  A new analytic formulation to express the time derivatives of the
instantaneous impact point (IIP) of a rocket is proposed. The geometric
relationship on a plane tangential to the IIP is utilized to decompose the
inertial IIP rate vector into the downrange and crossrange components, and a
systematic procedure to determine the component values is presented. The new
formulation shows significant advantages over the existing formulation such
that the procedure and final expressions for the IIP derivatives are easy to
understand and more compact. The validity of the proposed formulation was
demonstrated through numerical simulation.
