6 Vector Models for Adaptive Array Signals

Previous chapters have discussed the general nature of the signals associated with HF communications, indicating that the signals are typically narrowband but numerous; as a result, a desired signal is often simultaneously received with other signals not of interest that interfere with reception of the desired signal. In addition to such interference from other signals, the desired signal propagates from transmitter to receiver by means of ionospheric refraction, perhaps on several different routes, or modes, each with its own delay, frequency shift, and propagation-induced polarization change. When these multiple delayed and frequency-shifted copies of the same signal combine at the receive antenna, reception becomes garbled, perhaps making it unintelligible. This self-interference is a second major issue for HF sensing that can be addressed with adaptive array processing.