In partnership with Raytheon Intelligence and Space, Labsphere Inc. has been developing a technology demonstration system for a new type of on-board absolute radiometric calibration source. The Improved Radiometric Calibration of Imaging Systems (IRIS) addresses the need for reduced risk, cost, size, and mass for next generation Earth Observation (EO) satellites through paired onboard and vicarious calibration methods. In particular, the IRIS High-performance Integrated Flat Illuminator (HIFI) is a compact, combined VISNIR and SWIR (0.4 – 2.3μm), and MWIR-LWIR (3-14μm) Jones radiance source. Funded by the NASA Earth Science Technology Office (Grant to Raytheon #80NSSC20K1676), the IRIS Technology Demonstration Unit currently under test successfully meets significant program specifications for radiance, stability, adjustability, uniformity, and polarization. Development is ongoing to further improve system performance and achieve space flight qualification. This type of new technology additionally may provide a path to on-board calibration for small satellite architectures.
Labsphere has created automated vicarious calibration sites using convex mirror technology in the new FLARE (Field Line-of-sight Automated Radiance Exposure) Network. FLARE has been operational for over two years, with network expansion and performance validation against industry standards and common methods for calibration and validation (cal/val) of 350-2500nm optical Earth Observation Systems (EOS). The FLARE point sources provide absolute and traceable data, creating a new tool in harmonization of satellites with ground sampling distances (GSD) of 0.3m to 60m. This paper provides an overview of the FLARE system and presents findings and improvements in operational hardware and software performance. Once commissioned, all FLARE nodes have been repeatedly targeting Landsat 8, Landsat 9 (starting 2022), and Sentinel 2A/B. This has produced a multi-year archive of radiometric and spatial calibration imagery. Landsat and Sentinel are the premier reference programs for Earth Observation performance and utilize both on-board calibration equipment and on-ground reference sites such as RadCalNet and PICS. This work compares the results of the FLARE technique to current official radiometric coefficients and spatial performance metrics for these satellites. Discussion will center on new insights gleaned from the archive analysis and FLARE’s contribution to the community’s capability for data fusion, instrument harmonization, and the potential to support the concept of Analysis Ready Data (ARD) for easier data use and information extraction. Finally, the future progression of FLARE sites, capabilities, and activities will be outlined.
The Empirical Line Method (ELM) is a widely applied technique of achieving absolute atmospheric correction assuming a linear relationship between the raw Digital Numbers (DNs) or at-sensor radiance and surface reflectance measurements collected in-situ. The ELM measures reference targets of known reflectance in an image. Labsphere has created an automated vicarious calibration system using the SPecular Array Radiometric Calibration (SPARC) mirror technology in the new Field Line-of-sight Automated Radiance Exposure (FLARE) network. In the FLARE system the known reflectance targets are convex mirrors - because of that it is titled Mirror based Empirical Line Method (MELM). In this context, the objective of this work is to present the initial results of the MELM using one the FLARE network system. The FLARE system evaluated in this work is the Alpha Node located at Arlington, SD. Initially, the data collected in 2020 and 2021 with the Alpha FLARE concomitant with the OLI sensor overpass on-board the Landsat-8 satellite were used in the assessment. In summary, the surface reflectance image product available to download for OLI sensor were compared directly with the surface reflectance image resulting from the MELM method. The preliminary results showed the mean absolute error data between the surface reflectance from the OLI Level-2 product image and the surface reflectance from the MELM was lower than 0.01 for the Blue, Green, Red and SWIR-1 bands; lower than 0.03 for the for the NIR and SWIR-2 spectral bands; and around 0.05 for Coastal Aerosol band (all in reflectance units). These results suggest the MELM technique using FLARE has great potential for reflectance surface evaluation of orbital sensors.
Labsphere has created automated vicarious calibration sites using the SPecular Array Radiometric Calibration (SPARC) mirror technology in the new Field Line-of-sight Automated Radiance Exposure (FLARE) network. A short introduction to FLARE and SPARC will be given showing how arduous field ground calibrations can now be done remotely through FLARE nodes via an internet portal. Preliminary results of the performance of the system’s absolute radiometric and spatial calibration capability were published in 2020, demonstrating validation and uncertainty against current methods of remote calibration and spatial and geometric performance against edge and line targets. This paper will describe FLARE’s impact to ongoing evaluation and maturation of automated analysis processes for all data processing levels for space satellite and UAV imagers.
The SPecular Array Radiometric Calibration (SPARC) methodology uses convex mirrors to relay an image of the sun to a satellite, airborne sensor, or other Earth Observation platform. The signal created by SPARC can be used to derive absolute, traceable calibration coefficients of Earth remote sensing systems in the solar reflective spectrum. This technology has been incorporated into an automated, on-demand commercial calibration network called FLARE (Field Line-of-site Automated Radiance Exposure). The first station, or node, has been successfully commissioned and tested with several government and commercial satellites. Radiometric performance is being validated against existing calibration factors for Sentinel 2A and diffuse target methodologies. A radiometric uncertainty budget indicates conservative 1-sigma uncertainties that are comparable to or below existing vicarious cal/val methods for the VIS-NIR wavelengths. In addition to radiometric performance, SPARC and FLARE can be utilized for characterization of a sensor’s spatial performance. Line and Point Spread Functions, and resulting Modulation Transfer Functions, derived with SPARC mirrors are virtually identical to those measured with traditional diffuse edge targets. Ongoing development of the FLARE network includes improved radiometric calibration, web portal scheduling and data access, and planned expansion of the network to Railroad Valley Playa and Mauna Loa, Hawaii.
Spectralon® is a high reflectance excellent Lambertian diffuser used to reflect sunlight for use as a calibrator for on-orbit and ground instruments. Radiometric calibration of the reflective bands in the 0.4 to 2.5μm wavelength range is performed by measuring the sunlight reflected from Spectralon® . Reflected sunlight is directly proportional to the Bidirectional Reflectance Distribution Function (BRDF) of the Spectralon® . On-orbit exposure to sunlight results in solarization due to solar UV. Previously, the rate / amount of solarization has varied as observed from on orbit measurements as well as laboratory UV exposure testing of samples. A method for determining whether a particular batch of Spectralon® has low solarization has been developed. This method relies on hemispherical reflectance measurements in the 0.25- 0.5 μm wavelength range before and after Spectralon® bake out. This method is reliable for as-made Spectralon® , not for contamination verification after shipment. We have also determined that additional Spectralon® bake outs do not change the as-made Spectralon® solarization rate. Knowledge of possible Spectralon solarization is important prior to its shipment to customers and eventual deployment in satellite and ground-based instrument calibration.
Bioluminescence is a striking and ubiquitous source of light in the global ocean, utilized in a variety of ecologically important communication, camouflage, and predator deterrence functions. It can be prevalent in surface waters at night and at most times in mesopelagic waters (≈200-1000m) where ambient light approaches a weak, asymptotic radiance field. The propagation of bioluminescent signals, and therefore the distance at which these signals can be detected, is dependent upon the inherent optical properties (IOPs) of the water column. The effects of IOPs on the propagation of light from isotropic point sources embedded in bioluminescent layers were examined in terms of emitted signal against background radiance throughout the water column, i.e., a metric defining the required ability to detect the emissions.
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