Offsets In Time Research Articles

Real-time B0 compensation during gantry rotation in a 0.35-T MRI-Linac.

Rotation of the ferromagnetic gantry of a low magnetic field MRI-Linac was previously demonstrated to cause large center frequency offsets of ±400Hz. The B0 off-resonances cause image artifacts and imaging isocenter shifts that would preclude MRI-guided arc therapy. The purpose of this study was to measure and compensate for center frequency offsets in real time during gantry rotation on a 0.35-T MRI-Linac using a free induction decay (FID) navigator. A nonselective FID navigator was added before each 2D balanced steady-state free precession cine image acquisition on a 0.35-T MRI-Linac. Images were acquired at 7.3 frames per second. Phase data from the initial FID navigator (while the gantry was stationary) was used as a reference. The phase data from each subsequent FID navigator was used to calculate the real-time B0 off-resonance. The transmitter/receiver phase and the phase accrual over the adjacent image acquisition were adjusted to correct for the center frequency offset. Measurements were performed using an MRI-Linac dynamic phantom prior to and while the gantry rotated clockwise and counterclockwise. Image quality and signal-to-noise ratio (SNR) were compared between uncorrected and B0 -corrected MRIs using a reference image acquired while the gantry was stationary. Four targets in the phantom were manually contoured on the first image frame, and an active contouring algorithm was used retrospectively on each subsequent frame to assess image variations and calculate Dice coefficients. Additionally, three healthy volunteers were imaged using the same pulse sequences with and without real-time B0 compensation during gantry rotation. Normalized root mean square errors (nRMSEs) were calculated for the phantom and in vivo to assess the efficacy of the B0 compensation on image quality. The measured center frequency offsets from the volunteer and MRI dynamic phantom navigator data were also compared. The sinusoidal behavior of the center frequency offsets was modeled based on the gantry layout and long-time constant eddy currents resulting from gantry rotation. The duration of the FID navigator and processing was 4.5ms. The FID navigator resulted in a ≤11% drop in SNR in the phantom and in vivo (liver). Dice coefficients from the MRI-guided radiation therapy (MR-IGRT) phantom contour measurements remained above 0.8 with B0 compensation. Without B0 compensation, the Dice coefficients dropped below 0.8 for up to 21% of the time depending on the contour. Real-time B0 compensation resulted in mean reductions in nRMSE of 51% and 16% for the MR-IGRT phantom and in vivo, respectively. Peak-to-peak center frequency offsets ranged from 757 to 773Hz in the phantom and 760 to 871Hz in vivo. Dynamic real-time B0 compensation significantly improved image quality and reduced artifacts during gantry rotation in the phantom and in vivo. However, the FID navigator resulted in a small drop in the imaging duty cycle and SNR.

The 1.28 GHz MeerKAT DEEP2 Image

We present the confusion-limited 1.28 GHz MeerKAT DEEP2 image covering one primary-beam area with θ = 76 FWHM resolution and rms noise. Its J2000 center position α = 04h 13m 264, δ = −80° 00' 00'' was selected to minimize artifacts caused by bright sources. We introduce the new 64-element MeerKAT array and describe commissioning observations to measure the primary-beam attenuation pattern, estimate telescope pointing errors, and pinpoint (u, v) coordinate errors caused by offsets in frequency or time. We constructed a 1.4 GHz differential source count by combining a power-law count fit to the DEEP2 confusion P(D) distribution from 0.25 to 10 μJy with counts of individual DEEP2 sources between 10 μJy and 2.5 mJy. Most sources fainter than S ∼ 100 μJy are distant star-forming galaxies (SFGs) obeying the far-IR/radio correlation, and sources stronger than 0.25 μJy account for ∼93% of the radio background produced by SFGs. For the first time, the DEEP2 source count has reached the depth needed to reveal the majority of the star formation history of the universe. A pure luminosity evolution of the 1.4 GHz local luminosity function consistent with the Madau & Dickinson model for the evolution of SFGs based on UV and infrared data underpredicts our 1.4 GHz source count in the range .

Batch Localization Based on OFDMA Backscatter

OFDMA Wi-Fi backscatter can significantly improve the communication efficiency and meanwhile maintain ultra-low power consumption; however, the ground-up reworking on the core mechanism of traditional Wi-Fi system revolutionizes the basis of many existing Wi-Fi based mechanisms. In this paper, we explore how localization can be realized based on OFDMA backscatter, where a batch localization mechanism utilizing concurrent communication in the OFDMA backscatter system is proposed. We present a series of mechanisms to deal with the fundamental change of assumptions brought by the new paradigm. First, we process signals at the receiver in a finer granularity for signal classification. Then we remove phase offsets in real time without interrupting the communication. Finally, we propose an extended MUSIC algorithm to improve accuracy with limited localization information in OFDMA backscatter mechanism. We implement a prototype under the 802.11g framework in WARP, based on which we conduct comprehensive experiments to evaluate our propose mechanism. Results show that our system can localize 48 tags simultaneously, while achieving average localization errors within 0.49m. The tag's power consumption is about 55-81.3μW.

A Simple Offset “Calibration” Method for the Accurate Geographic Registration of Ship-Borne X-Band Radar Intensity Imagery

X-band radar remote sensing of the ocean surface using ground-based installations and radar intensity imagery is a mature technology for the determination of ocean wave properties. In order to transition the analytical methods to data collected from a moving vessel the accurate geo-registration of radar images must be performed. However, finite offsets in azimuth, range and time are generated by the physical installation of equipment aboard a vessel that may not be measurable. This paper details a simple, yet robust method to “calibrate” data recorded by an arbitrary equipment installation to allow accurate geo-registration of the radar imagery. Time-integrated radar images are generated using a set of time-stamped radar intensity images and high-frequency, high-accuracy vessel heading and position information (from global satellite navigation systems and inertial navigation systems). The time-integrated image sharpness is found to be maximal for correctly determined angular, range, and time offsets. The requirements to form an operational system from the proposed method are also discussed.

Improving method of real-time offset tuning for arterial signal coordination using probe trajectory data

In the environment of intelligent transportation systems, traffic condition data would have higher resolution in time and space, which is especially valuable for managing the interrupted traffic at signalized intersections. There exist a lot of algorithms for offset tuning, but few of them take the advantage of modern traffic detection methods such as probe vehicle data. This study proposes a method using probe trajectory data to optimize and adjust offsets in real time. The critical point, representing the changing vehicle dynamics, is first defined as the basis of this approach. Using the critical points related to different states of traffic conditions, such as free flow, queue formation, and dissipation, various traffic status parameters can be estimated, including actual travel speed, queue dissipation rate, and standing queue length. The offset can then be adjusted on a cycle-by-cycle basis. The performance of this approach is evaluated using a simulation network. The results show that the trajectory-based approach can reduce travel time of the coordinated traffic flow when compared with using well-defined offline offset.

Low-Loss Passive Si$_{3}$N $_{4}$ Serial-to-WDM Interface for Energy-Efficient Optical Interconnects

The increasing number of computational servers in data centers is imposing tighter constraints on the networking infrastructure. Scalable power efficient optical interconnect network becomes necessary to leverage the bandwidth capacity of current electronic switches or opto-electronic components. Hence, novel optical interconnect technology can enhance the network capacity by harnessing the feasibility of simultaneous processing of optical signal in the wavelength and time domains. In this paper, we present a four channel optical passive wavelength-striped mapping (PWSM) device, which passively time compresses/expands serial packets through optical wavelength multiplexing/demultiplexing. The PWSM device, which has a 1 $\times$ 4 channel optical wavelength demultiplexer with integrated optical delay lines, is designed in a low-loss Si $_{3}$ N $_{4}$ (propagation loss $\sim$ 3.1 dB/m) waveguide platform. The PWSM device multiplexes/demultiplexes four WDM channels and offsets in time the adjacent channels to optically serialize/de-serialize data packets. In this demonstration, a 64 ns long data packet is formed at the output of the device by combining four 16 ns data segments of the packet in time domain. Incremental optical insertion loss between adjacent channels is $\sim$ 9.7 dB due to the integrated passive optical delay waveguides. The data rate of the four segmented packets and the combined packet is 25 Gb/sec. We have measured a bit error rate performance below 1 $\times 10^{-9}$ for the 64 ns serial data packet regenerated by the PWSM device for a received optical power of 6.7dBm.

Real‐time inversions for finite fault slip models and rupture geometry based on high‐rate GPS data

We present an inversion strategy capable of using real‐time high‐rate GPS data to simultaneously solve for a distributed slip model and fault geometry in real time as a rupture unfolds. We employ Bayesian inference to find the optimal fault geometry and the distribution of possible slip models for that geometry using a simple analytical solution. By adopting an analytical Bayesian approach, we can solve this complex inversion problem (including calculating the uncertainties on our results) in real time. Furthermore, since the joint inversion for distributed slip and fault geometry can be computed in real time, the time required to obtain a source model of the earthquake does not depend on the computational cost. Instead, the time required is controlled by the duration of the rupture and the time required for information to propagate from the source to the receivers. We apply our modeling approach, called Bayesian Evidence‐based Fault Orientation and Real‐time Earthquake Slip, to the 2011 Tohoku‐oki earthquake, 2003 Tokachi‐oki earthquake, and a simulated Hayward fault earthquake. In all three cases, the inversion recovers the magnitude, spatial distribution of slip, and fault geometry in real time. Since our inversion relies on static offsets estimated from real‐time high‐rate GPS data, we also present performance tests of various approaches to estimating quasi‐static offsets in real time. We find that the raw high‐rate time series are the best data to use for determining the moment magnitude of the event, but slightly smoothing the raw time series helps stabilize the inversion for fault geometry.

Using Multiple Reference Points in Raman Based Distributed Temperature Sensor for Eliminating DC Interference

In this paper, the multiple reference points methods for eliminating dc interference in Raman scattering based distributed temperature sensors (DTSs) are presented and their performance comparison for eliminating dc interference is also made under two different test conditions. The experimental results show that the dual reference points temperature interrogating methods can calculate dc offsets in real time and improve the performance of DTS systems efficiently under mutative dc offsets condition. Meanwhile, more than two reference points cannot improve the system in performance, and the single reference point method is suitable for working under constant dc offsets condition.

Evaluation of Cold-Season Precipitation Forecasts Generated by the Hourly Updating High-Resolution Rapid Refresh Model

Abstract The hourly updating High-Resolution Rapid Refresh (HRRR) model is evaluated with regard to its ability to predict the areal extent of cold-season precipitation and accurately depict the timing and location of regions of snow, rain, and mixed-phase precipitation on the ground. Validation of the HRRR forecasts is performed using observations collected by the Automated Surface Observing System (ASOS) stations across the eastern two-thirds of the United States during the 2010–11 cold season. The results show that the HRRR is able to reliably forecast precipitation extent during the cold season. In particular, the location and areal extent of both snow and rain are very well predicted. Depiction of rain-to-snow transitions and freezing rain is reasonably good; however, the associated evaluation scores are significantly lower than for either snow or rain. The analyses suggest the skill in accurately depicting precipitation extent and phase (i.e., rain, snow, and mixed phase) depends on the size and organization of a weather system. Typically, larger synoptically forced weather systems are better predicted than smaller weather systems, including the associated rain-to-snow transition or freezing-rain areas. Offsets in space or time (i.e., causing misses and false alarms) have a larger effect on the model performance for smaller weather systems.

Estimation of the parameters of structures using acceleration measurements

This paper proposes a method that estimates the parameters of a seismically excited building. The acceleration measurements of the ground and of each floor are used for identification purposes. It is assumed that these measurements have offsets and noise. The proposed scheme is based on the Recursive Least Squares algorithm with forgetting factor and a parametrization of the structure using integrals over finite time intervals. These filters have finite impulse response, pass the typical frequency bandwidth of structures undergoing earthquake excitation, eliminate the offsets in a small finite time and attenuate the measurement noise. To confirm the effectiveness of the proposed method a simulation of a three-story building is presented. The results show that the estimated parameters converge to the true parameters in a short time.

Structures, Algorithms, and Algebraic Tools for Rhythmic Canons

STRUCTURES, ALGORITHMS, AND ALGEBRAIC TOOLS FOR RHYTHMIC CANONS EMMANUEL AMIOT 1. WHAT IS A RHYTHMIC CANON? HE AMBITIOUS PURPOSE of this paper is to fill in the gap between theory and practice of rhythmic canons. There is indeed quite a distance between musical canons, even rather intellectual ones like Bach’s in the Goldberg Variations or The Art of Fugue, and Vuza canons such as they are used by some modern composers. The basic idea of a canon is that some recognizable pattern is repeated with different offsets (usually with different instruments, or at least different voices). Sometimes this pattern (henceforth called the motif) is modified (say, augmented or retrograded). For rhythmic canons, we need only consider the occurrences of the musical events (notes, for instance), regardless of pitch, timbre, or dynamics. Hence, T 94 Perspectives of New Music with a motif A and transformations τi , i∈I (that may be only offsettings —i.e., translation in time), a canon will be the union C of the transforms ∪ i∈I τi (A). Say, for simplicity’s sake, that the fundamental beats are modeled by integers; e.g., some subset D⊂ℤ. If a motif is identified with its characteristic function 1A :D→{0,1}, then the superposition of all its copies appears as a sum C=∑ i 1τi(A) . For instance, in the common case when all transformations are just different offsets in time—i.e., τi =T i =(t ↦t+i)—we get ∑ i 1A+i , which is, in general, the characteristic function of a multiset, not a set. ILLUSTRATION 1. Let M be the tango or habanera rhythm {0,3,4,6}. An infinite canon can be made by offsetting M by -2 and 0 and repeating the sequence with period 8 (Example 1). Notice that C(4)=C(6)=2, while C(5)=0, for instance. A neater canon can be made, without gaps or coincidences, by also using retrogradation (Example 2). EXAMPLE 1: A CANON WITH A TANGO MOTIF EXAMPLE 2: A MOSAIC WITH A TANGO MOTIF M AND ITS RETROGRADATION R Structures, Algorithms, and Algebraic Tools for Rhythmic Canons 95 Obviously, if one chooses to consider as possible beats all possible quavers or semiquavers (say) in a definite time-span, then most musical canons in the classical sense will feature from nil to several notes on each beat. We can try to specialize more (theoretically) interesting kinds of canons: we will call them (as suggested to this author by Jon Wild) coverings and packings. A covering is a canon where every available beat features at least one note, maybe more. One gets a trivial covering by starting a new copy of the motif on every beat. In other words, with the above notation, ∀t∈D , C (t )≥1. A packing, conversely , is a canon where there is never more than one note on every possible beat. With the above notation, ∀t∈D , C (t )≤1. A trivial packing is made of only one (or even nil!) copy of the motif. Covering is not only trivial, but lumpy (several notes on the same beat); packing, conversely, leaves many gaps. From a mathematical point of view, the obvious way to get a welldefined and interesting problem1 is to demand one and only note per beat, as in Example 2. This is called a tiling (i.e., a mosaic with copies of one motif, maybe allowing some deformations—retrogradation and augmentation among other possibilities, apart from translation in time). Hence: mosaics/tilings = coverings ∩ packings: ∀t∈D , C(t)=1. Most studies (especially on the pure mathematics side) have been devoted to the simplest case of tiling with just one tile and some of its translates (i.e., mosaic rhythmic canons by translation). In the onedimensional case (filling every beat with one and only one note) it is equivalent to the problem of tiling ℤ (see Lagarias and Wang 1996; Vuza 1991–1993); if the tile is finite, it is equivalent to the tiling of a cyclic group ℤn , as we will develop below in Theorem 2. A simple case is shown in Example 3. EXAMPLE 3: A MOSAIC RHYTHMIC CANON DESIGNED BY GEORGES BLOCH FOR A GREETING CARD 96...

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STRUCTURES, ALGORITHMS, AND ALGEBRAIC TOOLS FOR RHYTHMIC CANONS EMMANUEL AMIOT 1. WHAT IS A RHYTHMIC CANON? HE AMBITIOUS PURPOSE of this paper is to fill in the gap between theory and practice of rhythmic canons. There is indeed quite a distance between musical canons, even rather intellectual ones like Bach’s in the Goldberg Variations or The Art of Fugue, and Vuza canons such as they are used by some modern composers. The basic idea of a canon is that some recognizable pattern is repeated with different offsets (usually with different instruments, or at least different voices). Sometimes this pattern (henceforth called the motif) is modified (say, augmented or retrograded). For rhythmic canons, we need only consider the occurrences of the musical events (notes, for instance), regardless of pitch, timbre, or dynamics. Hence, T 94 Perspectives of New Music with a motif A and transformations τi , i∈I (that may be only offsettings —i.e., translation in time), a canon will be the union C of the transforms ∪ i∈I τi (A). Say, for simplicity’s sake, that the fundamental beats are modeled by integers; e.g., some subset D⊂ℤ. If a motif is identified with its characteristic function 1A :D→{0,1}, then the superposition of all its copies appears as a sum C=∑ i 1τi(A) . For instance, in the common case when all transformations are just different offsets in time—i.e., τi =T i =(t ↦t+i)—we get ∑ i 1A+i , which is, in general, the characteristic function of a multiset, not a set. ILLUSTRATION 1. Let M be the tango or habanera rhythm {0,3,4,6}. An infinite canon can be made by offsetting M by -2 and 0 and repeating the sequence with period 8 (Example 1). Notice that C(4)=C(6)=2, while C(5)=0, for instance. A neater canon can be made, without gaps or coincidences, by also using retrogradation (Example 2). EXAMPLE 1: A CANON WITH A TANGO MOTIF EXAMPLE 2: A MOSAIC WITH A TANGO MOTIF M AND ITS RETROGRADATION R Structures, Algorithms, and Algebraic Tools for Rhythmic Canons 95 Obviously, if one chooses to consider as possible beats all possible quavers or semiquavers (say) in a definite time-span, then most musical canons in the classical sense will feature from nil to several notes on each beat. We can try to specialize more (theoretically) interesting kinds of canons: we will call them (as suggested to this author by Jon Wild) coverings and packings. A covering is a canon where every available beat features at least one note, maybe more. One gets a trivial covering by starting a new copy of the motif on every beat. In other words, with the above notation, ∀t∈D , C (t )≥1. A packing, conversely , is a canon where there is never more than one note on every possible beat. With the above notation, ∀t∈D , C (t )≤1. A trivial packing is made of only one (or even nil!) copy of the motif. Covering is not only trivial, but lumpy (several notes on the same beat); packing, conversely, leaves many gaps. From a mathematical point of view, the obvious way to get a welldefined and interesting problem1 is to demand one and only note per beat, as in Example 2. This is called a tiling (i.e., a mosaic with copies of one motif, maybe allowing some deformations—retrogradation and augmentation among other possibilities, apart from translation in time). Hence: mosaics/tilings = coverings ∩ packings: ∀t∈D , C(t)=1. Most studies (especially on the pure mathematics side) have been devoted to the simplest case of tiling with just one tile and some of its translates (i.e., mosaic rhythmic canons by translation). In the onedimensional case (filling every beat with one and only one note) it is equivalent to the problem of tiling ℤ (see Lagarias and Wang 1996; Vuza 1991–1993); if the tile is finite, it is equivalent to the tiling of a cyclic group ℤn , as we will develop below in Theorem 2. A simple case is shown in Example 3. EXAMPLE 3: A MOSAIC RHYTHMIC CANON DESIGNED BY GEORGES BLOCH FOR A GREETING CARD 96...

Displacement and stress Green's functions for a constant slip-rate on a quadrantal fault

SUMMARY I present, in a rigorous form, the Green’s functions that represent the dynamic displacement response of an infinite, homogeneous and isotropic medium to a constant slip rate on a quadrantal fault that continues perpetually after the slip onset. I also present analytical expressions for the corresponding stress Green’s functions that are a major simplification over previously known solutions. The equations derived can be utilized in the formulation of boundary element methods that discretize a fault plane into a set of small rectangular elements and assume the slip rate to be piecewise constant within each discrete element. 1I N T R O DUCTION In the boundary element method (BEM) modelling of the elastodynamic field created by slip on a fault in a three-dimensional (3-D) medium, the fault plane is often discretized into a set of small rectangular elements, on each of which the slip is assumed to follow certain types of spatio-temporal variation profiles. One of the simplest and most popular categories of such assumption is that of the so-called piecewise constant slip rate, which states that the slip rate takes a constant value everywhere and at every moment within a given rectangular element and within a given discrete time window. The elastodynamic field in the medium is calculated by convolving the discrete spatio-temporal profile of the fault slip with Green’s functions corresponding to a constant slip-rate that takes place on each discrete fault element and within each discrete time window. The knowledge of these Green’s functions therefore makes an indispensable part of the BEM modelling of fault dynamics. In mathematical terms, a constant slip-rate profile localized on a rectangular fault element and within a finite time window is equivalent to a superposition, with appropriate offsets in space and time and with appropriate sign reversals, of eight identical profiles of slip rate that takes a constant value everywhere on a quadrantal part of the fault and at every moment after the onset of slip (Fukuyama & Madariaga 1998). The question of calculating Green’s functions for a constant slip rate on a rectangular space‐time element thus reduces to the question of calculating the Green’s functions for a constant slip rate on a quadrantal fault that goes on perpetually after the onset of slip. The stress Green’s functions for such a quadrantal fault model placed in an infinite, homogeneous and isotropic medium were obtained analytically by Aochi (1999) and Aochi et al. (2000a) and were fully utilized in their numerical simulation of fault dynamics. The analytical expressions for the displacement Green’s functions have remained unknown, however, because of the complexity of the algebra involved. In the present study I have derived, in a rigorous form, Green’s functions that represent the displacement response of an infinite, homogeneous and isotropic medium to a constant slip rate on a quadrantal fault that continues perpetually after the slip onset. Although these Green’s functions can be evaluated numerically, the use of analytical expressions is highly beneficial from the viewpoint of computational efficiency. I have double-checked my equations by deriving them with two different methods and confirming their consistency with a corresponding two-dimensional (2-D) theory. I have also been able to substantially simplify the expressions for the stress Green’s functions over those obtained by Aochi (1999) and Aochi et al. (2000a).

On-line weld position control for fusion reactor welding

Infrared sensing techniques were investigated to assist remote welding systems to identify and correct weld-joint offsets in real time. During the welding process, the temperature distribution along a line normal to the joint and ahead of the arc was measured. A distinct drop in the measured temperature distribution was observed to coincide with a gap in the joints. In the temperature gradient profile, the gap was characterized by three changes in sign. The second change in sign of the temperature gradient profile was found to correspond to the gap center. The first and third changes in sign were found to coincide with the edges of the joint. These unique changes were used to successfully track curved contours of joints with a gap.

Underwater sound wave propagation in the presence of a randomly perturbed profile

The propagation of acoustic signals over long ranges in the deep ocean is influenced significantly by the following factors: (1) a deterministic sound speed profile possessing a “sound channel”; (b) highly anisotropic spatial random fluctuations; (3) temporal fluctuations due primarily to internal waves. The wave equation, with a randomly perturbed sound-speed profile, is initially considered, and a scaling is introduced which is typical of the ocean environment. A space—time parabolic equation is then obtained for the random product of complex pressures at two spatial and temporal points. Finally, a transport equation is derived for a suitably chosen transform of the pressure correlation function. This transport equation is general in the sense that it can model the decorrelation of acoustic signals due to offsets in range, azimuth, depth, and time. [Work supported by ONR.]