Eric W. Gill

The First-Order High Frequency Radar Ocean Surface Cross Section for an Antenna on a Floating Platform

The first-order high frequency surface wave radar (HFSWR) cross section of the ocean surface is derived for the case of the transmitting and receiving antenna being mounted on a floating, but otherwise fixed, ocean platform. It is assumed that the sway component of the platform or barge motion is responsible for observed differences in the cross section compared to that for the fixed antenna case. Based on earlier work, a general expression for the bistatically received first-order electric field, which consists of a two-dimensional spatial convolution, is presented and reduced to integral form. Then, it is assumed that the surface can be described by a Fourier series whose coefficients are zero-mean Gaussian random variables, and from there the analysis proceeds for the backscatter case. The integrals are taken to the time domain, with the source field being that of a barge-mounted omnidirectional vertically polarized pulsed dipole antenna. Subsequently, the first-order monostatic radar cross section is developed and found to consist of Bessel functions. Simulation results for the new cross section are also provided to show the effects of barge motion under different sea states and operating frequencies. It is seen that the results have important implications in the application of HFSWR technology to ocean remote sensing.

Automatic Target Recognition in Synthetic Aperture Radar Imagery: A State-of-the-Art Review

The purpose of this paper is to survey and assess the state-of-the-art in automatic target recognition for synthetic aperture radar imagery (SAR-ATR). The aim is not to develop an exhaustive survey of the voluminous literature, but rather to capture in one place the various approaches for implementing the SAR-ATR system. This paper is meant to be as self-contained as possible, and it approaches the SAR-ATR problem from a holistic end-to-end perspective. A brief overview for the breadth of the SAR-ATR challenges is conducted. This is couched in terms of a single-channel SAR, and it is extendable to multi-channel SAR systems. Stages pertinent to the basic SAR-ATR system structure are defined, and the motivations of the requirements and constraints on the system constituents are addressed. For each stage in the SAR-ATR processing chain, a taxonomization methodology for surveying the numerous methods published in the open literature is proposed. Carefully selected works from the literature are presented under the taxa proposed. Novel comparisons, discussions, and comments are pinpointed throughout this paper. A two-fold benchmarking scheme for evaluating existing SAR-ATR systems and motivating new system designs is proposed. The scheme is applied to the works surveyed in this paper. Finally, a discussion is presented in which various interrelated issues, such as standard operating conditions, extended operating conditions, and target-model design, are addressed. This paper is a contribution toward fulfilling an objective of end-to-end SAR-ATR system design.

Surface Current Measurement Under Low Sea State Using Dual Polarized X-Band Nautical Radar

In this paper, a comparison of surface current velocity extraction under low sea state from horizontal and vertical polarized X-band nautical radar image sequences is presented. Three different current retrieval algorithms including the classical least-squares (LS) fitting, a modified iterative least-square fitting routine and an improved normalized scalar product (NSP) method have been employed. An adaptive iteration termination criterion provides optimal times of iteration for the iterative LS method. Variable-search ranges and resolutions are proposed to reduce the computational cost for the NSP method. Field data from two X-band radars deployed during a short experiment at the Skerries Bight near St. Johns, Newfoundland is analyzed. Comparison of the results derived from the radar and in-situ buoy data shows that vertical polarization leads to better current measurements than horizontal polarization even under very low sea state. A performance comparison of LS, iterative LS and NSP algorithms indicates that the latter two provide reliable results, with the current measurements from the NSP method being the least variable.

On the Development of a Second-Order Bistatic Radar Cross Section of the Ocean Surface: A High-Frequency Result for a Finite Scattering Patch

The development of a model for the second-order bistatic high-frequency (HF) radar cross section on an ocean surface patch remote from the transmitter and receiver is addressed. A new approach is taken that allows a direct comparison with existing monostatic cross sections for finite regions of the ocean surface. The derivation starts with a general expression for the bistatically received second-order electric field in which the scattering surface is assumed to be of small height and slope. The source field is taken to be that of a vertically polarized dipole, and it is assumed that the ocean surface can be described, as is usually done, by a Fourier series in which the coefficients are zero-mean Gaussian random variables. Subsequently, a bistatic cross section of the surface, normalized to patch area, is derived. The result is verified by the following two means: 1) the complete form of the bistatic HF radar cross section in backscattering case is shown to contain an earlier monostatic result that has, itself, been used extensively in radio oceanography applications; and 2) the bistatic electromagnetic coupling coefficient is shown to reduce exactly to the monostatic result when backscattering geometry is imposed. The model is also depicted and discussed based on simulated data

HF radar wave and wind measurement over the Eastern China Sea

High-frequency (HF) radar can be employed to measure sea surface state parameters such as waveheight, wind field, and surface current velocity. This paper describes the application of the HF ground wave radar in remote sensing the surface conditions over the Eastern China Sea in October 2000. The radar, referred to as the OSMAR2000, was developed by Wuhan University. Preliminary wave spectra, waveheights, and wind fields estimated from the collected data are presented and compared with ship-recorded measurements where such are available. The range for wind direction sensing is up to 200 km. Wave information and wind speed can be provided up to a range of 120 km. The mean difference between radar- and ship-measured significant waveheight is 0.323 m; wind direction is measured within 20/spl deg/; and wind speed to within 0.6 m/s. With such agreement being fairly reasonable, the feasibility of the inversion algorithm and the ocean state real-time sensing capability of OSMAR2000 are demonstrated.

The Second-Order High Frequency Radar Ocean Surface Cross Section for an Antenna on a Floating Platform

The second-order high frequency (HF) radar cross section (RCS) of the ocean surface, normalized to the area of the scattering patch, is derived for the case in which the radar transmitting and receiving antennas are mounted on a swaying platform or barge. The second-order result includes both electromagnetic and hydrodynamic contributions. The derivation for the hydrodynamic patch scatter component, for time pulsed radars, is based on the first-order RCS found in the counterpart of this paper by replacing the first-order ocean wave spectrum with the second-order ocean wave spectrum. The electromagnetic patch scatter development begins with a general expression for the bistatically received second-order electric field in which platform sway is introduced. Based on an assumption that the ocean surface can be described as a Fourier series whose coefficients are random variables, the second-order monostatic RCS is developed. The resulting second-order cross section is found to consist of Bessel functions and no singularity exists in the newly derived electromagnetic coupling coefficient. Simulation results for the new RCS are also provided to indicate the effects of barge motion under a variety of sea states.

Measurement of Sea Surface Wind Direction Using Bistatic High-Frequency Radar

A method for extracting sea surface wind direction information from bistatic high-frequency (HF) radar Doppler spectra is presented. By analogy to the monostatic case, the ratio of the intensities of the positive and negative bistatic Bragg peaks is used to derive the (ambiguous) wind direction. For bistatic operation, the reference is taken with respect to the scattering ellipse normal rather than the radar beam direction. The method is shown to be valid based on simulated bistatic HF radar Doppler spectra. Wind direction is also extracted from the bistatic radar data collected on the Southern China coast. Comparison between the radar-measured wind directions and those obtained from the Advanced Scatterometer shows good agreement.

A Self-Adaptive Wavelet-Based Algorithm for Wave Measurement Using Nautical Radar

In this paper, a self-adaptive 2-D continuous-wavelet-transform-based algorithm for extracting wave information from X-band nautical radar images is presented. After investigating the 2-D continuous wavelet transform and its application for radar image processing, it is found that the wavelet scaling parameters will affect the results of wave field analysis. The relation of the scaling parameters to the minimum distinguishable wavenumber is developed using a calibration factor. Optimal empirical values of such calibration factors are determined from a series of simulation data tests for variable wave conditions. An iterative algorithm is then proposed that enables the system to automatically select the optimal calibration factor without requiring a reference to other instrumentation. The algorithm is evaluated using dual-polarized radar data collected on the east coast of Canada. Results of the proposed algorithm are analyzed and compared with in situ TRIAXYS wave buoy data as well as that obtained from the conventional 3-D fast Fourier transform (FFT)-based method. The impact of signal polarization on the results is explored. The agreement between the buoy and FFT results indicates that the proposed algorithm is practical and effective as an alternative to the classic 3-D FFT-based method for retrieving ocean wave information.

Measurement of ocean surface currents using a long-range, high-frequency ground wave radar

High-frequency (HF) ground wave radar (GWR) is emerging as a significant tool for monitoring ocean surface conditions at ranges well beyond the line-of-sight horizon that limits conventional systems. An experimental GWR system at Cape Race, Newfoundland, Canada that has been operational since 1991, has the ability to performing routine surveillance of oceanic surface parameters and surface target detection. Operating in the frequency range between 5 and 8 MHz, the frequency modulated interrupted continuous wave (FMICW) radar has a nominal range capability of 200 km. An experiment was performed during the period of October 20-November 21, 1992 to test the surface current measuring capability of the Cape Race system. Here, near real-time radial surface current information is extracted from the Doppler spectra of the radar time series data and a comparison is performed to the Lagrangian velocities derived from the position-time tracks of Accurate Surface Tracker (AST) drifters. A wide range of oceanic conditions were experienced during the experimental period, and favorable results were obtained from the comparison regardless of the sea state conditions. The analysis shows the standard deviation in the radar radial velocity component to be approximately 5 cm/s. >

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.