Scheduled for launch in January 2024, the PACE mission represents NASA’s next investment in ocean biology, clouds, and aerosol data records. A key feature of PACE is the inclusion of an advanced satellite radiometer known as the Ocean Color Instrument (OCI), a global mapping radiometer that combines multispectral and hyperspectral remote sensing. A critical requirement for OCI is the high-contrast or spatial crosstalk specification (also referred to as in-field stray-light response). The requirement states that for global top-of-atmosphere radiances based on measured MODIS radiances, the global average residual contamination shall be less than 0.4% for 350 nm, 360 nm, 385 nm, 555 nm, 583 nm, 820 nm and 865 nm and less than 0.20% for all other multispectral bands. Accurate resolution of high contrast in TOA radiance images is important to estimate stray light contamination due to clouds, for studying small scale features like ocean fronts and for working in coastal and estuarine areas where the scales are 1km. This occurs in all wavelengths in the spatial direction. Knowledge of high contrast resolution makes up part of the artifact budget. Accurate measurement of the high-contrast performance of OCI requires laboratory Ground Support Equipment (GSE) that projects a scene of sufficient quality that the unwanted stray light of the GSE itself is not confused with the stray light response of the telescope. This paper concerns the development, analyses and test of the GSE to ensure the quality of the projected image is sufficient to verify the OCI requirements. Optical models were developed for both the instrument as well as the GSE and laboratory environment. Simulation of various non-ideal parameters were critical to accurately predict performance. Measurements using COTS cameras and lenses were also made of the projected GSE image to reasonably verify the optical model predictions. Measured and modelled results from OCI are discussed.
Scheduled to launch in 2024, the Ocean Color Instrument (OCI) onboard the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission will collect hyperspectral data from 315 nm to 895 nm via two grating spectrometers (in both the blue and red spectral regions) and 9 multi-spectral bands in the short-wave infrared (940 nm to 2260 nm). The increased spectral resolution and radiometric accuracy is expected to improve upon data collected by heritage sensors such as SeaWiFs, MODIS, and VIIRS, allowing new applications in ocean color, aerosol, and cloud science. During ground testing, higher than expected spatial-spectral crosstalk was measured for the hyperspectral bands in the blue spectrograph. Using a monochromatic-collimated light source, light from a single science pixel (1km x 1km) was found to produce crosstalk signals over 31 pixels in the cross-track direction. This spatial augmentation is caused by the spectral crosstalk’s asynchronous spatial movement during Time Delay Integration (TDI). To fully characterized the magnitude and spectral dependency from this, a crosstalk model was developed by synthesizing data collected from monochromatic-collimated light and monochromatic light that filled the OCI optical aperture. The model was validated by showing good agreement between predicted values and other relevant test data collected using both monochromatic and white light sources.
Bidirectional Scattering Distribution Function (BSDF) measurements of selected specular samples were made using the Table-Top Goniometer (TTG) in the Diffuser Calibration Lab (DCL) at NASA GSFC in the support of NASA remote sensing instruments and programs. The same TTG system has also been used in the BRDF measurements for diffuse samples. The tunable laser-based TTG possesses the advantages of small incident beam profile and configuration flexibility and is able to meet various BSDF test requirements on specular samples with flat and curved surfaces. It also has a useful capability in characterizing instrument straylight due to surface roughness and in determining the scattering light distribution function of optical surfaces. The BSDF measurements on specular samples can be performed over 8 orders of linear dynamic range with correction of instrument signatures. In this paper, we present BSDF results on two types of specular samples: a witness flat fold mirror for the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) project and four Multi-Layer Insulation (MLI) samples for the Restore project at NASA GSFC. BRDF measurements in the viewing angle range of ± 90° were acquired at 500 nm, 700 nm, and 2000 nm and at incident angles of 0°, 8°, and 25° for the PACE sample, and at 500 nm, 633 nm, 700 nm, 900 nm, 1000 nm, 1550 nm, and 1800 nm at incident angles of 10° and 25° for the MLI samples. For both types of samples, the ABg model was applied to fit the BSDF data to generate the parameters for optical modeling. The ABg model is able to fit the BSDF data on the polished surface of the flat mirror very well. However, two scattering components were seen in the MLI BSDF fitting results attributed to wrinkle and surface morphology issues. Total Hemispherical Reflectance (THR) and Total Integrated Scatter (TIS) measurements were also made on the samples and were compared to the BSDF results. The details of the BSDF measurement setup and the methodology for realization of the BRDF scale for the specular samples are also described.
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