Browsing by Author "Stone, Thomas C."
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Item Evaluation of air-LUSI Measurements to Advance Lunar Modeling and the ROLO Lunar Calibration Reference(2021-10-20) Stone, Thomas C.; Turpie, Kevin; Brown, Steven; Maxwell, Stephen; Woodward, JohnThe airborne Lunar Spectral Irradiance (air-LUSI) project is dedicated to acquiring high-accuracy, spectrally resolved measurements of the Moon from the NASA ER-2 high-altitude aircraft, flying above more than 90% of Earth's atmosphere. The air-LUSI instrument is a non-imaging system designed specifically for measuring spectral irradiance of the Moon at wavelengths from ~350 nm to 1100 nm. The project aims to achieve absolute measurement uncertainty approaching 0.5% (k=1) with traceability to NIST radiometric standards and SI. These measurements can advance lunar calibration by constraining absolute scale of models that constitute the lunar radiometric reference, such as the USGS ROLO model. A 5-night flight campaign in November 2019 collected lunar measurements at phase angles ranging from 9.4 to 58.5 degrees after Full Moon. ROLO model outputs have been generated for the times and aircraft locations of each night's observations. Inter-night comparisons after normalizing by ROLO show inter-consistency of the measurements within 1.5%, despite a factor of 3.34 difference in lunar irradiance at 500 nm over the 5-night span. These results give no indication of an appreciable phase angle dependence in the ROLO model within the observed range. This talk will highlight implications of the high-accuracy air-LUSI measurements with regard to lunar calibration using irradiance measurements derived from lunar images acquired by space-based sensors.Item Improving the ROLO Lunar Calibration Reference with New Measurements of the Moon(2023-06-12) Stone, Thomas C.; Turpie, Kevin; Woodward, John; Maxwell, StephenThe Moon provides a calibration target that can serve as a common reference for all space-based Earth observing sensors. Lunar calibration has the potential to achieve very high accuracy, leading to important capabilities for Earth remote sensing such as inter-calibration of instruments in satellite constellations and building climate data records. Practical use of the Moon for radiometric calibration requires a model to account for the continuously changing lunar brightness and that can accommodate the various viewing geometries of sensors’ Moon observations. The USGS ROLO lunar calibration system operates with a model for the disk-integrated irradiance at reflected solar wavelengths, developed from an extensive set of ground-based observations. Although the ROLO model is widely used, to reach the full accuracy potential of lunar calibration requires collecting new characterization measurements of the Moon to apply toward refining and ultimately reformulating the analytic model that provides the lunar reference. The airborne Lunar Spectral Irradiance (air-LUSI) project is a currently operational system to acquire spectrally resolved measurements of the Moon from the NASA ER-2 high-altitude aircraft, flying above ~95% of the atmosphere.Item The irradiance instrument subsystem (IRIS) on the airborne-lunar spectral irradiance (Air-LUSI) instrument(IOP Science, 2022-03-23) Grantham, Steven E.; Turpie, Kevin R.; Stone, Thomas C.; Gadsden, S. Andrew; Larason, Thomas C.; Zarobila, Clarence J.; Maxwell, Stephen E.; Woodward, John T.; Brown, Steven W.The objective of the airborne lunar spectral irradiance (air-LUSI) project is to make low uncertainty, SI-traceable measurements of the LUSI in the visible to near-infrared region from an aircraft above most of the optically absorbing components of the atmosphere. The measurements are made from a NASA ER-2 aircraft, which can fly at altitudes of approximately 20 km above sea level. Air-LUSI measurements, corrected for residual atmospheric attenuation, are designed to provide a matrix of low uncertainty top-of-the-atmosphere lunar irradiances at known lunar phase and libration angles to be compared and combined with other lunar irradiance data sets to constrain the uncertainties in models of lunar irradiance and reflectance. The measurements are also expected to provide insight into the differences between models and satellite sensor measurements of lunar irradiance. This paper describes the development and characterization of the air-LUSI subsystem for acquiring lunar measurements, called the irradiance instrument subsystem, prior to flight.Item Measurements of Absolute, SI Traceable Lunar Irradiance with the Airborne LUnar Spectral Irradiance (air LUSI) Instrument(2021-10-20) Woodward, John T.; Brown, Steven W.; Grantham, Steve; Larason, Thomas C.; Maxwell, Stephen E.; Turpie, Kevin; Gadsden, S. Andrew; Newton, Andrew; Stone, Thomas C.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, its accuracy as an absolute reference may be limited to several percent and it is not SI-traceable. Advancing the model to be a more accurate absolute lunar reference requires new measurements. 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, above 95% of the atmosphere. To that end, the Air-LUSI system uses a non-imaging telescope system that robotically tracks the Moon in flight, fiber-optic coupled to a stable spectrometer housed in an enclosure providing a robustly controlled environment. The spectrometer measures about 350 to 1050 nm at 3.8 nm resolution, with 0.8 nm sampling. The instrument is reproducibly stable to 0.3% and rigorously calibrated before and after campaigns and flights using a similar transfer standard spectrograph. An on-board LED source is used to monitor the instrument response during flight ascent and descent. Air-LUSI successfully conducted a Demonstration Flight Campaign on five consecutive nights from 12 to 17 November 2019, corresponding to lunar phase angles of about 10°, 21°, 34°, 46° and 59°. Each night, the Air-LUSI system observed the Moon from above 68,000 feet altitude for 30 to 40 minutes. To reach a target uncertainty for lunar irradiance of 0.5% (k=1), processing the raw data to exo-atmospheric lunar spectral irradiance required accounting for various known behaviors of the instrument, such as thermal and stray light corrections. Additional measures were taken to address variances idiosyncratic to the campaign and were factored into the measurement error budget. The resulting error budget currently stands at less than 1% over most of the VNIR range. This paper reviews the steps taken towards high accuracy results for Demonstration Flight Campaign, how they factored in the error budget, and how our uncertainty target can be met in future campaigns.