The ABI (Advanced Baseline Imager) uses periodic star sensing to determine the absolute line of sight. Using a specifically tailored onboard star catalogue, the ABI can sense visible stars (0.64 ƒÝm) for image navigation and IR stars (3.90 ƒÝm) for coregistrtaion purposes. The 3ƒã location accuracy for stars of magnitude 7 is expected to be better than 2.5 ƒÝrad for the visible stars and 6 ƒÝrad for the IR stars. The ABI star sense algorithm simulates the star observation with the relevant mechanical and optical effects. It employs a centroiding technique and is robust against signal geometry, background noise, clutter, jitter and degradation. The onboard ABI star catalogue is prepared using appropriate selection criteria based on the ABI field of view, star isolation, and brightness.