Skywave Radar Research Articles

The sky-wave radar detection performance computing based on the dynamic ionospheric model

The time-varying, dispersion and non-stationary ionospheric characteristics cause the sky-wave radar echo spectra broaden and the layer-structure characteristics make the multimode and multipath propagation occur, which seriously affect the detection performance of the OTHR. Therefore, in this paper, using the ionospheric model provided by CCIR as reference and the data from the Hainan ionosonde, a dynamic ionospheric model is proposed to assess the detection performance of the sky-wave radar. We will study the detection performance of the sky-wave radar by the quality of the propagation path from three aspects. Firstly, the coverage of the sky-wave radar under the given ionospheric parameters and the working frequency is calculated. Secondly, whether the multimode phenomenon appeared in the propagation is considered. Thirdly, whether the multipath phenomenon occurred is studied. Then a program is developed to assess the radar detection capability based on the propagation path analysis, which can provide the working parameters for the sky-wave radar.

Modified Ocean Clutter Cancellation Algorithm in Sky-Wave Radar

In sky-wave over-the-horizon radar system, it is difficult to detect ship because the Doppler shift of the ship is very close to the strong ocean clutter. Traditional clutter cancellation algorithms suppress ocean clutter by estimating clutter parameters and removing the clutter signal in time domain. However, they estimate the clutter parameters by the maximum value of the post DFT spectrum and thus lead to the low accuracy. In this paper, a modified clutter cancellation algorithm is proposed, which corrects estimated frequency by utilizing the three maximum spectrum values and overcomes the poor cancellation performance caused by inaccurate estimation of clutter parameters. Compared with traditional clutter cancellation algorithm, the proposed algorithm can get more accurate clutter parameters and reduce the energy and expansion of the remaining clutter. Simulation demonstrated that the proposed algorithm had a better clutter cancellation performance compared to the traditional ones.

Time–frequency method to reduce contamination by ionospheric fast frequency modulation for HF skywave radars

The unstable and time-varying property of the ionosphere causes a frequency modulation on the high frequency sky-wave radar echo signals. When the modulation period is of 10 s or smaller, it is called the fast phase-path modulation. This sort of modulation will result in the broadening, the splitting and even the overlapping of the spectra, which makes target detection and sea-state remote sensing hard. Hence, it is necessary to decontaminate the echo signals before the coherent integration process. In this study, the pseudo Wigner-Ville distribution is proposed as a means of estimating the ionospheric frequency modulation. A frequency weighting method is used to obtain the instantaneous frequency (IF) from the time–frequency distribution and an adaptive low-pass filter is introduced to purify the IF. Then, the amplitude difference between the two first-order Bragg peaks is adopted in the weighting process to obtain the phase compensation function. To improve the compensation effect, an optimising process is proposed. The simulation results show that the method can improve the quality of the spectra for the target detection and remote sensing efficiently even in the cases of a high sea-state.

Skywave Radar Altitude Estimation Algorithm Based on Matched-field Processing

摘要: 天波超视距雷达通常采用距离-多普勒处理来检测海面上的目标。距离-多普勒处理可以获得目标的斜距和多普勒频率，而不能获得目标的高度信息。该文提出一种基于匹配场处理的算法来估计目标高度，算法利用单个驻留周期的回波数据来进行估计。仿真表明，在给定的仿真场景下，目标高度估计精度可达2000 m左右。与已提出的对数似然函数算法相比，该算法具有更好的稳健性。 关键词: 天波超视距雷达 / 高度估计 / 匹配场处理

A Cascaded Approach for Correcting Ionospheric Contamination with Large Amplitude in HF Skywave Radars

Ionospheric phase perturbation with large amplitude causes broadening sea clutter's Bragg peaks to overlap each other; the performance of traditional decontamination methods about filtering Bragg peak is poor, which greatly limits the detection performance of HF skywave radars. In view of the ionospheric phase perturbation with large amplitude, this paper proposes a cascaded approach based on improved S-method to correct the ionospheric phase contamination. This approach consists of two correction steps. At the first step, a time-frequency distribution method based on improved S-method is adopted and an optimal detection method is designed to obtain a coarse ionospheric modulation estimation from the time-frequency distribution. At the second correction step, based on the phase gradient algorithm (PGA) is exploited to eliminate the residual contamination. Finally, use the measured data to verify the effectiveness of the method. Simulation results show the time-frequency resolution of this method is high and is not affected by the interference of the cross term; ionospheric phase perturbation with large amplitude can be corrected in low signal-to-noise (SNR); such a cascade correction method has a good effect.

Inversion of Sweep Frequency Backscatter Ionogram From Monostatic HF Sky-Wave Radar

The Wuhan Ionospheric Oblique Backscattering Sounding System (WIOBSS) is a monostatic high-frequency sky-wave radar used for ionospheric remote sensing. The sweep frequency backscatter ionogram (SFBI) recorded by the WIOBSS contains both backscatter echo scattered by distant terrestrial surface and vertical incidence (VI) echo reflected by local ionosphere over the sounding station. The approach for SFBI inversion introduced in this letter requires input of leading edge and peak height derived from local VI echo. The final output of this SFBI inversion approach is the 2-D electron density distribution in a vertical plane aligned in the direction of sounding. In addition, the time and geographic variation of foF2 can be obtained from a series of SFBI inversion results. Two experiments have been utilized to validate the SFBI inversion approach, and the SFBI inversion results are found to be very close to the ionosonde data. The fast-converged feature of the approach makes it possible for applications in real time. The success of the SFBI inversion approach enables us to use the WIOBSS as a vehicle-mounted system to obtain ionosphere electron density profile over a large geographic area.

An Evaluation Scheme of Skywave Radar Coverage of Ship Target Based on GRG

An evaluation scheme of skywave radar coverage of ship target is researched. On the basis of fifteen kinds of indexes of the skywave radar coverage, a sort of model of coverage quantitative evaluation with three levels of hiberarchy is established by means of the modified analytical hierarchy process (AHP) based on grey relational grade (GRG), the quantitative calculation method of all level indexes is discussed, and the effectiveness ofthe model is verified. This method solve the problem in AHP effectively. The result shows that the evaluation scheme of skywave radar coverage is feasibility.

Suppression of Sea Clutter of Skywave Radar Based on AR Model

The prediction of chaotic time sequence based on AR model is studied. The forming mechanism of the first-order Bragg lines of sea clutter is analyzed. According to the signal characteristic of sea clutter, a sea clutter time sequence predictor and a skywave radar sea clutter suppression system based on AR model are presented. The cancelling of sea clutter is achieved by subtracting the predicted signal from the original signal. The simulation results show that the first-order Bragg lines in the signal were suppressed effectively after processed by this sea clutter suppression system, the detecting performance of skywave radar was increased dominatively.

A Novel Low-Power Multifunctional Ionospheric Sounding System

An ionosonde is a kind of high-frequency skywave radar developed for ionospheric research and HF channel management. A novel low-power multifunctional ionospheric sounding system, i.e., the Wuhan Ionospheric Comprehensive Sounding System (WICSS), is presented in this paper. Software-defined-radio-based power and low transmitting power are the two most significant features of the system. The system can achieve vertical-incidence, oblique backscatter, and oblique-incidence sounding within low transmitting power. The application of pulse compression and coherent integration allows WICSS to measure ionospheric parameters with high resolution and very low power. The background of ionospheric sounding is introduced. Conventional and modern ionospheric sounding theories are described. The radar waveforms, system structure, and major system parameters are depicted in detail. Typical experimental results are shown at the end of this paper.

A Novel Phase Distortion Correction Method Based on Temporal Correlation Matrix

Since the existence of the ionosphere contamination, results of high-frequency radar has been greatly affected. This paper addresses a new method based on temporal correlation matrix to correct phase distortion for HF sky-wave radars. Strong point echoes, such as target, clutter, transponder and so on, which are highly temporal correlated, can be used to structure temporal correlation matrix. And the matrix contains angles of echoes and phase distortions information. In this paper, we use two-dimension FFT to estimate angle of strong point echo, and then obtain the corrections. Computer simulations are conducted to show the validity of the new method, and some of performances of PGA were compared.

Coordinate Registration Method based on Sea/Land Transitions Identification for Over-the-Horizon Sky-Wave Radar: Numerical Model and Basic Performance Requirements

We propose an approach to the problem of range coordinate registration (CR) for HF band over-the-horizon sky-wave radar (OTHR-SW) signals on a single pulse basis. The approach is based on the a priori knowledge of the displacement of the sea/land transitions within the area illuminated by the radar antenna beam. It takes advantage of the geomorphological structure of the surveillance area, which is exploited to build a surface correlation mask that is in turn used as a geographic reference for the received radar echo. The method is based on the maximization of the cross-correlation between the received radar echo and the surface mask signatures. We describe first an extended numerical model of the whole HF OTHR-SW scenario for simulating the received signal, then we show the results of the application of the method under simplifying operative hypotheses that allow to point out the minimum requirements in terms of received signal-to-noise ratio (SNR) and differential sea/land backscattering coefficient for achieving given accuracies in the range estimates.

Remote sensing applications of HF skywave radar: The Australian experience

Australia has conducted research into over-the-horizon radar (OTHR) for almost sixty years. Early programs focused exclusively on military capabilities, such as the detection of aircraft, missiles and ships, but in 1974 a team was formed to design a new OTHR for which a remote sensing mission was proposed. Oceanic wind mapping experiments in 1977-78 yielded promising results so remote sensing became a recognised mission assigned to the new radar, known as Jindalee Stage B. This capability was progressively expanded over the period 1982-87 to include mapping of wave height and other oceanographic variables. A real-time data link to the Australian Bureau of Meteorology was set up in 1985, providing daily wind direction maps covering over one million square kilometres of the NW Indian Ocean, the Timor Sea and the Arafura Sea. In concert with user-focused remote sensing programs, investigations were undertaken to gain a detailed understanding of issues relating to propagation, system calibration, radar resource management and scheduling. Many theoretical studies and radar experiments were conducted to assess the radar's ability to measure ever more ambitious geophysical phenomena, ranging from oil spills and tropical rainfall to cyclogenesis and volcanic activity. Inevitably, as the radar evolved into a facility owned and operated by the Royal Australian Air Force, the remote sensing mission had to compete with surveillance tasks with higher priorities and the quality of the routine service provided to the Bureau of Meteorology deteriorated, eventually being terminated after a decade of activity. Despite this, the knowledge acquired and the practical lessons learned remain valid today and may be of value to other OTHR programs.

Antenna systems with beam forming and beam steering capabilities for HF skywave radars

Radiation characteristics of linearly phased, periodic, planar dipole arrays, which can be used as transmitting/receiving antenna systems for HF skywave radars, are investigated. Rectangular, triangular and trapezoidal arrays are proposed in obtaining different beam shapes in the desired directions. Beam steering is achieved by adjusting the inter-element phase increments coherently. The effects of vertical array-tilt with a desired take-off angle (TOA) \alpha are presented by a number of radiation pattern examples. The ground effect is investigated using the image theory by assuming the Earth's surface as perfectly electric conductor (PEC). Radiated fields are obtained using Floquet wave representations plus the synthetic aperture approach. Validation is done against element-by-element summation representations.

Adaptive Transient and CW RF Interference Mitigation in HF OTH Radar: Experimental Results

Adaptive Transient and CW RF Interference Mitigation in HF OTH Radar: Experimental Results

Ionospheric Decontamination and Sea Clutter Suppression for HF Skywave Radars

In this paper, a cascaded correction and suppression method of reducing ionospheric phase path contamination and sea clutter to enable detection of targets travelling at speeds near the Bragg Doppler is addressed. The Hankel rank reduction (HRR) technique based on singular value decomposition (SVD) has been used to estimate the ionospheric phase distortion and suppress the sea clutter. Simulation results show that such a technique is helpful for the worse conditions when the target masking effect happens even after ionospheric phase decontamination. Finally, an attempt to combine another phase decontamination algorithm based on the piecewise polynomial phase modeling with the clutter cancellation stage for faster phase fluctuation is discussed briefly and some results are given.

Ionosphere and HF skywave radar

Ionosphere and HF skywave radar

HF‐over‐the‐horizon radar ship detection with short dwells using clutter cancelation

Coherent sea‐clutter cancelation is shown to be an effective method for the exposure of ships with HF OTH (high‐frequency over‐the‐horizon) sky wave radar. A sinusoidal model has been successfully used to approximate and subtract the first‐order Bragg peaks, which spread and mask slow targets when using Fourier Doppler processing on short dwells (fewer than 10 s). (Short dwells are required for aircraft surveillance over the vast HF radar coverage area.) The new clutter canceling algorithm has been successfully applied to both synthetic and real OTH data from the U.S. Navy's Relocatable Over‐the‐Horizon radar (ROTHR) (AN/TPS‐71), and results are presented.

Applications of high‐frequency radar

Efforts to extend radar range by an order of magnitude with use of the ionosphere as a virtual mirror started after the end of World War II. A number of HF radar programs were pursued, with long‐range nuclear burst and missile launch detection demonstrated by 1956. Successful east coast radar aircraft detect and track tests extending across the Atlantic were conducted by 1961. The major obstacles to success, the large target‐to‐clutter ratio and low signal‐to‐noise ratio, were overcome with matched filter Doppler processing. To search the areas that a 2000 nautical mile (3700 km) radar can reach, very complex and high dynamic range processing is required. The spectacular advances in digital processing technology have made truly wide‐area surveillance possible. Use of the surface attached wave over the oceans can enable HF radar to obtain modest extension of range beyond the horizon. The decameter wavelengths used by both skywave and surface wave radars require large physical antenna apertures, but they have unique capabilities for air and surface targets, many of which are of resonant scattering dimensions. Resonant scattering from the ocean permits sea state and direction estimation. Military and commercial applications of HF radar are in their infancy.

A unified approach to detection, classification, and correction of ionospheric distortion in HF sky wave radar systems

HF sky wave radar signals are subjected to a wide variety of distortion and contamination effects as they propagate through the ionosphere. Signal processing techniques designed to remove or at least mitigate these effects have been in use for some years, but they do not address the problem of simultaneous occurrence of multiple distortion mechanisms, which may reduce drastically the efficacy of the ameliorative processing. In this paper we present an algorithmic structure for a global optimum distortion correction scheme which can function under arbitrary combinations of signal corruption phenomena. The scheme is based on multiply recursive estimation of distortion parameters. Results from a prototype implementation of this scheme show substantial gains over traditional processing sequences. The new approach is expected to enhance the remote sensing capabilities of HF sky wave radars significantly.

Mapping Wind Directions with HF Radar

BESIDES THE MEASUREMENT of ocean surface currents, high-frequency (HF) radar has also been demonstrated to be effective at measuring the wind direction at scales on the order of one to several kilometers and over areas of millions of square kilometers in the case of sky-wave HF radars (Georges et al. , 1993) and many hundreds of square kilometers in the case of groundwave HF radars. The capability to estimate the wind direction at high resolution over large areas makes HF radar unique in that it is ground based, yet is capable of collecting remotely sensed measurements of the oceanic wind field that could prove beneficial to mariners, weather forecasters, ocean and atmospheric modelers, offshore operators, and recreational users. Not only are such measurements useful to a variety of people, but they would be expensive and difficult, if not impossible, to obtain through other means, such as through the use of buoys at this resolution.