Hal Hellman, Controlled Guidance Systems, Howard W. Sams & Co., 1967


In passive homing there is no external target illumination whatever. All the energy used in detection must be generated by the target itself. This could of course lead to a problem. For example, a passive system might be designed to hone in on radio energy generated by the target. But if the enemy decides to maintain radio silence during the attack, this method obviously cannot be used.