  This paper investigates terahertz ultra-massive (UM)-MIMO-based angle
estimation for space-to-air communications, which can solve the performance
degradation problem caused by the dual delay-beam squint effects of terahertz
UM-MIMO channels. Specifically, we first design a grouping true-time delay unit
module that can significantly mitigate the impact of delay-beam squint effects
to establish the space-to-air THz link. Based on the subarray selection scheme,
the UM hybrid array can be equivalently considered as a low-dimensional
fully-digital array, and then the fine estimates of azimuth/elevation angles at
both UAVs and satellite can be separately acquired using the proposed
prior-aided iterative angle estimation algorithm. The simulation results that
close to Cram\'{e}r-Rao lower bounds verify the effectiveness of our solution.
