Evaluating the statistical performance of less applied algorithms in classification of worldview-3 imagery data in an urbanized landscape

In recent decade, analyzing the remotely sensed imagery is considered as one of the most common and widely used procedures in the environmental studies. In this case, supervised image classification techniques play a central role. Hence, taking a high resolution Worldview-3 over a mixed urbanized landscape in Iran, three less applied image classification methods including Bagged CART, Stochastic gradient boosting model and Neural network with feature extraction were tested and compared with two prevalent methods: random forest and support vector machine with linear kernel. To do so, each method was run ten time and three validation techniques was used to estimate the accuracy statistics consist of cross validation, independent validation and validation with total of train data. Moreover, using ANOVA and Tukey test, statistical difference significance between the classification methods was significantly surveyed. In general, the results showed that random forest with marginal difference compared to Bagged CART and stochastic gradient boosting model is the best performing method whilst based on independent validation there was no significant difference between the performances of classification methods. It should be finally noted that neural network with feature extraction and linear support vector machine had better processing speed than other.     

Regolith-derived ferrosilicon as a potential feedstock material for wire-based additive manufacturing

Ferrosilicon is a primary metallic alloy produced during the reduction of metal oxides contained in lunar and Martian regolith by a variety of techniques. This study examines the usefulness of ferrosilicon as a candidate feedstock material for wire-based 3D printers designed for in-space manufacturing. Alloys of composition ranging from pure iron to 12?wt% Si were synthesized and their electrical and mechanical properties characterized. The melts were cast into rods for mechanical testing to determine ultimate strength and ductility. It was determined that the samples above 3?wt% Si were too brittle to be pulled into wire and ruptured at low strain values. The 3?wt% Si sample and iron had comparable mechanical properties relative to samples of higher silicon content but with differences in ductility and ultimate strength. Microstructure and compositional studies revealed the differences between the ductile and brittle samples as being the complete ferrite phase presence on the iron and low-Si content samples. This study establishes an upper limit on the Si content at 3?wt% in ferrosilicon materials to be used in wire feedstock in additive manufacturing for in-space applications.     

Estimation of differential code biases with Jason-2/3 onboard GPS observations

With the continuous deployment of Low Earth Orbit (LEO) satellites, the estimation of differential code biases (DCBs) based on GNSS observations from LEO has gained increasing attention. Previous studies on LEO-based DCB estimation are usually using the spherical symmetry ionosphere assumption (SSIA), in which a uniform electron density is assumed in a thick shell. In this study, we propose an approach (named the SHLEO method) to simultaneously estimate the satellite and LEO onboard receiver DCBs by modeling the distribution of the global plasmaspheric total electron content (PTEC) above the satellite orbit with a spherical harmonic (SH) function. Compared to the commonly used SSIA method, the SHLEO model improves the GPS satellite DCB estimation accuracy by 13.46% and the stability by 22.34%, respectively. Compared to the GPS satellite DCBs estimated based on the Jason-3-only observations, the accuracy and monthly stability of the satellite DCBs can be improved by 14.42% and 26.8% when both Jason-2 and Jason-3 onboard observations are jointly processed. Compared with the Jason-2 solutions, the GPS satellite DCB estimates based on the fusion of Jason-2 and Jason-3 observations have an improved consistency of better than 18.26% and 9.71% with the products provided by the Center for Orbit Determination in Europe (CODE) and Chinese Academy of Sciences (CAS). Taking the DCB products provided by the German Aerospace Center (DLR) as references, there is no improvement in accuracy of the GPS satellite DCB estimates based on the fusion of Jason-2 and Jason-3 observations than the Jason-2 solutions alone. A periodic variation is found in the time series of both the Jason-3 and Jason-2 onboard receiver DCB estimates. Preliminary analysis of the PTEC distribution based on the estimated SH coefficients are also presented.     

Comparison of RO tropopause height based on different tropopause determination methods

The tropopause is an important boundary in the Earth’s atmosphere, and has been the subject of close attention from atmosphere and climate researchers. To monitor the global tropopause using radio occultation (RO) data, there are two primary methods, one is the widely used temperature lapse rate method, and the other is the bending angle covariance transform method which is unique to RO data. We use FengYun3-C (FY3C) and Meteorological Operational Satellite Program (MetOp) RO data and European Centre for Medium-Range Weather Forecasts (ECMWF) operational analysis data to determine differences in RO tropopause height calculated by these two methods. We compute biases of the RO lapse rate tropopause height (LRTH) and the RO bending angle tropopause height (BATH) relative to the ECMWF LRTH. The dependences of the tropopause height biases on tropopause height (TPH) retrieval method, latitude, season and RO mission are investigated. The results indicate that BATH show a consistent 0.8–1.2 km positive bias over the tropics and high latitude regions compared with LRTH, however, over 25° to 40° latitude in both hemisphere, BATH results are less stable. Furthermore, the mean bias between BATH and LRTH displayed a different symmetrical characteristic from 2017.12 to 2018.2 (DJF) compared to 2018.6–2018.8 (JJA). However, except for some bias over Antarctica, the mean value of both LRTH and BATH show a similar tropopause variation, indicating the consistency of both methods.     

Improvement and application of Y2O3 directional solidification crucible

In order to satisfy the drastic temperature change and high chemical activity in directional solidification of Nb–Si based alloys, Y_2O_3 crucible is demanded to possess high thermal shock resistance and erosion resistance. This paper improved the sintering degree and density of Y_2O_3 crucible by optimizing the sintering temperature and time, and its practical application performance was investigated. Y_2O_3 grains gathered with the increase of sintering temperature and time, and the contact area enlarged, resulting in the open pores being changed into closed pores.The higher density caused the improvement of erosion resistance of Y_2O_3 crucibles. However, excessive density weakened the thermal shock resistance. Considering high-temperature strength, erosion resistance, thermal shock resistance and costs, optimum sintering temperature and time of Y_2O_3 directional solidification crucible were 1800 °C and 120 min, respectively, and the porosity was20%. Improved Y_2O_3 crucible has been successfully applied to directional solidification of Nb–Si based alloys, and significantly reduced the oxygen contamination. Slight interaction occurred between Hf and Y_2O_3, but no obvious dissolution, penetration or erosion was found, showing good erosion resistance and thermal shock resistance.     

Analysis of BDS-3 distributed autonomous navigation based on BeiDou system simulation platform

Satellite autonomous navigation is an important function of the BeiDou-3 navigation System (BDS-3). Satellite autonomous navigation means that the navigation satellite uses long-term forecast ephemeris and Inter-Satellite Link (ISL) measurements to determinate its own spatial position and time reference without the support of the ground Operation and Control System (OCS) for a long time to ensure that the navigation system can normally maintain the time and space reference. This paper aims to analyze the feasibility of distributed autonomous navigation algorithms. For the first time, a ground parallel autonomous navigation test system (GPANTS) is built. The performance of distributed autonomous navigation is then analyzed using the two-way ISL ranging of BDS-3 satellites. First, the BDS simulation platform and the GPANTS are introduced. Then, the basic principles of distributed satellite autonomous orbit determination and time synchronization based on ISL measurements are summarized. Preliminary evaluation of the performance of the BDS-3 constellation autonomous navigation service under ideal conditions through simulation data. Then the performance of autonomous navigation for 22 BeiDou-3 satellites using ISL measurements is evaluated. The results show that when satellites operate autonomously for 50 days without the support of any ground station, the User Range Error (URE) of autonomous orbit determination is better than 3 m, and the time synchronization accuracy is better than 4 ns.     

Analysis of electron content variations over Japan during solar minimum: Observations and modeling

Comparative analysis of GPS TEC data and FORMOSAT-3/COSMIC radio occultation measurements was carried out for Japan region during period of the extremely prolonged solar minimum of cycle 23/24. COSMIC data for different seasons corresponded to equinox and solstices of the years 2007–2009 were analyzed. All selected electron density profiles were integrated up to the height of 700 km (altitude of COSMIC satellites), the monthly median estimates of Ionospheric Electron Content (IEC) were retrieved with use of spherical harmonics expansion. Monthly medians of TEC values were calculated from diurnal variations of GPS TEC estimates during considered month. Joint analysis of GPS TEC and COSMIC data allows us to extract and estimate electron content corresponded to the ionosphere (its bottom and topside parts) and the plasmasphere (h > 700 km) for different seasons of 2007–2009. Percentage contribution of ECpl to GPS TEC indicates the clear dependence from the time and varies from a minimum of about 25–50% during day-time to the value of 50–75% at night-time. Contribution of both bottom-side and topside IEC has minimal values during winter season in compare with summer season (for both day- and night-time). On average bottom-side IEC contributes about 5–10% of GPS TEC during night and about 20–27% during day-time. Topside IEC contributes about 15–20% of GPS TEC during night and about 35–40% during day-time. The obtained results were compared with TEC, IEC and ECpl estimates retrieved by Standard Plasmasphere–Ionosphere Model that has the plasmasphere extension up to 20,000 km (GPS orbit).     

基于北斗三号导航系统的卫星定位技术

对北斗三号体制下的传输链路、接收门限进行理论分析,得出捕获算法采用4次非相干累加,可以使捕获灵敏度达到-130 dBm,同时实现定位时间小于5 min的综合较优结果.接着以低轨太阳同步轨道卫星为例,利用STK建模进行全轨道周期仿真,结果显示全程收星数超过4颗,定位精度达到2.59 m,速度误差为0.00378 m/s....     

Assessment of the suitability of public mobile data networks for aircraft telemetry and control purposes

This paper provides a review of the state of the art of relevant work on the use of public mobile data networks for aircraft telemetry and control proposes. Moreover, it describes the characterisation for airborne uses of the public mobile data communication systems known broadly as 3G. The motivation for this study was to explore how this mature public communication systems could be used for aviation purposes. An experimental system was fitted to a light aircraft to record communication latency, line speed, RF level, packet loss and cell tower identifier. Communications was established using internet protocols and connection was made to a local server. The aircraft was flown in both remote and populous areas at altitudes up to 8500 ft in a region located in South East Queensland, Australia. Results show that the average airborne RF levels are better than those on the ground by 21% and in the order of −77 dbm. Latencies were in the order of 500 ms (1/2 the latency of Iridium), an average download speed of 0.48 Mb/s, average uplink speed of 0.85 Mb/s, a packet of information loss of 6.5%. The maximum communication range was also observed to be 70 km from a single cell station. The paper also describes possible limitations and utility of using such communications architecture for both manned and unmanned aircraft systems.     

3D打印技术在水冷板加工中的应用研究

目前,3D打印技术在各行业得到了广泛的应用,探讨和研究3D打印技术在空空导弹中的应用具有重要的意义.简要介绍了3D打印技术概念、原理和材料,重点介绍了水冷板进行3D打印加工过程,打印成功的水冷板通过检验、加工、装配,满足结构设计要求.3D打印能够缩短水冷板加工周期,降低加工成本.