Fast cooperative trajectory optimization and test verification for close-range satellite formation using Finite Fourier Series method

The process of formation reconfiguration for close-range satellite formation should take into account the risk of collisions between satellites. To this end, this paper presents a method to rapidly generate low-thrust collision-avoidance trajectories in the formation reconfiguration using Finite Fourier Series (FFS). The FFS method can rapidly generate the collision-avoidance three-dimensional trajectory. The results obtained by the FFS method are used as an initial guess in the Gauss Pseudospectral Method (GPM) solver to verify the applicability of the results. Compared with the GPM method, the FFS method needs very little computing time to obtain the results with very little difference in performance index. To verify the effectiveness, the proposed method is tested and validated by a formation control testbed. Three satellite simulators in the testbed are used to simulate two-dimensional satellite formation reconfiguration. The simulation and experimental results show that the FFS method can rapidly generate trajectories and effectively reduce the risk of collision between satellites. This fast trajectory generation method has great significance for on-line, constantly satellite formation reconfiguration.     

双卫星故障条件下的RAIM可用性研究

现有的关于接收机自主完好性监测（RAIM）算法大部分是基于单颗卫星故障的假设,但随着GPS技术的发展,多颗卫星发生故障的可能性已不能再被忽视,特别是两颗卫星同时发生故障的情况。本研究基于对水平定位误差保护级（HPL）的分析,从一颗卫星故障的可用性出发,推导出两颗卫星故障时的可用性。率先给出了在两颗卫星故障的情况下,天津区域内RAIM算法的可用性。仿真结果表明：两颗卫星故障的可用性低于单颗卫星故障的可用性。     

Design of Satellite Constellations for Optimal Continuous Coverage

A satellite-borne sensor can view a region at or above the Earth's surface. The size of this region depends on the satellite's altitude, the maximum range and scan angle of the sensor, the minimum above-the-horizon viewing angle required, the extent in altitude of the region to be viewed, and the maximum altitude of sensor obscuration by the atmosphere. Except for geosynchronous satellites this region moves relative to the Earth, so that constellations of satellites are generally necessary for continuous coverage. Satellite constellations which minimize the number of satellites required for continuous coverage are derived as a function of the angle subtended at the Earth's center by the coverage of a single satellite. This is done for single and triple continuous coverage of the entire Earth and of the polar regions extending to arbitrary latitude. Simple, cogent approximations for the configurations and numbers of satellites are found. Expressions which relate sensor capabilities and surveillance requirements to are presented. Examples are given to illustrate the use and accuracy of the results.     

Geostationary Satellite Navigation Systems

The concept of position determination using geostationary satellites as an alternative to the global positioning system (GPS) is studied. The advantage of a geostationary system is that only three, or at most four, satellites are required to cover the continental United States. A total of twelve satellites are sufficient for global coverage (excluding polar regions), or eight if only longitude and latitude, but not altitude, are measured. The system involves the determination of the range to either four geostationary satellites or, if the altitude is not measured, three geostationary satellites. The accuracy of the proposed systems are evaluated to obtain the rms error associated with position determination, and the concept for the implementation of measurements required by the systems is presented. The accuracy of the systems are adequate for civilian use in the continental United States; however, there is a degradation in accuracy as the location of the user approaches the equator.     

A Concept of Position Fixing by a Passive Mode Navigation Satellite System

A position fix in a passive mode using satellites usually necessitates an expensive computer or lengthy hand calculation. This is the largest drawback of passive navigation and it would be more desirable if the user could find his position by a mere glance at a chart and table, as one uses Loran. The first step toward this goal is to use a synchronous satellite because it simplifies the problem. The next step is to find the position of the user by a Loran type of chart, which is universal, and correct this apparent position by looking at a special table which is made according to the amount of perturbation of both the satellite and the user's position. An example of the position fix along the route between Yokohama to Hawaii is shown. The concept can be extended to orbiting satellites due to the rules which govern the motion of satellites, if the fix accuracy is in the order of 2 to 5 miles. This method should be more accurate than the common sextant and more practical due to the fact that the satellite can be used at any time and in any weather. As a total system, it will be better than Omega because it could provide additional navigation information such as communication or traffic control by using satellites.     

Detection of obstacles in the flight path of an aircraft

The National Aeronautics and Space Administration (NASA), along with members of the aircraft industry, recently developed technologies for a new supersonic aircraft. One of the technological areas considered for this aircraft is the use of video cameras and image-processing equipment to aid the pilot in detecting other aircraft in the sky. The detection techniques should provide high detection probability for obstacles that can vary from subpixel to a few pixels in size, while maintaining a low false alarm probability in the presence of noise and severe background clutter. Furthermore, the detection algorithms must be able to report such obstacles in a timely fashion, imposing severe constraints on their execution time. Approaches are described here to detect airborne obstacles on collision course and crossing trajectories in video images captured from an airborne aircraft. In both cases the approaches consist of an image-processing stage to identify possible obstacles followed by a tracking stage to distinguish between true obstacles and image clutter, based on their behavior. For collision course object detection, the image-processing stage uses morphological filter to remove large-sized clutter. To remove the remaining small-sized clutter, differences in the behavior of image translation and expansion of the corresponding features is used in the tracking stage. For crossing object detection, the image-processing stage uses low-stop filter and image differencing to separate stationary background clutter. The remaining clutter is removed in the tracking stage by assuming that the genuine object has a large signal strength, as well as a significant and consistent motion over a number of frames. The crossing object detection algorithm was implemented on a pipelined architecture from DataCube and runs in real time. Both algorithms have been successfully tested on flight tests conducted by NASA.     

Detecting near collisions for satellites

A highly efficient algorithm is presented for detecting near collisions involving satellites. Rather than examine all possible pairings of a satellite with another orbiting object, the new approach examines trajectories at a sequence of coarse time steps. Objects which are spatially isolated cannot be involved in a collision, and are discarded. The time steps, orbit calculations, and discarding are repeatedly refined until the only remaining objects experience near collisions.     

Receiver autonomous integrity monitoring (RAIM) capability forsole-means GPS navigation in the oceanic phase of flight

Receiver autonomous integrity monitoring (RAIM), a GPS integrity monitoring scheme that uses redundant ranging signals to detect a satellite malfunction that results in a large range error, involves two functions: detection of the presence of a malfunctioning satellite and identification of which satellite (or satellites) is malfunctioning. An analysis is presented of GPS RAIM capability for sole-means navigation in the oceanic phase of flight, where the position protection limit requirement for the integrity function is not as stringent as for nonprecision approaches, and yet both detection and identification function may be required if GPS is to be used as a sole-means system. For this purpose, a detection and identification algorithm is developed which takes advantage of the fact that for the oceanic phase of flight, a much larger position error is acceptable than for the nonprecision approach phase of flight. The performance of this algorithm and an algorithm proposed previously by others is estimated via simulation and compared. On the basis of the results, recommendations are made on how RAIM may be used if GPS is to be coupled with an inertial system to provide a sole-means capability in the oceanic phase of flight     

The Advent of Land Mobile Satellite Service Systems

The practicality of providing cellular-type communications service to underserved remote areas of the country is now possible through the use of satellites in geostationary orbit. The advent of high-power, land-mobile satellites, coupled with high-performance, low-cost ground receivers, makes it possible to provide mobile radio, mobile telephone, data communication, and other services to large numbers of rural and suburban users. A recent Federal Communications Commission (FCC) decision has allocated L-band (1.5 GHz, 1.6 GHz) spectrum to this service. Even though there is a significant amount of spectrum available at L-band, the expected demand for this service is high and spectral efficient means must be devised to maintain sufficient capacity. Expedient means used to increase capacity, in the absence of additional spectrum, are single channel per carrier, demand assignment multiple access (SCPC-DAMA) with voice, frequency reuse via multiple beams, and orbital reuse by using multiple satellites. Some of the operational, systemic, and technological considerations of the first generation land mobile satellite service (LMSS) that would provide thin-route services to large land masses of North America are considered here.     

Rosette Constellations of Earth Satellites

Satellite constellations having rosette (flowerlike) orbital patterns are described which exhibit better worldwide coverage properties than constellations previously reported in U.S. literature. The best rosettes with 5-15 satellites are identified and evaluated relative to prior results. In most cases, the best results are obtained by placing one satellite in each of N separate planes and by using inclined rather than polar orbits. Coverage properties of these constellations are analyzed in terms of the largest possible great circle range between an observer anywhere on the Earth's surface and the nearest subsatellite point. When evaluated in this manner, coverage properties are invariant with deployment altitude. As deployment altitude is reduced, however, higher order constellations must be used to maintain a fixed minimum viewing angle. Coverage properties are also invariant with deployment orientation relative to Earth coordinates, although specific orientations can cause the satellite patterns to appear quasi-stationary. Thus these constellations offer a promising alternative to the use of geostationary satellites. Rosette constellations can also be used to guarantee multiple satellite visibility on a continuous worldwide basis. It is shown that 5, 7, 9, and 11 satellites are the minimum numbers required for single, double, triple, and quadruple visibility, respectively. Examples of rosette constellations which achieve these bounds are given.     

A Novel Onboard-gateway-based Mechanism to Improve TCP Performance in Aeronautical Satellite Networks

The IP-based networks on aircraft serve to support Internet services via satellites. However, in aeronautical satellite hybrid net- works,the TCP protocol performance often deteriorates due to improper decreases and slow recovery of the congestion window. This paper proposes a window size determination and notification mechanism, onboard-gateway-based mechanism (OGBM), which is based on the onboard gateway in the networks on aircraft. A cross-layer approach is adopted by the onboard gateway to obtain the satellite link bandwidth information. And then, by the gateway, through changing the receiver’s advertised window field in ACK packets, TCP sources are notified of the window size of each TCP source calculated on the ground of bandwidth delay product and flow numbers. The mechanism is able to avoid improper changes of TCP window and serve multiple users. Simulation results show that the mechanism with the fairness index close to 1 improves TCP performance in aeronautical satellite networks.     

给定寿命下的疲劳裂纹尺寸分布

 本文论证了疲劳裂纹超值概率和疲劳寿命破坏率之间的等同性。由给定裂纹尺寸下疲劳寿命的分布推导出给定寿命下疲劳裂纹超值概率和疲劳裂纹尺寸分布,建立了三维空间的P-a-t曲面方程。     

磁强计数据辨识卫星自旋状态

针对近地低轨三轴稳定卫星在轨管理后期,除磁强计外其他姿态敏感器都无效情况下的姿态异常问题,分析了三轴稳定卫星失去姿态基准后,星体自旋状态下三轴磁强计测量数据的特点,提出了使用磁强计测量数据的矢量信息,以找到能够获取卫星状态的方法,从而建立了磁强计测量矢量与卫星自旋轴的几何关系,给出了处于自旋状态下的卫星自旋轴确定方法。通过此种辨识方法,获得了某气象卫星姿态异常翻转状态下的自旋矢量方向和自旋角速度,从而证明了该辨识方法快速、有效,可以作为姿态异常卫星自旋状态的辨识手段,为恢复卫星姿态提供了重要信息,具有一定的工程应用价值。     

Reliability and Availability of Redundant Satellite Orbit Systems

In order to find the optimum redundant satellite orbit system, the formulas are derived for reliability and availability of redundant systems composed of two parallel, three parallel, one functioning and one standby, and two parallel and one standby satellites, where both the probability of a start or switchover and the necessary delay time for a start or switchover are taken into consideration. The calculation by these formulas shows the relation among the reliability, availability, launch probability, and launch delay time, as illustrated by numerical examples of conventional rockets and space shuttles.     

基于变结构控制器的敏捷卫星姿态机动方法

随着遥感卫星观测能力的逐步提升,对卫星敏捷机动能力提出了更高的要求。针对敏捷卫星大角度姿态机动问题,以6个单框架控制力矩陀螺(SGCMG)组成五棱锥构型的姿态控制系统执行机构,在构建敏捷卫星姿态运动数学模型以及设计SGCMG系统操纵律的基础上,对卫星绕Euler轴进行姿态机动的角轨迹进行规划,并设计了一种基于误差四元数与误差角速度的变结构控制器。仿真及在轨验证结果表明,该控制器能够完成规划轨迹的良好跟踪且具有较强的鲁棒性,研究成果对敏捷卫星姿态控制系统的设计具有重要的参考意义。     

星座碰撞规避的迭代学习构型保持方法CSCD

针对低轨卫星星座运行中地球引力摄动的周期特性,基于迭代学习控制(ILC)方法,提出了星座碰撞规避的迭代学习构型保持方法。该方法由反馈控制和ILC两部分构成,分别抑制卫星运行过程中的非周期摄动和周期摄动对构型保持精度的影响,进而在地球非球形引力摄动未知条件下,通过相对构型的精确保持实现对星座卫星碰撞的有效规避。仿真结果表明,在地球J摄动影响下,与传统反馈控制相比,ILC方法以更小的控制输入实现了轨道保持精度的显著提升,进而在星座卫星轨道高度相近的情形下显著降低了碰撞风险,且控制器可在保证收敛性能的前提下,实现启动时间的灵活选择。     

Characterization of NAVSTAR GPS and GLONASS on-board clocks

Investigation into navigation satellite on board clock frequency references and performance are reported. The focus is on the stability of the clocks aboard the NAVSTAR GPS (Global Positioning System) and GLONASS satellites as well as those used by their respective maser control stations and associated time scales. Allan-variance techniques have been applied to determine the long-term time-domain behavior of satellite clocks in an attempt to identify different regions of power spectral density. Coupled with analysis of relative-frequency drift over a period of many weeks, this behavior allows the type of satellite onboard standard to be tentatively identified. The known nature of the GPS clocks has shown that the different types of clocks aboard the satellites (crystal, rubidium, and cesium) are distinguishable given a sufficient sample time. The same approach has been applied to the GLONASS satellites, and a comparison of the results obtained from GPS has allowed conjecture on the type of clock used by the GLONASS satellites. It appears that GLONASS has used clocks of the quality of rubidium atomic oscillators since at least 1986, and that the quality and performance of onboard standards have increased steadily with time. Some current satellites perform well enough in terms of frequency drift, flicker FM noise floor, and long-term stability to compare favorably with the cesium beam standards carried on NAVSTAR GPS satellites launched in 1983-84     

14.5-14.8 GHz Frequency Sharing by Data Relay Satellite Uplinks and Broadcasting-Satellite Uplinks

Data relay satellites are being developed to provide real-time data links between research satellites in low earth orbits and central data acquisition and processing facilities. Frequency assignments for data relay satellite links will be made in bands allocated internationally to the space research service. One of the bands which will be used lies between 14.5 and 15.35 GHz, where the space research service has had a frequency allocation as a secondary service since 1971. During the General World Administrative Radio Conference of the International Telecommunication Union, held in Geneva in 1979, a primary frequency allocation was made in the band 14.5-14.8 GHz to the fixed-satellite service, specifically for use by earth-to-space links of the broadcasting satellite service. The feasibility of shared band operation is evaluated between data relay satellite uplinks and broadcasting-satellite feeder links in the band 14.5-14.8 GHz. Relationships for predicting interference power levels are formulated, as functions of satellite separation and of earth station separation. Tradeoffs between satellite separation angle and earth station separation are explored, and conclusions are drawn regarding the feasibility of band sharing. Co-channel operation is demonstrated to be technically feasible for typical systems, provided appropriate separations are maintained.     

OPPORTUNITIES FOR MAGNETOSPHERIC RESEARCH WITH COORDINATED CLUSTER AND GROUND-BASED OBSERVATIONS

ESA's first multi-satellite mission Cluster is unique in its concept of 4 satellites orbiting in controlled formations. This will give an unprecedented opportunity to study structure and dynamics of the magnetosphere. In this paper we discuss ways in which ground-based remote-sensing observations of the ionosphere can be used to support the multipoint in-situ satellite measurements. There are a very large number of potentially useful configurations between the satellites and any one ground-based observatory; however, the number of ideal occurrences for any one configuration is low. Many of the ground-based instruments cannot operate continuously and Cluster will take data only for a part of each orbit, depending on how much high-resolution (burst-mode') data are acquired. In addition, there are a great many instrument modes and the formation, size and shape of the cluster of the four satellites to consider. These circumstances create a clear and pressing need for careful planning to ensure that the scientific return from Cluster is maximised by additional coordinated ground-based observations. For this reason, the European Space Agency (ESA) established a working group to coordinate the observations on the ground with Cluster. We will give a number of examples how the combined spacecraft and ground-based observations can address outstanding questions in magnetospheric physics. An online computer tool has been prepared to allow for the planning of conjunctions and advantageous constellations between the Cluster spacecraft and individual or combined ground-based systems. During the mission a ground-based database containing index and summary data will help to identify interesting datasets and allow to select intervals for coordinated studies. We illustrate the philosophy of our approach, using a few important examples of the many possible configurations between the satellite and the ground-based instruments.     

Spaceborne GPS receiver antenna phase center offset and variation estimation for the Shiyan 3 satellite

In determining the orbits of low Earth orbit (LEO) satellites using spaceborne GPS, the errors caused by receiver antenna phase center offset (PCO) and phase center variations (PCVs) are gradually becoming a major limiting factor for continued improvements to accuracy. Shiyan 3, a small satellite mission for space technology experimentation and climate exploration, was developed by China and launched on November 5, 2008. The dual-frequency GPS receiver payload delivers 1 Hz data and provides the basis for precise orbit determination within the range of a few centime-ters. The antenna PCO and PCV error characteristics and the principles influencing orbit determi-nation are analyzed. The feasibility of PCO and PCV estimation and compensation in different directions is demonstrated through simulation and in-flight tests. The values of receiver antenna PCO and PCVs for Gravity Recovery and Climate Experiment (GRACE) and Shiyan 3 satellites are estimated from one month of data. A large and stable antenna PCO error, reaching up to 10.34 cm in the z-direction, is found with the Shiyan 3 satellite. The PCVs on the Shiyan 3 satellite are estimated and reach up to 3.0 cm, which is slightly larger than that of GRACE satellites. Orbit validation clearly improved with independent k-band ranging (KBR) and satellite laser ranging (SLR) measurements. For GRACE satellites, the average root mean square (RMS) of KBR resid-uals improved from 1.01 cm to 0.88 cm. For the Shiyan 3 satellite, the average RMS of SLR resid-uals improved from 4.95 cm to 4.06 cm.