GPS时间比对数据的Vondrak平滑和三次样条插值处理

介绍了Vondrak平滑方法和三次样条插值方法。利用国家授时中心时间频率基准重点实验室的GPS时间比对数据,先进行预处理和规范化处理,再采用Vondrak平滑方法和三次样条插值方法进行处理,得到的结果表明:在实际中利用Vondrak平滑方法和三次样条插值方法对GPS时间比对数据处理是可行的,减弱了随机噪声的影响。     

A global survey of COSMIC ionospheric peak electron density and its height: A comparison with ground-based ionosonde measurements

With a network of ground-based ionosondes distributed around the world, the ionospheric peak electron density and its height measured by FORMOSAT-3/COSMIC satellites in terms of GPS radio occultation technique are extensively examined in this article. It is found that, in spite of the latitude, the mean values of the peak electron density measured by COSMIC satellites are systematically smaller than those observed by ground-based ionosondes. The discrepancy between them is dependent on the latitude, namely, it is small in low and mid-latitudes and large in high-latitude region. Moreover, statistical analysis shows that the slopes of the regression line that is best fitted to the scatter diagram of occultation-retrieved peak electron density (ordinate axis) versus ionosonde-observed peak density (abscissa axis) are universally less than one. This feature is believed to be the result of path average effect of non-uniform distribution of the electron density along the GSP ray during the occultation. A comparison between COSMIC-measured peak height and ionosonde-derived peak height hmF2 indicates that the former is systematically higher than the latter. The difference in the two can be as large as 20% or more in equatorial and low-latitude regions. This result implies that the peak height hmF2 derived from the virtual height through true height analysis based on Titheridge method seems to underestimate the true peak height. The correlation between COSMIC and ionosonde peak electron densities is analyzed and the result reveals that correlation coefficient seems to be dependent on the fluctuation of the occultation-retrieved electron density profile. The correlation will be higher (lower) for the electron density profiles with smaller (larger) fluctuations. This feature suggests that the inhomogeneous distribution of the electron density along the GPS ray path during the occultation plays an important role affecting the correlation between COSMIC and ionosonde measurements.     

Response of the EIA ionosphere to the 7-8 May 2005 geomagnetic storm

In this paper, response of low latitude ionosphere to a moderate geomagnetic storm of 7–8 May 2005 (SSC: 1920 UT on 7 May with Sym-H minimum, ∼−112 nT around 1600 UT on 8 May) has been investigated using the GPS measurements from a near EIA crest region, Rajkot (Geog. 22.29°N, 70.74°E, Geomag.14°), India. We found a decrease in total electron content (TEC) in 12 h after the onset of the storm, an increase during and after 6 h of Sym-H deep minimum with a decrease below its usual-day level on the second day during the recovery phase of the storm. On 8 May, an increase of TEC is observed after sunset and during post-midnight hours (maximum up to 170%) with the formation of ionospheric plasma bubbles followed by a nearly simultaneous onset of scintillations at L-band frequencies following the time of rapid decrease in Sym-H index (−30 nT/h around 1300 UT).     

用加权几何精度因子选星的GPS抗多径定位方法

在ＧＰＳ（Ｇｌｏｂａｌ ｐｏｓｉｔｉｏｎｉｎｇ ｓｙｓｔｅｍ）中普遍用几何精度因子（Ｇｅｏｍｅｔｒｉｃ ｄｉｌｕｔｉｏｎ ｏｆ ｐｒｅｃｉｓｉｏｎ，ＧＤＯＰ）来选星，但对近距差分ＧＰＳ，多径干扰已变成了主要误差源，传统的ＧＤＯＰ值选星不是最佳的。为减小多径误差，文中提出了一种新的加权ＧＤＯＰ值选星方法。在仿真验证过程中，分析了多径的几何传播机理，并考虑了抗多径天线的抑制效应。仿真结果表明，用加权Ｇ     

Vertical motions in Thailand after the 2004 Sumatra–Andaman Earthquake from GPS observations and its geophysical modelling

Following previous findings from ongoing GPS research in Thailand since 2004 we continue to exploit the GPS technique to monitor and model land motions induced by the Sumatra–Andaman Earthquake. Our latest results show that up to the end of 2010, Thailand has been co-seismically displaced and is subsequently undergoing a post-seismic horizontal deformation with total displacements (co-seismic plus post-seismic) ranging from 10.5 to 74.7 cm. We observed the largest horizontal displacements in the southern part of Thailand and moderate and small displacements in the central and northern parts. In addition to horizontal displacements throughout Thailand, continuous GPS measurements show that large parts of Thailand are subsiding at rates up to 1 cm/yr. It is the first time that such vertical post-seismic deformations at large distances (650–1500 km away from the Earthquake’s epicentre) have been recorded. We have investigated the physical processes leading to the observed subsidence. While after-slip on the subduction interface induces negligible or even slightly positive vertical motions, relaxation in the asthenosphere is associated with a sizable subsidence. Predictions from a 3D finite element model feature an asthenosphere with an effective viscosity of the order of 3 * 10¹⁸ Pas, fit the horizontal post-seismic data and the observed subsidence well. This model is then used to predict the subsidence over the whole seismic cycle. The subsidence should go on with a diminishing rate through the next two decades and its final magnitude should not exceed 10 cm in the Bangkok area.     

The effect of geocenter motion on Jason-2 orbits and the mean sea level

We compute a series of Jason-2 GPS and SLR/DORIS-based orbits using ITRF2005 and the std0905 standards ( Lemoine et al., 2010). Our GPS and SLR/DORIS orbit data sets span a period of 2 years from cycle 3 (July 2008) to cycle 74 (July 2010). We extract the Jason-2 orbit frame translational parameters per cycle by the means of a Helmert transformation between a set of reference orbits and a set of test orbits. We compare the annual terms of these time-series to the annual terms of two different geocenter motion models where biases and trends have been removed. Subsequently, we include the annual terms of the modeled geocenter motion as a degree-1 loading displacement correction to the GPS and SLR/DORIS tracking network of the POD process. Although the annual geocenter motion correction would reflect a stationary signal in time, under ideal conditions, the whole geocenter motion is a non-stationary process that includes secular trends. Our results suggest that our GSFC Jason-2 GPS-based orbits are closely tied to the center of mass (CM) of the Earth consistent with our current force modeling, whereas GSFC’s SLR/DORIS-based orbits are tied to the origin of ITRF2005, which is the center of figure (CF) for sub-secular scales. We quantify the GPS and SLR/DORIS orbit centering and how this impacts the orbit radial error over the globe, which is assimilated into mean sea level (MSL) error, from the omission of the annual term of the geocenter correction. We find that for the SLR/DORIS std0905 orbits, currently used by the oceanographic community, only the negligence of the annual term of the geocenter motion correction results in a – 4.67 ± 3.40 mm error in the Z-component of the orbit frame which creates 1.06 ± 2.66 mm of systematic error in the MSL estimates, mainly due to the uneven distribution of the oceans between the North and South hemisphere.     

Noise analysis of continuous GPS coordinate time series for CMONOC

The Crustal Movement Observation Network of China (CMONOC) is one of the major scientific infrastructures, mainly using Global Positioning System (GPS) measurements, to monitor crustal deformation in the Chinese mainland. In this paper, decade-long coordinate time series of 26 continuous GPS sites of CMONOC are analyzed for their noise content using maximum likelihood estimation (MLE). We study the noise properties of continuous GPS time series of CMONOC for the unfiltered, filtered solutions and also the common mode signals in terms of power law plus white noise model. In the spatial filtering, we remove for every time series a common mode error that was estimated from a modified stacking of position residuals from other sites within ∼1000 km of the selected site. We find that the common mode signal in our network has a combination of spatially correlated flicker noise and a common white noise with large spatial extent. We demonstrate that for the unfiltered solutions of CMONOC continuous GPS sites the main colored noise is a flicker process, with a mean spectral index of ∼1. For the filtered solutions, the main colored noise is a general power law process, indicating that a major number of the filtered regional solutions have a combination of noise sources or local effects. The velocity uncertainties from CMONOC continuous GPS coordinate time series may be underestimated by factors of 8–16 if a pure white noise model is assumed. In addition, using a white plus flicker noise model, the median values of velocity errors for the unfiltered solutions are 0.16 (north), 0.17 (east) and 0.58 (vertical) mm/yr, and the median values for the filtered solutions are 0.09 (north), 0.10 (east) and 0.40 (vertical) mm/yr.     

Ionospheric scintillations at Guilin detected by GPS ground-based and radio occultation observations

The occurrence of ionospheric scintillations with S4 ? 0.2 was studied using GPS measurements at Guilin, China (25.29°N, 110.33°E; geomagnetic: 15.04°N, 181.98°E), a station located near the northern crest of the equatorial anomaly. The results are presented for data collected from January 2009 to March 2010. The results show that nighttime amplitude scintillations only took place in February and March of the considered years, while daytime amplitude scintillations occurred in August and December of 2009. Nighttime amplitude scintillations, observed in the south of Guilin, always occurred with phase scintillations, TEC (Total Electron Content) depletions, and ROT (Rate Of change of TEC) fluctuations. However, TEC depletions and ROT fluctuations were weak during daytime amplitude scintillations, and daytime amplitude scintillations always took place simultaneously for most of the GPS satellites which appeared over Guilin in different azimuth directions. Ground-based GPS scintillation/TEC observations recorded at Guilin and signal-to-noise-ratio (SNR) measurements obtained from GPS-COSMIC radio occultation indicate that nighttime and daytime scintillations are very likely caused by ionospheric F region irregularities and sporadic E, respectively. Moreover, strong daytime amplitude scintillations may be associated with the plasma density enhancements in ionospheric E region caused by the Perseid and Geminid meteor shower activities.     

一种抑制控制力矩陀螺框架电机共模电压的方法

控制力矩陀螺作为执行机构,是保障航天器在轨寿命和工作性能的核心部件之一。为了抑制控制力矩陀螺外框电机的共模电压,提高控制力矩陀螺的在轨寿命,本文提出了一种三相四桥臂的拓扑结构与优化的SVPWM调制算法结合的共模电压抑制方法,调制过程中避免零矢量的使用,使驱动器可以保持在平衡状态,经过仿真试验的结果对比分析,验证了这种方法可以有效的抑制系统的共模电压。     

全遮挡导流支板对排气系统红外特性的影响

为了降低末级涡轮转子对排气系统尾向红外(IR)辐射贡献,对涡轮后导流支板进行低红外特征结构设计,通过模型试验的方法研究了基准轴对称排气系统和不同支板冷却状态的全遮挡导流支板(FSGS)排气系统的红外特性。研究结果表明:全遮挡导流支板表面温度低于末级涡轮表面温度,但全遮挡导流支板结构会使下游隔热屏和喷管壁面温度明显升高;全遮挡导流支板对排气系统尾向0~10°角域红外辐射有较好的抑制作用,正尾向红外辐射强度比基准轴对称排气系统低13.9%;支板壁面气膜冷能进一步降低排气系统尾向0~10°角域内的红外辐射强度,支板冷却气密流比(BR)为0.5、0.7和0.9时,全遮挡导流支板排气系统0°方向的红外辐射强度的降幅分别为18.1%、25.8%和34.5%。因此,带气膜冷却的全遮挡导流支板是抑制末级涡轮对排气系统红外辐射贡献的一种非常有效的手段。