SPARM Stabilized Platforms
for Airborne Solar Radiation Measurements
The horizontal stabilization platform SPARM (IfT/enviscope Stabilized Platform for Airborne Solar Radiation Measurements) has already been successfully installed by enviscope GmbH in co-operation with Institute for Tropospheric Research, Leipzig (IfT) on unpressurized, slow-flying research aircraft (type Partenavia P68B, the POLAR2 aircraft (Dornier DO228), the POLAR 5 (Basler BT-67), as well as on high flying jets, i.e. the Learjet 35A and the NCAR-NSF Gulfstream G-V HIAPER. In general the system involves an accurate roll and pitch angle measurement unit and the active horizontal adjustment system.
The horizontal stabilization unit keeps two radiation sensors (up- and downward looking) in a horizontal position during the flight with an accuracy of better than 0.2° over a range of pitch and roll angles of +-6°. The system works properly for angular velocities up to 3 degrees per second with a response time of the angle adjustment of 43 ms. Thus it can be applied even under turbulent atmospheric conditions. The limitations of the stabilization have been determinedby laboratory and in-flight performance tests (Wendisch, M., D. Müller, D. Schell, and J. Heintzenberg, 2001: An airborne spectral albedometer with active horizontal stabilization. J. Atmos. Oceanic Technol., 18, 1856-1866). As a result we find that the new horizontal stabilization system assures that misalignment-related uncertainties of the measured irradiances are less than 1% for solar zenith angles up to 70°.
To measure roll and pitch of the a/c an Inertial Measurement System (IMS; iMAR GmbH, St.-Ingbert, Germany) is used which is supported by GPS. The combined IMS and GPS data are processed and the resulting accurate position and attitude data are stored on the integrated data acquisition system. The processor unit transfers the analog attitude angle signals to a Computer, which is equipped with a Real-Time Motion Controller Card. This card drives two separate 2-dimensional tilt stages (zenith and nadir). Each of them is equipped with two servo motors, which realize the horizontal adjustment of the optical collectors. This set-up gives a maximum tilt range of approximately 6°. The real-time controller code and the data acquisition software has been developed under the National Instruments LabView environment.
This instrument was initially configured for low altitude measurements with un-pressurized aircraft. For high-altitude measurements aboard pressurized aircaraft the system had to be modified so that it ensures proper and save performance on such aircraft. The first deployment of a "pressurized" version was aboard the Learjet 35A, D-CGFD. Major modifications were:
- Development of an instrument box to carry those components which have to be mounted outside the pressurized cabin,
- Heating of mechanical components,
- Weather protection of the mechanical and electronic components, which have to be installed in the un-pressurized area of the a/c,
- Wind protection to reduce dynamic pressure upon and turbulent flow around the sensing unit and to ensure proper moving of the stabilized optical collector.
The most recent development of the system has been deployed on the NCAR-NSF aircraft HIAPER (Gulfstream V). The stabilized platforms on HIAPER are used for the horizontal stabilized spectral irradiance measurements. The technical challenge of the sensor stabilization is to precisely measure the roll and pitch angles on accelerated platforms and to use these measurements to compensate the aircraft attitude changes in real-time with a desired final adjustment accuracy of better than 0.2°. For scientific reasons it is required to expose the Light Collector (Optical Inlet) outside the aircraft cabin. For this reason, also the stabilization system with its tilt table construction and the corresponding servo units have to be installed outside the pressurized cabin. One major restriction while installing equipment on the upper fuselage is the max. allowed protrusion into the cabin, which was 2 inches for the installation on HIAPER. Consequently the outward protrusion of the instrument is approx. 3 inches. The other restriction was the limited horizontal space below the 7 x 10 inch aperture plate.
The stabilized platforms for HIAPER have been developed as part of the HIAPER Airborne Radiation Package (HARP) on behalf of the ACD-ARIM group at NCAR. The platforms have been tested during the HEFT-08 campaign in February 2008 in Boulder, Colorado.
Currently the system is being installed on the new german research aircraft G-550 HALO on behalf of the Institute for Atmospheric Physics of the University of Mainz, Germany.