Abstract-Low probability of intercept (LPI) communication generally relies on the presence of noise to obfuscate a covert signal through the use of spectral spreading or hopping. In contrast, this paper addresses the use of ambient interference from other man-made emissions as a means to mask the presence of covert communication. Specifically, the high power, wide bandwidth, and repeating structure of pulsed radar systems provide an advantageous framework within which to embed a communication signal. The operating paradigm considered here is that of an RF tag/transponder that is illuminated by the radar and intends to covertly communicate with the radar or some other desired receiver while being masked by the ambient radar backscatter to avoid detection by an intercept receiver. Communication takes place on an intra-pulse (or individual pulse) basis to maximize the data rate. The impact of multipath, and its exploitation using time reversal to achieve spatio-temporal focusing, is considered. The processing gain for the destination receiver and intercept receiver are derived analytically and subsequently used to optimize the parameterization of communication symbol design.

This paper examines some of the practical aspects of waveform design for intra-pulse radarembedded communications.Specifically, the generation of communication waveforms at the tag is considered for the case when forward scatter (i.e. multipath) is present thus altering the received version of the radar waveform at the tag and potentially producing a mismatch between the tag and desired receiver. Also, because it may be difficult to precisely determine the exact temporal length of the incident radar waveform (especially when multipath is present), the communication waveform design process may employ a larger than necessary temporal extent to ensure the incident waveform is completely captured. The impact of this temporal expansion of the communication waveforms is considered where it has been found to actually improve performance (in the absence of multipath). As such a rule-of-thumb for the total expanded time-bandwidth product is determined as a function of observed symbol error rate performance.

Abstract-LORAN-C is being examined as a viable backup to GPS for timing and navigation. Accuracy from the LORAN-C network has always been limited by variations in