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61.
A physical system is subject to a phase transition process when it shows a discontinuous change of a macroscopic feature of the system under a continuous change of a system’s state variable. 相似文献
62.
Stochastic assessment of GPS carrier phase measurements for precise static relative positioning 总被引:17,自引:11,他引:17
Global positioning system (GPS) carrier phase measurements are used in all precise static relative positioning applications.
The GPS carrier phase measurements are generally processed using the least-squares method, for which both functional and stochastic
models need to be carefully defined. Whilst the functional model for precise GPS positioning is well documented in the literature,
realistic stochastic modelling for the GPS carrier phase measurements is still both a controversial topic and a difficult
task to accomplish in practice. The common practice of assuming that the raw GPS measurements are statistically independent
in space and time, and have the same accuracy, is certainly not realistic. Any mis-specification in the stochastic model will
inevitably lead to unreliable positioning results. A stochastic assessment procedure has been developed to take into account
the heteroscedastic, space- and time-correlated error structure of the GPS measurements. Test results indicate that the reliability
of the estimated positioning results is improved by applying the developed stochastic assessment procedure. In addition, the
quality of ambiguity resolution can be more realistically evaluated.
Received: 13 February 2001 / Accepted: 3 September 2001 相似文献
63.
J. M. Léger J. Haines C. Chateau G. Bocquillon M. W. Schmidt S. Hull F. Gorelli A. Lesauze R. Marchand 《Physics and Chemistry of Minerals》2001,28(6):388-398
The structure of the cristobalite-like polymorph of phosphorus oxynitride PON has been refined using neutron powder diffraction
data. It is tetragonal, space group I&4macr;2d, Z=4. The four P–(O,N) distances are equal but the tetrahedron is compressed along c. In AX2 or ABX4 compounds, the tetragonal I&4macr;2d or I&4macr; structure is obtained when the average ratio of the cation to anion radius is below 1.186, whereas the tetragonal P41212 or orthorhombic C2221 structure is obtained at low temperatures for larger ratios. The cell parameters of this PON polymorph have been determined
as a function of hydrostatic pressure by in situ angle dispersive X-ray powder diffraction in a diamond-anvil cell. Under
truly hydrostatic pressure, a strong anisotropic behavior is observed with the c parameter being nearly incompressible. Very slight anisotropic stress strongly modifies the high-pressure behavior. According
to the pressure-temperature conditions of treatment, three phases, cristobalite-, moganite-, and quartz-like, have been obtained
by quenching experiments, and the P–T phase diagram of PON was derived. The high-pressure behavior of the α-quartz, moganite, and cristobalite-like polymorphs
of PON and SiO2 is discussed.
Received: 7 August 2000 / Accepted: 21 January 2001 相似文献
64.
G. J. Redhammer G. Roth W. Paulus G. André W. Lottermoser G. Amthauer W. Treutmann B. Koppelhuber-Bitschnau 《Physics and Chemistry of Minerals》2001,28(5):337-346
Single crystals of Li-aegirine LiFe3+Si2O6 were synthesized at 1573?K and 3?GPa, and a polycrystalline sample suitable for neutron diffraction was produced by ceramic sintering at 1223?K. LiFe3+Si2O6 is monoclinic, space group C2/c, a=9.6641(2)?Å, b= 8.6612(3)?Å, c=5.2924(2)?Å, β=110.12(1)° at 300?K as refined from powder neutron data. At 229?K Li-aegirine undergoes a phase transition from C2/c to P21 /c. This is indicated by strong discontinuities in the temperature variation of the lattice parameters, especially for the monoclinic angle β and by the appearance of Bragg reflections (hkl) with h+k≠2n. In the low-temperature form two non-equivalent Si-sites with 〈SiA–O〉=1.622?Å and 〈SiB–O〉=1.624?Å at 100?K are present. The bridging angles of the SiO4 tetrahedra O3–O3–O3 are 192.55(8)° and 160.02(9)° at 100?K in the two independent tetrahedral chains in space group P21 /c, whereas it is 180.83(9)° at 300?K in the high-temperature C2/c phase, i.e. the chains are nearly fully expanded. Upon the phase transition the Li-coordination changes from six to five. At 100?K four Li–O bond lengths lie within 2.072(4)–2.172(3)?Å, the fifth Li–O bond length is 2.356(4)?Å, whereas the Li–O3?A bond lengths amount to 2.796(4)?Å. From 57Fe Mössbauer spectroscopic measurements between 80 and 500?K the structural phase transition is characterized by a small discontinuity of the quadrupole splitting. Temperature-dependent neutron powder diffraction experiments show first occurrence of magnetic reflections at 16.5?K in good agreement with the point of inflection in the temperature-dependent magnetization of LiFe3+Si2O6. Distinct preordering phenomena can be observed up to 35?K. At the magnetic phase transition the unit cell parameters exhibit a pronounced magneto-striction of the lattice. Below T N Li-aegirine shows a collinear antiferromagnetic structure. From our neutron powder diffraction experiments we extract a collinear antiferromagnetic spin arrangement within the a–c plane. 相似文献
65.
GPS接收机天线相位中心偏差的检测 总被引:5,自引:0,他引:5
GPS接收机天线相位中心偏差是指GPS天线接收卫星信号的电气中心与基机械几何中心之差。在高精度测量中,这是不容忽视的。讨论了采用基线测量相对测定法确定天线相位中心在水平和垂直方向上的偏差的原理和方法,并给出了测量结果及建议。 相似文献
66.
67.
长期预报的相空间近邻等距法 总被引:6,自引:0,他引:6
考虑到确定性系统的外在随机因素和内在随机性(浑沌)所造成的长期预报的不准确性,本义将有关浑沌的理论和数理统计理论结合起来,提出了d维相空间距平符号传播的“正、反规则”假说和近邻等距模式.实例检验表明,经调试后距平符号的报准率一般为66%—80%,而相对误差一般不大于8%. 相似文献
68.
本文在分析自动脉冲数字测相基本原理的基础之上,推出了不仅与测距频率有关,而且还与填充脉冲频率及低频测相频率有关的新的频率改正公式,并给出新的频率改正公式与传统频率改正公式之间等同的条件,即有锁相环路(或频率跟踪电路)的正常工作来保证。 相似文献
69.
70.
H. Lemmens M. Czank G. Van Tendeloo S. Amelinckx 《Physics and Chemistry of Minerals》2000,27(6):386-397
Using minimum exposure techniques, it is feasible to perform high resolution electron microscopy on the α-cristobalite phase
of (Si0.9 Ge0.1)O2, which is extremely radiation sensitive. Such images reveal atomic scale information of twins and tridymite-like stacking
faults on (1 1 1)β planes, as well as of domain boundaries resulting from the β→α transition. Polytype structures are formed in certain cases.
Morphological features suggest that the phase transformation cristobalite → tridymite proceeds by means of a zonal dislocation
mediated synchro-shear process on (1 1 1)β planes; the geometry of this process is analyzed.
Received: 13 June 1999 / Accepted: 30 October 1999 相似文献