Earth observing satellites are responsible for a variety of essential operations, including communication, navigation, and scientific observation. In the harsh environment of space, however, on-orbit sensor degradation can cause inaccuracies over time. Using the Moon as a reference source, we can achieve reliable in-situ satellite calibration to address this. The objective of the airborne lunar spectral irradiance (air-LUSI) mission is to improve the value of the Moon in this regard, extending the accuracy of existing lunar models and observations through its establishment of an in-situ, SI-traceable, absolute calibration target. A non-imaging telescope is flown at 70,000 ft aboard an ER-2 aircraft, circumventing a significant fraction of lunar spectra measurement uncertainties due to Earth's atmosphere. The improvement in these measurements is projected to improve the widely accepted ROLO (GIRO) model such that it achieves absolute uncertainties of less than 1%. Due to erratic motion of the aircraft, a robotic telescope mount is deployed to stabilize the telescope along its line-of-sight, minimizing data loss. This paper discusses the development and in-lab performance of the tracking system for this instrument, referred to as the high-altitude aircraft-mounted robotic telescope mount (HAAMR). An overview of the robotic system is provided, and a static loading analysis is conducted to demonstrate airworthiness. This instrument was designed to be an improvement upon the ARTEMIS subsystem of the same purpose, for the series of flight campaigns from December 2024 to December 2025. The analysis demonstrates, from past flight data, that the tracking accuracy of the HAAMR will guarantee high pointing accuracy during turbulent flight.

A technique for characterizing and correcting the linearity of radiometric instruments is known by the names the 'flux-addition method' and the 'combinatorial technique'. In this paper, we develop a rigorous uncertainty quantification method for use with this technique and illustrate its use with both synthetic data and experimental data from a 'beam conjoiner' instrument. We present a probabilistic model that relates the instrument readout to a set of unknown fluxes via a set of polynomial coefficients. Maximum likelihood estimates (MLEs) of the unknown fluxes and polynomial coefficients are recommended, while a non-parametric bootstrap algorithm enables uncertainty quantification including standard errors and confidence intervals.The synthetic data represent plausible outputs of a radiometric instrument and enable testing and validation of the method. The MLEs for these data are found to be approximately unbiased, and confidence intervals derived from the bootstrap replicates are found to be consistent with their target coverage of 95%. For the polynomial coefficients, the observed coverages range from 91% to 99%. The experimental data set illustrates how a complete calibration with uncertainties can be achieved using the method plus one well-known flux level. The uncertainty contribution attributable to estimation of the instrument's non-linear response is less than 0.025% over most of its range.

The airborne lunar spectral irradiance (air-LUSI) mission is an inter-agency partnership between the US National Aeronautics and Space Administration and the US National Institute of Standards and Technology. Air-LUSI aims to make SI-traceable measurements of lunar spectral irradiance at visible to near-infrared wavelengths with unprecedented accuracy. To minimize uncertainty, lunar spectra are acquired above 90 % of the Earth’s atmosphere aboard NASA’s Earth Resources aircraft, a civilian descendant of the U-2 spy plane. The data collected by the air-LUSI instrument is poised to improve upon current lunar calibrations of Earth observing satellites. The air-LUSI team recently completed their Operational Flight Campaign in Palmdale, California in March 2022. In addition to the Engineering Flight Campaign of August 2018 and the Demonstration Flight Campaign of November 2019, the air-LUSI instrument has been successfully deployed on over ten lunar spectral measurement flights at altitudes of roughly 21 km. This paper presents the simplified double gimbal design that was capable of recently tracking the Moon with a root mean square tracking error of less than 0.1°.

The airborne lunar spectral irradiance (air-LUSI) instrument is designed to make low uncertainty measurements of the lunar spectral irradiance from an ER-2 aircraft from altitudes above 95% of the atmosphere. Measurements cover the visible and near infrared spectral region (350 nm to 1050 nm) and are traceable to the international system of units. Five demonstration flights were conducted in November 2019 at NASA’s Armstrong Flight Research Center. During that campaign, air-LUSI measured the spectral irradiance at lunar phase angles ranging from 10° to 60°. This work provides an overview of the air-LUSI instrument, the lunar irradiance measurements made during demonstration flights, a description of our calibration approach, and summary of the uncertainty budget. Based on the flight results and laboratory measurements, we estimate the instrument is capable of measuring lunar irradiance, propagated to the top-of-the atmosphere, with combined standard uncertainty of 1% ( k = 1) or less over the spectral region from 450 nm to 980 nm. An examination of the uncertainty budget leads to a path forward toward potentially achieving uncertainties of 0.6% in lunar irradiance over much of the spectral range for future flights.

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.

To monitor global environments from space, satellites must be calibrated accurately and consistently across time, missions and instruments. This requires the use of a stable, common reference that is continuously accessible to Earth observing satellites, whether they make up series of missions spanning long periods of time or comprise constellations acquiring many simultaneous observations across the planet. The Moon can serve well as such a common reference. Its surface reflectance is stable to within one part in 108. It is theorized that its radiant output with time changes repeatedly and very predictably with viewing and illumination geometry. In addition, it has a radiant flux more comparable to the Earth’s surface than the Sun and can be viewed directly by the instrument. Currently, 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. Current knowledge of the Moon's spectral irradiance is thought to be limited to 5-10% uncertainty. However, monitoring changing Earth environments calls for an absolute lunar reference with higher accuracy. The development of the ROLO model and subsequent attempts to better characterize the lunar spectral irradiance cycle were based on observations made from the Earth surface. This requires applying corrections to remove effects of the atmosphere, which limits the accuracy. The Airborne LUnar Spectral Irradiance (Air-LUSI) system was developed to make highly accurate, SI-traceable measurements of lunar spectral irradiance from NASA’s ER-2 aircraft flying at 21 km, above 95% of the atmosphere. 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. During November 2019, the Air-LUSI system was demonstrated with flights on five consecutive nights acquiring observations of the Moon at lunar phases of 10°, 21°, 34°, 46°, and 59°. Air-LUSI is now ready for operational use. This paper provides an overview of this new capability and how it, along with other efforts underway, can help transform how we monitor the Earth from space.

The air-LUSI (airborne LUnar Spectral Irradiance) mission is a program jointly sponsored by the National Aeronautics and Space Administration (NASA) and the National Institute of Standards and Technology (NIST) that aims to establish the moon as an absolute calibration source for space-based radiometric

We have designed a non-imaging telescope for measurement of the spectral irradiance of the moon. The telescope was designed to be integrated into a wing pod of a National Aeronautics and Space Administration ER-2 research aircraft to measure lunar spectral irradiance during flight. The telescope and support system were successfully flown in August 2018 at altitudes near 21 km and at speeds of ∼760 km/h. The wing pod in which the telescope is mounted has an opening through which the moon can be observed. The mount exposes the telescope to high winds, low pressures, temperatures near -60 °C, and vibrations both due to flight and due to the motion of the aircraft on the ground. This required a telescope design with high thermal stability and high resistance to shock. The optical design of the telescope is optimized to have high throughput and spatially uniform transmission from 380 nm to 1000 nm over a field of view about three times the angular size of the moon as viewed from the Earth. The final design resulted in a telescope with singlet design incorporating a 139.7 mm lens with an effective focal length of 377 mm and a field of view of 1.6°. The light from the telescope is introduced into an integrating sphere, which destroys the image and the polarization for measurement by a fiber-coupled spectroradiometer. Herein, we present an overview of the instrument and support system with emphasis on the telescope design.

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∘.

The imaging spectrometers of the second orbiting carbon observatory were radiometrically calibrated before launch during instrumentlevel ground testing. The gain and dark responses were characterized for each focal plane array detector element. An integrating sphere source with an integrated monitoring spectroradiometer illuminated the OCO-2 spectrometers at many light levels. Instrument output was compared with the calibrated output of the source to derive gain coefficients. This source was calibrated in situ with respect to the National Institute of Standards and Technology reference standards, and the instrument met its absolute performance requirement of 5%. Matching fields of view for the internal monitor detectors and the external instrument under test was found to be particularly important, as observed in the results and supported by modeling. Temperature-dependent dark offsets were corrected in a separate process. Solar spectra with varying neutral density filters were used to validate the linearity of the spectrometers.

Integrated path concentrations of ambient levels of carbon dioxide and methane have been measured during nighttime periods at NIST, Boulder (CO, USA), using a ground-based, eye safe laser system. In this contribution, we describe the transmitter and receiver system, demonstrate measurements of CO2 and CH4 in comparison with an in situ point sensor measurement using a commercial cavity ring-down instrument, and demonstrate a speckle noise reduction method.

In this article we describe a high-precision laboratory measurement targeting the R(6) manifold of the 2ν3 band of 12CH4. Accurate physical models of this absorption spectrum will be required by the Franco-German, Methane Remote Sensing LIDAR (MERLIN) space mission for retrievals of atmospheric methane. The analysis uses the Hartmann-Tran profile for modeling line shape and also includes line-mixing effects. To this end, six high-resolution and high signal-to-noise absorption spectra of air-broadened methane were recorded using a frequency-stabilized cavity ring-down spectroscopy apparatus. Sample conditions corresponded to room temperature and spanned total sample pressures of 40 hPa - 1013 hPa with methane molar fractions between 1 μmol mol-1 and 12 μmol mol-1. All spectroscopic model parameters were simultaneously adjusted in a multispectrum nonlinear least-squares fit to the six measured spectra. Comparison of the fitted model to the measured spectra reveals the ability to calculate the room-temperature, methane absorption coefficient to better than 0.1% at the on-line position of the MERLIN mission. This is the first time that such fidelity has been reached in modeling methane absorption in the investigated spectral region, fulfilling the accuracy requirements of the MERLIN mission. We also found excellent agreement when comparing the present results with measurements obtained over different pressure conditions and using other laboratory techniques. Finally, we also evaluated the impact of these new spectral parameters on atmospheric transmissions spectra calculations.

A rapid-scan remote-sensing spectrometer based on an arbitrary waveform driven electro-optic phase modulator for spectral scans over 37 GHz and a telescope/photon counting system for detection was used to measure long term ambient level concentrations of greenhouse gases from natural targets.

Optical frequency comb generators for rapidly re-configurable trace gas sensing are demonstrated using high-bandwidth low-Vπ electro-optic modulators and rapid-scanning microwave sources. These compact devices perform sensitive dual-comb spectroscopy with user-defined line spacing.

The Advanced Baseline Imager (ABI) is the next-generation imaging sensor for the National Oceanic and Atmospheric Administration's (NOAA's) operational meteorological satellites in geostationary orbit. One pathway for traceability to reference standards of the visible and near-infrared radiometric response for ABI is to a 1.65 m diameter integrating sphere source standard of spectral radiance. This source illuminates the full entrance pupil via the ABI Earth-view port, thus determining the absolute spectral radiance responsivity in the visible and shortwave infrared. The spectral radiance values of the large sphere are assigned by Exelis using a double monochromator and a 15.24 cm diameter integrating sphere source standard that is calibrated by NIST. As part of the ABI program, Exelis was required by NASA to have the spectral radiance values assigned by Exelis to the large sphere be validated by NIST. Here we report the results of that activity, which took place in April, 2013. During the week of April 8, Exelis calibrated the 1.65 m diameter sphere at all 24 levels that correspond to the ABI calibration protocol. During the week of April 15, the NIST validation exercise for five selected levels took place. NIST deployed a portable spectral radiance source, a filter radiometer restricted to the visible and near-infrared, and two spectroradiometers that covered from 350 nm to 2500 nm. The NIST sphere source served as the validation standard. The comparison results, which are reported at the ABI bands, agreed to within the combined uncertainties. We describe the methodology, results, and uncertainty estimates related to this effort.

Dual-drive Mach-Zehnder modulators were utilized to produce power-leveled optical frequency combs (OFCs) from a continuous-wave laser. The resulting OFCs contained up to 50 unique frequency components and spanned more than 200 GHz. Simple changes to the modulation frequency allowed for agile control of the comb spacing. These OFCs were then utilized for broadband, multiheterodyne measurements of CO2 using both a multipass cell and an optical cavity. This technique allows for robust measurements of trace gas species and alleviates much of the cost and complexity associated with the use of femtosecond OFCs produced with mode-locked pulsed lasers.