Monochromatic measurements of the JPSS-1 VIIRS polarization sensitivity

Uncertainty budget for detector-based absolute radiometric calibration with GLAMR

The accuracy of the absolute radiometric calibration (RadCal) for remote sensing instruments is essential to their wide range of applications. The uncertainty associated with the traditional source-based RadCal method is assessed at a 2% (k=1) or higher level for radiance measurement. To further improve the accuracy to meet the demands of climate studies, a detector-based approach using tunable lasers as a light source has been devised. The Goddard Laser for Absolute Measurement of Radiance, known as the GLAMR system, is a notable example of the incorporation of such technology. Using transfer radiometers calibrated at the National Institute of Standards and Technology as calibration standards, the absolute spectral response function of a remote sensing instrument is measured with its uncertainty traceable to the International System of Units. This paper presents a comprehensive uncertainty analysis of the detector-based absolute RadCal using the GLAMR system. It identifies and examines uncertainty sources during the GLAMR RadCal test, including those from the GLAMR system, the testing configuration, and data processing methodologies. Analysis is carried out to quantify the contribution of each source and emphasize the most influential factors. It is shown that the calibration uncertainty of GLAMR RadCal can be better than 0.3% (k=1) in the wavelength range of 350-950 nm and 0.6% (k=1) between 950 and 2300 nm, with the exception of regions with strong water absorption. In addition, recommendations are made to refine the calibration process to further reduce the uncertainty.

Absolute Radiometric Calibration of an Imaging Spectroradiometer Using a Laboratory Detector-Based Approach

The HyperSpectral Imager for Climate Science (HySICS) is the core instrument of the Climate Absolute Refractivity and Reflectance Observatory (CLARREO) Pathfinder (CPF) mission and is currently scheduled to be launched to the International Space Station (ISS) in 2023. HySICS is an Offner–Chrisp imaging spectrometer designed to meet an unprecedented radiometric uncertainty requirement of 0.3% (k = 1) over its entire spectral range of 350–2300 nm. The approach represents the need for significant improvement over the Radiometric Calibration (RadCal) of existing space-borne spectrometers. One strategy to demonstrate that HySICS achieves this level of accuracy is through an Independent Calibration (IndCal) effort that can provide an alternative referencing RadCal, which follows a traceability chain independent of the operational RadCal of ratioing approach. The IndCal relies on a pre-launch detector-based absolute RadCal of HySICS, using a tunable laser system as source, and the system planned for the HySICS absolute RadCal is the Goddard Laser for Absolute Measurement of Radiance (GLAMR). GLAMR was developed at NASA’s Goddard Space Flight Center and has been used to calibrate multiple operational remote sensing instruments, as well as the SOlar, Lunar Absolute Reflectance Imaging Spectroradiometer (SOLARIS), a calibration demonstration system developed for the CLARREO mission. In this work, the data of SOLARIS GLAMR RadCal conducted in 2019 are processed to derive the Absolute Spectral Response (ASR) functions and other key characterization parameters of SOLARIS detectors. The results are further analyzed with the goals to plan the HySICS GLAMR RadCal, in particular to optimize its configuration, to demonstrate the traceability route to the NIST standard, and to develop the error budget of the calibration approach. The SOLARIS calibration is also compared with other source- and detector-based calibrations to validate the absolute radiometric accuracy achieved.

NOAA-20 VIIRS polarization effect and its correction

The follow-on Visible Infrared Imaging Radiometer Suite (VIIRS) housed in the NOAA-20 satellite was launched on 18 November 2017. It has 22 spectral bands, among which 14 are reflective solar bands (RSBs) covering the wavelength range from 411 to 2258 nm. Prelaunch polarization sensitivity measurements have revealed that NOAA-20 VIIRS RSBs are much more sensitive to polarization of the incident light than its predecessor, the VIIRS on the Suomi National Polar-orbiting Partnership. For the short wavelength bands, i.e., M1-M4, the polarization sensitivities are out of specifications, especially for band M1, for which the polarization factors can be as large as ∼6%. The polarization effect induces striping in imagery along the track and radiometric bias both along the scan and along the track, resulting in much larger uncertainties in the environmental data records (EDR). In this paper, the polarization effect correction algorithms are described and applied to the NOAA-20 VIIRS RSBs for ocean scenes where the top-of-atmosphere radiance can be separated into the ocean normalized water-leaving radiance, the basis of the ocean color EDR, and the sunlight reflected by the atmosphere, which can be mostly described by the Rayleigh scattering radiance. The errors of the sensor data records (SDR or Level-1B radiance) due to the polarization effect can be as large as ∼1% for bands M1 and M2, and those in the ocean normalized water-leaving radiances are about 13% and 10% for wavelengths at 411 nm (band M1) and 445 nm (band M2), respectively. The polarization effect also induces strong striping in both NOAA-20 VIIRS RSB SDR and normalized water-leaving radiances. It is demonstrated that with the polarization correction applied, the aforementioned errors and artifacts are successfully removed.

Polarization sensitivity error analysis and measurement of a greenhouse gas monitoring instrument

Greenhouse gas monitoring instruments (GMI) are spatial heterodyne spectroscopy (SHS) sensors that monitor greenhouse gases (GHG) from space. Due to several kinds of polarization-sensitive optical elements in GMIs, to some extent, the instrument becomes a polarization-sensitive sensor. Its polarization sensitivity will reduce the radiometric accuracy and spectral inversion accuracy of GHG column concentration. Theoretical radiation response models for analyzing the polarization sensitivity of a GMI, which is mainly affected by a scanning mirror beam splitter and diffraction gratings, are presented in this paper. Based on these models and the polarization performance testing, the theoretical and experimental results of the main spectral band of a GMI, covering the wavelength range of 1.568-1.583 μm for carbon dioxide (CO₂) detection, have been given. The result shows that the linear polarization sensitivity is less than 0.65% and 1.32% in the nadir (45°, 0°) and in the oblique view direction (45±20°, ±31°), respectively, and that it meets the qualification requirement for an absolute radiometric calibration accuracy better than 5%. The absolute radiometric calibration accuracy directly affects the accuracy of GHG concentration retrieval.