Slave Satellite Research Articles

Development and Simulation of Motion Control System for Small Satellites Formation

In the paper, the problem of forming and maintaining the small satellites formation in the near-earth projected circular orbits is considered. The satellite formation reconfiguration and formation-keeping control laws are proposed by employing the passivity-based output feedback control. For the complete nonlinear and time-dependent dynamics of the relative motion of a pair of satellites in elliptical orbits, new combined control algorithms, including a consensus protocol, are proposed and analyzed. A comparison of the control modes using the passivity-based output feedback control and the proportional-differential controller with and without the consensus algorithm is given. On the basis of the passification method, the algorithm is obtained ensuring the stable motion of the slave satellite relative to the orbit of the master satellite. To improve the accuracy of the satellites’ positioning, a consensus protocol based on measurements of the relative positions of the satellites is proposed and studied. Computer simulations of the proposed algorithms for options to construct formations are provided for two projected circular orbits of 8 satellites, demonstrating the efficiency of the proposed control schemes. It is shown that the resulting passivity-based output feedback control provides better accuracy than the PD controller. It is also shown that the use of the consensus protocol further increases the positioning accuracy of the satellite constellation.

Coherence-Based Geosynchronous SAR Tomography Employing Formation Flying: System Design and Performance Analysis

Coherence-based synthetic aperture radar (SAR) tomography (TomoSAR) exploits the complex coherences of SAR images to achieve 3-D imaging. Utilizing two-sensor spaceborne SAR formation flying to realize coherence-based TomoSAR has attracted increasing attention because temporal decorrelation-free interferograms can be constructed; therefore, TomoSAR has excellent potential for inverting the vertical structures of natural scenes such as forests and glaciers. However, low earth orbit (LEO) TomoSAR is disadvantaged by limited data and nonuniform sampling in the elevation direction. Geosynchronous (GEO) TomoSAR can overcome these limitations owing to its short revisit time of no more than 24 h. For the first time, this article discusses coherence-based TomoSAR exploiting GEO SAR formation flying. The benefits of GEO-formation coherence-based TomoSAR, including the low cost of slave satellites, rich data sets, and uniform sampling, are noted. The key problems of system design, including the formation design and data acquisition, are discussed. A formation design method based on the minimum along-track baseline is proposed that can realize uniform elevation sampling. The geometric correlation of a general SAR observation geometry is derived; on this basis, an optimal data acquisition method based on the optimal height measurement Cramer-Rao lower bound (CRLB) is proposed. Finally, the performance of GEO-formation coherence-based TomoSAR is analyzed; in particular, the ambiguity height in the altitude direction, the Rayleigh resolution in the altitude direction, and the theoretical optimal geometric correlation are evaluated. Finally, computer simulations validate the proposed formation design method, data acquisition scheme, and performance analysis formula.

Study and simulation of heterodyne optical phase-locked loop for inter-satellite laser interferometer

Coherent transmission of optical signal is an important module of inter-satellite laser interferometer. Through heterodyne optical phase-locking technology, the coherent transmission of laser signals from the master spacecraft can be achieved. And in the case of ensuring the stability of the local laser frequency, amplifying of the optical output power is realized. This article introduces and analyzes the principle, modeling and simulation of heterodyne optical phase-locking technology, which provides a reference for the realization of laser signal’s coherent transmission from slave satellite.

Joint Master–Slave Yaw Steering for Bistatic Spaceborne SAR With an Arbitrary Configuration

Yaw steering is an important technique for bistatic spaceborne synthetic aperture radar (SAR), traditionally implemented separately at the master and slave satellites, such as the X-band TerraSAR-X/TanDEM-X system. This separate yaw steering method, however, will degrade the illumination synchronization between the master and slave satellites. With the increment in frequency band and master–slave distance, the degradation will get worse, leading to intolerable azimuth resolution degradation or even failure of imaging. To solve this problem, a new joint yaw steering (JYS) method is proposed for bistatic spaceborne SAR with an arbitrary configuration in this letter. The word “joint” means that the master and slave satellites <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cooperatively</i> work to always make their beams point to an identical point on the Earth’s surface. In this way, the best illumination synchronization can be achieved, contributing to the best available azimuth resolution. The presented approach has been evaluated through computer simulations.

Energy efficiency resource allocation based on spectrum-power tradeoff in distributed satellite cluster network

Distributed satellite cluster network is a novel structure in spatial information network. This paper investigates the spectrum-power tradeoff algorithm for distributed satellite cluster (DSC), which enables the master satellite (MS) to offload data traffic to the slave satellite (SS). The SS provides power resource to the master satellite users (MSUs) with bandwidth as compensation. The purpose of the proposed algorithm is to maximize the energy efficiency (EE) of the SS while ensuring the QoS of the DSC. Considering data rate and bandwidth constraints of the MS, the SS jointly allocates bandwidth and power resources based on the served MSU set, meanwhile, data rate and power constraints of the SS should be also satisfied. We first prove that compensating bandwidth of a MSU can only be used by one slave satellite user. Then objective function is simplified to the equivalent subtractive form that can be solved by convex optimization method. Besides, we discuss the MSU selection conditions and employ them to the proposed spectrum-power tradeoff algorithm. The numerical results validate the performance improvement of the proposed algorithm and demonstrate the impact of power, bandwidth and data rate on the EE.

Formation Design for Single-Pass GEO InSAR Considering Earth Rotation Based on Coordinate Rotational Transformation

The single-pass geosynchronous synthetic aperture radar interferometry (GEO InSAR) adopts the formation of a slave satellite accompanying the master satellite, which can reduce the temporal decorrelation caused by atmospheric disturbance and observation time gap between repeated tracks. Current formation design methods for spaceborne SAR are based on the Relative Motion Equation (RME) in the Earth-Centered-Inertial (ECI) coordinate system (referred to as ECI-RME). Since the Earth rotation is not taken into account, the methods will lead to a significant error for the baseline calculation while applied to formation design for GEO InSAR. In this paper, a formation design method for single-pass GEO InSAR based on Coordinate Rotational Transformation (CRT) is proposed. Through CRT, the RME in Earth-Centered-Earth-Fixed (ECEF) coordinate system (referred to as ECEF-RME) is derived. The ECEF-RME can be used to describe the accurate baseline of close-flying satellites for different orbital altitudes, but not limited to geosynchronous orbit. Aiming at the problem that ECEF-RME does not have a regular geometry as ECI-RME does, a numerical formation design method based on the minimum baseline error criterion is proposed. Then, an analytical formation design method is proposed for GEO InSAR, based on the Minimum Along-track Baseline Criterion (MABC) subject to a fixed root mean square of the perpendicular baseline. Simulation results verify the validity of the ECEF-RME and the analytical formation design method. The simulation results also show that the proposed method can help alleviate the atmospheric phase impacts and improve the retrieval accuracy of the digital elevation model (DEM) compared with the ECI-RME-based approach.

Energy cost analysis to station keeping for satellite formation type “TerraSAR-X - TanDEM-X”

The maintenance of a given configuration of the satellite formation of the “TerraSAR-X - TanDEM-X” type is considered. It is assumed that the master satellite performs only maneuvers to maintain the working orbit, and the slave satellite performs identical maneuvers to maintain the working orbit and additionally performs maneuvers to maintain a given relative configuration of the group. For the working orbit of the master satellite, the dependence of the total characteristic velocity costs for maintaining a large semi-axis, eccentricity, inclination, and their various combinations on the maintenance accuracy is studied. The minimum limits of accuracy at which maneuvering is not required are set for each of the elements. This study is general in nature and allows future missions to be planned, providing the necessary trade-offs between the accuracy of maintenance and the total characteristic speed costs that increase as maintenance accuracy increases. Also, a study of the energy costs of relative maintenance, provided that the engines of the master and slave satellites operate almost the same. It is shown that the relative maintenance requires significantly lower fuel costs, even with the accuracy required in the project. The software product developed for this study is of universal application and will be used to investigate the cost of maintaining a more complex system of four satellites, in which three satellites rotate relative to the base satellite.

Focusing the L-Band Spaceborne Bistatic SAR Mission Data Using a Modified RD Algorithm

LuTan-1 [(LT-1), i.e., TwinSAR-L] mission is an innovative spaceborne bistatic synthetic-aperture radar (BiSAR) mission focusing mainly on differential interferometry, which will be launched in 2020. This article introduces some important aspects of the LT-1 mission for the first time, including the formation configuration, imaging mode, application scenarios, and the efficient baselines between the master satellite and the slave satellite. To realize the high accurate topography and deformation measurements, a wide swath BiSAR focusing algorithm with phase reserving ability should be developed. This article proposes a modified bistatic range-Doppler algorithm based on 2-D principle of stationary phase spectrum, which reduces the phase error introduced by the root term expansion and has an excellent focus performance and a good phase reserving ability. Finally, the spaceborne bistatic simulation experiments using orbital parameters and imaging mode of the LT-1 mission, including point targets and scene targets, are utilized to illustrate the validity and accuracy of the proposed algorithm.

Satellite Formation Flying Control Using Image Processing

A problem of a formation flying control is observed. As an example a concept of a multysatellite configuration ("space commercial") is described. Space advertisement is realized by groups of specialized free flying and cable-connected satellites, and their combinations. Construction of a space commercial satellite and a set of a control system equipment is presented. Several methods for group configuration are presented: periodic impulse correction of each flying satellite position formation; method of a satellite positioning on nocoplanar orbits. Other methods include combinations of methods mentioned above. Configuration control using onboard video camera with a wide-angle image detector is presented. Characteristics of an optical system are shown. Navigation parameters of a using a received observed videoimage are estimated. A method of a relative position measurement is described. The method detects a video track of a moving ahead satellite on the matrix detector. According to the method lines of the matrix detector are positioned transversely and columns are positioned in parallel with vertical axis of the orbital coordinate system. Course of the slave satellite is proportional to the travel of the centre of the lider satellite mark on the matrix detector columns. Pitch of the slave satellite is proportional to the travel of the centre of the leader satellite mark on the matrix detector lines. A video-display processing algorithm is developed and researched. The algorithm contains procedures of a forming, localization of the leader satellite mark, deviation angle calculation. The algorithm uses structural method, detecting bright spots (image syntactic characteristics). Efficiency of the algorithm depends on the leader satellite mark brightness, size, light-striking, distance.

Inter-satellite laser link acquisition with dual-way scanning for Space Advanced Gravity Measurements mission.

Laser link acquisition is a key technology for inter-satellite laser ranging and laser communication. In this paper, we present an acquisition scheme based on the differential power sensing method with dual-way scanning, which will be used in the next-generation gravity measurement mission proposed in China, called Space Advanced Gravity Measurements (SAGM). In this scheme, the laser beams emitted from two satellites are power-modulated at different frequencies to enable the signals of the two beams to be measured distinguishably, and their corresponding pointing angles are determined by using the differential power sensing method. As the master laser beam and the slave laser beam are decoupled, the dual-way scanning method, in which the laser beams of both the master and the slave satellites scan uncertainty cones simultaneously and independently, can be used, instead of the commonly used single-way scanning method, in which the laser beam of one satellite scans and that of the other one stares. Therefore, the acquisition time is reduced significantly. Numerical simulation and experiments of the acquisition process are performed using the design parameters of the SAGM mission. The results show that the average acquisition time is less than 10 s for a scanning range of 1-mrad radius with a success rate of more than 99%.

Design and derivation of the dual transponder carrier ranging system

The accuracy of microwave ranging is mainly limited by the frequency instability of the oscillator that generates the carrier phase signal. A dual transponder carrier ranging method is used to minimize the oscillator noise by combining the reference and the to-and-fro measurements. This ranging approach together with pseudo-noise ranging or other means can be used to measure the inter-satellite distance with a high precision. The pseudo-noise ranging system or other ranging systems help to solve the integer circles while the dual transponder ranging system guarantees the accurate fractional circle. The two satellites work in the master-slave mode. The range measurements are derived on the master satellite while the slave satellite just coherently transfers the received signal, so that the dual transponder ranging system does not need to rely on the time tagging system to synchronize the two satellites. This study first describes the dual transponder carrier ranging system and shows how the system removes most of the oscillator noise components effectively. Then, a detailed design scheme on the frequency planning of the ranging system is presented and the supporting analysis illustrates the feasibility of this system. Based on the design innovation, a laboratory demonstration system is assembled to verify the realizability of the dual transponder ranging system. The experimental results demonstrate that a high level of accuracy (about 30 μm under laboratory circumstance) can be achieved by the use of the proposed dual transponder carrier ranging system.

State Modelling of the Land Mobile Propagation Channel with Multiple Satellites

We evaluate a new approach for multisatellite state modelling: the Master-Slave approach. By this concept slave satellites are modelled according to an existing master, whereas the correlation between multiple slaves is omitted. Master-Slave is therefore a generic name for a state modelling concept, for which different realisations are possible. As a possible realisation we present the Conditional Assembling Method. For modelling of only two satellites (one master and one slave), the Conditional Assembling Method enables an accurate resimulation of the correlation coefficient between the satellites and the probabilities of single and combined states. Based on this condition, the performance of Master-Slave for three, four, and five satellites is evaluated in terms of state probability modelling. Therefore, the correlation coefficients and the all bad-state probabilities with Master-Slave are compared with the measurements for different elevation angles and azimuth angle separations of the multisatellite system. Master-Slave has a high modelling error in case of small azimuth separation between the slave satellites (except that one slave has a small azimuth separation to the master). Furthermore, a master satellite with a high elevation provides a lower probability error compared to a master with low elevation.

AN ADVANCED DSS-SAR INSAR TERRAIN HEIGHT ESTIMATION APPROACH BASED ON BASELINE DECOUPLING

In the distributed small satellites synthetic aperture radar (DSS-SAR), baseline is usually coupled, that is, along-track baseline and across-track baseline exist simultaneously. However, coupling baseline makes it di-cult to distinguish phase difierences caused by terrain height and Doppler frequency difierence. In SAR interferometry (InSAR) geometric model, across-baseline is necessary to bring interferometric phase to estimate terrain height. Oppositely, along-track baseline will bring extra phase difierence and dramatically decrease the accuracy of terrain height estimation. Considering the aforementioned problem, this paper focuses on the study of baseline decoupling of DSS-SAR. We flrstly analyze the efiect of coupling baseline on terrain height estimation, and then propose the method of baseline decoupling through space projection theory. In order to realize baseline decoupling, equivalent slave satellite, equivalent baseline, and equivalent slant range are deflned through projecting slave satellite on range-height plane of master satellite. Furthermore, based on our baseline decoupling, an advanced approach of estimating terrain height is presented, which is more efiective than traditional InSAR geometric model. Simulation results illuminate that the baseline decoupling can eliminate along-track baseline efiect on terrain height estimation efiectively and conflrm the validity and e-ciency of terrain height estimation approach proposed in this paper.

Formationkeeping for a pair of satellites in a circular orbit

Certain future missions will require that a pair of satellites—ma ster and slave—fly in a fixed relative formation. Active control is required to maintain this formation in spite of disturbances such as aerodynamic drag and solar radiation pressure. This paper develops a closed-loop formationkeeping controller for satellites in any circular orbit using digital optimal control theory. A formationkeepi ng sensor concept, tradeoffs, design, brassboard demonstration, and modeling are discussed. The formationkeeping actuators are assumed to be chemical thrusters on the slave satellite. Two satellites flying in trail 700 m apart in geosynchronou s orbit are used as an example to present closed-loop formationkeeping simulation plots. The results show that formation is maintained to within the ±21 m allocated requirement in the in-track and out-of-plane directions.

Cambodia 1977: Gone to Pot

Cambodian. The adequate harvest of 1976 was succeeded by the drought of 1977. Although Prime Minister Pol Pot's visit to Peking and Pyongyang raised Cambodia's international profile, the armed conflict with Vietnam and Thailand became increasingly severe. The mysterious Revolutionary Organization (Angkar) gave way to a more familiar type of organization, the Cambodian Communist Party; however, as of late 1977 this institutionalization of power had not brought an end to the killings that have characterized Cambodian internal politics since April 1975. Finally, economic conditions combined with internal political machinations to produce a whole new wave of refugees fleeing from Democratic Cambodia. Political Developments In 1977 the fundamental goals and values of the new Cambodia became increasingly apparent from the public statements of its leaders.' The regime's ideology emphasizes: national defense and self reliance; radical egalitarian collectivism; puritanical morality; agricultural and industrial modernization; and the dictatorship of the proletariat. According to Pol Pot, Cambodia until April 1975 was not independent but only a half slave satellite of U.S. imperialism. The 1954 Geneva agreement sold out the revolution, Sihanouk's diplomacy compromised national integrity, and urban areas suffered from the deca