  Laser Guide Stars (LGS) have greatly increased the sky-coverage of Adaptive
Optics (AO) systems. Due to the up-link turbulence experienced by LGSs, a
Natural Guide Star (NGS) is still required, preventing full sky-coverage. We
present a method of obtaining partial tip-tilt information from LGSs alone in
multi-LGS tomographic LGS AO systems. The method of LGS up-link tip-tilt
determination is derived using a geometric approach, then an alteration to the
Learn and Apply algorithm for tomographic AO is made to accommodate up-link
tip-tilt. Simulation results are presented, verifying that the technique shows
good performance in correcting high altitude tip-tilt, but not that from low
altitudes. We suggest that the method is combined with multiple far off-axis
tip-tilt NGSs to provide gains in performance and sky-coverage over current
tomographic AO systems.
