This paper presents the design, development and implementation of a computing based simulation model for interceptor missile guidance for countering an anti-ship missile through a navigation law. It investigates the possibility of deriving, testing and implementing an efficient variation of the PN and RPN laws. A new guidance law [true combined proportional navigation (TCPN) guidance law] that combines the strengths of both the PN and RPN and has a superior capturability in a specified zone of interest is presented in this paper. The presented proportional navigation (PN) guidance law is modeled in a two dimensional planar engagement model and its performance is studied with respect to a varying navigation ratio (N) that is dependent on the ‘heading error (HE)’ and missile lead angle. The advantage of varying navigation ratio is: if N′ > 2, Vc > 0, Vm > 0, then the sign of navigation ratio is determined by cos (ε + HE) and for cos (ε + HE) ≥ 0 and N > 0, the formulation reduces to that of PN and for cos (ε + HE) < 0 and N < 0, the formulation reduces to that of RPN. Hence, depending upon the values of cos (ε + HE) the presented navigation guidance strategy is shuffled between the PN navigation ratio and the RPN navigation ratio. The theoretical framework of TCPN guidance law is implemented in two dimensional setting of parameters. An important feature of TCPN is the HE and the aim is to achieve lower values of the heading error in simulation. The presented results in this paper show the efficiency of simulation model and also establish that TCPN can be an accurate guidance strategy that has its own range of application and suitability. © 2017, The Institution of Engineers (India).