Pre-Launch JPSS-2 VIIRS Response versus Scan Angle Characterization

Abstract

On-orbit whisk-broom sensors have scanning mirror assemblies, whose reflectance variations with scan angle must be characterized prior to launch. One such instrument is the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Joint Polar Satellite System 2 (JPSS-2) platform. The scanning optics inside VIIRS includes a four mirror rotating telescope assembly (RTA) and a half angle mirror (HAM), rotating at half the speed of the RTA, which de-rotates the light before it enters the aft-optics assembly. The angle of incidence (AOI) on the HAM varies with scan angle; all of the other optical components in VIIRS have a fixed AOI with scan angle. In general, the reflectance of the HAM will vary with AOI. This parameter is difficult to quantify once in orbit and therefore must be characterized pre-launch. Ground measurements were performed in the summer of 2016 to determine the relative reflectance change of the instrument with scan angle, referred to as the response versus scan angle (RVS). This work will describe the RVS testing performed and the results obtained, including an atmospheric water vapor correction and an uncertainty analysis. Results indicate that the reflectance variation with scan angle is small for spectral bands between 0.4 µm and 4 µm (less than 2% over the full range of AOI); in contrast, the reflectance variation is between 3% and 10% for the spectral bands in the 8 µm to 12 µm range. Uncertainties are below 0.05% in the reflective solar spectral region and below 0.26% in the thermal emissive spectral region. Comparisons to previous VIIRS builds (on the SNPP and JPSS-1 satellites) show comparable performance.

A New Method for Characterizing NOAA-20/S-NPP VIIRS Thermal Emissive Bands Response Versus Scan Using On-Orbit Pitch Maneuver Data

“…In the VIIRS calibration algorithm, RVS is used to characterize the effect of the change of source radiance with AOI on the HAM. The relationship between AOI on the HAM and scan angle is given in Equation (4) [4,15]:…”

“…During the prelaunch test; band, detector, and HAM-side dependent RVS functions are measured using the Laboratory Ambient Blackbody, which is placed at various VIIRS scan angles to ensure a good characterization of RVS at the entire EV and SV scan angle range. The 2 nd order polynomial fit is used to model RVS variations to predict RVS at all AOIs [4,15]. In the NOAA operational processing, RVS LUT (VIIRS-SDR-RVF-LUT, where RVF stands for response versus frame, i.e., pixel index along the scan) is used to provide band, detector, and HAM-side dependent RVS values for all EV frames (pixels), SV, and OBCBB.…”

Pre-launch Radiometric Characterization of JPSS-2 VIIRS Thermal Emissive Bands

“…where the reflectance factor (ρ RTA ) represents the product of individual reflectances of the RTA and is the emissivity of the source. For VIIRS, the only optic that has a varying angle of incidence (AOI) is the HAM; RVS is the AOI dependent, relative reflectance of the HAM [14] and here it was at the AOI of either the source or the SVS. The radiances of the sources (L) are determined via Planck's law, convolved over the extended bandpass of the spectral transmittance of each band.…”

An Overall Assessment of JPSS-2 VIIRS Radiometric Performance Based on Pre-Launch Testing

“…This allowed the instrument to view external sources by rotating the instrument while keeping the illumination level constant. Data taken at twelve angles-of-incidence (AOI) were used to fit the RVS function, which is a quadratic polynomial in AOI, after correcting for source drift and background radiances [21,22]. Figure 10 shows J2 band-average RVS functions for HAM side A and comparison to J1.…”