How do I design a multi-channel direction finding receiver using synchronous SDR modules?

Designing a multi-channel direction finding (DF) receiver using synchronous SDR modules requires a coherent multi-channel receive system where multiple SDR channels simultaneously digitize the same signal arriving at different antenna elements of a calibrated array, and the inter-channel phase differences are used to estimate the signal's angle of arrival (AoA). The system consists of: a calibrated antenna array (2-16 elements in circular, linear, or L-shaped configuration, with element spacing typically lambda/2 at the highest operating frequency), a multi-channel coherent SDR (multiple synchronized receivers sharing a common clock and reference, such as 4x Ettus USRP N210 with MIMO cables, or an integrated multi-channel platform like KerberosSDR or the Ettus USRP N310 with 4 phase-coherent channels), digital signal processing for AoA estimation (correlating the signals across channels and applying direction finding algorithms such as phase interferometry, MUSIC, ESPRIT, or Capon beamforming), and a calibration procedure (measuring the array response to known-direction signals to create a correction table that accounts for cable length differences, channel gain/phase mismatches, and antenna element mutual coupling). The fundamental measurement is the phase difference between channels: for a plane wave arriving at angle theta to a two-element array with spacing d, the phase difference is delta_phi = 2 pi d sin(theta) / lambda.