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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.