Development of a heliostat facility for solar-radiation-based calibration of earth observing sensors

A new heliostat facility at Ball Aerospace and Technologies Corporation (BATC) in Boulder, CO will allow the use of the sun as the source in the calibration of earth observing sensors. The solar spectrum is the basic energy source for such instruments; therefore it is advantageous to perform initial ground radiometric calibrations using the sun. Using this method for preflight radiometric calibration reduces uncertainties caused by the spectral mismatch between the preflight and in-flight calibration, especially in the case in which a solar diffuser is the in-flight calibration method. This method also reduces stray light concerns as the instrument diffuser is measured in situ with the same radiance level it sees on orbit. This paper presents the design of a heliostat test facility which tracks the sun and directs the solar beam into a thermal vacuum chamber, allowing the instrument under test to be kept in a safe, clean and controllable environment. Design considerations that affect the uniformity and transmission of the system are discussed. The opto-mechanical logistics of creating a heliostat that will deliver a 13-inch solar beam into a thermal vacuum chamber are also presented. This facility is currently under construction at BATC and is expected to be operational by the end of 2008.