Autonomous descent and soft-landing of spacecraft is of utmost importance in
asteroid missions that are
gaining more attention from space agencies across the world​. In literature,
optimal control-based guidance and optimal sliding mode-based guidance have been
studied for this problem. However, most
of the methods in existing literature are formulated in a linearized engagement
dynamics and require accurate time-to-go estimate, which is usually very
challenging to obtain. To overcome these drawbacks, in this
thesis, Guidance schemes that do not depend on the time-to-go estimates are
developed considering full non-linearity of the engagement dynamics.



First, a Sliding Mode Control-based guidance (SMCG) scheme is presented
for soft landing under a planar engagement set-up formulated in a
landing-site-fixed reference frame. The developed SMCG algorithm involves two
sliding variables based on the notion of collision course
from classical Guidance theory for precise landing and the constraint of desired
end-velocity to ensure a sufficiently small terminal velocity of the spacecraft for
its soft landing. Simulation
studies show that the performance of the developed SMCG scheme is similar to that
of constrained terminal velocity guidance (CTVG), a widely-referred soft landing
guidance
formulated in landing-site-fixed reference frame. However,
the SMCG suffers from certain disadvantages in terms of optimality and the
requirement of initial geometry-specific tuning. To obviate these drawbacks, a
Sliding Mode Control-based instantaneously optimal (SMC-IO) guidance is
subsequently presented in this
thesis under the same planar engagement set-up, in which the heading error is still
retained as a sliding variable,
while a pre-defined maximum allowable range-dependent velocity profile is utilized
for controlling the velocity profile of the spacecraft to ensure a soft landing. A
static optimization problem with square of instantaneous guidance command as the
performance
index and the sliding variable dynamics, velocity envelop profile, equations of
engagement and thrust limit as constraints is solved to obtain the guidance command
at every time-instant. Comparative simulation studies reveal the superior
performance of the
SMC-IO guidance over the CTVG algorithm.



However, the SMCG and SMC-IO guidance schemes developed so far in the thesis do not
consider the variation of asteroid
gravity with the position of the spacecraft. To this end, and moreover, to
accommodate the effect of angular rotation of the asteroid on the engagement
dynamics, asteroid-centered
body-fixed coordinate system is found as more suitable than earlier considered
landing-site-fixed frame and hence exploited to formulate the 3D SMC-IO guidance
that expands the SMC-IO guidance to a
three dimensional engagement set-up. Simulation studies considering different
landing scenarios show the effectiveness of this method in comparison
with ZEM/ZEV (Zero-Effort-Miss/Zero-Effort-Velocity) feedback guidance, a
well-referred soft landing guidance formulated in asteroid-centered
body-fixed frame.