Abstract
As a near orbit space platform, the International Space Station (ISS) has been increasingly used for Earth observing applications. This paper presents a quaternion-based forward geolocation algorithm for Earth observing sensors onboard the ISS. The input parameters include the orbital state and attitude information of the ISS and the look vector of the sensor. The proposed algorithm agrees with the commercial navigation product, Satellite Tool Kit®, within 0.5 m in ideal situations. The inherent uncertainties in ISS attitude and state determinations, and the International Space Station Agriculture Camera (ISSAC) tilting angle were estimated to introduce an error less than 800 m. However, the actual geolocation error evaluated using the images obtained by the ISSAC is roughly 4 km, much greater than the inherent uncertainty and mainly due to (a) delay caused by the Windows® operating system in acquiring images, and (b) the misalignment of the ISSAC sensor coordinate system with the ISS body-fixed coordinate system. A preliminary cal/val process using the Google Earth™ as reference was performed to quantify these two errors, the correction of which improved the geolocation accuracy to 500 m, well within the inherent uncertainty.
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