An airborne spectral albedometer with active horizontal stabilization

An airborne albedometer including a low-cost, precise, and fast sensor head horizontal stabilization system was developed to measure spectral down- an d upward irradiances between 400- and 1000-nm wavelength. It is installed o n a small research aircraft (type Partenavia P68-B), but it can easily be m ounted on other aircraft as well. The stabilization unit keeps the two radi ation sensor heads (up- and downward looking) of the albedometer in a horiz ontal position during the flight with an accuracy of better than +/-0.2 deg rees over a range of pitch and roll angles of +/-6 degrees. The system work s properly for angular velocities up to 3 degrees s(-1) with a response tim e of the horizontal adjustment of 43 ms. Thus it can be applied even under turbulent atmospheric conditions. The limitations of the stabilization have been determined by laboratory and in-flight performance tests. As a result it is found that the new horizontal stabilization system ensures that misa lignment-related uncertainties of the measured irradiances are less than 1% for solar zenith angles up to 70 degrees. This does not include uncertaint ies due to deviations from the ideal cosine response of the optical inlet s ystem and measurement errors resulting from absolute calibration problems. An example of downward spectral irradiances measured under cloudless condit ions above and within a distinct boundary layer with enhanced aerosol parti cle concentrations shows the potential of the new instrument for improved r adiative budget measurements in the atmosphere.