Off-Axis Response of Limb Emission Sensors Derived from Lunar Scans

Deriving atmospheric state parameters from Earth limb emission demands accurate knowledge of off-axis response. Sounding the upper atmosphere (>60km) can require knowledge of the off-axis(>1.0 degrees from on-axis) to an accuracy of 10-5 or better, relative to on-axis. This level of sensitivity is very difficult and expensive to achieve in the laboratory. We describe the use of lunar scans to determine the off-axis response of the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument on-board the NASA TIMED (Thermosphere, Ionosphere, Mesosphere Energy and Dynamics) satellite. SABER is a 10-channel emission sensor that performs high resolution scans of the Earth limb for studies of the mesosphere and thermosphere. The off-axis calibration method bootstraps from the precise on-axis FOV measured in the
laboratory, by characterizing the lunar source function, which is then used to infer the very weak distant low-resolution off-axis response. Procedures and results are presented.