Yaw Bias Model for Earth Observation Spacecrafts

Most of the remote sensing spacecrafts carry payload with staggered CCD configuration (odd and even pixels spread linearly in 2 rows). Prime requirement is to capture the same line of scan by both arrays of pixels for smooth imaging. However, due to time difference in the arrival of the two CCD arrays over imaging line, the same line can’t be scanned by the two arrays as the earth has rotated in that duration. As, radius of the greater circle on the surface of earth is variable due to its oblateness and non symmetric structure and so computation of yaw bias as a compensation for earth’s rotation is very complicated due to large number of harmonics, if spherical earth model is considered. In this paper, yaw bias (rotation about nadir axis) model with respect to sub satellite latitude is derived, considering flat earth model which is very well valid for low earth orbit spacecrafts with swath coverage of about few hundred meters. The model is simple, easy to understand and is derived using orbit geometry of the spacecraft and basic trigonometric identities.