Min Ho Ka

Model-Associated Forest Parameter Retrieval Using VHF SAR Data at the Individual Tree Level

A simple statistical extension at the individual tree level for an earlier-developed very high frequency forest backscatter model is proposed. This extended model treats trunk volumes as random quantities. A concept of random forest reflection coefficient is also introduced to characterize radar returns from individual trees. Based on the extended model, a set of algorithms for estimating the mean trunk (stem) volume from synthetic aperture radar data at the individual tree level is developed assuming that the areal tree density is known. The algorithms are specified for different scenarios related to a priori information on parameters of statistical distributions for the trunk volume and fluctuations of the forest reflection coefficient. An approximate lower bound on the standard deviation in the unbiased estimation of the mean trunk (stem) volume is proposed. This bound can be readily obtained by means of computer simulation for any specified statistical distribution for the trunk volume and fluctuations of the forest reflection coefficient. Performance analysis for the proposed algorithms is numerically performed by means of Monte Carlo simulation for a variety of scenarios. This analysis has shown that the algorithms provide nearly unbiased and efficient estimates, and the proposed lower bound is a very accurate approximation. The results of the study have demonstrated that the approach and methods developed in this paper suggest promising solutions in accurate forest parameter retrieval.

Effect of Look Angle on the Accuracy Performance of Fixed-Baseline Interferometric SAR

The effect of look angle on the accuracy performance of fixed-baseline interferometric synthetic aperture radar is studied. It is shown that there exists an optimal look angle that minimizes the variance of the surface height estimate for a resolution cell, and it depends upon the system as well as surface parameters. Numerical analysis confirming the existence of the optimal look angle is presented

Selection of Pulse Repetition Frequency in High-Precision Oceanographic Radar Altimeters

A readily computable formula to perform versatile analysis of the nonstationary pulse-to-pulse correlation properties of ocean altimeter returns is presented. Using this formula, the correlation coefficient and correlation interval depending on the reference time, sea state, and system parameters have been analyzed. The results have suggested that the pulse repetition frequency should adaptively be adjusted to the sea state to ensure precise altitude measurements.

Precision Near-Field Reconstruction in the Time Domain via Minimum Entropy for Ultra-High Resolution Radar Imaging

Ultra-high resolution (UHR) radar imaging is used to analyze the internal structure of objects and to identify and classify their shapes based on ultra-wideband (UWB) signals using a vector network analyzer (VNA). However, radar-based imaging is limited by microwave propagation effects, wave scattering, and transmit power, thus the received signals are inevitably weak and noisy. To overcome this problem, the radar may be operated in the near-field. The focusing of UHR radar signals over a close distance requires precise geometry in order to accommodate the spherical waves. In this paper, a geometric estimation and compensation method that is based on the minimum entropy of radar images with sub-centimeter resolution is proposed and implemented. Inverse synthetic aperture radar (ISAR) imaging is used because it is applicable to several fields, including medical- and security-related applications, and high quality images of various targets have been produced to verify the proposed method. For ISAR in the near-field, the compensation for the time delay depends on the distance from the center of rotation and the internal RF circuits and cables. Required parameters for the delay compensation algorithm that can be used to minimize the entropy of the radar images are determined so that acceptable results can be achieved. The processing speed can be enhanced by performing the calculations in the time domain without the phase values, which are removed after upsampling. For comparison, the parameters are also estimated by performing random sampling in the data set. Although the reduced data set contained only 5% of the observed angles, the parameter optimization method is shown to operate correctly.

Signal Processing for a Multiple-Input, Multiple-Output (MIMO) Video Synthetic Aperture Radar (SAR) with Beat Frequency Division Frequency-Modulated Continuous Wave (FMCW)

In this paper, we present a novel signal processing method for video synthetic aperture radar (ViSAR) systems, which are suitable for operation in unmanned aerial vehicle (UAV) environments. The technique improves aspects of the system’s performance, such as the frame rate and image size of the synthetic aperture radar (SAR) video. The new ViSAR system is based on a frequency-modulated continuous wave (FMCW) SAR structure that is combined with multiple-input multiple-output (MIMO) technology, and multi-channel azimuth processing techniques. FMCW technology is advantageous for use in low cost, small size, and lightweight systems, like small UAVs. MIMO technology is utilized for increasing the equivalent number of receiving channels in the azimuthal direction, and reducing aperture size. This effective increase is achieved using a co-array concept by means of beat frequency division (BFD) FMCW. A multi-channel azimuth processing technique is used for improving the frame rate and image size of SAR video, by suppressing the azimuth ambiguities in the receiving channels. This paper also provides analyses of the frame rate and image size of SAR video of ViSAR systems. The performance of the proposed system is evaluated using an exemplary system. The results of analyses are presented, and their validity is verified using numerical simulations.

Enhancement of the sensitivity of the dual-frequency correlation radar over sea surface roughness in a small satellite

The possibility of sea surface roughness estimation using the dual-frequency cross-correlation function of reflected signals with the correlation radar in a small satellite was analyzed. We showed the possibility of reducing the influence of the decorrelation factor on the dual-frequency correlation function, by synthesizing the antenna aperture in each frequency channel and by constructing a cross-track linear antenna array which is perpendicular to the flight direction of the satellite. Finally, we achieved enhancement of the sensitivity of the measurement characteristic with an array of small antennas, which is particularly important for the remote sensing system in a small satellite.

Wideband Grating-Lobe Suppression by Rotation of the Phased Array Stations in the SKA Low-Frequency Sparse Aperture Array

A simple cost-effective technique of wideband grating-lobe (GL) suppression by rotation of the phased array antennas is developed for application in a system of low-frequency sparse aperture array (LF-SAA) antennas of the square kilometer array (SKA) radio-telescope. The technique implies using of equal sparse planar circular antennas with the well-tried square lattice (SqL) distribution of the station antenna array sensors, which is known as the best in terms of production costs and maintenance charges. The peak levels of residual GLs in the fringe pattern of most pairs of the rotated antenna array stations do not exceed value of -17 dB over frequencies ranged from 70 to 450 MHz in the case of uniform antenna array element weighting, whereas the maximum levels of residual GLs are reduced to -25 dB when the Hamming weight tapering is applied.

Estimation of the Sea-Surface Slope Variance Based on the Power Spectrum Width of a Radar Scatterometer

In this letter, we introduce the possibility of the sea-surface slope measurements using second-order statistics of return waveforms from sea surface in a continuous-wave radar scatterometer. It has been shown that the estimation of the variance of sea-surface slopes can be obtained by measuring the signal power or the power-spectrum width of return waveforms. We found that the power-spectrum-width-based estimation approach is more sensitive to the variance of the sea-surface slope for weak sea-surface disturbances in contrast to the power-based one.

Measurement of the Rotation Center From the Received Signals for Ultrahigh-Resolution Radar Imaging

In this letter, we propose a method for estimating the center of rotation on an experimental radar imaging. The experiment is performed with tomographic approach to obtain the radar cross section images of the target on a rotation table. In an ultrahigh-resolution imaging using tomographic reconstruction, precise knowledge of rotation center of the turntable position is necessary since this has a big influence on the image quality. The position of rotation center can be physically measured, but the result is not accurate due to the delay introduced by the RF components and cables. Thus, we present a method to precisely determine the rotation center based on the rotational symmetry of the received data. The proposed method is verified by doing imaging of several targets using ultrawideband radar with picosecond-length pulses. The system bandwidth is larger than 30 GHz, and the imaging results have subcentimeter resolution.

Analysis of Microwave-Induced Thermoacoustic Signal Generation Using Computer Simulation

Computer simulations were conducted to demonstrate the generation of microwave-induced thermoacoustic signal. The simulations began with modelling an object with a biological tissue characteristic and irradiating it with a microwave pulse. The time-varying heating function data at every particular point on the illuminated object were obtained from absorbed electric field data from the simulation result. The thermoacoustic signal received at a point transducer at a particular distance from the object was generated by applying heating function data to the thermoacoustic equation. These simulations can be used as a foundation for understanding how thermoacoustic signal is generated and can be applied as a basis for thermoacoustic imaging simulations and experiments in future research.