UAV and UFB Detection Capability of an L-Band Long-Range Air Surveillance Radar: Geometric and RCS Constraints for LSS Targets

The spread of unmanned aerial vehicles (UAVs) and unmanned free balloons (UFBs) has made ground-based air surveillance more difficult, especially for low, slow, and small (LSS) targets. Such targets often combine low radar cross-section (RCS), low altitude, small radial velocity, and strong coupling to ground clutter. This study provides a focused assessment of the detection constraints expected for a RAT-31DL-type long-range L-band surveillance radar against small UAVs and radiosonde-type light UFB payloads. The work combines simplified RCS estimates, literature-based UAV RCS data, finite element method (FEM) simulation, radar-horizon geometry, elevation-beam intersection analysis, and low-Doppler considerations. Idealized broadside reference RCS values are calculated at 1.5 GHz. Published 26–40 GHz UAV RCS data are used as comparison references and are back-scaled to the L-band to illustrate frequency-scaling uncertainty. A simplified FEM model of a trademark Meteomodem M20 radiosonde is simulated at 1.5 GHz and at 26 GHz for comparison, to examine aspect- and polarization-dependent scattering. The simulated radiosonde cross-polarized RCS values vary from approximately −36.49 to −23.45 dBsm at 1.5 GHz. For a 30 m radar installation and 60–140 m target altitudes, the smooth-Earth horizon-limited visibility range is approximately 55–71 km. Low-altitude coverage can be further limited by positive-elevation beam geometry. Taken together, the results indicate that LSS detectability is strongly scenario-dependent and is governed by RCS variability, geometric visibility, clutter, Doppler behavior, and radar-specific processing choices.