Mitigation of Range Ambiguity via Encrypted Radar Pulse Coding

The research presented here investigates the use of pseudo-random binary phase shift keying (BPSK) in radar as a modulation method that reduces and possibly eliminates target range ambiguity. AES-192, a well known encryption algorithm, is used to generate the pseudo-random bits used for the baseband BPSK encryption (E-BPSK) sequence. It was found that the matched filter response to targets resulting from E-BPSK modulated signals have a single large and very narrow target main-lobe due to its non-cyclic properties. However, this method does produce undesirable side-lobes that are periodic and require additional signal processing to be removed. A CLEAN type algorithm is used to to eliminate these ambiguous side-lobes from the matched filter response. Prior work in this area has focused on mitigating range ambiguity by using an Ipatov-Barker Hybrid BPSK sequence (H-BPSK). The HBPSK sequencing effectively extended the unambiguous range by the Ipatov sequence number times the normal unambiguous range. The results found in this research are compared to that found using the H-PBSK sequences with identical transmission parameters (bandwidth, PRF, duty cycle, and CPI). It was found that prior to the use of the CLEAN algorithm, the Ipatov-Barker sequences had better target discrimination than the E-BPSK sequences. However, the Ipatov-Barker sequences only extended the maximum unambiguous range by a factor proportional to the Ipatov sequence length. The Ipatov-Barker sequences also require a mismatched matched filter for processing specific length data signals. When the CLEAN algorithm is used as the matched filter pre-processor for the E-BPSK sequence, there is no unambiguous range since the signaling is not periodic. The E-BPSK modulation could effectively be used for radar using very high PRFs and/or long CPIs in noisy environments. It should also be noted that using the pseudorandom E-BPSK signal is secure and not predictable when compared to the periodic H-BPSK sequence.