The Stabilized Radiometer Platform ( STRAP)-An Actively Stabilized Horizontally Level Platform for Improved Aircraft Irradiance Measurements

Measurements of solar and infrared irradiance by instruments rigidly mounted to an aircraft have historically been plagued by the introduction of offsets and fluctuations into the data that are solely due to the pitch and roll movements of the aircraft. The Stabilized Radiometer Platform ( STRAP) was developed to address this problem. Mounted on top of an aircraft and utilizing a self-contained, coupled Inertial Navigation System-GPS, STRAP actively keeps a set of uplooking radiometers horizontally level to within +/- 0.02 degrees for aircraft pitch and roll angles of up to approximately +/- 10 degrees. The system update rate of 100 Hz compensates for most pitch and roll changes experienced in normal flight and in turbulence. STRAP was mounted on a Twin Otter aircraft and its performance evaluated during normal flight and during a series of flight maneuvers designed to test the accuracy, range, and robustness of the platform. The measurements from an identical pair of solar pyranometers-one mounted on STRAP and the other rigidly mounted nearby directly to the aircraft-are compared to illustrate the accuracy and capability of the new platform. Results show that STRAP can keep radiometers level within the specified pitch and roll range, that it is able to recover from flight maneuvers outside of this range, and that it greatly increases the quantity of useful radiometer data from any given flight. Of particular note, STRAP now allows accurate measurements of the downwelling solar irradiance during spiral ascents or descents of the aircraft, greatly expanding the utility of aircraft radiometer measurements.