The design of a communication link to have low probability of intercept (LPI) capability is predicated on the requirement for it to operate in a hostile environment wherein the enemy tries to detect the presence of the communicator s transmission. A typical scenario might consist of an underwater or surface vehicle (the communicator) attempting to correspond with a satellite (the receiver) via transmission of a short message, and an enemy search aircraft (the interceptor) whose mission it is to detect the presence of RF energy corresponding to this communicated message. The key words in the last sentence are detect and energy since typically the interceptor s performance is judged by his ability to determine (detect) that the communicator has initiated a transmission (energy) and not by what might occur following this accomplishment, e.g., location of the communicator, decoding of the message, etc. To further emphasize this point, the interceptor is ordinarily acquainted only with such communicator information as his frequency band, modulation format, and those spread-spectrum (SS) modulation characteristics such as bandwidth and code (hop) rate, which are insufficient by themselves to allow for decoding of the message. Thus, in the absence of information related to the time of message initiation and the secure codes used to generate the SS modulation, the interceptor s detection strategy is limited to employing some form of energy detector.

A large part of this chapter shall...