  In any spacecraft landing mission, precision soft landing in a fuel-efficient
way while also avoiding nearby hazardous terrain is of utmost importance. Very
few existing literature have attempted addressing both the problems of
precision soft landing and terrain avoidance simultaneously. To this end, an
optimal terrain avoidance landing guidance (OTALG) was recently developed,
which showed promising performance in avoiding the terrain while consuming
near-minimum fuel. However, its performance significantly degrades in the face
of external disturbances, indicating lack of robustness. In order to mitigate
this problem, in this paper, a novel near fuel-optimal guidance law is
developed to avoid terrain and land precisely and softly at the desired landing
site, under atmospheric perturbations and thrust deviations and constraints.
Expanding the OTALG formulation by using sliding mode control with multiple
sliding surfaces (MSS), the presented guidance law, named `MSS-OTALG', improves
in terms of precision soft landing accuracy. Further, the sliding parameter is
designed to allow the lander to avoid terrain by leaving the trajectory
enforced by the sliding mode, and eventually returning to it when the terrain
avoidance phase is completed. And finally, the robustness of the MSS-OTALG is
established by proving practical fixed-time stability. Extensive numerical
simulations are also presented to showcase its performance in terms of terrain
avoidance, low fuel consumption and accuracy of precision soft landing.
Comparative studies against existing relevant literature validates a balanced
trade-off of all these performance measures achieved by the developed
MSS-OTALG.
