  This paper addresses the pursuit-evasion problem involving three agents -- a
purser, an evader, and a defender. We develop cooperative guidance laws for the
evader-defender team that guarantee that the defender intercepts the pursuer
before it reaches the vicinity of the evader. Unlike heuristic methods, optimal
control, differential game formulation, and recently proposed time-constrained
guidance techniques, we propose a geometric solution to safeguard the evader
from the pursuer's incoming threat. The proposed strategy is computationally
efficient and expected to be scalable as the number of agents increases.
Another alluring feature of the proposed strategy is that the evader-defender
team does not require the knowledge of the pursuer's strategy and that the
pursuer's interception is guaranteed from arbitrary initial engagement
geometries. We further show that the necessary error variables for the
evader-defender team vanish within a time that can be exactly prescribed prior
to the three-body engagement. Finally, we demonstrate the efficacy of the
proposed cooperative defense strategy via simulation in diverse engagement
scenarios.
