What is the Watson-Watt direction finding technique and how does it work?

The Watson-Watt technique is an amplitude-comparison direction finding method that uses two orthogonal antenna pairs (or loops) and an omnidirectional reference antenna to determine the bearing of an incoming signal in a single measurement: (1) Antenna configuration: two crossed loop antennas (or Adcock pairs), oriented at 90° to each other (North-South and East-West). One omnidirectional antenna (vertical whip or monopole) as a reference. Each loop produces a cosine-shaped response: loop 1 (N-S) output: V_NS = V_0 × cos(θ), where θ is measured from North. Loop 2 (E-W) output: V_EW = V_0 × sin(θ). The omnidirectional antenna output: V_omni = V_0 (same amplitude, independent of direction). (2) Bearing calculation: the bearing is computed from the ratio of the two loop outputs: θ = arctan(V_EW / V_NS). The omnidirectional reference resolves the 180° ambiguity (the loop outputs have two nulls per revolution; the reference determines which semicircle the signal is in). Alternatively: use the sign of the reference signal relative to the loops to resolve the ambiguity. (3) Advantages: instantaneous measurement (all three signals are measured simultaneously; no scanning or rotation needed), simple processing (one arctangent calculation), works well at HF and VHF frequencies (where loop antennas are practical and compact), and no phase-coherent receivers needed (only amplitude ratios are required). (4) Disadvantages: accuracy limited to 2-10° RMS (due to antenna pattern imperfections, multipath, and site effects), requires accurate orthogonality between the two loop pairs (any angular error between the loops directly biases the bearing), susceptible to vertically-polarized signals coupling into the loops differently than horizontally-polarized signals, and not suitable for very high frequencies (> 1 GHz) because loop antenna performance degrades.