Abstract: A guidance system for a homing missile having an observer filter which utilizes signals representative of range, range rate, acceleration normal to the line-of-sight of the missile, acceleration along the line-of-sight of the missile, and line-of-sight rate of the missile to a target is described. The filter includes means, responsive to a lateral acceleration signal, a measured range rate signal and a range signal for providing an estimated line-of-sight rate signal. The measured line-of-sight rate signal and the estimated line-of-sight rate signal are used to produce a difference output signal. The difference output signal is used to produce an error signal which is applied to the providing means to minimize the difference output signal. The estimated line-of-sight rate signal provides a more accurate estimate of the actual line-of-sight rate of the missile to the target than that which can be obtained with a standard proportional navigation system.