LPI IN PULSED-LASER SPACE COMMUNICATIONS

The use of space optics with lightwave beams is being considered for v arious types of long range communication links. Most of these links ar e being designed with pulsed laser transmissions, usually in the form of a coded pulse position modulated (PPM) format. Closely associated w ith the design of the communication link itself may be a corresponding requirement for LPI (low probability of intercept). This occurs when an intercepting optical receiver is present to determine the source lo cation by interrogating the light field. This paper formulates a basic LPI study for a pulsed laser PPM optical link with a co-located direc t detection intercepting receiver. The latter is considered to be a fo rm of optical radiometer using continuous threshold testing to detect the presence of the transmitting laser. The various design alternative s of the coded PPM link, and their effect on the communication and LPI performance, are examined. It is shown that the use of optical pulse power spreading via multiple frame symbol coding is vital for combatin g the interceptor performance. This can be achieved by transmitting me ssage symbols as repeated PPM symbols using a laser pulse of sufficien tly low peak power so that the detected pulse energy is much less than the detected background energy. The possibility of both wideband and narrowband radiometers by the interceptor further limits the communica tors code design choices. Some equations defining effective LPI gain p arameters for the communication link are developed.