An Airborne Spectral Albedometer with Active Horizontal Stabilization

TLDR

The authors developed an airborne albedometer equipped with a low‑cost, precise, fast horizontal stabilization system to measure spectral down‑ and up‑welling irradiances from 400 to 1000 nm. The stabilization unit, mounted on a Partenavia P68‑B (and adaptable to other aircraft), keeps the up‑ and down‑looking sensor heads horizontal within ±0.2° over ±6° pitch/roll, as verified by laboratory and in‑flight tests. The system responds within 43 ms, tolerates angular velocities up to 3° s⁻¹, keeps misalignment‑related uncertainties below 1 % for solar zenith angles up to 70°, and demonstrates improved radiative‑budget measurements even in turbulent, cloud‑free conditions.

Abstract

An airborne albedometer including a low-cost, precise, and fast sensor head horizontal stabilization system was developed to measure spectral down- and upward irradiances between 400- and 1000-nm wavelength. It is installed on a small research aircraft (type Partenavia P68-B), but it can easily be mounted on other aircraft as well. The stabilization unit keeps the two radiation sensor heads (up- and downward looking) of the albedometer 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° s-1 with a response time 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 misalignment-related uncertainties of the measured irradiances are less than 1% for solar zenith angles up to 70°. This does not include uncertainties due to deviations from the ideal cosine response of the optical inlet system and measurement errors resulting from absolute calibration problems. An example of downward spectral irradiances measured under cloudless conditions above and within a distinct boundary layer with enhanced aerosol particle concentrations shows the potential of the new instrument for improved radiative budget measurements in the atmosphere.