuttle itself.

All aerodynamic investigations on this complex engineering were performed during a Master Thesis by one of our junior scientists in collaboration with the “Institut für Strömungslehre und Aerodynamik” at the “Technische Universität Darmstadt”. The results of this Master Thesis were detailed studies in aerodynamic behavior of the 2013 AIRTOSS configuration performed with numerical simulations in Computational Fluid Dynamics (CFD). The gathered information was used for constructive changes to the towed probe to ensure maximum flight stability over a wide range of possible airspeeds and flight altitudes.

Simulation results showing the airflow velocity distribution and streamlines close to the fuselage in a two-dimensional layer-cut (upper panel) and a three-dimensional velocity distribution on an iso-surface (lower panel, Q-criteria for identifying turbulent airflow). The results are based on ICAO-standard atmosphere conditions at 25.000 feet altitude and 120 m/s true airspeed.

AIRTOSS Flight Test Program

Due to the aerodynamic changes made to the sensor shuttle and in order to expand the certification of the tandem platform Learjet/AIRTOSS for high level flights, a comprehensive flight test program at a restricted areal over the North Sea was performed.

The flight tests were part of a “Major Change” to the Learjet 35A managed by our partner Gomolzig Flugzeug und Maschinenbau GmbH (GFM) and supervised by the Luftfahrt-Bundesamt (LBA). We explicitely thank both parties for their great support of this challenging project, especially because it took place within a tight timeframe. The flight testing was very successful, for the very first time a towed atmospheric probe with a payload of more than 60 kg was deployed in altitudes of up to 39.000 feet. The new AIRTOSS configuration showed ideal flight characteristics with a great flight stability even in hard aerodynamic flight maneuvers like the “Engemann corner-flight”.

SMART-AIRTOSS close before docking maneuver.

Towed SMART-AIRTOSS during flight test.

Test-campaign (May 2013)

Subsequent to the AIRTOSS flight test program a test-campaign for the new set-up of the tandem platform, fully equipped Learjet 35A D-CFGD and new sensor shuttle “SMART-AIRTOSS” was completed in the first week of May 2013, in cooperation with our partner “Gesellschaft für Flugzieldarstellung” (GFD) .

The Learjet instrumentation on this campaign was:

  • Forward Scattering Spectrometer Probe, FSSP-100 (MPI Mainz, IPA Mainz)
  • Backscatter Probe, BCP (IEK-8, Research Centre Jülich)
  • Fast In-Situ Stratospheric Hygrometer, FISH (IEK-7, Research Centre Jülich)
  • VIS and NIR spectrometers for down- and upwelling radiance and irradiance measurements (LIM) installed in a new revision of the enviscope SPARM stabilized platform.
  • DENCHAR (Development and Evaluation of Novel Compact Hygrometer for Airborne Research), (IEK-8, Research Centre Jülich)
  • FABLE, LICOR (IPA Mainz)
  • Standard Learjet instrumentation (enviscope)

Flight trajectory during test-campaign May 2013.

Within only three days five measuring flights of the tandem platform Learjet/AIRTOSS were performed over the North Sea in altitudes up to 36.000 feet. The ideal environmental conditions for cirrus clouds provided valuable measuring data for the participating scientists and the test-campaign turned out to be a quiet good main-campaign.

Main campaign (August/September 2013)

The main campaign was scheduled for beginning of September to be sure to have cirrus clouds below the maximum altitude for the Learjet 35A equipped with AIRTOSS. Because of construction works at the runway in Hohn we had to transfer the fully equipped Learjet 35A D-CGFD to the near airfield in Jagel. The main campaign started on August 26 at Jagel with wonderful sunny weather – to sunny for measurement flights, because there were no clouds in the restricted areas near Helgoland and Rügen, we had to use for AIRTOSS operation.

On the fourth day of the main campaign we finally started our first flight out of six in the Helgoland area with perfect measuring conditions and high cirrus clouds above 30.000 feet. In the next days we completed four more successful flights in both areas over the North and Baltic Sea in German airspace.

For the final flight there was a high pressure system over northern Germany, so we equipped the Learjet with two wingpods and performed a 2300 km flight to Kevlavik, Iceland and back to Jagel. On this flight the Learjet measured a wide band of high cirrus clouds as well as dry air in the stratosphere.

Overall the campaign was very successful with nearly 20 hours of measuring flights.