Abstract The realization of clock synchronization and syntonization via inter-satellite link is of vital importance for navigation, space-based VLBI experiments and precision tests of fundamental physics. We study the accuracy and stability of time and frequency transfer via inter-satellite link in cis-lunar space. A relativistic time transfer model using microwave dual one-way ranging is developed. Taking the DRO-LEO inter-satellite link as an instance, sub-nanoseconds level accuracy is achieved. We analyze the error in orbit determination of LEO satellite and DRO satellite. With the models of relative velocity correction, relativistic frequency shift and Shapiro delay, the stability of time transfer is studied. The result shows the DOWR microwave link would support clock synchronization with a time stability of better than 14.2ps over 1000s, better than 100.1ps over one day, with the accuracy constraints on the orbit determination of the LEO satellite 10cm and DRO satellite 50m in position. If the longer time stability of hardware delay reaches ps level, the performance of DOWR time transfer link can be further improved to support the distribution of the time-frequency scale established by an active hydrogen maser with a frequency stability of 2×10-15 over one day.We estimated that high-performance DRO-LEO time and frequency comparisons may support the gravitational redshift tests at a 10−6 level and the space-based VLBI experiments to improve the orbit determination of deep-space probes by one to two orders of magnitude.
Given the high-precision modern space mission, a precise relativistic modeling of observations is required. By solving the eikonal equation with the post-Newtonian approximation, the light propagation is determined by the iterative method in the gravitational field of an isolated, gravitationally bound N-body system. Different from the traditional $N$ bodies that are independent with each other in the system, our system includes the velocities, accelerations, gravitational interactions and tidal deformations of the gravitational bodies. The light delays of these factors then are precisely determined by the analytical solutions. These delays are significant and are likely to reach a detectable level for the \emph{strong} gravitational fields, such as binary pulsars and some gravitational wave sources. The result's application in the vicinity of the Earth provides a relativistic framework for modern space missions. From the relativistic analysis in the TianQin mission, we find the possible tests for the alternative gravitational theories, such as a possible determination for the post-Newtonian parameter $\gamma$ in the level of some scalar-tensor theories of gravity.
This paper presented fieldbus SOPC-based FSB (fast serial bus) controller using FPGA technology for CNC application, which used H/S(hardware/software) codesign method. This controller completed control of AL (application layer), DLL(data link layer) and PL(physical layer). In the controller, CAN-compatible hardware designed by VHDL handled with DLL and PL of FSB, while software run in soft processor implemented FSB AL protocol. The experimental results indicate that the SOPC-based controller is efficient and flexible, and the design cycle and cost of CNC system with FSB is reduced by using this fieldbus controller.
Abstract Laser interferometry plays a crucial role for laser ranging in high-precision space missions such as GRACE (Gravity Recovery and Climate Experimen) Follow-On-Like missions and gravitational wave detectors. For such accuracy of modern space missions, a precise relativistic model of light propagation is requires. With the post-Newtonian approximation, we utilize the Synge world function method to study the light propagation in the Earth’s gravitational field, deriving the gravitational delays up to order c -4 . Then, we investigate the influences of gravitational delays in the three inter-satellite laser ranging techniques, including one-way ranging, dual one-way ranging, and transponder-based ranging. By combining the parameters of Kepler orbit, the gravitational delays are expanded up to the order of e 2 ( e is the orbital eccentricity). Finally, considering the GRACE Follow-On-Like missions, we estimate the gravitational delays to the level of picometer. The results demonstrate some high-order gravitational and coupling effects, such as c -4 -order gravitational delays and coupling of Shapiro and beat frequency, may be non-negligible for higher precision laser ranging in the future.
There exist multitudes of cloud performance metrics, including workload performance, application placement, software/hardware optimization, scalability, capacity, reliability, agility and so on. In this paper, we consider jointly optimizing the performance of the software applications in the cloud. The challenges lie in bringing a diversity of raw data into tidy data format, unifying performance data from multiple systems based on timestamps, and assessing the quality of the processed performance data. Even after verifying the quality of cloud performance data, additional challenges block optimizing cloud computing. In this paper, we identify the challenges of cloud computing from the perspectives of computing environment, data collection, performance analytics and production environment.
There are four conventionally accepted fundamental interactions in nature—gravitational, electromagnetic, strong, and weak forces. The gravitational force, based on Einstein’s general theory of relativity, is described as a continuous classical field. The other three, part of the standard model of particle physics, are described as discrete quantum fields, and their interactions are each carried by a quantum, an elementary particle. Two theoretical frameworks upon which all modern physics rests have been developed and have been able to withstand almost all the experimental tests so far individually, but they are mutually incompatible—they cannot both be right. It is a puzzle in physics that the two perfect theories are not compatible, and how to reconcile quantum theory with general relativity is still an open question. Our understanding on the nature of gravity is the key issue. In this article, we first introduce interpretation of gravity in the framework of general relativity, in which gravity is not a real force but a representation of the curved spacetime. On the other hand, the standard model gave a different interpretation of gravity, in which gravity is effects of exchange of gravitons between two masses. Graviton is the fundamental particle, whose mass is zero and spin is two, but it has not been found in any experiments. Then, the contradiction when scientists attempt to unify general relativity and quantum theory is introduced. The superstring theory, a quantum theory not of point particles and a possible candidate in pursuit of a theory of everything in nature, seems to give an unique solution to solve the contradiction, in which all of the particles and interactions of nature are modelled as the vibrations of tiny supersymmetric strings, and the price to pay is unusual features such as six extra dimensions of space in addition to the usual three. The size of the string is about 10-35m, the Plank scale. String and its vibration could construct everything, which also includes gravity, and the physical properties of forces and particles were decided by the frequencies of the string, such as spin and mass. The superstring theory had a puzzle that supersymmetry could be drew into string theory to form five different superstring theories by five different methods. M-theory was born with the development of superstring theory, which unified five different forms of superstring theory. Finally, the experimental tests of the nature of gravity is briefly introduced.
Linux kernel introduces the memory control group (memcg) to account and confine memory usage at the process-level. Due to its flexibility and efficiency, memcg has been widely adopted by container platforms and has become a fundamental technique. While being critical, memory accounting is prone to missing-account bugs due to the diverse memory accounting interfaces and the massive amount of allocation/free paths. To our knowledge, there is still no systematic analysis against the memory missing-account problem, with respect to its security impacts, detection, etc.
The theory of general relativity has an ultraviolet(UV) problem that can be ameliorated by gravity with higher derivatives. The four-derivative gravity as an effective gravitational theory at UV scalars has two Yukawa-like corrections with parameters ${\ensuremath{\lambda}}_{0}$ and ${\ensuremath{\lambda}}_{2}$. By the analysis of experiments of HUST-2015, Newman's group, lake experiment, and Cassini spacecraft, we obtain the strong-bound regions for these parameters at submillimeter-to-millimeter, centimeter-to-meter, tens-of-meter, and solar-system scales, in which the properties of potential are clearly shown. Recently, the ghostfree and singularityfree gravity, a more potential modified gravity theory, introduces the novel conception of nonlocality. We test the scale of gravitational nonlocality ${\ensuremath{\lambda}}_{\mathrm{m}}<2.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\text{ }\text{ }\mathrm{m}$ by the torsion pendulum experiment HUST-2015. The predicted decaying spatial oscillations are visible meaning possible violation at shorter ranges. Our result provides useful information for gravitational interaction at microscopic ranges.
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