  This paper presents cooperative target defense guidance strategies using
nonlinear model predictive control (NMPC) framework for a
target-attacker-defender (TAD) game. The TAD game consists of an attacker and a
cooperative target-defender pair. The attacker's objective is to capture the
target, whereas the target-defender team acts together such that the defender
can intercept the attacker and ensure target survival. We assume that the
cooperative target-defender pair do not have perfect knowledge of the attacker
states, and hence the states are estimated using an Extended Kalman Filter
(EKF). The capture analysis based on the Apollonius circles is performed to
identify the target survival regions. The efficacy of the NMPC-based solution
is evaluated through extensive numerical simulations. The results show that the
NMPC-based solution offers robustness to the different unknown attacker models
and has better performance than CLOS and A-CLOS based strategies.
