Synthetic Aperture Interferometric Radiometer Research Articles

A PASSIVE MILLIMETER-WAVE IMAGER USED FOR CONCEALED WEAPON DETECTION

An 8mm-band passive millimeter-wave imager BHU-2D has been developed by Beihang University. This imager is designed for detecting concealed weapons on human body. The imager adopts two-dimensional synthetic aperture interferometric radiometer (SAIR) technique which avoids the radiation risk to human body. Compared with scanning technique, SAIR technique could obtain images of a larger fleld of view (FOV) while achieving high imaging rate, which are necessary for security monitoring. In this paper, the imaging principle of SAIR is stated flrstly. Secondly, background cancellation method designed for BHU-2D is interpreted. The technique is used to reduce the complexity as well as the dimensional requirements of the receiving elements. Thirdly, system conflguration is illustrated in detail. Then external point source calibration method is introduced and discussed speciflcally for security check applications. Finally, imaging experiments on a person with concealed weapon are conducted by which the design and image calibration algorithms are verifled. To conclude, experimental results prove that BHU-2D could be employed in security check applications.

NEAR FIELD IMAGE RECONSTRUCTION ALGORITHM FOR PASSIVE MILLIMETER-WAVE IMAGER BHU-2D-U

A passive millimeter-wave imager BHU-2D-U based on synthetic aperture interferometric radiometer (SAIR) technique has been developed by Beihang University. The imager is designed for detecting concealed weapons on human body and operated under the near-field condition of the antenna array, thus the conventional Fourier imaging theory does not apply. In this paper, an accurate numerical image reconstruction algorithm using regularization theory is proposed. By means of adding a prior information of desired brightness temperature image, the influences of measurement noise and focusing error on the reconstructed image have been reduced. Numerical simulations and experiments on BHU-2D-U have been performed to verify the superiorities of the proposed algorithm over the corrected Fourier method and the Moore-Penrose pseudo inverse method. The results demonstrate that the proposed method is an advantageous imaging algorithm for near-field millimeter-wave SAIR.

LOCAL OSCILLATOR UNCORRELATED PHASE NOISE ANALYSIS FOR MILLIMETER-WAVE PASSIVE IMAGER BHU-2D FREQUENCY SYNTHESIZER

In this paper, a nontrivial local oscillator uncorrelated phase noise analysis is proposed for frequency synthesizer of a pas- sive millimeter-wave Synthetic Aperture Interferometric Radiometer (SAIR) imager BHU-2D designed for concealed weapon detections on human bodies with high imaging rates. The frequency synthesizer pro- vides local oscillator signals for both millimeter-wave front-ends and intermediate frequency I/Q demodulators for the receivers. The in- ∞uence of local oscillator uncorrelated phase noise in difierent ofiset frequency ranges on the visibility phase errors have been systemati- cally investigated, and the corresponding system-level visibility speci- flcations are drawn. The integrated RMS phase error has been applied to set uncorrelated phase noise requirements in the most critical ofiset frequency range for visibility error control. The synthesizer design is given, and measurement results have proved that the visibility phase error requirement is achieved by the PN analysis method proposed with system-level visibility error tests performed. To conclude, the phase noise efiects on SAIR visibility phase errors are investigated by the- ory, and are properly limited by the PN requirement analysis method to ensure that the system-level visibility phase error speciflcation is satisfled.

Optimum Intercalibration Time in Synthetic Aperture Interferometric Radiometers: Application to SMOS

Interpolation strategies for calibration of the Soil Moisture and Ocean Salinity (SMOS) mission of the European Space Agency are tested and compared. Calibration strategy (how and how often) is critical in achieving the required performance of any instrument, but it is even more important in very complex instruments such as the new family of synthetic aperture interferometric radiometers and, in particular, in the Microwave Imaging Radiometer by Aperture Synthesis instrument aboard the SMOS mission. On one hand, frequent calibration reduces the available observation time. On the other hand, the calibration requirements for soil moisture applications are more relaxed than those for ocean salinity, so the intercalibration time requirements are very different. Since SMOS drifts are stationary, half-orbit information is available to perform different interpolation strategies. In this letter, these approaches are tested to estimate the calibration parameters between consecutive calibrations. The average root-mean-square phase error is then used to find the optimum interpolation strategy and intercalibration time. On the other side, in real-time instruments, the “future” calibration data are not available at the time of taking the measurements, and predictors are required to estimate the evolution of the calibration parameters from past data only. For these systems, the extended Kalman filter can be used. The intercalibration time in a real-time instrument is evaluated, and the requirements and performances are compared to offline instruments.

Calibration, Performance, and Imaging Tests of a Fully Digital Synthetic Aperture Interferometer Radiometer

This work presents the calibration, characterization, and imaging tests of the Passive Advanced Unit-Synthetic Aperture instrument (PAU-SA). PAU-SA is a fully digital Y-shaped two-dimensional synthetic aperture interferometric radiometer operating at the Global Positioning System (GPS) L1 band (fL1 = 1.57542 GHz ), conceived as a test-bed for potential technological improvements in future instruments. Calibration of instrumental offsets is performed by looking to a microwave absorber and the “cold” sky. Since the instrument operates in the GPS L1 band, GPS satellites are imaged therefore, a new way to compute the Flat Target Response (FTR) has been devised. Internal phase/amplitude calibration is performed using a new technique that consists of injecting pseudo-random noise signals. Different paths from the input switch to the antennas are calibrated by means of an external beacon. A near-field to far-field transformation is applied to compensate for differences in the propagation paths. Finally, absolute amplitude calibration is achieved by imaging the GPS satellites constellation when pointing to the zenith. Evaluation of the images' quality in terms of angular resolution, radiometric resolution and precision, show the goodness of the techniques applied to compensate for instrumental errors, the imaging capabilities of the instrument, and demonstrate their applicability in future missions.

STATISTICAL POWER MEASUREMENT UNIT FOR AN 8 MM-BAND TWO DIMENSIONAL SYNTHETIC APERTURE INTERFEROMETRIC RADIOMETER BHU-2D

An 8mm-band two-dimensional Synthetic Aperture Inter- ferometric Radiometer (SAIR) called BHU-2D has been developed by Electromagnetics Engineering Laboratory of Beihang University. The radiometer could obtain images in real-time beneflting from the adop- tion of a 1bit/2level FPGA-based correlator array. The correlator array requires a group of Power Measurement Units (PMUs) to de- normalize the correlation coe-cients into visibility function samples. The design and implementation of the PMU in BHU-2D is presented in this paper. The PMU adopts a novel method based on probability statistics. The principle and quantitative error analysis of this power measurement method is presented. In order to verify the principle of the design, a sample board is manufactured and a series of validation experiments have been conducted. Measurement results have proved that the performance of the PMU could meet the requirements of SAIR systems. The PMU has been applied to BHU-2D and the result is sat- isfactory.

Gyrosynchrotron Radiation of Interplanetary CMEs

CMEs (Coronal Mass Ejections) are an important means of energy release in the solar corona. Solar Polar Orbit Radio Telescope (SPORT) is a mission being proposed for observing the propagation of interplanetary CMEs from solar polar orbit. The main payload onboard SPORT is a synthetic aperture interferometric radiometer, which receives radio emission of interplanetary CMEs. It is identified that there are mainly three radio emission mechanisms of CMEs, <I>i.e.</I>, bremsstrahlung, gyrosynchrotron emission and plasma emission. Among these emission types, bremsstrahlung emission is the main emission mechanism of the high-density plasma clouds of interplanetary CMEs. Gyrosynchrotron emission is the continuous emission generated by high-energy electrons from CMEs, while plasma emission is the main mechanism of transient radio bursts from CMEs. In this paper, the gyrosynchrotron emission of interplanetary CMEs is focused on. Firstly, the mechanism of gyrosynchrotron emission is reviewed. Secondly, a review of the physical parameter models of background solar wind and interplanetary CMEs is presented. After these, the brightness temperature and polarization of gyrosynchrotron emission of interplanetary CMEs are calculated and analyzed. Finally, the detectability of gyrosynchrotron emission of interplanetary CMEs by radio meters is discussed briefly.

Design of 183.31GHz Interferometer with Two Elements

Microwave emissions at various frequencies are known to contain information on atmospheric chemistry. Information about water vapor is of great interest at 183.31GHz. Interferometric synthetic aperture radiometer avoids the disadvantages of manufacturing and carrying large antennas at the expense of complexity. This paper has discussed the design of the 183.31GHz interferometer system with two elements based on imaging theory and retrieving algorithm of two-dimensional synthetic aperture radiometer. And then several important experimental results are presented.

Imaging algorithm for synthetic aperture interferometric radiometer in near field

With the benefits of digital IC technology development, the synthetic aperture interferometric radiometer (SAIR) technique is growing fast and expanding to more and more application areas. The near field imaging detection is a potential application which has received increasing demand recently. Because the Fourier imaging theory of the traditional SAIR is based on far-field approximation, it will be invalid for near-field condition. This paper is devoted to establishing a new accurate imaging algorithm for near-field SAIR imaging. Firstly, the visibility function in near field is deduced and the relationship of which to far-field visibility function is analyzed. Then, a numerical method based on pseudo inverse and focal plane approximation is developed. The effectivity of this method is tested with imaging simulation of point source and extended source, and the superiority is also demonstrated by comparing with the existing phase-modified Fourier transform method. At last, the field experiment with one-dimensional SAIR instrument is performed to validate the practical feasibility of this method.

Modeling and Analysis of Polarimetric Synthetic Aperture Interferometric Radiometers Using Noise Waves

Polarimetric synthetic aperture interferometric radiometers are analyzed, including the polarization leakage and other front-end non-idealities that can be measured and represented with S-parameters. The analysis utilizes the noise wave concept in order to account for the amplitude and phase properties of the front-end. The method is applied to a model of an entire synthetic aperture interferometric radiometer. The simulations can be used to examine and retrieve requirements for the system parameters. The feasibility of the method is demonstrated and examples of end-to-end simulation results are also given in this paper.

Near Field Imaging of Synthetic Aperture Radiometer

The near field applications of synthetic aperture interferometric radiometer (SAIR) axe receiving great interest in the recent years. Because the traditional far field Fourier imaging theory can not stand any longer in near field condition, developing the associate imaging theory will be an important objective of SAIR technique. This work is devoted to explore an effective near field imaging method for SAIR systems with traditional far field plane antenna array. The discrete numerical inversion method with focus plane approximation is developed. The Tikhonov regularization method is introduced to deal with the ill condition and measurement errors.

Evaluation of Imaging Performance for Sub-Y-type Interferometric Synthetic Aperture Radiometer

The two-dimensional interferometric synthetic aperture radiometer measures the brightness temperature distribution using two-dimensional antenna array without scanning. It has been reported that Y-type array configuration with equally spaced antennas is optimal in terms of a narrow 3dB beamwidth and wide synthesized field-of-view (FOV). The sub-Y-type array was suggested to improve the spatial resolution than that of conventional Y-type array with the same number of antenna elements. To evaluate it, 37 GHz two-channel interferometric radiometer demonstration model is implemented. The angular resolution of sub-Y-type array was compared with that of Y-type array with the same number of antennas. In this paper, the performance of sub-Y-type is evaluated under the some two-dimensional targets such as the contiguous square objects. The images of sub-Y-type and Y-type array were simulated. The imaging features were compared. The sub-Y-type has higher spatial resolution than Y-type in case of the contiguous target as well as a single point source.

Calibration and experimental results of a two‐dimensional interferometric radiometer laboratory prototype

In recent years, Earth observation by means of aperture synthesis radiometry has received special attention by some space agencies as a possible solution to achieve high radiometric accuracy and spatial resolution at low microwave frequencies (L band), where the apparent brightness temperature is much more sensitive to soil moisture and sea surface salinity. This paper presents the characterization and calibration procedure, as well as some synthetic images measured with an X band experimental Y‐shaped Synthetic Aperture Interferometric Radiometer prototype developed at the Polytechnic University of Catalonia. The instrument is composed of a single pair of antennas that can be moved along the arms of an Y structure to synthesize a set of baselines. An experimental procedure is proposed to evaluate and then calibrate offset, inphase, quadrature, and amplitude errors generated by receivers and correlators.

Digital correlators for synthetic aperture interferometric radiometry

A numerical simulator is developed to assess various design implications of a digital correlator used by a synthetic aperture interferometric radiometer (SAIR). The simulator permits control of the type of digitization, the digitization thresholds with respect to noise power, and the degree of correlation between the two antenna signals which are being cross correlated. Digitization schemes are considered which use 2, 3, 4, and 8 levels. Estimates are made of the increase in inherent radiometer noise (/spl Delta/T) due to the digitization. The increase in /spl Delta/T is found to depend strongly on the degree of correlation, with higher correlations suffering less increase. In most cases, 3 level digitization is recommended based on this sensitivity consideration. Two levels perform significantly noisier and four levels only slightly cleaner. Several case studies are also considered regarding the need to control the signal level relative to the digitizer thresholds. Automatic gain control circuitry prior to digitization is found not to be necessary, provided the thresholds are preset within a fairly broad region of minimum sensitivity to variations in signal power, and provided the system noise temperature of the radiometer is monitored with reasonable accuracy. It is also found that, under conditions of very high correlation between the two signals, digital correlators have better SNR performance than analog. Reasons for this behavior are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Error analysis of image reconstruction by a synthetic aperture interferometric radiometer

An error analysis is presented for the image reconstruction algorithm currently employed by the electronically steered thinned array radiometer aircraft instrument. This instrument samples components of the Fourier transform of the brightness temperature distribution at 1.4 GHz. Sources of error are identified, including systematic offsets and stochastic noise in the measurements, errors in the interference pattern antenna calibration, multipath scattering from the antenna support structure, mutual coupling between nearby array elements, and variations in the antenna patterns of the array elements. These errors are shown to produce artifacts in the image which appear as “streaks” running along the instrument flight line. Possible hardware corrections are discussed. These include a reduction in the profile of the antenna support structure and a decoupling of adjacent and other nearby array elements using corrugated “skirts.” Also discussed are several image enhancement techniques which detect and correct for some of the streaking. These techniques include a renormalization of the interference patterns based on the measured element patterns, a perturbation based interpretation of image contrast, and a covariance analysis of noise present in the image.