Presence Of Offsets Research Articles

Global mantle convection models produce transform offsets along divergent plate boundaries

The presence of offsets, appearing at intervals ranging from 10s to 100s of kilometres, is a distinct characteristic of constructive tectonic plate margins. By comparison, boundaries associated with subduction exhibit uninterrupted continuity. Here, we present global mantle convection calculations that result in a mobile lithosphere featuring dynamically derived plate boundaries exhibiting a contrasting superficial structure which distinguishes convergence and divergence. Implementing a yield-stress that governs the viscosity in the lithosphere, spreading boundaries at the top of a vigorously convecting mantle form as divergent linear segments regularly offset by similar length zones that correlate with a large degree of shear but comparatively minimal divergence. Analogous offset segments do not emerge in the boundaries associated with surface convergence. Comparing the similarity in the morphologies of the model plate margins to the Earth’s plate boundaries demonstrates that transform-like offsets are a result of stress induced weakness in the lithosphere owing to passive rupturing.

Precision of continuous GPS velocities from statistical analysis of synthetic time series

Abstract. We use statistical analyses of synthetic position time series to estimate the potential precision of GPS (Global Positioning System) velocities. The synthetic series represent the standard range of noise, seasonal, and position offset characteristics, leaving aside extreme values. This analysis is combined with a new simple method for automatic offset detection that allows an automatic treatment of the massive dataset. Colored noise and the presence of offsets are the primary contributor to velocity variability. However, regression tree analyses show that the main factors controlling the velocity precision are first the duration of the series, second the presence of offsets, and third the noise level (dispersion and spectral index). Our analysis allows us to propose guidelines, which can be applied to actual GPS data, that constrain velocity precisions, characterized as a 95 % confidence limit of the velocity biases, based on simple parameters: (1) series durations over 8.0 years result in low-velocity biases in the horizontal (0.2 mm yr−1) and vertical (0.5 mm yr−1) components; (2) series durations of less than 4.5 years are not suitable for studies that require precisions lower than mm yr−1; (3) series of intermediate durations (4.5–8.0 years) are associated with an intermediate horizontal bias (0.6 mm yr−1) and a high vertical one (1.3 mm yr−1), unless they comprise no offset. Our results suggest that very long series durations (over 15–20 years) do not ensure a significantly lower bias compared to series of 8–10 years, due to the noise amplitude following a power-law dependency on the frequency. Thus, better characterizations of long-period GPS noise and pluri-annual environmental loads are critical to further improve GPS velocity precisions.

Dynamics modeling and model selection of space debris removal via the Tethered Space Robot

This work proposes a scheme to select a proper dynamics model for space debris removal which is captured by a Tethered Space Robot. A proper dynamics model is crucial for the parameters estimation and controller design in a Tethered Space Robot mission, in particular, for the retrieval or de-orbiting of uncooperative target. A new dynamics model of the system is derived by treating the base satellite and the space debris as rigid bodies in the presence of offsets, and with the effect of the tether’s flexibility and elasticity. Then the equations of motion are simplified based on the attitude analysis and numerical simulations in different cases. It is concluded that in the Tethered Space Robot’s mission, the strong coupled attitude motions among the base, target satellites, and the tether cannot be ignored during the retrieval, which is totally different from the traditional tethered satellite system. The attitude motions of the system in different conditions are discussed respectively, and a method of the model selection of the system during post-capture and retrieval/de-orbiting phase is proposed, which is a balance of the accuracy and facility. Finally, a control scheme is used to prove this conclusion.

Adaptive control for space debris removal with uncertain kinematics, dynamics and states

As the Tethered Space Robot is considered to be a promising solution for the Active Debris Removal, a lot of problems arise in the approaching, capturing and removing phases. Particularly, kinematics and dynamics parameters of the debris are unknown, and parts of the states are unmeasurable according to the specifics of tether, which is a tough problem for the target retrieval/de-orbiting. This work proposes a full adaptive control strategy for the space debris removal via a Tethered Space Robot with unknown kinematics, dynamics and part of the states. First we derive a dynamics model for the retrieval by treating the base satellite (chaser) and the unknown space debris (target) as rigid bodies in the presence of offsets, and involving the flexibility and elasticity of tether. Then, a full adaptive controller is presented including a control law, a dynamic adaption law, and a kinematic adaption law. A modified controller is also presented according to the peculiarities of this system. Finally, simulation results are presented to illustrate the performance of two proposed controllers.

Detecting offsets in GPS time series: First results from the detection of offsets in GPS experiment

The accuracy of Global Positioning System (GPS) time series is degraded by the presence of offsets. To assess the effectiveness of methods that detect and remove these offsets, we designed and managed the Detection of Offsets in GPS Experiment. We simulated time series that mimicked realistic GPS data consisting of a velocity component, offsets, white and flicker noises (1/f spectrum noises) composed in an additive model. The data set was made available to the GPS analysis community without revealing the offsets, and several groups conducted blind tests with a range of detection approaches. The results show that, at present, manual methods (where offsets are hand picked) almost always give better results than automated or semi‒automated methods (two automated methods give quite similar velocity bias as the best manual solutions). For instance, the fifth percentile range (5% to 95%) in velocity bias for automated approaches is equal to 4.2 mm/year (most commonly ±0.4 mm/yr from the truth), whereas it is equal to 1.8 mm/yr for the manual solutions (most commonly 0.2 mm/yr from the truth). The magnitude of offsets detectable by manual solutions is smaller than for automated solutions, with the smallest detectable offset for the best manual and automatic solutions equal to 5 mm and 8 mm, respectively. Assuming the simulated time series noise levels are representative of real GPS time series, robust geophysical interpretation of individual site velocities lower than 0.2–0.4 mm/yr is therefore certainly not robust, although a limit of nearer 1 mm/yr would be a more conservative choice. Further work to improve offset detection in GPS coordinates time series is required before we can routinely interpret sub‒mm/yr velocities for single GPS stations.

Mechanics of fluid‐driven fracture growth in naturally fractured reservoirs with simple network geometries

A numerical model has been developed for fluid‐driven opening mode fracture growth in a naturally fractured formation. The rock formation contains discrete deformable fractures, which are initially closed but conductive because of their preexisting apertures. Fluid flow that develops along fractures depends on fracture geometry defined by preexisting aperture distribution, offsets along a fracture path, and intersections of two or more fractures. The model couples fluid flow, elastic deformation, and frictional sliding to obtain the solution, which depends on the competition between fractures for permeability enhancement. The fractures can be opened by fluid pressure that exceeds the normal stress acting on them and by interactions with intersecting closed fractures experiencing Coulomb‐type frictional slip. The Newtonian fluid is assumed to flow through the conductive fractures according to a lubrication equation that relates the cube of an equivalent hydraulic aperture to fracture conductivity. The rock material is assumed to be impermeable and elastic. This paper provides the governing equations for the multiple fracture systems and the solution methods used. Flow distribution and fracture growth in conductive fracture sets are simulated for a range of geometric arrangements and hydraulic properties. Numerical results show that elastic interaction between fracture branches plays a controlling role in fluid migration, although initial apertures can give rise to a preferential fluid flow direction during the early stage. In the presence of offsets, fracture segments subject to strong compression are difficult to open hydraulically, and their resulting smaller permeability can increase overall upstream fracture pressure and opening. The patterns of fluid flow become more complicated if fractures intersect each other. A portion of injected fluid is lost into closed empty fractures that cut across the main hydraulic fracture, and this delays the pressure increases required for fracture growth past the crosscutting fracture. The nonlinear fluid loss rate depends on the geometric complexities of the fracture sets and on the fluid viscosity. Sometimes fracture growth can be accelerated by the fast fluid transport along an intersected, relatively conductive natural fracture.

General relations between IP2, IP3, and offsets in differential circuits and the effects of feedback

In the presence of offsets, all balanced circuits show an apparent second-order distortion. Differential feedback lowers third-order nonlinearity and also these second-order effects. The results are important for the baseband circuits of zero-IF wireless receivers, which often need a very large second-order intercept point. It is shown that a published analysis of distortion in a bipolar double-balanced mixer is a special case of these general relationships.

Dynamics and control of the tethered satellite system in the presence of offsets

A mathematical model is proposed here for studying the dynamics of the Tethered Satellite System (TSS) that consists of a plate-type Space Station from which a tether supported subsatellite is deployed or retrieved. The rigid body dynamics of the tether, subsatellite and Space Station are analyzed accounting for the mass of the tether as well as a three-dimensional offset of its point of attachment. Controllability of the linearized equations is established numerically and a comparative study of three different control strategies conducted. The strategies employ thrusters, tension in the tether line or motion of the offset of the attachment to achieve control of the system subjected to a relatively large initial disturbance. Results suggest that, in the stationkeeping mode, the tension control strategy damps a given disturbance in the shortest time, however, at an expense of the energy. On the other hand, the offset control proves to be the most efficient in terms of energy consumption, but now the response to disturbance persists over a long duration. In addition, the performance of the thruster control and tension control strategies are analyzed during retrieval of the tether from different initial lengths.

Dynamics and control of the tethered satellite system in the presence of offsets

A mathematical model for studying the dynamics of the Tethered Satellite System (TSS) is proposed that accounts for a three-dimensional offset of the point of attachment. The system chosen consists of a plate-type space station from which a tether supported subsatellite is deployed or retrieved. The mass of the tether as well as the rigid body dynamics of the tether, subsatellite and space station are considered. Controllability of the linearized equations is established numerically and the control is achieved through simple velocity feedback using thrusters and momentum wheels.