**1. Introduction**

Remote sensing of our geophysical environment by means of radio waves in the HF band is now a truly global activity, with decametric radars operating in scores of countries, and on every continent [1]. In a number of instances, international collaborations facilitate the integration of the outputs from individual radars to yield regional or even basin-scale products, thereby increasing the quality, diversity, and utility of the derived information [2].

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 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, often 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. Bistatic HF radars with very specific missions have been deployed since the 1960s, predominantly in defense applications, but the convenience of monostatic designs and the adequacy of their standard remote sensing products have tended to discourage wider adoption of bistatic configurations.

Once we allow for the separation of transmit and receive facilities, many possible configurations emerge. Each of these subsystems can be located on land, at sea, in the air, or even in space, with a range of propagation mode combinations possible for the signal paths from transmitter to target and target to receiver. Of these, line-of-sight, ground wave (we shall use the term *surface wave* throughout this paper, though strictly it refers to only one component of the total field−the dominant one at over-the-horizon ranges), and skywave modes are by far the most common, though more exotic propagation mechanisms have been explored. Figure 1 presents a taxonomy of the main configurations; those that are understood to have been implemented, or at least reached the advanced design and experimentation phase [3], are indicated by the colored dots (E. Lyon, personal communication, May 19, 2015).


**Figure 1.** A taxonomy of HF radar configurations. The conventional monostatic surface wave and skywave radars are indicated with blue and green markers, respectively; the topical hybrid sky–surface wave configuration is shown by the magenta marker.

An obvious generalization of these single radar configurations is the deployment of multiple radars to interrogate a common area of interest. This is the standard modus operandi of current mapping HF surface wave radars (HFSWR) such as the CODAR SeaSonde [4] and the Helzel Messtechnik WERA [5], where two or more measurements of radial velocity are combined to yield a resultant vector. We note that measurements from these two distinct radar designs—based on direction-finding and beam-forming, respectively—can be combined to expand network coverage and reduce down-time [6]. Skywave radar networks with overlapping coverage have been operational in Australia (JORN) [7] and the United States (ROTHR) [8] for decades; not surprisingly, there are many issues to be taken into account when designing such configurations [9,10]. The term stereoscopic has been used to describe these multi-monostatic configurations; other applications include ship target dynamic signature analysis and excitation of nonlinear scattering mechanisms.

Another generalisation is the use of relay stations; that is, combined receive–transmit facilities that acquire the signal radiated by the primary radar transmitter, amplify it, possibly with additional modulation, and then reradiate it, thereby extending the range of the system or facilitating other radar functions.

This diverse array of system geometries offers many opportunities for remote sensing. In particular, the ability to extend the range of Bragg resonant scattering to lower wavenumbers opens the way to observing some environmental phenomena to which monostatic radars are insensitive. One example of this is the determination of sea ice parameters. Short sea waves are rapidly attenuated as they enter the marginal ice zone; only long waves penetrate to useful distances into the ice field. The sea ice

properties are encoded in the radar Doppler spectrum, most visibly in the first-order peaks [11]. For a monostatic radar to observe these peaks, it would need to operate at a very low frequency, below those employed by present-day HF radars, but a bistatic geometry enables the returns from longer waves to be measured. Another example is the investigation of the physics of the ionosphere via analysis of impressed phase modulation [12,13], wavefront distortion [14], and polarization transformation [15] of oblique (bistatic) radar reflections; these are largely inaccessible to monostatic radars.

In this paper, we explore many of the issues that arise with bistatic HF radar configurations, basing our analysis on the formal radar process model presented in the following section. After examining the implications for the component elements of the radar observation process, we proceed to describe some specific radar missions that benefit from the physics of bistatic scattering and/or hybrid propagation modes. The term hybrid is often applied to configurations where the outbound and inbound propagation modalities are different; that is, they lie off the diagonals in the boxes of Figure 1. Along the way, we describe and illustrate some practical techniques that can serve as a guide to bistatic HF radar design and deployment. In particular, we look at the problem of site selection, a challenge that is compounded by the need to address multiple radar missions.
