9 Direction Finding Techniques for HF Applications

The bounds described in the previous chapter provide measures of the direction-finding performance that can be expected from an array with a specified geometry and size, at the frequency associated with a particular signal. This chapter covers many of the important methods for direction finding; that is, estimating line of bearing (LOB). Recall from chapter 7 that the arrays of concern are small relative to the propagation distances involved. This observation motivates the view that a single plane wave reaching an array excites all elements with the same amplitude, and if the array elements are not identical, the responses to this excitation will differ. If only one signal is present, all array elements are identical, and element-to-element mutual coupling is modest, phase provides the only clue to signal direction; if the array elements differ in their gain response as a function of arrival angle, then relative gain may also provide clues to the signal direction. One should also note that most receivers perturb the element signals with additive noise, usually with zero mean and Gaussian statistics. This observation informs the analysis presented here, providing a tractable model for estimating an LOB. Finally, it is worth noting that this topic has been the focus of many theoretical and experimental research efforts, leading to the rich discourse of the current chapter.