International Journal of Antennas and Propagation

Volume 2017, Article ID 4964267, 5 pages

https://doi.org/10.1155/2017/4964267

## Analysis of Ship RCS Detected by Multifrequency HFGWR

Radar and Signal Processing Laboratory, Electronic Information School, Wuhan University, Wuhan 430072, China

Correspondence should be addressed to Biyang Wen; nc.ude.uhw@newyb

Received 22 January 2017; Revised 28 March 2017; Accepted 2 May 2017; Published 18 May 2017

Academic Editor: Pierfrancesco Lombardo

Copyright © 2017 Ke Sun et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

One of the important applications of high frequency-ground wave radar (HFGWR) is to detect offshore ships. A proper method should be used to obtain the ship radar cross section (RCS), which is a key parameter of the ship. This paper proposes a method based on an automatic information system (AIS). The relationship of the ship RCS versus bearing for different frequencies is analyzed by processing multifrequency HFGWR data. With this new method, bearing information is taken into consideration, which is not the case in traditional empirical formulas. The results provide prior knowledge for ship detection and tracking; therefore, the probability of detection is significantly improved.

#### 1. Introduction

Over-the-horizon radar, HF ground-wave radar (HFGWR), has emerged in the past few decades for both monitoring ocean kinetic parameters (e.g., current, wave, and wind) and detecting moving targets on the ocean surface [1–5]. Such systems are relatively inexpensive and convenient. Vertically polarized HF (3–30 MHz) electromagnetic (EM) waves can propagate over the horizon along the sea surface with low loss and resonate with targets at sea. HFGWR provides over-the-horizon, all-weather, continuous, and real-time detection of a large sea area. The radar cross section (RCS) provides the ability to scatter EM waves transmitted by radar, which is fundamental for target recognition and detection. Targets are generally confined to ships offshore. In field experiments, targets from tens of meters to a hundred meters in size can be detected in the resonance region of the EM waves in the HF band. Small targets can be detected more easily at the upper end of the HF band. However, we need to detect targets that will cover a range of bearings and can be tracked for a long time. Therefore, relatively large targets are of interest in this study.

There is no simple formula applicable for calculating the RCS in the resonance region. The RCS is typically obtained by numerical simulation and experimental measurements. However, due to experimental conditions and cost constraints, it is challenging for HFGWR to obtain a large number of ship RCSs with experimental measurements. Certain numerical simulation methods, for example, the method of moments (MOM) and the finite difference time domain method (FDTD), have been applied to analyze RCSs in the HF bands [6, 7]. Detailed ship information (e.g., structure, material, and size) is required to simplify the ship model. The effect of a rough ocean surface, randomly fluctuating waves, and the interaction of nearby ships should also be taken into consideration. Therefore, these numerical simulation methods are relatively cumbersome [8]. Researchers in related fields have extracted empirical formulas by analyzing a large amount of data. For example, (1) has primarily been used to calculate the RCS in X, S, and L band radar applications [9], and Ponsford applied it to HFGWR [10].where is the operating frequency of the HFGWR (in MHz) and is the full displacement (in kilotons). This equation is a static description of the RCS, which is a constant for a certain ship when is stable.

By analyzing experimental data of HFGWR, Barrick proposed that the target echo is proportional to the sixth power of the height of the mast [11], which remained consistent with the static characteristic of the RCS. The above studies provide rough descriptions of the RCS and fit experimental measurements obtained from specific situations. However, these methods do not address the effect of bearing on the RCS, which is critical in real radar applications. For example, ship echoes may disappear suddenly when the ship changes course.

Because of the limitations of previous RCS studies, a more convenient and comprehensive method should be developed. This paper proposes a method based on experimental HFGWR data with the help of automatic information system (AIS) information. A large number of targets are analyzed, and the relationship of the RCS versus the bearing for a variety of frequencies is obtained.

#### 2. Method of Data Processing

The radar equation for HFGWR iswhere is the range of the target, is the average transmission power, is the receiving power, is the transmitting antenna gain, is the receiving antenna gain, is the RCS, is the Norton attenuation factor, is the EM wavelength, and is the system attenuation factor [12].

When the HFGWR operates in steady state, , , , and can be considered constant. Equation (2) can be expressed aswhere is a constant for a particular radar deployment. is also an invariant when the radar frequency is stable. Thus, can be expressed as

The attenuation factor is related to the frequency, polarization, propagation path, and scattering of rough ocean surfaces, among other factors. Barrick calculated the influence of a rough ocean surface on EM wave propagation attenuation based on EM field theory [13, 14]. The data used in this paper were collected over a short time when the HFGWR had been operating steadily and the sea state changed minimally in beam width. In this case, other factors have minimal influences on except for the distance of EM wave propagation.

Both the echo power of ships and Bragg waves decline when distance increases. In the case of a fully developed sea surface, Barrick proposed that the RCS of Bragg wave echo per unit area is invariant [15] that can be expressed as

Based on this equation, the RCS of the Bragg wave echo at different ranges is invariant. From (3), the power differences of the Bragg wave echo at different ranges are caused by and . Then, the rule of can be obtained. fluctuates significantly when processing different HFGWR data. In actual experimental conditions, the assumption of a fully developed ocean is no longer established; therefore, may fluctuate. An analysis of HFGWR data shows that the difference in echo power is small for one target at a certain bearing in close distance (no more than 2 range bins). In this case, can be considered an invariant. The following studies are based on this type of target, which is defined as a matched target.

The AIS information provides characteristics of offshore ships including location, velocity, and heading. By matching AIS information and HFGWR data, some matched targets can be obtained. The bearings to the radar site of these targets can be calculated with the location of the radar site, the targets, and the heading of the targets (defining the bearing to be 0° when the bow is facing the radar site and 180° when the stern is facing the radar site), as illustrated in Figure 1. The arrow represents the heading of this ship. is calculated from the locations of the radar site and the ship. represents the heading of the ship. The bearing to the radar site of this ship is . This value changes when the ship is positioned at different locations or has a different heading.