SpecSentry: Micro-power Wideband Spectrum Surveillance for Ephemeral Transmissions

Covert radio-frequency exfiltration using short, low-duty-cycle transmissions highlights the need for continuous, low-latency spectrum monitoring capable of capturing fleeting emissions. Modern RF environments increasingly feature bursty, opportunistic signals that appear only for milliseconds, making them difficult for traditional receivers to detect. Conventional wideband sensing architectures depend on high-speed ADCs, local oscillators (LO), and mixer chains that draw significant power, while swept-LO and sequential scanning approaches suffer from limited real-time visibility. Parallel filter-bank designs improve instantaneous coverage but remain bulky and energy-intensive. We present SpecSentry, a low-power spectrum-monitoring architecture that maintains wideband visibility while reducing analog complexity by more than an order of magnitude. SpecSentry implements a "mark-fold-capture-detect" workflow in which the RF front end passively imprints frequency-dependent codes onto any active signal and folds the result into a compact representation sampled at rates more than 20× below the Nyquist rate. A two-stage neural regression pipeline analyzes this compressed snapshot to infer key signal characteristics, achieving median detection errors of 0.3 MHz for the bandwidth and 7.89 MHz for the center-frequency of the transmission over a 400 MHz band. This hybrid analog-ML design enables persistent, wideband, ultra-low-power monitoring suited for mobile, unattended, and maintenance-free deployments where detecting ephemeral transmissions is essential.