A Generalized Combinatorial Technique for Linearity Calibrations Applied to Optical Detectors and Spectrographs

For many quantities, indicating instruments are calibrated only at a limited number of values, and the extension of the calibrations to higher or lower values must rely upon the linearity of the instruments. A method for calibrating or determining the linearity of instruments that exploits the combinatorial properties of a set of different-valued, and mostly uncalibrated, artefacts is described. The presentation describes the underlying principles of the method, its limitations, and examples of the application of the method to very different quantities: mass balances, resistance bridges, optical detectors and spectrographs. The resulting uncertainty due to linearity can be assigned from the residuals of the fitted functional form of the linearity function to the measured signals. The implementation of this combinatorial method with the NIST Beamconjoiner apparatus is described, and calibrations of visible and infrared photodiodes, and spectrographs for internal and external customers are shown. This method is shown to be capable of determining linearities in the visible and infrared wavelength region to uncertainties of 200 ppm or 0.02 % (k=2). Linearities of spectrographs at a set integration time and as a function of integration times can be measured using this approach. Experimental setups to characterize focal plane arrays placed in cryo-vac chambers will be discussed

Airborne Lunar Spectral Irradiance (air-LUSI) Missioni Capability Demonstration

The Moon is a very useful calibration target for Earth-observing sensors in orbit because its surface is radiometrically stable and it has a radiant flux comparable to Earth scenes. To predict the lunar irradiance given an illumination and viewing geometry, the United States Geological Survey (USGS) has developed the Robotic Lunar Observatory (ROLO) Model of exo-atmospheric lunar spectral irradiance. The USGS ROLO model represents the current most precise knowledge of lunar spectral irradiance and is used frequently as a relative calibration standard by space-borne Earth-observing sensors. However, instrument calibration teams have expressed the need for an absolute lunar reference with higher accuracy. The objective of the airborne LUnar Spectral Irradiance (air-LUSI) mission is to make highly accurate, SI-traceable measurements of lunar spectral irradiance in the VNIR spectral region from NASA’s high-altitude ER-2 aircraft. To that end, the air-LUSI system employs an autonomous, robotic telescope system that tracks the Moon in flight, and a stable spectrometer housed in an enclosure providing a robustly controlled environment. These instrument subsystems are situated in a wing pod of the ER-2 aircraft with a small dorsal view port. Through this port, the telescope can observe the Moon from above 95% of the Earth’s atmosphere. air-LUSI successfully conducted a Demonstration Flight Campaign on five consecutive nights from 12 to 17 November 2019. Each night, the air-LUSI system observed the Moon at about 68,000 feet altitude. Each observation period lasted 30 to 40 minutes and measured the lunar spectral irradiance at wavelengths from about 380 to 1000 nm. The five flights corresponded to lunar phase angles of 10°, 21°, 34°, 46° and 59°. The measurement uncertainty is currently estimated to be about 0.8% or less through the mid-visible range. With this new capability, the air-LUSI team plans to acquire additional lunar spectral irradiance measurements and apply this state-of-the-art data set to improve the accuracy of ROLO predictions. This paper will summarize the air-LUSI objectives and provide an overview of the Demonstration Flight Campaign and lessons learned that could further improve air-LUSI accuracy in future flights

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

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