  In this paper, a robust autopilot is designed for a missile autopilot, such
that the system stability is guaranteed in low altitude and short-range
conditions. First, using the v-gap metric, the system is linearzed around the
equilibrium point. Then, the robust $H_\infty$ loop shaping controller is built
for the linear model. The proposed approach does not utilize the gain
scheduling method, and guarantees the system stability throughout the flight
envelope. Particle Swarm Optimization (PSO) algorithm is used along with the
control approach to reduce the complicated tuning process of the weight
functions. The weighting functions are optimized throughout the evolutionary
algorithm to maximize the stability margin. From the simulations, it is proved
that the stability margins achieved guarantees the stability of interceptor
throughout the whole flight envelope.
