Identifying Broadband Radio Technosignatures (BRATS) in deep, wide-field radio surveys.

The search for extraterrestrial intelligence (SETI) has traditionally focused on detecting powerful, narrowband electromagnetic beacons. However, terrestrial technology is increasingly evolving toward low-power, distributed, wideband digital infrastructure. The strict adherence to narrowband filtering that characterises most SETI surveys, risks discarding the aggregate leakage signatures of advanced civilisations by systematically misclassifying them as unstructured noise. We investigate the feasibility of detecting such planetary-scale broadband radio technosignatures (BRaTs) using a hierarchical observational framework. In this tiered approach, wide-field radio surveys conducted by next-generation arrays (such as the SKA and its precursors) perform the initial deep-field observations, with targeted Very Long Baseline Interferometry (VLBI) providing high-resolution follow-up. Because broadband continuum emission is largely insensitive to Doppler drift, long-duration "SETI Deep Fields'' are observationally viable, extending the accessible detection volume for Kardashev Type I leakage to ~100 pc. To distinguish these signals from other astrophysical confounders (of which there are many!), a multi-wavelength, multi-parameter diagnostic framework is presented. Candidate technosignatures are identified through a convergence of high brightness temperatures, negligible circular polarisation, spectral non-uniformity, interstellar scintillation, and (probable) sub-milliarcsecond astrometric co-motion with nearby Galactic stars/exoplanets.