HF Ground Wave Research Articles

Revisiting HF Ground Wave Propagation Losses Over the Ocean: A Comparison of Long‐Term Observations and Models

[ Understanding variations in the received power levels for land-based high frequency radar systems is critical to advancing radar-based estimates of winds and waves. We use a long-term record of one-way high frequency radar power observations to explore the key factors controlling propagation losses over the ocean. Observed propagation loss was quantified using an 8-month record of radio frequency (RF) power from a shore-based transmitter, received at two locations: an offshore tower and a nearby island. Observations were compared to environmental factors such as wind speed and air temperature as well as models of path loss incorporating smooth and rough surface impedances and varying atmospheric properties. Significant differences in the observations at the two sites existed. One-way path loss variations at the tower, a wavelength above mean sea level, were closely related to atmospheric forcing, while variations at the distant island site were dominated by wind-driven surface gravity wave variability. Seasonal variability in ocean conductivity had no significant effect on over-ocean path losses. Simplistic analytical models of path loss were found to have more skill than either ground wave propagation models or more complex numerical models of field strength in matching the observations, due in part to under-observation of the atmosphere but also the differences in rough surface impedance between models of ocean waves. ]

The study of single station inverting the sea surface current by HF ground wave radar based on adjoint assimilation technology

This paper introduces the assimilation technology in an ocean dynamics model and discusses the feasibility of inverting the sea surface current in the detection zone by assimilating the sea current radial velocity detected by single station HF ground wave radar in ocean dynamics model. Based on the adjoint assimilation and POM model, the paper successfully inverts the sea surface current through single station HF ground wave radar in the Zhoushan sea area. The single station HF radar inversion results are also compared with the bistatic HF radar composite results and the fixed point measured results by Annderaa current meter. The error analysis shows that acquisition of flow velocity and flow direction data from the single station HF radar based on adjoint assimilation and POM model is viable and the data obtained have a high correlation and consistency with the flow field observed by HF radar.

HF ground wave propagation over a curved rough sea surface in the presence of islands

For a vertically polarized line source, in the context of HF (3–30 MHz) ground wave propagation over a curved rough sea surface in the presence of islands, this paper adapts the FB-SA (Forward–Backward Spectral Acceleration) method to compute the attenuation function over sea–land (island)–sea mixed paths for different shapes and heights of the islands. The rigorous FB-SA numerical method is based on the method of moments and was originally developed for scattering from rough surfaces and is especially efficient in solving a huge problem, which is required to predict the ground wave propagation over a long surface. In addition, for zero island height, this method is compared with an analytical approach expressed from a (residue) series, in which the roots of a differential equation, depending on the Airy function, must be calculated. In addition, from an intuitive approach and from the work of Barrick and Ishimaru, this analytical approach is extended to include the sea roughness and then validated from the FB-SA.

HF ground wave propagation over a curved rough sea surface

For a vertically polarized line source, in the context of HF (3–30 MHz) ground wave propagation over a curved rough sea surface, this paper presents different asymptotic and rigorous methods to compute the attenuation function. When the Earth's curvature is taken into account, the attenuation function is expressed as a series, in which the roots of a differential equation, depending on the Airy function, must be calculated. In addition, from Taylor series expansions, different closed-form expressions can be obtained. For a smooth sea surface, the purpose of this paper is to compare these different formulations with fast rigorous numerical methods, such as the BMIA-CAG (Banded-Matrix-Iterative Approach CAnonical-Grid) and FB-SA (Forward-Backward Spectral-Acceleration) methods, based on the method of moments and originally developed for rough surfaces. These methods are especially efficient to solve a problem with huge unknowns, which is required to predict the ground wave propagation over a long surface. In addition, from a partial fraction expansion of the attenuation function in the Laplace domain, the Bremmer asymptotic expansion is extended to any order by including the surface roughness.

Estimation of detection threshold in multiple ship target situations with HF ground wave radar

A credible method of calculating the detection threshold is presented for the multiple target situations, which appear frequently in the lower Doppler velocity region during the surveillance of sea with HF ground wave radar. This method defines a whole-peak-outlier elimination (WPOE) criterion, which is based on in-peak-samples correlation of each target echo spectra, to trim off the target signals and abnormal disturbances with great amplitude from the complex spectra. Therefore, cleaned background noise samples are obtained to improve the accuracy and reliability of noise level estimation. When the background noise is nonhomogeneous, the detection samples are limited and often occupied heavily with outliers. In this case, the problem that the detection threshold is overvalued can be solved. In applications on experimental data, it is verified that this method can reduce the miss alarm rate of signal detection effectively in multiple target situations as well as make the adaptability of the detector better.

Estimation of the direction of arrival using virtual matrix method for HF ground wave radar

According to the characteristic of the echo of high-frequency ground wave radar(HF GWR), which is one-dimensional narrow band signal, a virtual direction of arrival(DOA) matrix is constructed at first, then the DOA of target evaluation is achieved by the method of resolving equations for two-dimensional DOA matrix. And this method bases on the redundancy information of a linear two-row array of antennae. Both the simulation process and the treatment results of measured data (in the case of low SNR echoes and short data series) are given at the end of this paper. By comparing with GPS data of the targets, the validity and practical applicability of the method in this paper is verified.

Modification of polarization filtering technique in HF ground wave radar

The polarization filter using three orthogonal linear polarization antennas can suppress more disturbances than the polarization filter using two orthogonal linear polarization antennas in HF ground wave radar. But the algorithm of the threedimension filter is relatively complicated and not suitable for real-time processing. It can't use linear and nonlinear polarization vector translation technique directly. A modified polarization filter which is simple and has same suppressing ability as the three-dimension polarization filter is given. It only has half parameters of the primary one. Some problems about estimation of polarization parameters and selection of disturbances are discussed. A method of holding the phase of radar backscatter signal constantly is put forward so that unstationary disturbance signal can be processed.

Ionospheric effect of HF surface wave over‐the‐horizon radar

The use of HF surface wave radar as an effective method for inspecting the environment of the ocean in the beyond‐the‐horizon area has been developing in recent years. However, because the radiating beam of the transmitting antenna is of a certain width and its electromagnetic radiating power propagates not only along the sea surface but also to the upper space, as a result, after interacting with the ionosphere or a scattering object on its path, the backscattered signal then returns to the radar along the radiative path. Thus, if we analyze the echo signal received by the radar, we can obtain some information about the ionosphere. This paper presents a method of sensing the ionosphere using HF ground wave radar, which can give information about the altitude of the ionosphere, Doppler, meteor showers, etc.

Design of electrically small ground planes for HF ground wave transmission

Electrically small spiral ground planes for a monopole and an electrically small anienna have been designed for HF ground wave transmission. Prototype ground planes have been built and tested. For the monopole, the transmission loss of the designed ground plane is 6 dB better than that of the same size ground radials. For the electrically small antenna, the transmission loss of the designed ground plane is 7 dB better than that of the same size solid ground plane at the antenna's operating frequency.

Measuring Surface Wind Direction by Monostatic HF Ground-Wave Radar at the Eastern China Sea

The extraction of full wind vectors from data obtained by single-site (monostatic) high-frequency ground-wave radar (HFGWR) is an ongoing challenge because of the inherent directional ambiguities. Here, a new algorithm for resolving the ambiguity of wind direction from monostatic data is presented. The true wind direction is determined by minimizing the sum of the difference among three wind directions derived from three different radar look angles. The wind directions estimated by applying the algorithm to data obtained from the OSMAR2000 HFGWR situated at the Eastern China Sea are compared with values obtained from ship-borne instrumentation. The mean difference between the ground-truthed values and those obtained from the radar data is approximately 20/spl deg/. The distance limit for wind direction sensing using the OSMAR2000 is about 200 km, which is the range for which signal-to-noise ratios typically exceed about 23 dB in the relevant first-order portions of the backscatter spectra.

Application of ESPRIT in broad beam HF ground wave radar sea surface current mapping

HF surface wave radar system OSMAR2000 is a broad-beam sea-state detecting radar. ESPRIT (Estimation of Signal Parameters via Rotational Invariance Technique) algorithm is proposed to apply in DOA (direction of arrival) determination of sea echoes. The algorithm of ESPRIT is briefly introduced first. Then discussions are made on the technique for application in the OSMAR2000 framework. Numerical simulation results are presented to demonstrate the feasibility of radial current mapping based on this method. The algorithm manifests significant performance and computational advantages compared with that of MUSIC. Data acquired by OSMAR2000 are processed to give radial current map and the synthesized vector currents are compared with the in-situ measurement with traditional means. The results show the validity of ESPRIT application in DOA determination for broad-beam radar.

Electrically small antenna for maximising transmission into HF ground waves

A top-loaded, inductively-coupled, electrically small antenna is designed for HF ground wave transmission. A genetic algorithm is used to optimise the antenna parameters. Two prototype antennas have been built and the test results match well with simulations. Both antennas have an electrical size kr of 0.2, greater than 65% efficiency and transmission performance within 1 dB of a commercial whip.

Synchro controller of radar on FPGA

This paper mainly represents the realization of synchro controller based on the programmable logic devices FPGA by request of HF ground wave radar synchro controller under the instance of making the best of the virtues of FPGA. This design introduces the data communication between PC and synchro controller byI2C Bus, which can carry the synchronous signals’ parameters to RAM of synchro controller, then according to the theory that the result of comparing counter value with signals’ parameters is the needed wave, we produce all waves HF ground wave radar needs, moreover all waves are produced time-sharing in order to save resources.

Waveform analysis for HF ground wave radar

According to general radar designing principles, this paper discusses the concepts and processing techniques concerning the radar waveform analysis. For the need in developing HF ground wave system, as requested by a key oceanic project in the national 863 plans, basic theories and parameter design techniques on Frequency Modulated Interrupted Continuous Wave (FMICW) have been studied. This study serves as the waveform design basis for the HF ground wave radar OSMAR2000.

Some problems in general scheme for HF ground wave radar engineering

Some problems in general scheme for HF ground wave radar engineering

A new MF and HF ground-wave model for urban areas

Following discussions (Luo Lichun 1997) on the propagation and regularities of HF ground waves in urban areas in China, and processing the HF experimental data obtained from 1993 through 1997 and MF data obtained in 1995 and 1997 in Beijing, a new model for MF and HF ground-wave propagation in urban areas is proposed in this paper. The key novelty in the proposed model is the inclusion of three building-complex parameters, r/sub b/, r/sub s/ and r/sub e/, and a new height-gain factor, G/sub h/. A qualitative derivation of these is offered. The method of selecting these building parameters is discussed. Some HF and MF ground-wave propagation losses, which occur under different conditions of the operating frequencies, heights, and locations of the transmitting antennas, are calculated by means of the new model and, for contrast, by means of some earlier models. The new model can be used to calculate either the coarse prediction of ground-wave loss, or to calculate fine predictions, even for a point site. The accuracy of the calculation using the new model is superior to that of the earlier models.

Ship target detection for HF groundwave shipborne OTH radar

The spreading of the first order Bragg lines that result from movement of the radar platform will obscure targets with Doppler frequencies within the range of this spreading. This is the main difficulty for HF groundwave OTH radar in detecting ship targets in a shipboard environment. In the paper, the properties of the spread spectrum are analysed with the aid of synthetic aperture theory, the possibility of detecting targets in the spread spectrum is shown, and the technique for target detection is presented. The main factors affecting performance of target detection are studied. The experimental results further demonstrate the effectiveness of the technique.

Improved numerical solution of Ott's integral equation for HF ground wave attenuation

An alternative solution is presented for Ott's (1970) integral equation for the attenuation of a high-frequency vertically polarized radio signal propagating over rough inhomogeneous terrain. The method circumvents the numerical stability occurring with an earlier method at steep slopes and/or high radar frequencies. The method is illustrated by application to a steep profile at 20.08 MHz. The obtained values for the attenuation function agree well with experimental measurements. >

A review of methods of remote sensing of sea-surface conditions by HF radar and design considerations for narrow-beam systems

Present achievements in HF ground-wave radar remote mapping of surface currents and wave characteristics are reviewed, and the features of the different systems that are in use are described. Particular emphasis is given to the problems remaining to ensure effective mapping of nondirectional wave parameters and wave directional spectra over a sector of sea extending up to a range of 150-200 km from a coastal or island site. As an example of current progress, measurements in the United Kingdom of first-moment wave period by single-site radar have achieved measurements to within 15 percent of buoy data over a range from 4 to 9 s. Measurements of significant wave height gave standard deviations of 15 and 20 percent for wave heights less than and greater than 2 m, respectively. In the latter case, a bias of 40 percent was found, which later work suggests is reducible to about 14 percent, using a model-fitting technique. To achieve directional wave spectrum measurement with a single radar requires some 40-dB ratio between the Bragg line return and the noise floor of the Doppler spectra. A two-site radar has been shown to yield advantages for directional spectrum measurement. The design of steerable narrow-beam radars in the 6-25-MHz band to achieve such performance is discussed. Suitable transmitted waveforms to achieve good rejection of interference and to be acceptable in the HF band are put forward. The operational experience in the United Kingdom, obtained with a particular FM interrupted continuous-wave radar system associated with a steerable array, is reported. Results of simulating the effect of the antenna radiation pattern on the observed Doppler spectra are illustrated. The principal effects are on the weaker Bragg line and the second-order continuum around it. This can result in errors in deduced surface wind direction and directional wave spectrum. Finally, some useful results on propagation monitoring of a skywave radar with the aid of a range-Doppler graphical output are reported.

Evaluation of HF ground-wave radar on the east coast of Canada

HF ground-wave radar runs of roughly ten days duration were carded out in two tidally dominated regions off the Canadian East Coast in conjunction with oceanographic experiments. The surface current deduced from the HF radar observations are here compared with Eulerian velocities from various moored current meters and Lagrangian velocities derived from surface drifters. The agreement with the current meters is generally better than 0.05 m/s. Real current shear in the top 15 m of the water column of up to 0.1 m/s, lasting for up to 12 h, was observed on several occasions, coincident with wind reversals. The agreement between moored instruments and shallow-draft rectangular drifters was good, and with deeper draft cylindrical drifters it was excellent.