GSM frequency scanner: sweeps a band (GSM850/900/1800/1900) for active base-station carriers by locking onto their FCCH/SCH bursts, reporting each ARFCN, its power and the radio's clock-frequency offset (ppm). The kalibrate-rtl fork drives an RTL-SDR; sibling forks exist for HackRF and UHD. The first step before any capture: it tells you which ARFCNs have a live BTS.