Crosseye jamming is a method used to create electronic decoys outside a platform in order to deflect attacking missiles. The effect of wave propagation in complicated environments has been studied to investigate how precisely a crosseye system must be constructed to handle incoming missiles. Radar waves emitted by the missile will be scattered, especially over sea, and this may affect the delicate phase balance required in a crosseye system. The level of phase error that will invalidate crosseye jamming has been estimated by employing the principle of reciprocity, which shows that a system may work even in complicated environments. Quantitative results are also derived concerning the requirements on components used in crosseye systems. The principle of reciprocity is invoked in this analysis by defining an "ideal missile seeker" and an "ideal reciprocal crosseye system", which are used to obtain the practical limits imposed on crosseye jamming.