What is the two-channel interferometer for coarse direction finding in an ESM system?

The two-channel interferometer for coarse direction finding in an ESM system measures the angle of arrival (AoA) of an intercepted radar signal by comparing the phase difference between two spatially separated antennas. Two antennas (typically identical wideband antennas such as spirals or Vivaldi elements) are separated by a distance d (the baseline). A plane wave arriving at angle theta relative to the array broadside creates a path length difference: delta_L = d × sin(theta). This path length difference produces a phase difference between the two received signals: delta_phi = (2 × pi × d × sin(theta)) / lambda = (2 × pi × f × d × sin(theta)) / c. By measuring delta_phi (from the phase difference between the two coherent receiver channels) and knowing f and d: theta = arcsin(delta_phi × c / (2 × pi × f × d)). For unambiguous measurement: the baseline d must satisfy d < lambda/2 at the highest frequency (to avoid phase wrapping). At 18 GHz: lambda/2 = 8.3 mm (very short baseline, low accuracy). For higher accuracy: use a longer baseline (d >> lambda) but accept the ambiguity (multiple possible angles for a given phase). Resolve the ambiguity using: multiple baselines (a short baseline for coarse, unambiguous AoA, and a longer baseline for fine accuracy; the coarse measurement resolves the ambiguity of the fine measurement), or amplitude information (amplitude comparison between the two antennas helps resolve the ambiguity). AoA accuracy: sigma_theta approximately lambda / (2 × pi × d × cos(theta) × sqrt(2 × SNR)). For d = 10 cm, f = 10 GHz (lambda = 3 cm), SNR = 20 dB (100): sigma_theta approximately 0.03/(2 × pi × 0.1 × 1 × 10) = 0.005 rad = 0.27 degrees.