*Article* **Measuring the Directional Ocean Spectrum from Simulated Bistatic HF Radar Data**

**Rachael L. Hardman 1,†, Lucy R. Wyatt 1,2,\*,† and Charles C. Engleback <sup>1</sup>**


Received: 19 December 2019; Accepted: 14 January 2020; Published: 18 January 2020

**Abstract:** HF radars are becoming important components of coastal operational monitoring systems particularly for currents and mostly using monostatic radar systems where the transmit and receive antennas are colocated. A bistatic configuration, where the transmit antenna is separated from the receive antennas, offers some advantages and has been used for current measurement. Currents are measured using the Doppler shift from ocean waves which are Bragg-matched to the radio signal. Obtaining a wave measurement is more complicated. In this paper, the theoretical basis for bistatic wave measurement with a phased-array HF radar is reviewed and clarified. Simulations of monostatic and bistatic radar data have been made using wave models and wave spectral data. The Seaview monostatic inversion method for waves, currents and winds has been modified to allow for a bistatic configuration and has been applied to the simulated data for two receive sites. Comparisons of current and wave parameters and of wave spectra are presented. The results are encouraging, although the monostatic results are more accurate. Large bistatic angles seem to reduce the accuracy of the derived oceanographic measurements, although directional spectra match well over most of the frequency range.

**Keywords:** HF radar; remote sensing; inversion; radar cross section; bistatic radar; directional wave spectrum
