Spectral Stability of the Libya 4 site using EO-1 Hyperion

To assess the on-orbit radiometric stability of imaging sensors, pseudo-invariant calibration sites (PICS) have been utilized and recommended by the Committee on Earth Observation Satellites (CEOS). In the African continent, the Libya 4 test site has been frequently used to detect long-term changes because of its spatial uniformity, temporal stability, and low atmospheric aerosol loading. The nominal location of the Libya 4 test site is centered at the latitude and longitude of +28.55 degrees and +23.99 degrees with a suggested usable area of 75 x 75 km. Previous cross-calibration studies have used subset areas of the test site around the CEOS suggested center coordinate regardless of the spectral characteristic of the site. In this study, yearly hyper-spectral collections from 2004 to 2012 around day of the year 170 were used from Earth Observing One (EO-1) Hyperion sensor over the Libya 4 site. To evaluate the spectral stability over a broad range over the Libya 4 site, a series of Region of Interest (ROI) covering approximate areas of 3 by 3 km were selected in the along-track direction from +28.07 degrees to 29.55 degrees of latitude (about 170 km long), which is also a subset of the CEOS reference site. The reflectance variations in other ROIs are initially estimated by average deviation and compared with the spectral angle mapper (SAM) method using ROI number '0' in year 2004 collection as a reference spectrum. To consider a practical application, SAM and average deviation metrics are limited by the wavelength ranges of Landsat 7 (L7) Enhanced Thematic Mapper Plus (ETM+) Relative Spectral Response (RSR), instead of using the entire range of the Hyperion spectrum. These ETM+ RSR's focused SAM values are considered to be the sensor specific spectral stability of the Libya 4 site. The Libya 4 CEOS test site is shown to be a spectrally stable target where SAM angles are less than 1.6 degrees and average deviations are less than 5% reflectance level based on the reference spectrum in 2004 within ETM+ RSR Full-Width at Half-Maximum (FWHM) ranges.