Antenna Array Beamforming for Low Probability of Intercept Radars
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A radar system's focus on low probability of intercept (LPI) performance has become increasingly important as systems designed for electronic support measures (ESM) and electronic counter measures (ECM) continue to become more prevalent. Due to the inherent two-way versus one-way propagation loss of a transmitted signal, radar systems are often highly visible to intercept receivers, and thus have a high probability of detection. A novel transmit array beamforming approach has been introduced that offers significant LPI performance gains for radar systems using a one-dimensional phased antenna array. This method replaces the traditional high-gain scanned beam with a set of low-gain, spoiled beams scanned across the same observation area. A weighted summation of these spoiled beams can result in a return equivalent to that of the traditional high-gain pattern. As a result, the antenna performance of the radar system remains unchanged while the peak gain of the transmitted signal is reduced considerably. This LPI technique is expanded for the case of a two-dimensional antenna array. With this added dimension, the computational complexity of the method is increased, as the pattern now changes with respect to both theta and phi. Simulation results show that the developed technique is still applicable for a two-dimensional array. A carefully calculated set of complex coefficients can be applied across the set of low-gain basis patterns, which are simply the high-gain patterns spoiled by a certain phase shift, in a weighted summation. The results of this summation can be shown to provide nearly identical returns when compared to that of a traditional high-gain single beam scanned across the observation area. The high-gain transient power is replaced by lower power signals with an increased integration time, resulting in the same total energy on the target, and thus the same detection performance. The simulation results show that the intercept area, the area in which a hostile intercept receiver can detect the transmitted signal, can be reduced significantly due to the low gain of the transmitted spoiled patterns. For example, the intercept area is reduced by as much as 96 in the case of a 32x32 element array. The LPI benefits of this technique - significantly reducing the range at which a hostile receiver can intercept the radar beam while maintaining the range at which the radar can detect the target - are of obvious benefit in the ongoing battle of electronic warfare.