Frequency management support for remote sea-state sensing using the JINDALEE skywave radar

Successful operation of a skywave (over-the-horizon, OTH) radar in a remote sea-state sensing mode is critically dependent upon the application of comprehensive frequency management techniques. In addition to the problem of selecting a frequency yielding an adequate signal-to-noise ratio in the geographical area under investigation, attention must be paid to minimization of ionospheric multimode and other phenomena capable of distorting or convoluting the sea backscatter spectrum. This paper describes the manner in which these problems have been addressed in the JINDALEE skywave radar, and relates the practical difficulties inherent in the task of frequency management in support of an OTH radar involved in sea-state sensing. Measurement techniques include backscatter and oblique-incidence sounding, HF spectral surveillance, and a low-powered frequency-agile "miniradar" capable of operating in either a conventional backscatter or alternate oblique-incidence mode. In addition to providing the main radar with real-time frequency management advice, a principal emphasis has been the acquisition of a synoptic data base suitable for off-line statistical analysis.     

A model for Doppler peak spectral shift for low grazing angle sea scatter

A model is formulated for Doppler spectral characteristics of radar sea scatter for low grazing angles, and is compared with previous radar measurements reported in the literature. The Doppler model is based upon the two-scale model for radar scatter, with scatterer motions hypothesized as due to the orbital wave velocity of the large-scale waves, Stokes and wind drift currents, and the phase velocity of the small-scale Bragg scatterers. Expressions for Doppler shifts due to these motions are derived, and are given as a function of wave height, wave period, and wind speed. Although this model appears to account for the peak Doppler shift of the sea-scatter Doppler spectrum for vertical polarization, it is insufficient to describe horizontally and cross-polarized data, which have larger mean Doppler shifts. However, these two cases are found to scale very closely with the nearly simultaneous vertically polarized data for the variety of environmental conditions reported. Implications of the extension of these results to higher-angle remote-sensing applications are discussed.     

Doppler radar radial winds in HIRLAM. Part I: observation modelling and validation

An observation operator for Doppler radar radial wind measurements is developed further in this article, based on the earlier work and considerations of the measurement characteristic. The elementary observation operator treats radar observations as point measurements at pre-processed observation heights. Here, modelling of the radar pulse volume broadening in vertical and the radar pulse path bending due to refraction is included to improve the realism of the observation modelling.
The operator is implemented into the High Resolution Limited Area Model (HIRLAM) limited area numerical weather prediction (NWP) system. A data set of circa 133 000 radial wind measurements is passively monitored against the HIRLAM six-hourly background values in a 1-month experiment. No data assimilation experiments are performed at this stage. A new finding is that the improved modelling reduces the mean observation minus background (OmB) vector wind difference at ranges below 55 km, and the standard deviation of the radial wind OmB difference at ranges over 25 km.
In conclusion, a more accurate and still computationally feasible observation operator is developed. The companion paper (Part II) considers optimal super-observation processing of Doppler radar radial winds for HIRLAM, with general applicability in NWP.     

Operational Wave, Current, and Wind Measurements With the Pisces HF Radar

This paper presents results of a trial of a Pisces HF radar system aimed at assessing its use as a component of a wave-monitoring network being installed around the coasts of England and Wales. The radar system has been operating since December 2003 and the trial continued to June 2005. The data have been processed in near-real time and displayed on a website. Radar measurements of the directional spectrum and derived parameters are compared with those measured with a directional waverider and with products from the Met Office, United Kingdom, operational wave model. Radar measurements of currents and winds are also compared with Met Office model products and, in the case of winds, with the QuikSCAT scatterometer. Statistics on data availability and accuracy are presented. The results demonstrate that useful availability and accuracy in wave and wind parameters are obtained above a waveheight threshold of 2 m and at ranges up to 120 km at the radar operating frequencies (7-10 MHz) used. Waveheight measurements above about 1 m can be made with reasonable accuracy (e.g., mean difference of 2.5% during January-February 2004). Period and direction parameters in low seas are often contaminated by noise in the radar signal. The comparisons provide some evidence of wave model limitations in offshore wind and swell conditions     

Shallow gas and fluid migration in the northern Arafura Sea (offshore Northern Australia)

A variety of shallow gas indicators and fluid migration pathways have been interpreted from new sub-bottom profiler, multibeam bathymetry, side-scan sonar, and echo-sounder data together with geochemical analyses of sampled sediments from the northern Arafura Sea, offshore Northern Australia. This study provides new geological data and a seismic stratigraphy of the youngest units in the Money Shoal Basin, in an area that has not been closely studied in over 30 years. The shallow gas indicators include pockmarks, low frequency enhanced reflectors and acoustic blanking. These indicators are supported by gas within shallow cores. Geochemistry indicates that this gas has a microbial origin but deeper fluid movement is also suggested by the presence of interpreted hydrocarbon slicks based on synthetic aperture radar data. A region of likely hydrocarbon seepage is indicated by the clear coincidence of shallow gas indicators with an area of increased faulting and a zone of poor quality seismic data (recognised on conventional seismic data), together with anomalies (interpreted on remote-sensing data) aligned with mapped deep structures.     

Possibilities of X-band nautical radars for monitoring of wind waves near the coast

We present the results of development and testing of a coastal X-band radar system for monitoring wind waves and currents at the Black Sea (near Gelendzhik) created on the basis of nautical radars. Radar measurements of wave heights were validated by data from a wave buoy and a moored acoustic Doppler current profiler (ADCP). The conditions for successful radar measurements of waves in the coastal environment have been determined. It was shown that a radar with an aperture 1° could successfully measure wave heights at a distance of 1.2 km from the radar, when waves arrive at an angle of ±31° to the main sensing direction. In this case, for wave height measurements, the correlation coefficient between the radar and independent data is 0.82 and the standard deviation is 0.26 m.     

An Empirical Method to Derive Ocean Waves From Second-Order Bragg Scattering: Prospects and Limitations

High-frequency (HF) radar wave processing is often based on the inversion of the Barrick-Weber equations, introduced in 1977. This theory reaches its limitations if the length of the Bragg-scattering wave raises to the order of the significant waveheight, because some assumptions are no longer met. In this case, the only solution is moving to lower radar frequencies, which is not possible or desirable in all cases. This paper describes work on an empirical solution which intends to overcome this limitation. However, during high sea state, the first-order Bragg peaks sometimes could not be clearly identified which avoids the access to the second-order sidebands. These cases cause problems to the algorithm which have not been solved yet and currently limit the maximum significant waveheight to about the same values as reported for the integral inversion method. The regression parameters of the empirical solution calibrated from the European Radar Ocean Sensing (EuroROSE) data set are constant values for the complete experiment and when applied to the HF radar data they reconstruct the measurements by a colocated wave buoy quite well. When including a radar-frequency-dependent scaling factor to the regression parameters, the new algorithm can also be used at different radar frequencies. The second-order frequency bands used for the empirical solution are sometimes disturbed by radio interference and ship echoes. Investigations are presented to identify and solve these situations     

江苏沿海滩涂资源及发展趋势遥感分析

遥感技术用于广袤宽阔、复杂多变而难以实地勘测的潮间带资源调查是科学和最有效的选择。作者以三期低潮位卫星遥感数据图像为基础，对江苏沿海潮间带滩涂资源的总量、分布特点、发展趋势及开发利用前景进行了分析，指出了江苏滩涂资源总量呈减少趋势，匡围量大于可供匡围的滩涂资源生长量，应加强对滩涂资源开发利用规划和统一管理。     

Tidal Current Measurements Using VHF Radar and ADCP in the Normand Breton Gulf: Comparison of Observations and Numerical Model

Performance and operational feasibility of very high-frequency (VHF) Doppler radar have been demonstrated in a region dominated by strong tidal currents. An analysis of remote measurements of sea surface currents acquired by Courants de Surface MEsureacutes par Radar (COSMER)-pulsed Doppler radar during Evaluation et Preacutevision de l'Environnement Littoral (EPEL) experiment (supported by the French Navy) is presented in this paper. The VHF COSMER radar was deployed to provide continuous sea surface current measurements within an area of about 25 km times 25 km in the Normand Breton Gulf, France. This paper presents VHF measurement comparisons with observations such as acoustic Doppler current profiler (ADCP), as well as comparisons with numerical model TELEMAC 2-D. Results of tidal waves extraction, using harmonic analysis and residual currents, are shown in this paper. We also present a case where radar method is limited, due to the presence of additional peaks in the Doppler spectrum     

Polarimetric analysis and modeling of multifrequency SAR signaturesfrom Gulf Stream fronts

Using airborne synthetic aperture radar data from the 1990 Gulf Stream Experiment, this paper investigates the polarization and wavelength dependence of radar signatures for narrow fronts with converging flows occurring within the Gulf Stream. The signal-to-background ratios of the cross-polarization backscatter return from a convergent front were found much higher than those of copolarization returns, when the flight path is crossing the front. However, a second convergent front, imaged at 45°, showed that the signal-to-background ratios are nearly equal for co- and cross-polarizations. A polarimetric procedure, which has been successfully used to measure terrain slopes and to generate elevation maps, is applied to the convergent front to explain the polarization and imaging geometry dependence of these radar responses. A theoretical modeling of radar modulation using an ocean wave model and a composite-Bragg scattering model, which incorporates the effect of breaking waves, was developed. Calculations with the model agree reasonably well with the radar measurements at various polarizations for three radar frequencies: P-band (68 cm in wavelength), L-band (24 cm), and C-band (5.7 cm)     

On the performance of a shipborne current mapping HF radar

High-frequency (HF) radars have been developed to map surface currents offshore by means of land-based stations. Presently available radar systems use frequencies between 25 and 30 MHz and allow a spatial resolution of 1 km and ranges of up to 50 km. This paper reports on the experience with a shipborne radar and discusses problems which arise for the azimuthal resolution on a metal ship, the correction for the ship's speed, and limitations due to pitch-and-roll motions. Current measurements during cruises to the North Atlantic are presented. It has been found that, with the support of the satellite-supported Global Positioning System, the shipborne HF radar can measure surface current velocities with an accuracy of some 5 cm·s^-1     

Induced-current effects on microwave backscatter

Microwave measurements have been made with a coherent radar in a wind-wave tank to determine the effect of induced current on backscatter. Perturbations were introduced into the wave structure by inducing a current in the water that flowed either with or against the wind. The effect of current on radar cross section was slight; the effect on the Doppler was much more pronounced. It was found that the wave components responsible for radar backscatter are predominantly free waves (that is, waves which travel at the dispersion velocity) rather than waves which are parasitic (or locked) to the dominant waves.     

AAFE RADSCAT 13.9-GHz measurements and analysis: Wind-speed signature of the ocean

About 10 years ago, the advanced application flight experiment radiometer scatterometer (AAFE RADSCAT) made its first successful measurements of ocean radar scattering cross section from a NASA C-130 aircraft. This instrument was developed as a research tool to evaluate the use of microwave frequency remote sensors (particularly radars) to provide wind-speed information at the ocean's surface. The AAFE RADSCAT flight missions and analyses helped establish the feasibility of the satellite scatterometer for measuring both wind speed and direction. Probably the most important function of the AAFE RADSCAT was to provide a data base of ocean normalized radar cross-section (NRCS) measurements as a function of the surface wind vector at 13.9 GHz. NRCS measurements over a wide parametric range of incidence angles, azimuth angles, and winds were obtained in a series of RADSCAT aircraft missions from 1973 to 1977. Presented herein are analyses of data from the 26 RADSCAT flights during which the quality of the sensor and the surface wind measurements were felt to be understood. Subsets of this data base were used to model the relationship between theKu-band radar signature and the ocean-surface wind vector. The models developed partly from portions of this data base, supplemented with data from the Seasat (JASIN Report), were used for inversion of the Seasat-A Satellite Scatterometer (SASS) radar measurements to vector winds. This paper summarizes results from a comprehensive analysis of the RADSCAT/ocean wind signature deduced from this complete data set.     

南海珊瑚岛礁遥感分类体系和解译标志

南海珊瑚岛礁远离大陆且面积较小,高分辨率遥感技术已成为大范围进行珊瑚岛礁监测的重要手段,而建立适用的遥感分类体系和解译标志是进行珊瑚岛礁遥感监测的基础。本文综合考虑珊瑚岛礁的形成机制、珊瑚礁形态和高分辨率遥感影像上珊瑚岛礁图斑的可解译程度等因素,制定应用于珊瑚岛礁高分遥感监测的分类体系,将珊瑚岛礁分为珊瑚岛、沙洲、干出珊瑚礁、珊瑚暗礁、珊瑚暗沙和珊瑚暗滩6个一级类型;并在现场调查数据的辅助下,从遥感影像上的色、形、位等特征方面建立珊瑚岛礁不同类型的遥感解译标志,结果可服务于南海珊瑚岛礁的动态监测,进而为珊瑚岛礁的保护、可持续利用和安全管理提供数据支撑。     

Web-Based GIS as a Tool for Supporting Marine Research

Collecting marine data during hydroacoustic research surveys is a complex task, which not only consists of measurements but also analysis, validation, and interpretation of information acquired by various sensors. Automatic integration, visualization, and processing of collected data allow for a more precise investigation of the researched phenomena. The paper presents a dedicated Web-based system for fusion and dissemination of data from satellite imagery, hydrological measurements using CTD probes, acoustic surveys by multibeam systems, side-scan sonars and singlebeam echosounders, as well as live radar feed and oil spill spread models. The system has been designed as a tool for supporting research using the aforementioned techniques.     

利用高频雷达海面回波中奇异峰估计浪高的方法

The popular methods to estimate wave height with high-frequency(HF) radar depend on the integration over the second-order spectral region and thus may come under from even not strong external interference. To improve the accuracy and increase the valid detection range of the wave height measurement, particularly by the smallaperture radar, it is turned to singular peaks which often exceed the power of other frequency components. The power of three kinds of singular peaks, i.e., those around ±1,±2~(1/2) and ±1(2~(1/2)) times the Bragg frequency, are retrieved from a one-month-long radar data set collected by an ocean state monitoring and analyzing radar,model S(OSMAR-S), and in situ buoy records are used to make some comparisons. The power response to a wave height is found to be described with a new model quite well, by which obvious improvement on the wave height estimation is achieved. With the buoy measurements as reference, a correlation coefficient is increased to 0.90 and a root mean square error(RMSE) is decreased to 0.35 m at the range of 7.5 km compared with the results by the second-order method. The further analysis of the fitting performance across range suggests that the peak has the best fit and maintains a good performance as far as 40 km. The correlation coefficient is 0.78 and the RMSE is 0.62 m at 40 km. These results show the effectiveness of the new empirical method, which opens a new way for the wave height estimation with the HF radar.     

An Instantaneous-Frequency Filtering Technique for High-Frequency Radar Oceanography

High-frequency (HF) radar systems are remote sensing tools that can be used to measure oceanographic parameters. Problems can occur when using the conventional periodogram (PG) method for computing power spectral estimates from backscattered radar signals. Temporal and spatial inhomogeneities within the radar measurement region can cause distortion in the spectra. This paper describes an instantaneous-frequency (IF) filtering technique that has been developed to measure the first-order modulation contained within the radar signal. Successful removal of this modulation is shown to yield an increased quality and quantity of ocean measurements     

A minimum variance unbiased estimator for ocean surface currentsbased on multifrequency microwave radar observations

This paper presents a minimum variance unbiased (MVU) estimator for estimating an ocean surface current using the multifrequency microwave radar technique. In this technique the current information is obtained by finding the difference between the measured phase velocities of some specific surface gravity waves and the phase velocities calculated from the dispersion relation for still water. By defining the problem as a linear estimation problem, we develop an unbiased estimator for the current component along the radar look direction, which has a variance that is inversely proportional to the sum of the squared wavenumbers of the gravity waves used in the measurements. We also study the performance of an MVU vector estimator based on radar observations along two directions. Our analysis shows that the confidence region of this estimator has the shape of an elongated ellipse with semi-axes and orientation which are dependent on the angle between the observation directions, but independent on the true current vector. Furthermore, the theoretical models are thoroughly tested using both simulated and real radar data, and these tests show very good agreement with the model predictions     

A three-frequency scatterometer technique for the measurement ofocean wave spectra

A microwave technique for the measurement of ocean wave spectra has been compared with wave gauge output during extensive field testing. The method is based on the dual-frequency technique for detecting long ocean waves by matching the modulation of short waves with the beat wavelength between two transmitted microwave frequencies. The new method, however, utilizes three microwave frequencies in order to reduce mean backscatter not related to short-wave modulation. Two prototype scatterometers have been built using three frequencies at L-band and at K_u-band. Wave spectra have been measured by both radar systems which, when properly normalized, agree well with simultaneous in situ measurements taken by conventional wave gauges at the pier site. Thirteen sets of spectra have been computed, five of which correspond to a situation in which a local wind sea was generated and then decayed. The present experiment does not demonstrate the directionality of this new technique     

A comparison of sea-state parameters from nautical radar images and buoy data

Nautical radar and scalar buoy measurements of ocean wind generated waves have been analysed to compare the spectral parameters estimated from both sensors. The time series of different sea-state parameters and the differences and ratios of the values obtained from radar and buoy data using different analysis methods are compared. It has been observed that main differences between the sea-state parameters derived by using measurements obtained from both sensors result both from device characteristics and from the method of spectral estimation. In particular, it is shown that the Nyquist frequency has an important effect on the value of the sea-state parameters depending on spectral moments of order higher than zero.