Development of auroral electrojets and low-latitude geomagnetic disturbances during strong magnetic storms on November 7–11, 2004

The influence of auroral electojets and solar wind parameters on variations in low-latitude geomagnetic disturbances and D _st during strong magnetic storms on November 7–8, 2004 with D _st ≈ −380 nT and on November 9–10, 2004 with D _st ≈ −300 nT is studied on the basis of global geomagnetic observations. It is found that the impulsive variations of the western electrojet intensity with a duration of Δt ≈ 1–2 h (probably, substorm disturbances) lead to positive low-latitude disturbances of ΔH at Φ′ ≈ 10°–30° and to disturbances of the same durations with an amplitude +ΔH ∼ 30–100 nT at latitudes of the polar cap (Φ′ ≈ 75°–80°). More durable (with Δt ≥ 10 h) convection electrojets whose centers are shifted to latitudes of ∼50°–55° in the process of storm development are the main cause of the increase in negative values of ΔH at low latitudes and D _st . It is shown that meridional dynamics of position of the center of electrojets and the equatorial boundary of the auroral oval is governed by variations (increase or decrease) in the intensity of negative values of the IMF B _z component. It is assumed that in these storms the intensification of the magnetospheric partially ring current closes the circuit to the ionosphere with the help of field-aligned currents at the equatorial boundary of the auroral oval is the main cause of the magnetic field depression at low latitudes.     

离散航空制造业MES系统设计与实施

航空制造业属于典型离散式生产,其产品结构十分复杂,零件数量非常庞大,尤其大量的结构件需要机械加工和装配,同时还需要生产大量的工装、样板等辅助生产设备,使得生产过程复杂且生产周期长.     

基于相位补偿的FCCH捕获及频偏估计算法

文章分析了GSM公共控制信道的帧结构以及FCCH和SCH脉冲结构，介绍了一种捕获FCCH信道并进行频偏估计的算法。根据FCCH调制信号在相位上的特点，使用文章介绍的算法可以检测到FCCH的起始位置，而且能够根据该算法得到信号的频率偏移值，消去频偏对需要解调的SCH脉冲的影响。     

SPACE DEBRIS RESEARCH IN CHINA(2000-2002)

Space debris is is referred to as the human made pollution in the space environment.Ever since the first launch of man made satellite in 1957, millions of kilograms of objects have been sent into the space. Large space objects that are traceable and cataloged by ground base telescope and radar reach around 9 000, becoming grave threat to the social development of human beings as well as astronautics.     

辐射温度计分度方法探讨

研讨了辐射温度计的标定方法，提出了１种新的标定法－分度函数法；并以Ｇｅ光电池为例，导出其分度函数的具体形式，同时估算了分度误差。     

A novel predictive algorithm for double difference observations of obstructed BeiDou geostationary earth orbit (GEO) satellites

Transmission link disturbances and device failure cause global navigation satellite system (GNSS) receivers to miss observations, leading to poor accuracy in real-time kinematic (RTK) positioning. Previously described solutions for this problem are influenced by the length of the prediction period, or are unable to account for changes in receiver state because they use information from previous epochs to make predictions. We propose an algorithm for predicting double difference (DD) observations of obstructed BeiDou navigation system (BDS) GEO satellites. Our approach adopts the first-degree polynomial function for predicting missing observations. We introduce a Douglas-Peucker algorithm to judge the state of the rover receiver to reduce the impact of predictive biases. Static and kinematic experiments were carried out on BDS observations to evaluate the proposed algorithm. The results of our navigation experiment demonstrate that RTK positioning accuracy is improved from meter to decimeter level with fixed ambiguity (horizontal?<?2?cm, vertical?<?18?cm). Horizontal accuracy is improved by over 50%, and the vertical accuracies of the results of the static and kinematic experiments are increased by 47% and 27% respectively, compared with the results produced by the classical approach. Though as the baseline becomes longer, the accuracy is weakened, our predictive algorithm is an improvement over existing approaches to overcome the issue of missing data.     

Near real-time BDS GEO satellite orbit determination and maneuver analysis with reversed point positioning

The Geostationary Earth Orbit (GEO) satellite is a crucial part of the BeiDou Navigation Satellite System (BDS) constellation. However, due to various perturbation forces acting on the GEO satellite, it drifts gradually over time. Thus, frequent orbit maneuvers are required to maintain the satellite at its designed position. During the orbit maneuver and recovery periods, the orbit quality of the maneuvered satellite computed with broadcast navigation ephemeris will be significantly degraded. Furthermore, the conventional dynamic Precise Orbit Determination (POD) approach may not work well, because of a lack of publicly available satellite information for modeling the thrust forces. In this paper, a near real-time approach free of thrust forces modeling is proposed for BDS GEO satellite orbit determination and maneuver analysis based on the Reversed Point Positioning (RPP). First, the station coordinates and receiver clock offsets are estimated by GPS/BDS combined Single Point Positioning (SPP) with single-frequency phase-smoothed pseudorange observations. Then, with the fixed station coordinates and receiver clock offsets, the RPP method can be conducted to determine the GEO satellite orbits. When no orbit maneuvers occur, the proposed method can obtain orbit accuracies of 0.92, 2.74, and 8.30?m in the radial, along-track, and cross-track directions, respectively. The average orbit-only Signal-In-Space Range Error (SISRE) is 1.23?m, which is slightly poorer than that of the broadcast navigation ephemeris. Using four days of GEO maneuvered datasets, it is further demonstrated that the derived orbits can be employed to characterize the behaviors of GEO satellite maneuvers, such as the time span of the maneuver as well as the satellite thrusting accelerations. These results prove the efficiency of the proposed method for near real-time GEO satellite orbit determination during maneuvers.     

Ground-based and satellite DC-ULF electric field anomalies around Wenchuan M8.0 earthquake

The ground-based and satellite DC-ULF electric field data were analyzed around Wenchuan M8.0 earthquake on May 12, 2008 in China. The results show that ground electric field anomalies occurred at 3 stations located to the north and south of the epicenter with the amplitude of 3–100 mV/km. The change shapes and their amplitude of ground electric field anomalies are different largely due to their individual underground layer conductivity, water level and so on. The analysis of long time series illustrates that the abnormal geoelectric field started since March 2008. Onboard the DEMETER satellite, the ULF waveforms of electric field were collected and processed by wavelet transform method. The disturbances in the ionosphere were about 3–5 mV/m at a frequency band lower than 0.5 Hz. When the ground and space electric field anomalies were compared, their occurrence time and spatial distribution points are consistent with each other, including the long time anomalies from March 2008 and the short term ones 1–2 days before the Wenchuan earthquake. Finally, the coupling mechanism was discussed.