Measurement of the frequency stability of responders in aircraft

Measurement on an aircraft orbit, such as a satellite launching orbit, is made by the responder in the aircraft along with several remote track stations on the ground. During the launching, the system is required to have precise time synchronization and frequency accuracy. At the same time, accurate measurement of aircraft velocity requires high frequency stability of the system. However, atomic frequency standards in the ground stations supply time and frequency reference standard with excellent long term and short term frequency stability for the above-mentioned goals. The stability of responder is also an important factor affecting the performance of the system and there are more requirements for the corresponding time/frequency measurements. In the system, the responders do not use continuous wave (CW) but narrow pulse modulated wave; consequently, the characterization theory of their stability is more complicated and the measurement technique is more difficult for pulsed wave than that for CW. A systematic characterization theory of the frequency stability for pulsed wave is demonstrated and the measuring methods are discussed. The measurement systems, which have been set up in Beijing Institute of Radio Metrology and Measurement (BIRMM) and can be used to test the frequency stability of pulse coherent responders in time domain and frequency domain with high sensitivity and accuracy, are described. Using these measurement systems, successful measurements for the responders were made with which the satellite launching orbits were precisely obtained and tracked

A Conservative Finite Difference Scheme for Poisson-Nernst-Planck Equations

A macroscopic model to describe the dynamics of ion transport in ion channels is the Poisson-Nernst-Planck(PNP) equations. In this paper, we develop a finite-difference method for solving PNP equations, which is second-order accurate in both space and time. We use the physical parameters specifically suited toward the modelling of ion channels. We present a simple iterative scheme to solve the system of nonlinear equations resulting from discretizing the equations implicitly in time, which is demonstrated to converge in a few iterations. We place emphasis on ensuring numerical methods to have the same physical properties that the PNP equations themselves also possess, namely conservation of total ions and correct rates of energy dissipation. We describe in detail an approach to derive a finite-difference method that preserves the total concentration of ions exactly in time. Further, we illustrate that, using realistic values of the physical parameters, the conservation property is critical in obtaining correct numerical solutions over long time scales

Experimental investigation and fracture simulation of welded steel connections

This thesis aims to investigate the mechanical properties and fracture performance of welded steel connections. First, the effects of stress triaxiality and Lode angle on fracture of steels, are discussed based on the experimental data collected from the literature and test results of two newly designed specimen types. It is observed that the void growth exponential function can accurately describe the stress triaxiality effect in the whole stress state, and the Lode angle effect can be considered using an exponential function, based on which a new fracture model is proposed and termed Lode angle modified void growth model (LMVGM). Second, the mechanical properties in the weld zone and the weld residual stresses (WRS) are measured from welded connections. The mechanical properties of the weld, heat affected zone (HAZ) and base metal, including their stress-strain relationships and fracture strains under various stress states, are measured in longitudinal and transverse directions by conducting tensile tests on miniature coupons extracted from butt-welded plates. The test results show that the strength and ductility of the HAZ are generally in-between those of the base metal and the weld. The magnitude and distribution of WRS of fillet welds with different weld sizes and butt welds are measured from welded cruciform specimens and an end-plate connection using the neutron diffraction method. Finally, experimental investigation and numerical simulation of the fracture in welded cruciform specimens are conducted, and it is concluded that (ⅰ) material inhomogeneity in the weld zone has a significant effect on the performance of the welded connections, and (ⅱ) ignoring the properties of the weld and HAZ will underestimate the capacity of the connection. Based on the measurement results of the mechanical properties in the weld zone, the fracture process of the cruciform specimens is simulated using the calibrated LMVGM

Variation of the Atmospheric Boundary Layer Height at the Eastern Edge of the Tibetan Plateau

This paper utilized the high temporal and spatial resolution temperature profile data observed by the multi-channel microwave radiometer at the Large High Altitude Air Shower Observatory (LHAASO) on the eastern slope of the Tibetan Plateau from February to May and August to November 2021, combined with the ERA5 reanalysis data products for the whole year of 2021, to study the daily, monthly, and seasonal variations of the atmospheric boundary layer height (ABLH). The results are as follows: (1) The ABLH on sunny days showed obvious fluctuations with peaks and valleys. The ABLH continued to rise with the increase of surface temperature after sunrise and usually reached its maximum value in the afternoon around 18:00, then rapidly decreased until sunset. (2) The average ABLH in April was the highest at about 1200 m, while it was only around 600 m in November. The ABLH fluctuated greatly during the day and was stable at around 400 m at night. The ABLH results obtained from ERA5 were slightly smaller overall but had a consistent trend of change with the microwave radiometer. (3) The maximum ABLH appeared in spring, followed by summer and autumn, and winter had the lowest value, with all peaks reached around 14:00-15:00. These results are of great significance for understanding the ABLH on the eastern slope of the Tibetan Plateau, and provide reference for the absolute calibration of photon numbers of the LHAASO telescope and the atmospheric monitoring plan, as well as for evaluating the authenticity and accuracy of existing reanalysis datasets