Producing Exo atmospheric Fiduciary Reference Measurements of Lunar Spectral Irradiance from the Airborne Lunar Spectral Irradiance (air LUSI) March 2022 Flight Campaign

Abstract

In March of 2022 air-LUSI made four flights aboard a NASA ER-2 high-altitude aircraft and measured the lunar spectral irradiance from above ~95% of the Earth’s atmosphere. Measurements were made at lunar phases of -60.3°, -37.0°, -25.0° and -12.9° with a flight scheduled for -48.8° canceled due to high winds. The measurements are traceable to the SI through artifacts calibrated at NIST and used to calibrate air-LUSI while on the aircraft. An LED-based monitoring system then verifies the calibration during flight. In addition to calibration, both the transfer spectrograph and the air-LUSI instrument were characterized for their linearity and change in response with temperature. A tunable laser was used to measure their bandpass and correct for stray light. We will discuss the calibration approach and the measurement chain that establishes the SI-traceability of these measurements. A pipeline developed in Python incorporates the characterization results with measurements taken at each stage of the calibration chain to obtain a series of at-sensor lunar irradiances for each flight. To achieve top-of-the atmosphere (TOA) irradiance the flight telemetry data was used to correct for the residual atmospheric losses using MODTRAN. The spectra were normalized to a single time point using the ROLO model to correct for the relative change in lunar irradiance during forty minutes of data collection. The result is an SI-traceable TOA lunar spectrum for each flight. Our approach to developing an uncertainty budget will also be discussed.