A low probability of intercept (LPI), or low probability of detection (LPD) communication technique known as cyclic code shift keying (CCSK) is described. We discuss the basic concepts of CCSK and describe a system based on the use of random or pseudorandom codes for biphase modulation. We use simulation to show that the bit error rate (BER) for CCSK can be closely estimated by using existing equations that apply to M-ary orthogonal signaling (MOS). Also, we show that significantly fewer computations are required for CCSK than for MOS when the number of bits per symbol is the same. We show that using biphase modulation results in waveforms that have a large time-bandwidth product and very low input signal-to-noise ratio (SNR) and thus inherently have an LPI by a radiometer. We evaluate detection by a radiometer and show that LPI can be achieved by using codes of lengths greater than about 2 12 (i.e., by transmitting more than about 12 bits per symbol). Results illustrate the effect that the CCSK symbol length and error probability, and the radiometer integration time and probability of false alarm (PFA), have on detection by a radiometer. We describe a variation of CCSK called truncated CCSK (TCCSK). In this system, the code of length 2 k is cyclically shifted, then truncated and transmitted. Although shortened, the truncated code still represents k bits of information, thus leading to an increased data rate. We evaluate radiometer detection of TCCSK and it is shown that the probability of detection is increased compared with the detection of CCSK.

Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. A. A. Beex Virginia TechApproved for public release; distribution is unlimited. SSC San DiegoSan Diego, CA 92152-5001 Executive SummaryThis document describes a specular multipath simulation for antennas located close to a sea surface. The antennas are circularly polarized but the ray-trace implementation readily accommodates any other polarizations. The transmitted signal s T (t) arrives at the receiver's antenna along the direct path and from multiple re°ections o® the sea surface. The surface-level antenna turns the multipath into multiplicative noise|assuming the receiver is narrow band. Under this narrow-band assumption, the received signal s R (t) is approximated by modulating the transmitted signal s T (t) with this multiplicative noise:The multiplicative noise fa(t)g is determined by the sea surface, the elevation angle of the transmitted signal path, and the receiver's antenna. By sweeping over various realizations of the sea surfaces, elevation angles, and various antenna heights and speeds, the systems engineer can estimate sea-surface specular multipath e®ects on the surface-to-satellite link.iii

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