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The time maintenance accuracy of the navigation constellation determines the user positioning and timing performance. Especially in autonomous operation scenarios, the performance of navigation constellation maintenance time directly affects the duration of constellation autonomous navigation. Among them, the frequency stability of the atomic clock onboard the navigation satellite is a key factor. In order to further improve the stability of the navigation constellation time-frequency system, combined with the development of high-precision inter-satellite link measurement technology, the idea of constructing constellation-level synthetic atomic time has gradually become the development trend of major GNSS systems. This paper gives a navigation constellation time scale generation framework, and designs an improved Kalman plus weights (KPW) time scale algorithm and time-frequency steer algorithm that integrates genetic algorithms. Finally, a 30-day autonomous timekeeping simulation was carried out using the GPS precision clock data provided by CODE, when the sampling interval is 300 s, the Allan deviation of the output time scale is 5.73 × 10−14, a 71% improvement compared with the traditional KPW time scale algorithm; when the sampling interval is 1 day, the Allan deviation is 9.17 × 10−15; when the sampling interval is 1 × 106 s, the Allan deviation is 8.87 × 10−16, a 94% improvement compared with the traditional KPW time scale algorithm. The constellation-level high-precision time scale generation technology proposed in this paper can significantly improve the stability performance of navigation constellation autonomous timekeeping.

NASICON-structured NaTi2(PO4)3 (NTP) is an attractive anode material for aqueous rechargeable sodium-ion batteries (ARSIBs) thanks to its three-dimensional open framework and appropriate negative voltage window. Nevertheless, the lack of flexible and high-performance binder-free NTP-based anodes remains stumbling blocks to the development of wearable ARSIBs. Herein, hollow-structure NTP evenly encapsulated in cross-linked porous N-doped carbon nanofiber (HNTP@PNC) is prepared through electrospinning technology and subsequent carbonization treatment, directly acting as binder-free anode for flexible ARSIBs. Benefiting from its unique hollow structure, continuous conductive network and favorable synergistic effect, the HNTP@PNC electrode displays as high as of 108.3 mAh g−1 rate capacity at 5.50 A g−1 and an impressive cycling stability of 97.2% capacity retention after 3000 cycles. Further, theoretical calculations reveal that NTP with NC coating significantly enhances electronic conductivity and accelerates Na+ diffusion kinetics. Pairing with potassium zinc hexacyanoferrate free-standing cathode, a prototype quasi-solid-state ARSIB with a high-voltage discharge plateau of 1.6 V is successfully constructed, achieving a high volumetric capacity of 24.5 mAh cm−3 and an admirable energy density of 39.2 mWh cm−3, outperforming most reported flexible aqueous rechargeable energy-storage devices. These exciting results provide valuable intuition into the design of novel binder-free NTP-based anodes for next-generation wearable ARSIBs. Agri-Food and Veterinary Authority of Singapore (AVA) Submitted/Accepted version

Aerogels with extraordinary mechanical properties attract a lot of interest for their wide spread applications. However, the required flexibility is yet to be satisfied, especially under extreme conditions. Herein, a boron nitride nanoribbon aerogel with excellent temperature‐invariant super‐flexibility is developed by high temperature amination of a melamine diborate precursor formed by hydrogen bonding assembly. The unique structure of the aerogel provides it with outstanding compressing/bending/twisting elasticity, cutting resistance, and recoverable properties. Furthermore, the excellent mechanical super‐flexibility is maintained over a wide temperature range, from liquid nitrogen temperature (−196 °C) to higher than 1000 °C, which extends their possible applications to harsh environments.

The cross-spectral estimation methods are efficient in estimating the parameters of sinusoidal signals embedded in colored noise. But up to now, only FFT and cross-periodogram methods are used in this field. The modern auto-spectral estimation method is introduced into cross-spectral estimation in this paper, meanwhile the cross-correlation based Yule-Walker equation is proposed theoretically and the moment and singular-value decomposition (SVD)) algorithms for cross-spectral estimation have been developed. Finally, a numerical example is given for comparing the presented methods with the well-known Cadzow's SVD method.

Graphene-metal compound structure has been reported as a novel and outstanding component used in electrical and optical devices. We report on a first-principles study of graphene-cu compound structure, showing its capacity of converting laser energy into electrical power and storing the harvested energy for a long time. A real-time and real-space time-dependent density functional method (TDDFT) is applied for the simulation of electrons dynamics and energy absorption. The laser-induced charge transfer from copper layer to graphene layer is observed and represented by plane-averaged electron difference and dipoles. The effects of laser frequency on the excitation energy and charge transfer are studied as well. The enhancement of C-C {\sigma}-bond and decreasing of electron density corresponding to {\pi}-bond within graphene layer indicate the way in which the transferred-charges are stored. In addition, the shift and oscillations of dipole along z-direction after the application of laser pulse offer a concept that the compound structure has the ability of storing the harvested energy for a long time.

Smart antenna technology can significantly improve the coverage radius, increase channel capacity, reduce transmitting power as well as overcome multipath effects by adaptive beamforming. The performance of digital beamforming is greatly affected by the amplitude-phase inconsistency among the channels. We propose a method of calibrating amplitude-phase errors of RF channels of DBF transmitting antenna together with its realization architecture. A maximum likelihood estimation method is used to calculate the amplitude-phase calibration coefficients of the channels and a compensation matrix is constructed in the FPGA so as to accomplish the calibration procedure. In order to get rid of the nonlinear operation in the estimation algorithm, adjustment is made and thus improves the dynamic range of the algorithm. Prototype of a 61-channel digital beamforming transmitting antenna system with a calibration loop is developed and experiment is carried out under different error conditions. The result of the experiment demonstrates that the proposed algorithm and its realization architecture is of high performance and very economic.

Frame efficiency is key point for burst communication systems, and long-time preamble is needed for LEO satellite DSSS signal acquisition and coherent synchronization caused by large range Doppler shift. Many papers do works on shortening 2-dimension acquisition by using FFT or shortening convergence time by using adaptive loop filter bandwidth. In this paper a new modem is proposed, which eliminates the preamble spent by synchronization process definitely, and improves burst frame efficiency markedly.

In this paper, we propose inline DRR (deficit round robin) scheduling algorithm due to the limitation of the satellite switch processing speed and memory capacity. We build the model in OPNET. The results show that inline DRR processing delay is 26ms lower than DRR. The inline DRR complexity is O(1). According to the traffic and network structure of low earth orbit satellite system, we adopt CICQ as satellite on board switch structure and apply inline DRR in switch. At the same time, we build the model of the satellite on board switch in OPNET. The simulated results demonstrate that inline DRR algorithm switch delay is lower than 6ms and the algorithm is applicable for satellite switch.

The effect of IF (intermediate frequency) analog AGC on QPSK/OQPSK signal detection is evaluated for LEO satellite system where Doppler drift, large dynamic range of signal power and low Eb/No condition are difficult to deal with. The way of setting reference value of logarithmic power based AGC is discussed. Coherent accumulation and non-coherent accumulation are used for detecting, while signal's PDF and performance vs. accumulation times are derived mathematically. Experiment shows that with 60 KHz Doppler offset, Eb/No=0dB under QPSK/OQPSK, only 16 bits was consumed to detect the signal with 99.9% detection probability and 0.1% false alarm probability.

The stability of coiled carbon nanotubes under C ion irradiation is investigated by molecular dynamics simulations. The defect statistics shows that small curvature coiled carbon nanotubes have better radiation tolerance than normal straight carbon nanotubes. To understand the effect of the curvature on defect production, the threshold displacement energies for the upper and lower walls, as well as those for the side parts, are calculated. The results show that the lower wall has better radiation tolerance than the upper wall. For the upper wall, a small increase in the curvature of nanotube axis gives rise to an increase in the radiation tolerance and then a decrease with the curvature becomes larger. However, for the lower wall, as the curvature of the nanotube axis increases, the radiation tolerance increases as the bonds compressed slightly in our simulation.