Bistatic HF Radar for Coastal Ocean Remote Sensing: System Implementation and Validation in Taiwan

Bistatic configurations are increasingly important for extending observation geometry in coastal HF radar systems. In this study, we implement a bistatic HF radar testbed in Taoyuan, Taiwan, based on the open-source Generic High Frequency Doppler Radar architecture developed by the University of Hawai‘i. The site supports both monostatic and bistatic operations, enabling direct, real-time comparisons under identical environmental conditions.

Monostatic HF radar observations are inherently constrained by viewing geometry and are further limited in practice by hardware redundancy requirements, site availability, and electromagnetic interference. Bistatic configurations provide an effective means to expand observation geometry and spatial coverage. We report practical experience from the deployment of a bistatic HF radar system in Taiwan, with emphasis on cross-site time synchronization and bistatic signal processing.

Accurate time synchronization between transmitting and receiving sites is a critical challenge in bistatic operation. Although monostatic systems typically rely on temperature-compensated crystal oscillators (OCXO), operational tests show that residual clock drift can degrade phase coherence in bistatic measurements. To address this issue, two synchronization strategies are implemented: (1) a GPS-disciplined oscillator (GPSDO) with pulse-per-second (PPS) signals and DDS-based phase-lock feedback to achieve progressive convergence toward a target timing accuracy, and (2) highly stable atomic clocks combined with PPS calibration to ensure long-term timing stability during continuous operation.

On the processing side, the monostatic framework is extended to bistatic geometry. Following established bistatic scattering theory, the inversion procedure includes scattering-point localization using elliptical geometry, formulation of bistatic Bragg frequency relationships, and estimation of velocity components. A bistatic current inversion scheme is further developed to enable cross-validation between monostatic and bistatic measurements and to synthesize vector surface current fields.

Overall, this work demonstrates the feasibility of bistatic HF radar systems for overcoming key limitations of monostatic observations. The presented hardware synchronization strategies and processing framework provide a practical foundation for future multi-station collaboration, system validation, and expanded coastal monitoring applications.