Yaoting Zhu

A Method for Seeking Low-Redundancy Large Linear Arrays in Aperture Synthesis Microwave Radiometers

For one-dimensional aperture synthesis microwave radiometers, the optimal placement of antenna elements in a low-redundancy linear array (LRLA) is difficult when large numbers of elements are involved. In this paper, the general structure of large LRLAs is summarized first, and then a novel stochastic optimization technique, ant colony optimization (ACO), is applied to the search for low redundancy arrays. By combining the general structure with the ACO procedure, an efficient method is proposed for a rapid exploration for optimal array configurations. Numerical studies show that the method can generate various large LRLAs with lower redundancy than the previous algorithms did and the computational cost is greatly reduced. Based on the method, several analytical patterns for LRLAs are further derived, which can yield various array configurations with very low redundancy in nearly zero computation time. Both the method and the resulting configurations can be utilized to facilitate antenna array design in synthetic aperture radiometers with high spatial resolution.

A General Platform for Millimeter Wave Synthetic Aperture Radiometers

A general platform for millimeter wave synthetic aperture radiometers is introduced in this paper. The platform consists of hardware and simulation software, and has a flexible and open structure. Simulation and experiment results show the platform is suitable for simulation of synthetic aperture radiometer performance, algorithm performance, error influence on radiometer performance, and calibration performance. The platform can be used for radiometry simulation and design of synthetic aperture radiometers.

One-Dimensional Mirrored Interferometric Aperture Synthesis

Aperture synthesis technique can provide high spatial resolution without requiring very large and massive real aperture. However, for a large aperture synthesis system, which will involve hundreds of antennas and thousands of correlators, system and its calibration are very complicated. In this letter, mirrored interferometric aperture synthesis (MIAS) is proposed, which can achieve the same spatial resolution as a large traditional aperture synthesis system but needs fewer antennas. First, the imaging principle of MIAS is presented. A new concept, named Cosine Visibility, is proposed to replace the visibility function used in traditional aperture synthesis. The relationship between the brightness temperature and the cosine visibility is a Cosine Transform. Inverse Cosine Transform can be applied to reconstruct the image of brightness temperature. Then, an example for MIAS is given, and a method is proposed to handle the problems of baselines missing and the rank defect associated with the underdetermined linear equations which relate the cross correlation between signals collected by pairs of antennas with the cosine visibility. Moreover, MIAS with a small array can provide higher and higher spatial resolution by combining more and more linear equations with the help of adjusting the distance from the array to the reflector. The simulation results demonstrate the validity of MIAS and the improvement of spatial resolution. The study in this letter provides a means to reduce the complexity of system and calibration of large traditional aperture synthesis systems.

One-Dimensional Mirrored Interferometric Aperture Synthesis: Performances, Simulation, and Experiments

Mirrored interferometric aperture synthesis (MIAS) can reduce the complexity of large IAS systems. In this paper, UV sampling, spatial resolution, sensitivity, and image reconstruction of 1-D MIAS are analyzed, which are not addressed before. Numerical simulation and experiments are carried out to illustrate the performances of MIAS. The results demonstrate the validity of MIAS and the improvement of spatial resolution.

A Sparse Antenna Array with offset Parabolic Cylinder Reflector at Millimeter Wave Band

In order to meet the spatial resolution requirement of passive microwave remote sensing from space, aperture synthesis radiometer is used. An antenna array is an important part in synthetic aperture radiometer. A Sparse Antenna Array with Parabolic Cylinder Reflector at Millimeter Wave Band is presented in this paper. The overall requirements for antenna array are listed, followed by design and characterization of the entire antenna array architecture and each part of it including offset reflector geometry, sparse feed array and support structures. The theoretical computation and measured results of electrical parameters of the antenna array are also summarized.

A general signal-level simulator for aperture synthesis radiometers

Aperture synthesis technique is a feasible means to provide high spatial resolution at low frequencies. But due to the impacts of system errors, time varying scenes, and different inversion algorithms, analytic studies of system performances are difficult or impossible, and the need of numerical simulation appears. In this paper, a signal-level simulator is presented. It covers brightness temperature generator, the process of antenna array collecting thermal radiation and transforming thermal radiation signal to RF signal, amplifying and filtering of receivers, complex cross-correlation, and inversion. Simulations are conducted, and the results demonstrate the validity of the simulator. The simulator provides a means to assess measurement accuracy and sensitivity, as well as to evaluate the calibration approach, investigate the inversion robustness in front of errors and so on.

Statistical Distribution of Visibilities in Digital Aperture Synthesis Radiometry

The statistical characteristics of visibilities measured by digital complex correlators in aperture synthesis radiometry are investigated. First, the analytic probability density function of visibilities is derived under the assumption that the successive samples are Gaussian distributed and statistically independent. However, in practice, the assumption may be not satisfied due to non-Nyquist sampling and quantizing. Then, to account for such impacts, an effective sample size is proposed. By compensating with it, the statistical distribution of visibilities is well represented in closed form. The experimental results by using an aperture synthesis millimeter-wave radiometer demonstrate the validity of the improved distribution. The results of this letter can be applied to sensitivity estimation and high-accuracy image reconstruction in digital aperture synthesis radiometry.

The Synchronization Testing for Multi-Channel Data Acquisition System of SADR

The synthetic aperture digital radiometer (SADR) can achieve the finest resolving power with an antenna array that uses a relatively small number of individual elements, in which the phase coherence is essential to image of SADR. Due to jitter of sampling clock and trigger signal, the multi-channel data acquisition system (MCDAS) is very difficult to sample data simultaneously of different receivers, which will result in correlation loss and blur the imaging of SADR. This paper uses orthogonal transformation method to test the sampling error of MCDAS, to correct the sampling skew by the phase calibration method. Experiment result showed: sampling skew between different channels reduces to 20 ps below after phase calibration.

An improved spatial impulse-response matrix for error calibration of aperture synthesis radiometers

The system imperfections have great influence on the performance of aperture synthesis radiometers (ASRs). An improved spatial impulse-response matrix (G matrix) is presented for error calibration of ASRs. Most receiver errors except for the baseline fringe-washing error can be eliminated with the improved G matrix in the image reconstruction of ASR without need for complex hardware calibration system. Simulations show the improved G matrix can improve image quality of inversed image of ASRs. Experiments validate the performance of the improved G matrix.

Analysis of statistic model of digital radiometer detector

The digital radiometer detector (DRD) is a digital signal processing detector, of which the essential elements hardware consist of a high-speed, high-resolution analog-to digital converter (ADC). A practical ADC will have an uneven step size (differential nonlinearity) between quantization thresholds, and changes the probability distribution of the sampled data and further distorts the input signal, statistics models may also be limited by deterministic interference coming from local oscillator (LO) leakage. This paper is to implement a high speed data acquisition instrument that can record the digital data stream in real time, develops an approach using high-order moment to measure kurtosis of the signal of DRD.