**Preface to "Bistatic HF Radar"**

The proliferation of HF radar systems for ocean remote sensing and maritime surveillance continues apace, with hundreds of such radars now deployed around the world. The overwhelming majority of these radars operate in the conventional monostatic configuration, with the transmitting and receiving systems collocated or closely spaced (the term "quasi-monostatic" is often used in this case). This simple geometry has obvious advantages in terms of cost, siting requirements, communications, maintenance, signal processing, and echo interpretation, and it has been adopted by HF radars exploiting line-of-sight, surface wave, and skywave propagation modalities.

All these considerations notwithstanding, in some circumstances there can be compelling reasons to implement bistatic configurations, defined as geometries in which the separation between transmitter and receiver is comparable with the range to the zones being interrogated. Factors that can drive this decision include energy budget, desire to exploit hybrid propagation modes, scattering characteristics of the targets of interest, properties of the clutter, survivability, and covertness.

While the literature on the design and application of monostatic HF radars continues to thrive, the same does not hold for the literature on bistatic configurations. Motivated by our desire to expand the palette of missions that can be addressed by HF radar, especially some that cannot be addressed by monostatic radars, we have compiled this Special Issue of Remote Sensing.

The issue contains nine papers, embracing contributions from authors in a dozen centers of HF radar research in Australia, Canada, China, the UK, and the USA. The opening paper, by Anderson, catalogs the many possible bistatic configurations according to the propagation modes involved and describes a number of radar missions where the bistatic geometry yields enhanced radar capability. Next, there are three papers dealing with generalizations of well-known perturbation-theoretic methods of HF scatter from the sea surface. Chen et al. treat the case of signals incident at grazing incidence from a shore-based transmitter and scattered upwards to be received by an airborne receiver; they compute the spectra to second order. Yao et al. consider the situation where the radar transmitter is mounted on a floating platform subject to motion with 6 degrees of freedom and explore different options for receiver placement and the resulting impact on the echo spectral structure. Silva et al. address the problem of high sea states, where the standard perturbation-theoretic models break down, and derive expressions for the modified first-order spectrum under various conditions.

The following paper, by Hardman et al., deals with the inverse problem of estimating the directional wave spectrum from the HF radar Doppler spectrum. They generalize the Seaview monostatic inversion method to handle bistatic geometries and assess its performance on simulated data.

While remote sensing of ocean currents and sea state is often the primary mission of HF radar, ship detection and tracking are of increasing interest, and the next three papers focus on this surveillance mission. Ji et al. examine the effects of ship motion on the bistatic first-order clutter returns. They develop the relevant theory and present simulated results for various configurations, then support the modeling with measurements carried out with two radars, one mounted on a cooperating vessel. Next, Sun et al. describe a newly-developed multistatic HFSWR, one with a single transmitter but two receiving stations, and demonstrate the improved tracking performance that can be achieved with such a configuration. This immediately raises the question of a reciprocal design, one with multiple transmitters and a single receiver. Liu et al. explore this concept in their paper, reporting a passive radar system that uses multiple GPS satellites as illuminators. Although this system operates in a much higher frequency band than HFSWR, it serves to illustrate some of the problems that arise when multiple transmitted signals need to be separated and processed at the receiving station; we anticipate that equivalent problems would arise with an analogous HFSWR configuration. Finally, Zhang et al. point out that, in practice, ship tracking is far from straightforward, with track fracture arising from a combination of many factors, including highly maneuverable vessels, dense channels, target occlusion, strong clutter/interference, long sampling intervals, and low detection probabilities. They describe a sophisticated tracking technique—an interacting multiple model extended Kalman filter combined with a machine learning architecture—and demonstrate its efficacy using real data from a stereoscopic HFSWR system.

The diversity of HF radar configurations represented in this Special Issue does not exhaust all the possibilities, as cataloged in the taxonomy shown in the first paper, but the impressive variety of bistatic HF radar systems now in operation, and the special capabilities that they offer, will ensure their continuing proliferation and the development of new concepts and missions.

> **Stuart Anderson** *Editor*
