Calibration Lessons Learned from Hyperion Experience

The use of hyperspectral imagers to provide climate-quality data sets, such as those expected from the solar reflective sensor on the Climate Absolute Radiance and Refractivity Observatory (CLARREO), requires stringent radiometric calibration requirements. These stringent requirements have been nearly met with broadband radiometers such as CERES, but high resolution spectrometers pose additional challenges. A review of the calibration processes for past space-based HSIs provide guidance on the calibration processes that will be needed for future sensors. In November 2000, the Earth Observer-1 (EO-1) platform was launched onboard a Boeing Delta II launch vehicle. The primary purpose of the EO-1 mission was to provide a technological testbed for spaceborne components. The platform has three sensors onboard, of which, the hyperspectral imager (HSI) Hyperion, is discussed here. The Hyperion sensor at the time had no comparable sensor in earth orbit, being the first grating-based, hyperspectral, civilian sensor in earth orbit. Ground and on-orbit calibration procedures including all cross-calibration activities have achieved an estimated instrument absolute radiometric error of 2.9% in the Visible channel (0.4 - 1.0 microns) and 3.4% in the shortwave infrared (SWIR, 0.9 - 2.5 microns) channel (EO-1/Hyperion Early Orbit Checkout Report Part II On-Orbit Performance Verification and Calibration). This paper describes the key components of the Hyperion calibration process that are applicable to future HSI missions. The pre-launch methods relied on then newly-developed, detector-based methods. Subsequent vicarious methods including cross-calibration with other sensors and the reflectance-based method showed significant differences from the prelaunch calibration. Such a difference demonstrated the importance of the vicarious methods as well as pointing to areas for improvement in the prelaunch methods. We also identify areas where lessons learned from Hyperion regarding improved methodologies during sensor development and additional capabilities of for on-orbit calibration systems that will enable reduction of the error terms. A key lesson learned was the approach taken to combine the various calibration results of Hyperion which differed by more than uncertainties in the methods.