Sea Echo Research Articles

RETRIEVING EVAPORATION DUCT HEIGHTS FROM POWER OF GROUND-BASED GPS OCCULTATION SIGNAL

Evaporation duct is a manifest phenomenon that can afiect microwave propagation seriously with low elevation angles near the sea surface. As an important parameter to describe the characteristic of the evaporation duct, duct height can be estimated from radar sea echo using a technique called as \refractivity from clutter. In this study, we proposed a novel approach to estimating the evaporation duct height. The signal power received by a ground- based GPS receiver is used when the GPS satellites rise or set at the local horizon over the sea. A forward propagation model and genetic algorithm are adopted to implement this method. The performance is evaluated via numerical simulation for inferring evaporation duct with difierent height. The results showed that the proposed method is well efiective, especially for the conditions with higher evaporation duct height.

A Modified STAP Estimator for Superresolution of Multiple Signals

A modified space-time adaptive processing (STAP) estimator is described in this paper. The estimator combines the incremental multiparameter (IMP) algorithm and the existing beam-space preprocessing techniques yielding a computationally cheap algorithm for the superresolution of multiple signals. It is a potential technique for the remote sensing of the ocean currents from the broadened first-order Bragg sea echo spectrum of shipborne high-frequency surface wave radar (HFSWR). Some simulation results and real-data analysis are shown to validate the proposed algorithm.

A New Super Resolution Algorithm in Ship-Borne Radar

In ship-borne radar, because of the influence of interference factors such as the correlation of background array noise and the coherence of targets and so on, the performance of high-resolution algorithms such as MUSIC is degraded. In this document by pre-whitening of background array color noise, de-correlation of coherent targets, compensation of amplitude phase mismatch, pre-whitened-Constrained-MUSIC algorithm is put forward, which can effectively resolute ship target and first-order sea echo in ship-borne radar. Furthermore, the algorithm performance is compared with other algorithms, result shows that pre-whitened-Constrained-MUSIC can be applied effectively in high-resolution processing in ship-borne radar.

Automatic antenna pattern estimation for high-frequency surface wave radars

Distortions of antenna patterns may cause big errors in direction of arrival (DOA) estimation for high-frequency surface wave radars using loop antennas and thus should be calibrated in the direction finding process. The conventional methods to acquire the actual patterns are to measure it, by either a transponder or a monotone beacon borne on a ship moving through all the look angles, leading to increased system complexity and cost. To address this, a novel method for automatic antenna pattern estimation is proposed, which is only involved with some soft computations, i.e. recursively approaching the actual patterns by the amplitude and direction parameter pairs extracted from the single-DOA sea echoes. Both simulations and real radar data processing results show the validity of the proposed method.

Statistical Modeling of the Interference Noise Generated by Computing Platforms

The trend of electronic systems toward higher integration and higher performance has also brought new challenges to the design of radio transceivers. The decrease of switching times accompanied by the increase of clock speeds, data rates, and interconnection speeds contribute to improve the overall system performance. At the same time, they also affect wireless communications due to an increment of the emissions of electromagnetic radiation. In this paper, we show that the statistics of the baseband components of the noise follow the K-distribution. The one-sided version of this distribution has been derived before as a model for the envelope of sea echo in radar and sonar, however, we show here that its two-sided version can also be used to model the interference noise that affects a transceiver located inside a computer platform. The model shows good agreement with experimental measurements. Also, a closed-form expression of the bit error rate (BER) and some bounds are computed.

An Adaptive Objective Function for Evaporation Duct Estimations from Radar Sea Echo

In the process of atmospheric refractivity estimation from radar sea echo, the objective function that calculates the match between the predicted and observed field plays an important role. To reduce the effect of noises from long ranges on the objective function, we present a selection method of final ranges for inversion. An adaptive objective function is introduced with a linear distance weight added to the least squares error function (LSEF). Through an evaporation duct height (EDH) retrieving process, the performance of the adaptive objective function is evaluated. The result illustrates that the present method performs better than the LSEF in EDH inversions from clutters with different clutter-to-noise ratios.

VERIFICATION OF A MODIFIED BAYESIAN METHOD FOR ESTIMATING DIRECTIONAL WAVE SPECTRA FROM HF RADAR

A Modified Bayesian Method (MBM) for estimating directional wave spectra from Doppler spectra obtained by HF radar is examined using field data which were employed in the verification of Bayesian Method (BM). Applicability, validity and accuracy of the MBM are demonstrated compared with the directional wave spectra estimated by BM and observed by buoy acquired from the reliable field data obtained from Surface Current and Wave Variability Experiments (SCAWVEX) project. The necessary conditions of the Doppler spectral components to be used to estimate a reliable directional spectrum are correspondingly estimated by BM. The results clearly demonstrate that directional wave spectra can be estimated by MBM on the basis of Doppler spectra. In addition, though BM shows very time consuming in computations, BM is more robust against the presence of noise than MBM.
 
 
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Parameter estimation of K-distributed sea clutter based on fuzzy inference and Gustafson–Kessel clustering

The detection performance of maritime radars is restricted by the unwanted sea echo or clutter. Although the number of these target-like data is small, they may cause false alarm and perturb the target detection. K-distribution is known as the best fit probability density function for the radar sea clutter. This paper proposes a novel approach to estimate the parameters of K-distribution, based on fuzzy Gustafson–Kessel clustering and fuzzy Takagi–Sugeno Kang modelling. The main contribution of the proposed method is the ability to estimate the parameters, given a small number of data which will usually be the case in practical applications. This is achieved by a pre-estimation using fuzzy clustering that provides a prior knowledge and forms a rough model to be fine tuned using the least square method. The algorithm also improves the calculations of shape and width of membership functions by means of clustering in order to improve the accuracy. The resultant estimator then acts to overcome the bottleneck of the existing methods in which it achieves a higher performance and accuracy in spite of small number of data.

Extraction of mixed-order multicomponent ship target signals from broadened sea clutter in bistatic shipborne surface wave radar

A dynamic geometry model was built to describe the Doppler-broadening characteristics of the first-order Bragg lines for bistatic shipborne surface wave radar. They are time-varying because the Doppler frequency shifts of sea echoes are simultaneously modulated by the velocity components projected from the unavoidably different motions of both platforms, which is more complex than its counterpart in the monostatic mode. The geometrical relation is used to obtain the received signal model: strong first-order sea clutter interferences and submerged ship targets with constant and non-constant speeds are considered as mixed-order multicomponent polynomial-phase signals (mc-PPSs) and both second-order sea clutter continuum and atmospheric noise as additive noise. Then, a scheme based on the product high-order ambiguity function is proposed to extract the targets; it is a recursive procedure in which the first-order sea clutter is removed by an existing time–space cascaded filtering method. Monte Carlo simulations show the validity and efficiency of the proposed scheme.

Occurrence of medium-scale travelling ionospheric disturbances identified in the Tasman international geospace environment radar observations

The occurrence characteristics of medium-scale travelling ionospheric disturbances (MSTIDs) were investigated using the Tasman International Geospace Environment Radar (TIGER). From the occurrence study of sea echoes, we found two maxima, one pre-noon and the other after noon. They are less obvious with increase of magnetic activities, and more obvious when B z is northwards. It is suggested that this maxima were related to fore- and after-noon maxima in the distribution of net field-aligned currents flowing from the magnetosphere to the ionosphere, and that these two regions were sources of atmospheric gravity waves (AGWs) due to enhancement of Hall conductivities in the ionosphere. The Lorentz force is suggested to be a possible mechanism for the excitation of MSTIDs in the dayside ionosphere.

衛星画像と音響測深機を利用したアマモ場分布域の把握―霧多布港周辺海域―

In the eastern part of Hokkaido's Kushiro Subprefecture lies Kiritappu Port, resting across two bays and connecting them with a channel. In recent years, sediments, which have accumulated in Kiritappu Port due to littoral drift, have led to the hindrance of the passage of ships at low tide. Since the accumulation of sediments in the passage will continue under the current conditions, measures against littoral drift are being considered. Such measures, however, must be environmentally friendly because Hokkai shrimp and other economically useful marine species live in and around the port as mentioned before. Under such circumstances, it was considered important to understand the distribution area of eelgrass beds before implementing countermeasures. In this study, the quantification of the distribution area was conducted through the analysis of satellite images, echo sounding, underwater photography and submersible research to gauge two-dimensional distribution. As a result, it was found that satellite images are effective in locating algae zones in shallow sea areas and echo sounding is useful in determining the presence of algae zones in sea areas with turbidity.

Automatic Estimation Method for Surface Currents from Doppler Spectrum Measured by DBF Marine Radar

We develop a method automatically identifying peak frequencies in the sea echo Doppler spectrum measured by DBF Marine Rader system to estimate coastal surface currents, and then evaluate the accuracy and Iimitation of this method. The velocities from the Radar system estimated with this method are compared well with near-surface data with ADCP. The processing time of this method is short in about 40 seconds.These results suggest that this method allows one to estimate remotely coastal surface current in real timeusing DBF Marine Radar system.

Extraction of Ocean Wave Spectra From Simulated Noisy Bistatic High-Frequency Radar Data

An algorithm is developed for the inversion of bistatic high-frequency (HF) radar sea echo to give the nondirectional wave spectrum. The bistatic HF radar second-order cross section of patch scattering, consisting of a combination of four Fredholm-type integral equations, contains a nonlinear product of ocean wave directional spectrum factors. The energy inside the first-order cross section is used to normalize this integrand. The unknown ocean wave spectrum is represented by a truncated Fourier series. The integral equation is then converted to a matrix equation and a singular value decomposition (SVD) method is invoked to pseudoinvert the kernel matrix. The new algorithm is verified with simulated radar Doppler spectrum for varying water depths, wind velocities, and radar operating frequencies. To make the simulation more realistic, zero-mean Gaussian noise from external sources is also taken into account

On the Singular Nature of the Second-Order Peaks in HF Radar Sea Echo

Electromagnetic (EM) scattering from the sea surface concerned with second-order Doppler spectra for finite-length pulses is theoretically analyzed in the scattering regime typical of, but not confined to, high-frequency (HF) radars. The Doppler spectra of the second-order cross section consist of three different parts: continuum and two pairs of peaks-the second harmonic and corner reflection peaks. This paper is the first investigation of fundamental properties of these peaks from the perspective of their use in measurements of surface currents. It is shown that these peaks are of singular nature in the following sense: The main (singular) contribution is due to particular pairs of waves, despite the fact that waves of many different wavelengths and directions contribute to these peaks. This fact opens a possibility to employ these peaks in remote sensing of vertical profiles of surface currents. Using the number of waves in the pulse (the pulse length) LGt1 as a large parameter, an asymptotic description of these peaks is developed. Height, width, and position of the peaks are explicitly found in terms of L. The peak positions, to the leading order, do not depend on the pulse length, although a small explicitly found O(L-1) shift has to be taken into account for the corner reflection peaks. The heights are ~lnL and ~L1/2 for the second harmonic and corner reflection peaks, respectively. The results open the way for wider use of the second-order peaks for probing surface currents

Adaptive ionospheric clutter suppression based on subarrays in monostatic HF surface wave radar

Coherent sidelobe cancellation (CSLC) based on subarrays has emerged as a key technology for improving the performance of radar systems required to operate in the presence of severe interference which generally includes clutter as well as jamming. While the theory of CSLC is well known, practical issues such as how to set up the subarrays and how to avoid diminishing the desired signal when the clutter is rejected need to be addressed when it comes to implementing CSLC in operational radar systems. A new adaptive ionospheric clutter suppression algorithm based on subarrays for monostatic HF surface wave radar (HFSWR) is introduced and analysed. Detailed comparisons between the proposed algorithm and conventional CSLC are performed. Simulation and real data results confirm that the general and robust algorithm can achieve effective ionospheric clutter suppression, while not decreasing the strength of the first-order sea echo using the data recorded by the OSMAR2003 (Ocean State Monitor and Analysis Radar, manufactured in 2003), located near Zhoushan in Zhejiang, China.

Studies of medium scale travelling ionospheric disturbances using TIGER SuperDARN radar sea echo observations

Abstract. Seasonal and diurnal variations in the direction of propagation of medium-scale travelling ionospheric disturbances (MSTIDs) have been investigated by analyzing sea echo returns detected by the TIGER SuperDARN radar located in Tasmania (43.4° S, 147.2° E geographic; –54.6°Λ). A strong dependency on local time was found, as well as significant seasonal variations. Generally, the propagation direction has a northward (i.e. equatorward) component. In the early morning hours the direction of propagation is quite variable throughout the year. It then becomes predominantly northwest and changes to northeast around 09:00 LT. In late fall and winter it changes back to north/northwest around 15:00 LT. During the other seasons, northward propagation is very obvious near dawn and dusk, but no significant northward propagation is observed at noon. It is suggested that the variable propagation direction in the morning is related to irregular magnetic disturbances that occur at this local time. The changes in the MSTID propagation directions near dawn and dusk are generally consistent with changes in ionospheric electric fields occurring at these times and is consistent with dayside MSTIDs being generated by the Lorentz force. Key words. Ionosphere (ionospheric disturbances; wave propagation; ionospheric irregularities; signal processing)

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.

Ocean Surface Current Detection by HF Surface Wave Radar at the Eastern China Sea

AbstractHF surface wave radar system OSMAR2000 adopts Frequency Modulated Interrupted Continuous Waveform (FMICW) and its antenna array is transmitting/receiving co‐used. MUSIC and MVM are applied to obtain sea echo's direction of arrival (DOA) when extracting current information. Verification tests of OSMAR2000 ocean surface dynamics detection against in‐situ measurements had been accomplished on Oct. 23~29, 2000. This paper introduces the radar system and the applied DOA estimation methods in the first, and then presents comparison results of surface current measurements in the verification tests. The results indicate the validity of the developed radar system and the effectiveness of the applied signal processing methods.

Applying a unified directional wave spectrum to the remote sensing of wind wave directional spreading

Models for directional wind wave spectra can be used to assist in interpretation and analysis of Doppler sea echo spectra to determine wind direction over the oceans by remote sensing techniques. This paper compares and contrasts the data analysis required at different electromagnetic wavelengths. At large wavenumbers the observed spreading depends not only on the directional spreading at the Bragg wavenumber, but also on the weighted integrals over all shorter wavenumbers. This is discussed in terms of microwave and VHF Doppler spectra. At small wavenumbers (HF) the observed directional spreading should be directly predicted by the model. It is in this simple case that there is an inconsistency between the models and the observations, and this has a potential impact on the use of the models at all wavenumbers.

VHF and HF sea echo Doppler spectrum for a finite illuminated area

The prediction of the sea echo Doppler spectrum of VHF and HF radio waves is important for the ocean wave remote sensing by VHF and HF radars. The authors extended the theory of the sea echo Doppler spectrum. In previous theories it is assumed that the scale of the illuminated area is much larger than the dominant wavelength. However, the effect of the finite illuminated area may not be negligible. The authors considered the effect to predict second‐order sea echo Doppler spectrum and extended previous theories. Furthermore, the authors developed the numerical technique to compute radar cross sections of the extended theory. As a result, the agreement between measured and predicted Doppler spectra is improved.