The FLARE Network: Radiometric and Spatial Vicarious Calibration - Overview and Initial Results

Abstract:

Low-uncertainty radiometric calibration is critical to accurate retrieval of remotely sensed Earth observation products, particularly for hyperspectral data. Extensive pre-flight characterization is the standard for large government and commercial missions, but may not be feasible for small platforms. Currently, in-flight calibration is carried out using on-board and vicarious techniques including solar diffusers, light sources, or viewing large, well characterized sites on Earth.

The SPecular Array Radiometric Calibration method employs convex mirrors to relay an image of the solar disk for deriving absolute radiometric coefficients in the VIS-NIR with a sub-pixel source. The combination of these arrays with a targeting station and NIST-traceable solar radiometer is the basis for a new, on-demand commercial network called FLARE. This network will provide a unique tool for radiometric and image quality characterization with capabilities relevant to high spatial/spectral resolution remote sensing. A large dynamic range of at-sensor radiance can be tailored to varying dpatial and radiometric requirements. Data have 2.4 nm resolution from 350 - 1000 nm, and are multispectral from 1050 – 2500 nm. FLARE is capable of tying large or small satellites, manned aerial, and UAV-based platforms to a single calibration site. Projected uncertainty for on-orbit calibration (~3%) is similar to current methodologies. A high-altitude site is in development, designed to provide uncertainty on the order of 1.5% by reducing atmospheric effects.

The spatial performance of the sensor and any resampling performed in the processing chain can become a significant error contributor to the radiometry of small targets. Factors include illumination non-uniformity, pixel to pixel response, spread functions, and post-processing impacts. FLARE can be used to oversample the sensor’s in-flight point spread function (PSF), assisting analysis of sensor MTF and small target calibration.

Results from initial field deployments are presented, including a direct comparison of performance metrics for Landsat 8 and Sentinel 2B derived from FLARE and traditional diffuse targets. Characterization has been made of small-sat constellations. Developing capabilities relevant to current and future platforms will be discussed.