Doppler spectra GPS HF radar HFSWR History of engineering & technology OTH radar bistatic HF radar bistatic HFSWR bistatic configuration bistatic radar compact HFSWR cyclic cross ambiguity function data fusion directional wave spectrum electromagnetic scattering experiment verification extended Kalman filter extreme learning machine first-order sea clutter interacting multiple model inversion multiple satellites collaboration passive detection radar cross section radar cross section (RCS) radio oceanography remote sensing scattering coefficient sea clutter shipborne HFSWR