International Journal of Antennas and Propagation

Volume 2017 (2017), Article ID 2754831, 8 pages

https://doi.org/10.1155/2017/2754831

## Ocean Surface Current Observation with a Dual Monopole-Cross-Loop Antenna Array

School of Electronic Information, Wuhan University, Wuhan 430072, China

Correspondence should be addressed to Hao Zhou

Received 21 July 2017; Revised 7 November 2017; Accepted 22 November 2017; Published 18 December 2017

Academic Editor: Amerigo Capria

Copyright © 2017 Yeping Lai 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

The high-frequency radars (HFRs) receiving the sea echoes backscattered from the fluctuating ocean surface to remotely sense ocean surface currents are a popular and powerful tool in oceanic observation. Dominant error source in current measurement for HFR systems has been recognized to be the direction of arrival (DOA) determination of the sea echoes. To eliminate this error and therefore improve the performance of direction-finding HFR system in current measurement, we have investigated a dual monopole-cross-loop (MCL) antenna array in current observation. Simulations indicated that the dual MCL antenna array has a better performance than the conventional single MCL antenna system in current mapping, especially for the complex current profile. And comparisons of radar field data and buoy measurements suggested that the *RMSE* value was larger than 15 cm/s for the conventional MCL antenna. But it decreased to 12.64 cm/s for the dual MCL antenna array. Moreover, the temporal coverage rate also showed the benefit of using this antenna system in current mapping. The results demonstrated that it is advisable to adopt the dual MCL antenna array in operational applications.

#### 1. Introduction

The HFRs operating at a frequency range of 3 MHz to 30 MHz have been extensively used to provide ocean surface current in real time [1]. These radars may sense current velocity up to a range of 300 km from the shore relying on the parameters of the radar configuration. And the data products derived from these radars can be used in many fields, including oceanographic and meteorological researches, monitoring the dispersal of pollution and other floating objects, as well as coastal and harbor management.

The current observation HFRs can be roughly divided into two types based on the method employed to determine the bearing of the radial currents: beam forming (e.g., WERA [2]) and direction finding (e.g., CODAR [3] and OSMAR-S [4, 5]). Beam forming radars electronically steer a linear phased array of receiving antennas toward a patch of the ocean surface. This type of radar can provide an excellent angular resolution to separate the sea echoes scattered from different patch efficiently but with a cost of occupying a large space in practice, while the direction-finding method is usually adopted by transportable radar systems, which are equipped with a MCL antenna comprising one monopole and two loops [6]. These radars exploit the directional properties of the conventional MCL antenna to determine bearing using the multiple signal classification (MUSIC) [7] algorithm. Because of the small size, this type of HFR has been widely used across the world.

Many studies of HFR surface current measurements have validated the capacity of the direction-finding HFR to remote sensing the ocean surface currents through comparisons with in situ current measurements, such as [8–12]. These studies demonstrated that there is a 7–20 cm/s differences between the current measurements derived from the direction-finding HFRs and those from the in situ instrument. And the DOA determination error is the dominant contributor to these differences.

To alleviate the DOA determination error and improve the performance of direction-finding HFR system in current measuring, a dual MCL antenna array, composed of two MCL elements, was investigated in this study. Because of the special structure of this antenna system, only the MUSIC direction-finding algorithm can be used to determine the DOA of the sea echoes. Thus, the relationship between the DOA estimation performance and the relative position of the two MCL elements was investigated, and we found that the spacing between the two antenna elements is not limited to the conventional half-wavelength condition due to the amplitude directional properties of the MCL element. And the simulation results suggested that the dual MCL antenna array has a better performance than the conventional single MCL antenna in current measuring, especially for the complex current pattern. Moreover, this performance improvement of the dual MCL antenna array relative to the conventional MCL antenna is also confirmed by the field experiment.

#### 2. Dual Monopole-Cross-Loop Antenna Array

A MCL antenna array is composed of multiple MCL antenna elements. And a MCL antenna element consists of a monopole and two orthogonal loops. The antenna pattern of the MCL antenna element can be expressed as where , , and represent the monopole and the two loops. Thus, the steering vector for signal coming from the direction of is On the other hand, the steering vector for uniform liner array consisting of identical omnidirectional element with a spacing of can be given as where is the number of antenna elements; is the phase shift for adjacent elements; and is the wavelength. Actually, the MCL antenna array is a synthesis of the MCL antenna and the linear antenna array. Thus, the steering vector of this hybrid array can be written as

For a dual MCL array, the steering vector can be reduced to

The amplitude directional properties denoted by in (3) and (4) result in the difference between MCL array and the conventional uniform linear array. In conventional uniform linear array, the spacing, , must be no more than half wavelength. If the spacing goes against this criterion, there will be ambiguity in DOA determination, because the condition leading to ambiguity is with and that is, with being an integer. Thus, the ambiguity condition is equivalent to where is an integer. And it is straightforward to rewrite (5) as

If the absolute value of the right side in (6) is larger than 0.5, being smaller than 0.5 will result in no solutions for and in (6), that is, no ambiguity in DOA determination. On the contrary, if is more than 0.5, the ambiguity will appear. Therefore, the spacing of adjacent element in conventional uniform linear array has to be no more than half wavelength for avoiding ambiguity in DOA estimation. But this is not the case for the MCL array. The ambiguity condition for MCL array is which is equivalent to simultaneously satisfy the following:

Clearly, there is no and satisfying (7) due to the presence of so there is no ambiguity in DOA estimation even for arbitrary spacing of the adjacent elements and for 360-degree look angle space.

But the performance of direction-finding algorithm is always related to the configuration of the antenna array, so that investigation of the effects of the number of elements and the spacing of the MCL array on DOA estimation in terms of MUSIC direction-finding algorithm is significant. In fact, Stoica and Nehorai [13] have proven that the estimated DOA, in MUSIC for arbitrary antenna system is a Gaussian distribution with a mean value being equal to the actual DOA, , and the variance given by where is the actual DOA of the incident signal; is the steering vector; is the number of samples; SNR is signal-to-noise ratio; and with Thus, in the case of a MCL array composed of identical MCL elements with a uniform spacing of , we have and which gives where Figure 1 shows the DOA estimation error varying with the number of the MCL elements. The simulation results shown in this figure are achieved by a Monte-Carlo simulation of 300 independent runs with 50 snapshots for each trial, while the theoretical results are directly calculated by (9) with the same parameters used in the simulation. To make the unit of the results obtained from (9) being the same as the unit of the DOA, the square root of the the variance (standard deviation), which is equal to the root-mean-square error due to the mean value of the DOA estimation error being zero, is used in Figure 1. These results displayed in Figure 1 indicate that the DOA estimation error decreases with the antenna element increases expectedly. Besides, the rate of the decrease gradually slows down and, eventually, the DOA estimation accuracy levels off at almost the same level for different signal-to-noise ratio (SNR). Taking the occupation of space of an antenna system into consideration, we suggest that an MCL antenna array composed of two or three MCL elements is an optimal scheme in practice. But in this study, we only focus on dual MCL antenna array composed of only two MCL elements.