Phase Bias Research Articles

Array-based satellite phase bias sensing: theory and GPS/BeiDou/QZSS results

Single-receiver integer ambiguity resolution (IAR) is a measurement concept that makes use of network-derived non-integer satellite phase biases (SPBs), among other corrections, to recover and resolve the integer ambiguities of the carrier-phase data of a single GNSS receiver. If it is realized, the very precise integer ambiguity-resolved carrier-phase data would then contribute to the estimation of the receiver’s position, thus making (near) real-time precise point positioning feasible. Proper definition and determination of the SPBs take a leading part in developing the idea of single-receiver IAR. In this contribution, the concept of array-based between-satellite single-differenced (SD) SPB determination is introduced, which is aimed to reduce the code-dominated precision of the SD-SPB corrections. The underlying model is realized by giving the role of the local reference network to an array of antennas, mounted on rigid platforms, that are separated by short distances so that the same ionospheric delay is assumed to be experienced by all the antennas. To that end, a closed-form expression of the array-aided SD-SPB corrections is presented, thereby proposing a simple strategy to compute the SD-SPBs. After resolving double-differenced ambiguities of the array’s data, the variance of the SD-SPB corrections is shown to be reduced by a factor equal to the number of antennas. This improvement in precision is also affirmed by numerical results of the three GNSSs GPS, BeiDou and QZSS. Experimental results demonstrate that the integer-recovered ambiguities converge to integers faster, upon increasing the number of antennas aiding the SD-SPB corrections.

Coherent Detection-Based Automatic Bias Control of Mach–Zehnder Modulators for Various Modulation Formats

In this paper, a method to automatically control the bias voltages of LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Mach-Zehnder modulator (MZM) is proposed and demonstrated experimentally. The automatic bias control (ABC) method is based on low cost coherent detection employing 3 × 3 optical coupler, which enhances the dither detection sensitivity. By minimizing the monitored dither frequency components which represents the residual carrier component in the optical signal, the modulator biases are optimized. A 0-π/2 square-wave phase modulation is introduced to mitigate the detection fluctuation induced by the imbalance of 3 × 3 coupler and improve the monitoring accuracy. Experiments show that the proposed method is universal for different modulation formats. High bias control precision is achieved for inner in-phase/quadrature (I/Q) branches and outer phase bias of optical quadrature modulator. The penalty caused by the proposed ABC is negligible for both single carrier (SC) modulation and optical orthogonal frequency division multiplexing modulation. The tolerance to temperature disturbance is also demonstrated.

Azimuth resolution improvement for spaceborne SAR images with quasi-non-overlapped Doppler bandwidth

The azimuth resolution improvement problem is solved via a coherent combination of synthetic aperture radar (SAR) images with the quasi-non-overlapped Doppler bandwidth. Prior to the spectra combination, SAR images should be co-registered, while phase biases induced by topography, atmospheric propagation delays and baseline measurement errors should be calibrated. However, the coregistration accuracy suffers from large Doppler decorrelation caused by the quasi-non-overlapped Doppler bandwidth. Furthermore, the method used to estimate phase biases from interferogram of azimuth pre-filtered SAR image pairs will fail when there is no overlapped spectrum. The fringe simulation and maximum sharpness optimization are adopted to deal with the problems. Accordingly, a novel algorithm to coherently synthesize SAR images is presented. The experiment with the Terra SAR X-band (TerraSAR-X) satellite data validates the performance of the presented method.

Global and Regional Ionospheric Corrections for Faster PPP Convergence

Rapid convergence of precise point positioning (PPP) solutions to cm-level precision is a key factor for many applications. One means of accelerating this convergence is to exploit the benefit of information on the ionosphere. In order to preserve the integer nature of carrier-phase ambiguities in PPP, it is imperative that ionospheric corrections be provided with a set of compatible satellite phase biases. When using the decoupled-clock model, global ionospheric maps (GIMs) currently provided by the International GNSS Service are not directly applicable to PPP with ambiguity resolution. This paper describes a methodology for incorporating external ionospheric corrections into this model. It is shown that the use of both GIMs and ambiguity resolution can potentially reduce the convergence time of PPP to 10-cm horizontal accuracies from 30 to 4.5 minutes (68th percentile), while a regional network with inter-station spacing of 150 km can reach this threshold instantaneously under favorable ionospheric conditions. © 2014 Her Majesty the Queen in Right of Canada. NAVIGATION. © 2014 the Institute of Navigation.

Experimental investigation of bias current and load modulation effects in phase distortion of GaN HEMTs

The relationships between phase distortion, bias point and load modulation in power amplifiers are experimentally evaluated for two different GaN HEMT technologies. Measurements on devices with fixed and modulated load, to emulate the dynamic working conditions of the main stage of a Doherty amplifier, have been carried out through an advanced source/load-pull setup. Almost flat phase distortion against output power can be obtained by the proper bias point when the load remains constant, whereas it is shown that this possibility is not compatible with load modulation, inherently present in Doherty power amplifiers.

Coherent suppression of electromagnetic dissipation due to superconducting quasiparticles.

Owing to the low-loss propagation of electromagnetic signals in superconductors, Josephson junctions constitute ideal building blocks for quantum memories, amplifiers, detectors and high-speed processing units, operating over a wide band of microwave frequencies. Nevertheless, although transport in superconducting wires is perfectly lossless for direct current, transport of radio-frequency signals can be dissipative in the presence of quasiparticle excitations above the superconducting gap. Moreover, the exact mechanism of this dissipation in Josephson junctions has never been fully resolved experimentally. In particular, Josephson's key theoretical prediction that quasiparticle dissipation should vanish in transport through a junction when the phase difference across the junction is π (ref. 2) has never been observed. This subtle effect can be understood as resulting from the destructive interference of two separate dissipative channels involving electron-like and hole-like quasiparticles. Here we report the experimental observation of this quantum coherent suppression of quasiparticle dissipation across a Josephson junction. As the average phase bias across the junction is swept through π, we measure an increase of more than one order of magnitude in the energy relaxation time of a superconducting artificial atom. This striking suppression of dissipation, despite the presence of lossy quasiparticle excitations above the superconducting gap, provides a powerful tool for minimizing decoherence in quantum electronic systems and could be directly exploited in quantum information experiments with superconducting quantum bits.

The Compensation of Y Waveguide Temperature Drifts in FOG with the Thermal Resistor

The lithium niobate integrated optical phase modulator (Y waveguide) is the key device in the digital closed-loop fiber optic gyroscope. However, the half-wave voltage of the lithium niobate changes with the environment temperature, which produces the phase bias drift and ultimately decreases the accuracy of FOG. In this manuscript, the thermal resistor is introduced in the amplification part in the driving circuits of Y waveguide. Due to the characteristic of the thermal resistor, the magnitude of driving voltage on Y waveguide changed with temperature to compensate the electro-optic effects temperature drift of the lithium niobate. This method was proved to improve the performance of fiber optic gyroscopes conveniently in experiment.

Performance analysis of a federated ultra-tight global positioning system/inertial navigation system integration algorithm in high dynamic environments

The Doppler frequency changes rapidly due to high dynamics of vehicle, which leads to the loose lock and even the abnormal performance of global positioning system (GPS) receiver. To solve this problem, a federated ultra-tight integration algorithm based on pre-filters is proposed to optimal estimate both receiver tracking control commands and inertial navigation system (INS) navigation solutions. Firstly, the INS error model and GPS receiver tracking loop structure are built to present the fundamental architecture of the proposed ultra-tightly coupled system. Meanwhile, in order to reduce the load of the integrated filter, the pre-filters are incorporated to the ultra-tightly coupled system, and the state variables are fed into the integrated Kalman filter. Secondly, the intrinsic relevance between the phase and frequency biases of replica signals and INS states is analyzed to accomplish the deep fusion of INS and tracking loop. Finally, semi-physical simulations are performed by using a GPS signal simulator to generate signals of two high dynamic trajectories. The experimental results indicate that the proposed ultra-tight integration algorithm can achieve a good performance on reliable positioning and robust tracking in high dynamic environments, compared with the conventional approaches such as tightly coupled integration strategy and third-order phase-locked loops.

A Novel Channel Phase Bias Estimation Method for Spaceborne Multi-channel High-resolution and Wide-swath SAR

方位多通道合成孔径雷达(SAR)可有效地抑制多普勒模糊以实现高分辨率宽测绘带(High-Resolution Wide-Swath HRWS)成像。通道间幅相误差将影响多普勒模糊抑制性能。基于星载SAR系统回波特性雷达回波在多普勒带宽外的能量通常远小于多普勒带宽内能量，该文提出一种多通道HRWS SAR系统通道间相位偏差估计新方法。该方法假定多普勒带宽外的信号近似为零，将单星多通道HRWS SAR系统相位偏差估计问题转换为恒模二次规划问题，然后利用经典二次规划问题求解方法特征值松弛法(Eigen-Value Relaxation, EVR)及半正定松弛法(Semi-Definite Relaxation, SDR)求解该问题，获取HRWS SAR系统通道间的相位偏差估计值。理论分析结果表明，该文算法可视为一种自适应加权最小二乘相位偏差估计算法。多通道SAR仿真实验结果验证了该文算法的有效性。

“Smile” Gap in the Density of States of a Cavity between Superconductors

The density of Andreev levels in a normal metal (N) in contact with two superconductors (S) is known to exhibit an induced minigap related to the inverse dwell time. We predict a small secondary gap just below the superconducting gap edge-a feature that has been overlooked so far in numerous microscopic studies of the density of states in S-N-S structures. In a generic structure with N being a chaotic cavity, the secondary gap is the widest at zero phase bias. It closes at some finite phase bias, forming the shape of a "smile". Asymmetric couplings give even richer gap structures near the phase difference π. All the features found should be amendable to experimental detection in high-resolution low-temperature tunneling spectroscopy.

Optical Voltage Sensor Using Single Fresnel Rhomb ${\rm Bi}_{4}{\rm Ge}_{3}{\rm O}_{12}$ Crystal

A novel optical voltage sensor is proposed by the use of single Fresnel rhomb bismuth germanate crystal. The optical sensing unit is only composed of one voltage sensing crystal and two prism polarizers. The distinguished feature is that the voltage sensing crystal is simultaneously used as a Fresnel rhomb quarter waveplate. An optical phase bias of π/2 is produced by two times of total internal reflection of light wave in the sensing crystal itself, thus additional quarter waveplate usually used in conventional voltage sensor can be removed. Besides the compact optical configuration, the proposed voltage sensor demonstrates larger linear measurement range and good temperature stability in experiments. The ac voltage in the range of 2000 V was measured with a nonlinear error <;0.82% and a measurement sensitivity of 2.2 mV/V, and corresponding temperature drift of the output voltage was <;0.9% for ambient temperature change in the range of (-10 ~ +50) °C.

Refined Filtering of Interferometric Phase From InSAR Data

Radar interferometry has been widely applied in measuring the terrain height and its changes. The information about surface can be derived from phase interferograms. However, the inherent phase noise reduces the accuracy and reliability of that information. Hence, the minimization of phase noise is essential prior to the retrieval of surface information that is embedded in an interferometric phase. This paper presents a refined filter based on the Lee adaptive complex filter and the improved sigma filter that was originally developed for amplitude image filtering. The basic idea is to adaptively filter the interferometric phase according to the local noise level to minimize the loss of signal for a particular pattern of fringes, including such extreme cases as involving broken fringes, following the removal of undesired pixels. Ultimately, the goals are to preserve the fringe pattern, to reduce phase bias and deviation, to reduce the number of residues, and to minimize the phase error. The preservation of the fringe pattern is particularly of concern in areas of high frequency of fringe and large phase gradient corresponding to steep terrains. The proposed refined filter was validated using both simulated data and real interferometric data. Results demonstrate that the filtering performance is better than that of commonly used filters.

LC phase bias investigation of ASG-EUPOS stations

Abstract Monitoring of permanent stations that make up the reference frame is an integral part of the geodesists work. Selection of reference stations is based on analysis of parameters characterizing them (hardware, coordinates’ stability, mounting, location). In this paper, we took into account phase residual as an indicator of unmodelled signal. Phase residuals were computed based on ASG-EUPOS and EPN observation processing. The results show the connection between the method of mounting the antenna and the residuals. We have reviewed multipath effect at ASG-EUPOS stations, and chosen those which are characterized by the highest value of phase residual. The results show that LC phase residual is a good factor to characterize site’s solutions’ reliability. For majority of sites RMS values were less than 10 mm. Modulations associated with multipath effect were observed for few ASG-EUPOS sites only. Phase residuals are distributed specifically for sites, which antennas are mounted on pillars (more common for EPN sites). For majority of analysed sites phase residual distribution was similar for different days and did not depend directly on atmosphere condition.

High-speed and broad optical bandwidth silicon modulator

A high-speed silicon modulator with broad optical bandwidth is proposed based on a symmetrically configured Mach—Zehnder interferometer. Careful phase bias control and traveling-wave design are used to improve the high-speed performance. Over a broadband wavelength range, high-speed operation up to 30 Gbit/s with a 4.5 dB–5.5 dB extinction ratio is experimentally demonstrated with a low driving voltage of 3 V.

Optical electric-field sensor based on angular optical bias using single β-BaB2O4 crystal

A novel optical electric-field sensor is proposed and demonstrated in experiment by use of a single beta barium borate (β-BaB2O4, BBO) crystal. The optical sensing unit is only composed of one BBO crystal and two polarizers. An optical phase bias of 0.5π is provided by using natural birefringence in the BBO crystal itself. A small angle (e.g., 0.6°) between the sensing light beam and principal axis of the crystal is required in order to produce the above optical bias. Thus the BBO crystal is used as the electric-field-sensing element and quarter waveplate. The ac electric field in the range of (1.4-703.2) kV/m has been measured with measurement sensitivity of 1.39 mV/(kV/m) and nonlinear error of 0.6%. Compared with lithium niobate crystal used as an electric-field sensor, main advantages of the BBO crystal include higher measurement sensitivity, compact configuration, and no ferroelectric ringing effect.

Efficient Between-Satellite Single-Difference Precise Point Positioning Model

This paper describes a new model for precise point positioning (PPP), which overcomes the drawbacks of existing PPP models. This model is based on between-satellite single-difference (BSSD), which cancels out the receiver clock error, receiver initial phase bias, and receiver hardware delay. The decoupled clock (DC) corrections, which were provided by Natural Resources Canada (NRCan), were applied to account for code and carrier-phase satellite clock corrections, satellite hardware delay, and satellite initial phase bias. The results from three PPP models (namely, undifferenced-DC, BSSD, and BSSD-DC) were compared with the traditional (i.e., undifferenced) PPP solution. It is shown that the proposed BSSD models improves the PPP convergence time by up to 50% and improves the solution precision by more than 60% in comparison with the traditional PPP model.

Simultaneous unbalanced shared local oscillator heterodyne interferometry for high signal-to-noise-ratio, minimally destructive dispersive detection of time-dependent atomic spins

We demonstrate &#x201C;simultaneous unbalanced shared local oscillator heterodyne interferometry&#x201D; (SUSHI) a new method for minimally destructive, high signal-to-noise-ratio (SNR) dispersive detection of atomic spins. In SUSHI a dual-frequency probe laser interacts with atoms in one arm of a Mach&#x2013;Zehnder interferometer, and then beats against a bright local oscillator (LO) beam traversing the other arm, resulting in two simultaneous, independent heterodyne measurements of the atom-induced phase shift. Measurement noise due to mechanical disturbances of beam paths is strongly rejected by the technique of active subtraction, in which antinoise is actively written onto the LO beam via an optical phase-locked loop. In SUSHI, technical noise due to phase, amplitude, and frequency fluctuations of the various laser fields is strongly rejected (i)&#xA0;for any mean phase bias between the interferometer arms, (ii)&#xA0;without the use of piezo actuated mirrors, and (iii)&#xA0;without signal balancing. We experimentally demonstrate an ultralow technical-noise-limited sensitivity of 51&#x2009;&#x2009;nrad/Hz over a measurement bandwidth of 60&#xA0;Hz to 8&#xA0;kHz using a 230&#xA0;&#x3BC;W probe, and stay within &#x223C;3&#x2009;&#x2009;dB of the standard quantum limit as probe power is reduced by more than 5 orders of magnitude to as low as 650&#xA0;pW. SUSHI is therefore well suited to performing quantum nondemolition measurements for preparing spin-squeezed states and for high-SNR, truly continuous observations of ground-state Rabi flopping in cold atom ensembles.

Calibration of the clock-phase biases of GNSS networks: the closure-ambiguity approach

In global navigation satellite systems (GNSS), the problem of retrieving clock-phase biases from network data has a basic rank defect. We analyse the different ways of removing this rank defect, and define a particular strategy for obtaining these phase biases in a standard form. The minimum-constrained problem to be solved in the least-squares (LS) sense depends on some integer vector which can be fixed in an arbitrary manner. We propose to solve the problem via an undifferenced approach based on the notion of closure ambiguity. We present a theoretical justification of this closure-ambiguity approach (CAA), and the main elements for a practical implementation. The links with other methods are also established. We analyse all those methods in a unified interpretative framework, and derive functional relations between the corresponding solutions and our CAA solution. This could be interesting for many GNSS applications like real-time kinematic PPP for instance. To compare the methods providing LS estimates of clock-phase biases, we define a particular solution playing the role of reference solution. For this solution, when a phase bias is estimated for the first time, its fractional part is confined to the one-cycle width interval centred on zero; the integer-ambiguity set is modified accordingly. Our theoretical study is illustrated with some simple and generic examples; it could have applications in data processing of most GNSS networks, and particularly global networks using GPS, Glonass, Galileo, or BeiDou/Compass satellites.

Frequency-dependent admittance of a short superconducting weak link

We consider the linear and nonlinear electromagnetic responses of a nanowire connecting two bulk superconductors. The Andreev states appearing at a finite phase bias substantially affect the finite-frequency admittance of such a wire junction. Electron transitions involving the Andreev levels are easily saturated, leading to the nonlinear effects in photon absorption for the subgap photon energies. We evaluate the complex admittance analytically at an arbitrary frequency and arbitrary, possibly nonequilibrium, occupation of the Andreev levels. Special care is given to the limits of a single-channel contact and a disordered metallic weak link. We also evaluate the quasistatic fluctuations of admittance induced by fluctuations of the occupation factors of the Andreev levels. In view of possible qubit applications, we compare properties of a weak link with those of a tunnel Josephson junction. Compared to the latter, a weak link has smaller low-frequency dissipation. However, because of the deeper Andreev levels, the low-temperature quasistatic fluctuations of the inductance of a weak link are exponentially larger than of a tunnel junction. These fluctuations limit the applicability of nanowire junctions in superconducting qubits.

Evolution of dielectric properties in BaZrxTi1−xO3 ceramics: Effect of polar nano-regions

BaZrxTi1−xO3 (x=0.2, 0.25, 0.3) ceramics were prepared by a citrate method. The ferroelectric phase transition behavior and bias electric-field-induced dielectric nonlinearity of the ceramics were investigated. With increasing zirconium content, the phase transition behavior of the ceramics changed from a diffusion phase transition to relaxor-like behavior. Applying bias electric field in diverse manners led to differing effects on the dielectric nonlinearity, depending on the zirconium content of the ceramics. These dielectric phenomena were related to the polarization response of polar nano-regions (PNRs) embedded in the ceramics, which are macroscopically in the paraelectric state at room temperature. The characteristic parameters of PNRs were determined by fitting the dielectric constants under bias electric field to a multipolarization mechanism model. It was found that the size of PNRs was decreased with increasing zirconium content. The evolution of the phase transition behavior and dielectric nonlinearity with zirconium content was interpreted in relation to the size change of PNRs.