An Improved Pushbroom Scanner Model for Precise Georeferencing of Alos Prism Imagery

One of the instruments on board of the ALOS satellite, launched by the Japanese Aerospace Exploration Agency (JAXA) in 2006, is the Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM). PRISM has three cameras with different viewing directions (nadir, forward, backward), recording imagery with a ground resolution of 2.5 m. A main characteristic of raw ALOS PRISM imagery is that depending on the observation mode each scene is split into four or six separate strips, each related to an individual CCD chip. Basic imagery is delivered as one image data file per strip. We have developed a pushbroom sensor model that is capable of dealing with individual CCD chips sharing some orientation parameters and that can thus be applied to ALOS PRISM imagery. It has previously been shown that pixel-level results can be achieved for georeferencing of ALOS PRISM imagery using this sensor model and a moderate number of ground control points. However, the distribution of resulting residuals suggested that the parameters describing the relative alignment of the individual CCD chips provided by JAXA might not be perfect. Thus, the sensor model was expanded to be capable of self-calibration of these CCD alignment parameters. In this paper, the sensor model will be outlined and the new self-calibration technique described. The effectiveness of self-calibration will be assessed as well as the calibration process carried out by JAXA, in the latter case comparing a set of CCD alignment parameters calibrated in October 2006, and thus representing a very early stage of system calibration, to an updated parameter set obtained in July 2007. Three scenes (forward, backward, nadir) covering a test field in Melbourne (Australia), consisting of more then 100 points surveyed by kinematic GPS, were used for this assessment. Our results show that self-calibration changes the relative alignment of the CCD chips by up to two pixels. If the original calibration data are used, self-calibration can improve the accuracy of the results by 33% and from pixellevel to sub-pixel level. The updated calibration parameters provided by JAXA yield considerably better results than the original ones. In this case, self-calibration essentially helps to increase the height accuracy by about 20%. * Corresponding author.