Air-LUSI: Development of a pointing and tracking control system for lunar spectral measurements

Abstract

The airborne lunar spectral irradiance mission is an inter-agency partnership between the US National Aeronautics and Space Administration and the US National Institute of Standards and Technology that aims to make SI-traceable measurements of lunar spectral irradiance at visible to near-infrared wavelengths with unprecedented accuracy. This information is vital to using the Moon as a calibration source for Earth observing satellites. To minimize uncertainty, the lunar measurements are made above 90% of the Earth's atmosphere from an Earth Resources 2 aircraft, a civilian descendant of the U-2 spy plane. Situated in a large wing pod, a custom-designed telescope automatically tracks the Moon and the measurements are fed into a spectrometer. This information is being used to develop an extremely accurate model that can be used to calibrate satellites. An Engineering Flight Campaign was completed in August 2018 and a Demonstration Flight Campaign in November 2019, which demonstrated autonomous lunar acquisition and tracking as well as measurements of the Moon's spectral irradiance from an altitude of approximately 21 km. This article presents the simplified double gimbal control system design that was used to manipulate the telescope, and was capable of targeting the Moon with a root mean squared tracking error of about 0.1∘.