Space weather monitoring is essential for characterizing and mitigating the effects of solar-driven geomagnetic disturbances which can disrupt critical infrastructure such as power grids, satellite navigation, and communication systems. These applications demand magnetometers that can deliver high sensitivity, wide bandwidth, and excellent long-term stability. We report a portable optically pumped magnetometer (OPM) operating in the free-induction-decay (FID) modality, providing absolute magnetic field measurements in ambient environments. The system features a compact, fully fiber-coupled design that employs a rapid fiber-optic switch to implement a dual-beam pump–probe scheme using a single laser source. Real-time data acquisition and signal processing are executed via a Python-based pipeline utilizing a Hilbert transform method for fast and precise frequency estimation. OPM performance was validated in controlled laboratory conditions, and in field trials at Eskdalemuir Geophysical Observatory (Scotland), including long-term comparisons with commercial fluxgate and proton precession magnetometers (PPMs) operated by the British Geological Survey (BGS). The two systems showed excellent agreement, with the Allan deviation reaching below 10 pT at 10 s integration and plateauing between 100–200 pT over long timescales. These results demonstrate the potential of FID-based OPMs as calibration-free, high-performance alternatives for next-generation space weather monitoring networks.