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There are two bias schemes, namely current-bias and voltage-bias, for the directly coupled readout circuit of direct current superconducting quantum interference device (dc SQUID). In order to find out the difference between two bias schemes, we present the theoretical analyses on their equivalent circuits, working point locations, and noise performances. The theoretical analysis and experimental validation prove that the voltage-bias scheme achieves a larger transfer coefficient and exhibits a better noise performance than the current-bias scheme in the reading of conventional dc SQUID. [ABSTRACT FROM AUTHOR]

It has been found that when superconducting quantum interference device magnetometers operate in a geomagnetic field environment, its output will occasionally show small momentary jumps with the change of the external magnetic field, which seriously affects the measurement accuracy of the geomagnetic field and needs to be eliminated. A magnetic flux jump model called the parasitic superconducting ring model was built to explain the causes of these small jumps, and Helmholtz coils were set up to study the jump and verify this model in the laboratory. The experimental results showed that the position and amplitude of the jumps were consistent with the model. It is believed that this model can guide the design of superconducting chips in geophysical applications. [ABSTRACT FROM AUTHOR]

Practical superconducting quantum interference devices (SQUIDs) have been developed into multiloop hybrid electric circuits with Josephson junctions and normal elements such as resistor, inductor and capacitor. To find a common circuit analysis and simulation method for different types of SQUID hybrid circuits consist of both superconducting and normal components, we introduce a mesh-current method, which can derive both the general system model and uniform circuit equations directly from the equivalent circuit. The mesh-current method is demonstrated in the simulation of a SQUID additional positive feedback circuit. The numerical simulation results are presented and verified with the measurements. Compared with the conventional nodal analysis method, the mesh-current method is excellent in dealing with the relations including external flux, self and mutual inductances in multiloop hybrid circuit analysis; its final circuit equations directly exhibit the physical theorems; its general system model reveals the quantum interference mechanism inside SQUID circuits, and is applicable for further dynamics studies. [ABSTRACT FROM AUTHOR]

Direct current superconducting quantum interference device (dc SQUID) can be regarded as a hybrid system with two nonlinear Josephson currents driving a linear network consists of conventional resistors (R), inductors (L), and capacitors (C). There must be LC resonances inside the SQUID loop with influences on its static current-voltage characteristics. To study the working principle of the LC resonance, we build an equivalent dynamic system model transformed directly from the circuit equations of dc SQUID. With dynamic system analyses in both dc and alternating current (ac) domains, we derive the analytical expressions of the resonance point with only the RLC circuit parameters. The analytical derivation proves that the LC resonance inside a symmetrical dc SQUID with resonance frequency ω = 1/√LC results in the flux modulation suppression with I–V curves concentrated on one resonance point. Based on those mathematical expressions, the resonance point is easily indentified from the measured I–V curves, and can be utilized for practical SQUID parameters characterization. [ABSTRACT FROM AUTHOR]

To implement a flux quanta counting (FQC) scheme in the wide range magnetic field measurement using a superconducting quantum interference device (SQUID), we present a SQUID proportional feedback amplifier (PFA) circuit. It is a simplified version of the direct readout flux-locked loop (FLL) electronics without an integrator. It achieved a periodically repeated (with a period of 1Φ0), quasi-linear flux-to-voltage characteristics with an input range extended to 2Φ0. Its working point is reset spontaneously with 1Φ0 of flux jump caused by the over range flux input. The resultant wide range measurement is achieved by the synthesis of the PFA voltage output and the flux jumps recorded using the FQC scheme. We simulated and experimentally verified the working principles of SQUID PFA. The FQC scheme using SQUID PFA was demonstrated with measurements in both shielded and unshielded environment. SQUID PFA is simple and easy-to-operate suited for applications in the Earth's magnetic field. [ABSTRACT FROM AUTHOR]

Nonlinearity with multiple working points limits the practical applications of a two-stage superconducting quantum interference device (SQUID) readout circuit, which is promising for low-noise performance. To solve this problem, we proposed a simple two-stage SQUID readout circuit with two conventional dc SQUIDs improved with proportional feedback schemes, where transfer coefficient of the first-stage SQUID circuit is enhanced by additional positive feedback circuit; transfer characteristics of the second-stage SQUID circuit are linearized by an additional negative feedback circuit. Two proportional feedback schemes are cooperated together based on matching conditions to achieve both noise suppression and working point matching. Working principles were verified in experiment results. The overall transfer characteristics achieved have only one working point and are simply adjusted according to the voltage swing of an amplifier. Transfer coefficient of the first-stage SQUID circuit was raised up to 10 (20 dB), and the total flux noise of two-stage SQUID readout circuit was reduced below 1 μΦ0/√Hz, where Φ0 = 2.07 ×10−15 Wb. Our two-stage SQUID readout circuit is easy-to-operate and suited for high-performance practical SQUID systems. [ABSTRACT FROM AUTHOR]

This paper proposes a new scheme of motion parameter estimation and relocation for airborne three-channel circular stripmap synthetic aperture radar (CSSAR)-ground moving target indication (GMTI) systems. Compared with the conventional straight-path SAR, the parameter estimation of a target is more challenging because the target’s range history and signal model are more complicated due to the complexity of the relative motion between CSSAR and ground moving target. In this paper, the signal model of a ground moving target and the expression for its along-track interferometric (ATI) phase from the environment of airborne three-channel CSSAR are derived. The coupling effect among the target’s motion and position parameters is also figured out. Then, a scheme of motion parameter estimation and relocation is proposed. The proposed scheme utilizes the ATI phase and the quadratic-term coefficient in the range equation to estimate the target’s motion and position parameters and utilizes an iterative strategy to address the coupling effect among these parameters. Numerical simulations are conducted to validate the satisfactory performance achieved by the proposed algorithm. [ABSTRACT FROM AUTHOR]

For colocated multiple-input multiple-output (MIMO) radar target detection in colored noise, we propose two adaptive detectors according to the Rao andWald test criteria. These detectors do not need training data and possess constant false alarm rate properties. We investigate the manner in which they work. From a detection viewpoint, we show that there is no need for matched filtering for colocated MIMO radar.We derive the statistical distributions of the proposed detectors and then obtain the analytical expressions for the probabilities of false alarm and detection for both deterministic and random signals. Numerical examples are provided to compare the detection performance of the Rao andWald tests with an existing detector. [ABSTRACT FROM PUBLISHER]

The SQUID Bootstrap Circuit (SBC) for direct-coupled readout of SQUID signals in voltage bias mode was recently demonstrated. In addition to the conventional dc SQUID, the SBC incorporates a shunt resistor Rs, and two coils coupled to the SQUID via mutual inductances M1 and M2. In this paper, basic equations of SBC are formulated based on its equivalent circuit model. The expression of equivalent flux noise from the preamplifier is also given. The effect of the three adjustable parameters (M1, M2 and Rs) on the characteristics of SBC and the preamplifier noise suppression are numerically simulated. The SBC combines current and voltage feedbacks in one circuit, allowing for an effective suppression of the preamplifier voltage noise through increased flux-current transfer coefficient and dynamic resistance. In contrast to other direct-coupled schemes, it offers not only a good noise performance, but also tolerance to a wide range of adjustable parameters. [ABSTRACT FROM AUTHOR]

Range-dependent clutter suppression is a challenging problem in nonsidelooking, bistatic, and conformal antenna array airborne radar, especially in the presence of range ambiguity. Superimposed stepped frequency (SSF) radar employs a small frequency increment across the stepped signals in each pulse, which introduces a dimension related to slant range. In this article, an airborne radar framework is established, which takes SSF signal as the transmit waveform. Thus, a two-stage adaptive clutter suppression method is proposed, which utilizes the degrees of freedom in range, space, and time domains provided by airborne SSF radar. In the first stage, the secondary range dependence compensation approach is adopted to distinguish the clutter of each range region in the carrier frequency domain. Then, a target-free covariance matrix is estimated by the compensated data, and the covariance matrix is used for the range-ambiguous clutter separation in the carrier frequency domain. Thus, the space-time snapshot of each range region can be extracted. In the second stage, a clutter segmentation processing method is devised for residual clutter suppression. For the near-range unambiguous clutter, the traditional clutter compensation approach is applied to further align the spectrum distribution of clutter, while the conventional space-time adaptive processing is utilized to suppress far-range clutter directly. Since the proposed method takes full advantage of the range dimension and the two stages of adaptive processing are data-dependent, excellent clutter suppression performance can be obtained. The proposed method can also extract the real range of the target since each range region is processed separately. Simulation results demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Describes the design and experimental investigation of a magnetic torquing system for a north finder using an electrostatic gyroscope (ESG). Components of the magnetic torquing system; Test of the magnetic torquing system; Investigation of north-seeking; Conclusion.

As part of the efforts to establish a reliable fabrication process of superconducting integrated circuits at Center for Excellence in Superconducting Electronics (CENSE) in China, we investigated the influences of various deposition conditions on the properties of molybdenum (Mo) thin films. Thin-film resistors were fabricated based on these films by using a lift-off process. It was found that Mo structures made from films peeled off easily from the substrate after lift-off. We have optimized the deposition conditions to grow Mo films with compressive or weakly tensile stress. Mo thin-film resistors with a sheet resistance of several ohm/square and surface roughness of less than 1 nm were successfully fabricated and were used in the fabrication of superconducting quantum interference devices and small-scale superconducting integrated circuits. [ABSTRACT FROM AUTHOR]

Compared with conventional two-dimensional space-time adaptive processing (2D-STAP) methods, the elevation-azimuth-Doppler three-dimensional space-time adaptive processing (3D-STAP) method has the advantage of suppressing nonstationary clutter. Thereby, it is suitable for nonsidelooking airborne radar (non-SLAR) applications. However, its huge training data requirements and computational load are often beyond radar's ability in practical clutter environments. In this correspondence, we develop a simple but efficient reduced-degree-of-freedom (DOF) 3D-STAP method that significantly reduces the required training data and the computational complexity while maintaining the suboptimal clutter suppression performance. The proposed method transforms the planar array data into linear array data in azimuth and in elevation, respectively, thereby beamforming an equivalent cross-shape array prior to STAP. In consequence, only a few spatial DOFs, including azimuth and elevation dimension, are used for STAP directly resulting in the potential advantage for nonstationary clutter suppression and drastically reducing training data requirements and computational load. Furthermore, the clutter rank estimation rules of the planar array and the transformed cross-shape array are derived, and the required elevation DOFs of the proposed method are further discussed in detail. Simulations for clutter suppression of non-SLAR show that the proposed STAP method outperforms the state-of-the-art 3D-STAP method in terms of convergence and computational complexity. [ABSTRACT FROM AUTHOR]

The recognition of the interference environment for adaptive radar detection is addressed in this article. Typically, detectors are designed in one specific scenario which may not be appropriate for the varying interference environment, especially for the airborne and space-based radar system. In this article, the considered recognition task is cast in terms of multiple hypothesis tests and the theory of model order selection (MOS) techniques are exploited to devise suitable decision rules. The interference environments are divided into homogeneity, partial homogeneity, and spherically invariant random process. Three MOS techniques, namely, the Akaike information criterion (AIC), generalized information criterion, and corrected AIC, are adopted. At the analysis stage, illustrating examples for the influence of the environment model parameters on the recognition accuracy of the MOS rules are presented. Numerical experiments show the AIC rule has the most robust recognition performance. [ABSTRACT FROM AUTHOR]

A superconducting quantum interference device (SQUID) planar gradiometer is an extremely sensitive sensor for magnetic gradient measurements. It has been shown to have potential applications for aeromagnetic detection. The major challenge when operating an aeromagnetic SQUID system in actual environment is the motion noise, including the inherent response resulting from gradiometer imbalance and from magnetic interferences. Three orthogonal reference magnetometers are usually adopted to improve the gradiometer balance. However, magnetic interference coming from the system itself also needs to be compensated. In this article, a mathematical model of the motion noise picked up by the gradiometer is derived from the traditional magnetic total field compensation method. Based on the model, the signals from the triaxial magnetometer can also be used to compensate the eddy current contribution to the magnetic interference. To verify the compensation method, a SQUID planar gradiometer system was set up and used for flight trials. The gradient field noise and imbalance of our homemade gradiometer were measured to be 100 fT/m/rt(Hz) and 2E-4 in the lab. In flight, the motion-induced peak-to-peak output of the gradiometer was reduced from 170 to about 0.1 nT/m, so that a magnetic anomaly signal of about 2 nT/m from a cargo ship was clearly recognized. [ABSTRACT FROM AUTHOR]

Transient electromagnetic method (TEM) is an effective way for mineral resource prospecting if an induction coil is used as the traditional sensor. Superconducting quantum interference device (SQUID) obtains excellent sensitivity at low frequency as compared to coil. Thus, the exploration depth and the accuracy of TEM can be greatly improved. However, an ultrahigh slew rate (>10 mT/s) is needed for transient primary field recording, which is a great challenge for a single high-sensitivity SQUID receiver. In this paper, two SQUIDs with different sensitivities (1.5 nT/Φ0 and 99 nT/Φ0) are employed to record the whole TEM decaying signal. The high-sensitivity SQUID (1.5 nT/Φ0) is in reset status when transmitting and is locked after tens of microseconds after transmitting has been cut off. The low-sensitivity SQUID (99 nT/Φ0) remains locked during the whole procedure with high slew rate (∼198 mT/s) to record the fast-switching transmitting field. The two channel SQUID outputs are combined together to form a complete decaying curve. A field test was carried out. The decaying curve with a noise level of 100 fT@DC-25 kHz and a fast-switching field of ∼630 nT cutting off in tens of microseconds (slew rate up to ∼56 mT/s) was recorded after stacking. This dual-channel SQUID-based TEM system obtains high stability, which is useful for small transmitting loop (<50 m) configuration. [ABSTRACT FROM AUTHOR]

The scene text interpretation is a critical part of the natural scene interpretation. Currently, most of the existing work is based on high-end graphics processing units (GPUs) implementation, which is commonly used on the server side. However, in Internet of Things (IoT) application scenarios, the communication overhead from the edge device to the server is quite large, which sometimes even dominates the total processing time. Hence, the edge-computing oriented design is needed to solve this problem. In this paper, we present an architectural design and implementation of a natural scene text interpretation (NSTI) accelerator, which can classify and localize the text region on pixel-level efficiently in real-time on mobile devices. To target the real-time and low-latency processing, the binary convolutional encoder–decoder network is adopted as the core architecture to enable massive parallelism due to its binary feature. Massively parallelized computations and a highly pipelined data flow control enhance its latency and throughput performance. In addition, all the binarized intermediate results and parameters are stored on chip to eliminate the power consumption and latency overhead of the off-chip communication. The NSTI accelerator is implemented in a 40 nm CMOS technology, which can process scene text images (size of 128 × 32) at 34 fps and latency of 40 ms for pixelwise interpretation with the pixelwise classification accuracy over 90% on ICDAR-03 and ICDAR-13 dataset. The real energy-efficiency is 698 GOP/s/W and the peak energy-efficiency can get up to 7825 GOP/s/W. The proposed accelerator is 7 times more energy efficient than its optimized GPU-based implementation counterpart, while maintaining a real-time throughput with latency of 40 ms. [ABSTRACT FROM AUTHOR]

In this part of the paper, we continue to study the problem of detecting a double subspace signal in Gaussian noise. Precisely, we address the detection problem in partially homogeneous environments, where the primary and secondary data share the same covariance matrix up to an unknown scaling factor. We derive the generalized likelihood ratio test (GLRT), Rao test, Wald test, and their two-step versions. We also introduce three spectral norm tests (SNTs). All these detectors possess the constant false alarm rate (CFAR) property. Moreover, various kinds of special cases of these detectors are given. At the stage of performance evaluation, we consider two cases. One is the case of no signal mismatch. The other is more general, namely, the case of signal mismatch, including the column-space signal mismatch and row-space signal mismatch. [ABSTRACT FROM PUBLISHER]

In this two-part paper, we consider the problem of adaptive multidimensional/multichannel signal detection in Gaussian noise with unknown covariance matrix. The test data (primary data) is assumed as a collection of sample vectors, arranged as the columns of a rectangular data array. The rows and columns of the signal matrix are both assumed to lie in known subspaces, but with unknown coordinates. Due to this feature of the signal structure, we name this kind of signal as the double subspace signal. Part I of this paper focuses on the adaptive detection in homogeneous environments, while Part II deals with the adaptive detection in partially homogeneous environments. Precisely, in this part, we derive the generalized likelihood ratio test (GLRT), Rao test, Wald test, as well as their two-step variations, in homogeneous environments. Three types of spectral norm tests (SNTs) are also introduced. All these detectors are shown to possess the constant false alarm rate (CFAR) property. Moreover, we discuss the differences between them and show how they work. Another contribution is that we investigate various special cases of these detectors. Remarkably, some of them are well-known existing detectors, while some others are still new. At the stage of performance evaluation, conducted by Monte Carlo simulations, both matched and mismatched signals are dealt with. For each case, more than one scenario is considered. [ABSTRACT FROM PUBLISHER]

We recently presented a direct readout technique for the dc Superconducting QUantum Interference Device (SQUID) without flux modulation (FM), operated in voltage bias mode, and named it the SQUID Bootstrap Circuit (SBC). The SBC combines additional voltage and current feedbacks to minimize the room-temperature preamplifier noise. The main point of this paper is to compare the flux noise performance of the SBC readout with that of the FM scheme using a sine wave modulation signal. Several liquid-helium-cooled SQUID magnetometers with different layouts and loop inductances were characterized using these two readout schemes. Measured noise was comparable to or even lower than that measured by FM electronics. Furthermore, the SBC noise performance was evaluated as function of resistance which, when properly adjusted, permits us to nearly fulfill the critical noise suppression condition. We believe SBC to be a promising candidate for multi-channel SQUID systems. [ABSTRACT FROM AUTHOR]