Time filter

Source Type

News Article | October 26, 2016
Site: www.nature.com

Abolition would see 'official' time unmoored from the Sun. "The times," sang Bob Dylan, "they are a-changin'." His words could become literal truth in January, when the World Radiocommunication Conference of the International Telecommunication Union in Geneva, Switzerland, will vote on whether to redefine Coordinated Universal Time (UTC) and pull our clock time out of synchronization with the Sun's location in the sky. At issue is whether to abolish the 'leap second' — the extra second added every year or so to keep UTC in step with Earth's slightly unpredictable orbit. UTC — the reference against which international time zones are set — is calculated by averaging signals from around 400 atomic clocks, with leap seconds added to stop UTC drifting away from solar time at a rate of about one minute every 90 years. But "leap seconds are a nuisance", says Elisa Felicitas Arias, the director of the Time Department at the International Bureau of Weights and Measures (BIPM) in Sèvres, France. They cannot be preprogrammed into software because they are typically announced only six months in advance by the International Earth Rotation and Reference Systems Service in Frankfurt, Germany. If the seconds get implemented inconsistently in different systems, clocks can briefly go out of synch, potentially leading to glitches that can stall computers and leave international financial markets vulnerable to attack. Still, some countries — principally China, Canada and the United Kingdom — want to keep leap seconds to maintain the link with solar time, in part for philosophical reasons. "Most Chinese scholars think it is important for timekeeping to have a connection to astronomical time because of traditional Chinese culture," says Chunhao Han of the Beijing Global Information Center of Application and Exploration, who adds, however, that China has yet to decide how it will vote in January. Last week, scientists and government representatives met at the Kavli Royal Society International Centre near Milton Keynes, UK, to discuss the issue, but they failed to reach a consensus, making the outcome of the January vote hard to predict. Arias, who co-organized that meeting, argues that leap seconds are obsolete now that global navigation systems, which set their own internal timescales, have replaced solar time for navigation and precision scientific measurements such as the motion of tectonic plates and how Earth's mass warps space-time. Adding an extra second inconsistently to multiple clocks across satellite networks could cause a system to fail for long enough to cause an air disaster, says Włodzimierz Lewandowski, a physicist at the BIPM. The US Global Positioning System ignores leap seconds for just this reason, and Russia's GLONASS system has had problems in the past incorporating the leap. Europe's Galileo system, which launched its first two satellites last month, and China's developing BeiDou system will also mark time with their own internal clocks. But Markus Kuhn, a computer scientist at the University of Cambridge, UK, says that most problems could be overcome by having a consistent prescription for adding extra seconds. Linux operating systems, for example, have experienced problems because they add the whole second in one abrupt jump at midnight, which confuses the software. In September, Google announced that it would use an alternative 'soft-leap' strategy, in which operating systems add portions of the second smoothly over an extended period. "This should be the standard approach," says Kuhn. Peter Whibberley, a physicist at the National Physical Laboratory in Teddington, UK, says that despite ten years of debate, "there's no convincing evidence that anything serious would happen if you made a mistake introducing a leap second into a system". Abolishing leap seconds only defers any problems, he adds. "A century down the line, we'll need to introduce a 'leap minute', and nobody has any sensible arguments for why that won't be a worse issue."

Yi G.,Beijing Global Information Center | Yi G.,Beihang University | Ming F.,Beihang University
Lecture Notes in Electrical Engineering | Year: 2012

Space-time interference suppression technique shows great predominance compared with other anti-jamming techniques. LCMV (Linearly Constrained Minimum Variance) criterion is often used in space-time adaptive processing. Beaming steering and null steering methods were deeply analyzed on LCMV criterion. A new space-time anti-jamming method is proposed combining beam steering and null steering. The method reduces the signal loss of satellite signal DOA estimation error compared with beaming steering and improves the output SINR compared with null steering. Numerical simulation demonstrates that the proposed method is effective and efficient. © 2012 Springer-Verlag.

Fan M.,No 2 Institute Of Engineering Corps | Guo Y.,Beijing Global Information Center | Yun C.,Beihang University | Song D.-Z.,Beihang University | Gao Z.-H.,Beihang University
Xitong Fangzhen Xuebao / Journal of System Simulation | Year: 2013

Dealing with a dynamic routing planning problem of the intelligent access system used in pharmacy, the mathematical formulations were proposed and an hybrid algorithm for this problem was designed. The designed algorithm incorporated the genetic algorithm and particle swarm algorithm and recommended dynamic adjustment and self-adaptive evolution strategies. The previous of the particle algorithm, with dynamic changing of the inertia weight, other parameters also made adjustments to improve search efficiency and search accuracy. In the latter part of the genetic algorithm, with self-adaptive parameter adjustment strategy, the amount of routing information could be maximized according to each iteration, and the search could be driven towards diverse regions of the solution space. The performance of the proposed algorithm was tested on some intelligent access system instances. It proves to be more effective and efficient than other algorithm.

Guo Y.,Beihang University | Guo Y.,Beijing Global Information Center | Fan M.,Beihang University | Kong M.,Beijing Global Information Center
2012 5th International Congress on Image and Signal Processing, CISP 2012 | Year: 2012

Spoofing interference suppression technique for GNSS receiver was deeply discussed. Space-time process and MMSE criterion were applied to anti-spoofing. The combination of PD(Partial Dispreading) and MIRR (Multistage Iterative Rank-reduced) method was proposed to simplify the processing. The proposed method can suppress spoofing interference and significantly reduce the complexity of space-time process. Meanwhile, it also can overcome the effects of Doppler shift and improve the steady-state and transient performance of the filter. Numerical simulation demonstrates that the proposed method is effect and efficient. © 2012 IEEE.

Zhang T.,Tsinghua University | Zhao H.,Beijing Global Information Center | Cui X.,Tsinghua University | Lu M.,Tsinghua University
Lecture Notes in Electrical Engineering | Year: 2012

With the application of satellite navigation becoming more and more widely, increasingly complex electromagnetic environment needs to be dealt with. The anti-jamming capabilities of satellite navigation receivers have got more and more attention, whether in the field of military or civilian. The most important and most widely used class of anti-jamming technology is pre-correlation anti-jamming mean. However, the upper bound of overall anti-jamming capability is less studied when the anti-jamming filter inserted into a digital satellite receiver. In this paper the upper bound of anti-jamming capability for pre-correlation anti-jamming receivers is studied. First of all, the quantization loss is theoretically derived, when the input interference is a continuous wave (CWI). A near-optimal quantization interval criterion is proposed through analyzing the minimize quantization loss. Also, according to the calculation of signal-to-noise ratio (SNR) chain in the receiver, the maximum anti-jamming capability is obtained under the constraint of specified quantization bits and quantization interval. The analysis of the upper bound of anti-jamming capability is very important for the design of anti-jamming receivers, and also improves the anti-jamming theory for all spread spectrum signal receivers. © 2012 Springer-Verlag.

Zhang X.,Tsinghua University | Zhang X.,Beijing Global Information Center | Yao Z.,Tsinghua University | Lu M.,Tsinghua University
Science China: Physics, Mechanics and Astronomy | Year: 2011

Satellite navigation is playing an important role in social life. The performances of the services a navigation system provides are the concern of the builders and users. The signal structure determines the inherent ability of a satellite navigation system to provide these services. Therefore, it is necessary and reasonable to optimize the signal in the initial design. The waveforms of signals in satellite navigation chips are an important aspect of the signal's structure, which impact the characteristics of the autocorrelation function and power spectrum in the time domain and frequency domain. The range accuracy of a navigation system is usually described by its Gabor Bandwidth. The Gabor Bandwidth of the Binary Offset Carrier (BOC) modulation in next-generation GNSS signal systems is not optimal. This paper presents a signal expression based on Multilevel Coded Symbol (MCS) signals, which can describe the fine structure of navigation signals. With this expression, we can optimize the Gabor Bandwidth of the signal, and find out the corresponding MCS vector and chip waveform, so as to get higher range accuracy. The method in this paper, in theory, is of great value in the design of next-generation navigation signals. © 2011 Science China Press and Springer-Verlag Berlin Heidelberg.

Li L.,Beijing Global Information Center | Zhou W.,China Aerospace Science and Technology Corporation | Tan S.,Beijing Global Information Center
IET Conference Publications | Year: 2011

The binary offset carrier (BOC) modulation offers improved performance and the opportunity for spectrum sharing while retaining implementation simplicity. However its drawback is the ambiguity of the autocorrelation function (ACF), which induces a complexity in the acquisition process, and a risk of biased measures while tracking. The traditional techniques for the classic BPSK modulated signal have to be revised. This paper summarizes the matching receiver techniques for BOC modulation, in order to avoid missed detection and false tracking.

Tan S.,Beijing Global Information Center | Li L.,Beijing Global Information Center | Xie C.,Beijing Global Information Center
Lecture Notes in Electrical Engineering | Year: 2012

This paper researches the radio frequency compatibility (RFC) of Beidou user equipment systemically. Firstly, the interference margin of the Beidou user equipment is analyzed. Secondly, the protection demand for the satellite navigation signal at the user equipment level is raised. Lastly, the RFC index of user equipment is studied out. Based on the RFC index, alter the Beidou user equipment rationally, and then the RFC issue can be resolved preferably. © 2012 Springer-Verlag.

Tan S.,Beijing Global Information Center | Zhou B.,Beijing Global Information Center | Guo S.,Beijing Global Information Center | Liu Z.,Beijing Global Information Center
Zhongguo Kongjian Kexue Jishu/Chinese Space Science and Technology | Year: 2011

The signals of global navigation satellite system of China must obey many constraints, and should be coordinated with other navigation signals as well. The service type, carrier frequency, modulation scheme and spreading codes are essential for the coordination. The signal structure and parameters of COMPASS excerpted from the reference assumptions document(RAD) were presented. The correlation performances of the spreading codes of the COMPASS open signals were consistent with that of GPS. Base on the signal structure and basic parameters of the RADs, COMPASS and GPS, the L band frequency coordination was completed in accordance with the frames established by ITU in September, 2010.

Li L.,Beijing Global Information Center | Zhou W.-H.,China Aerospace Science and Technology Corporation | Tan S.-S.,Beijing Global Information Center
Tien Tzu Hsueh Pao/Acta Electronica Sinica | Year: 2011

In this paper, the interference suppression of spread spectrum systems is treated as constrained minimum mean output energy (MMOE). After an orthogonal decompose procedure for transforming the MMOE problem to unconstrained minimum mean square error (MMSE) one and a construction for state equation and observation equation, and then the blind Kalman filtering (BKF), i. e., absence of a desire signal, is developed and analyzed. The research results indicate that the BKF algorithm is insensitive to variations in the correlation matrix, fast response to the changing environment, and close to level of optimization with iterative operation. Hence, it is superior to the blind least mean square (BLMS) and blind recursive least squares (BRLS) algorithms.

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