Robustness analysis method for orbit control

The accuracy of time synchronization can be significantly increased by enhancing the performance of atomic clocks. Future-generation time-frequency loads will be equipped with the latest ultrahigh-precision atomic clocks (with a day stability better than 10−17) and will leverage advantages of the space environment such as microgravity and low interference to operate a new generation of high-performance time-frequency payloads on low-orbit spacecraft. Moreover, using the high-precision time-frequency system of ground stations, low-time-delay high-performance time-frequency transmission networks, which have the potential to achieve ultrahigh-precision time synchronization, will be constructed. By considering full link error terms above the picosecond level, this paper proposes a new space-to-ground microwave two-way time synchronization method for scenarios involving low-orbit spacecraft and ground stations. Using the theoretical principles and practical application scenarios related to this method, a theoretical and simulation verification platform was developed to research the impact of the attitude, phase center calibration, and orbit determination errors on the single-frequency two-way time synchronization method. The effectiveness of this new method was verified. The results showed that when the attitude error is less than 72 arc seconds (0.02°), the phase center calibration error is less than 1 mm, and the precision orbit determination (POD) error is less than 10 cm (three-axis). After disregarding nonlink error terms such as equipment noise, this method can attain a space-to-ground time synchronization accuracy of better than 1.5 ps, and the time deviation (TDEV) of the transfer link is better than 0.7 ps @ 100 s, which results in ultrahigh-precision space-to-ground time synchronization.

The design of a L-Band shaped beam choked horn antenna for providing accurate position of the geostationary satellite is illustrated in this paper. The chokes of the antenna helps to provide a φ-symmetric conical pattern with peak gain at |θ| = 21 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> and low gain for |θ| = 9 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> . This antenna is capable of receiving GPS signals in a geostationary orbit and also has low immunity towards the noise temperature from hot earth. Designed antenna is right circularly polarized with 10dB cross-pol isolation at |θ| = 21 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> , to minimize errors in orbit determination. The design details and the simulation results of the antenna and its components are discussed in detail.

An impulsive multi-delayed feedback control strategy to control the period-doubling bifurcations and chaos in an n dimensional discrete system is proposed. This is an extension of the previous result in which the control method is applicable to the one-dimensional case. Then the application of the control method in a discrete prey–predator model is studied systematically, including the dynamics analysis on the prey–predator model with no control, the bifurcations analysis on the controlled model, and the bifurcations and chaos control effects illustrations. Simulations show that the period-doubling bifurcations and the resulting chaos can be delayed or eliminated completely. And the periodic orbits embedded in the chaotic attractor can be stabilized. Compared with the existed methods, a milder condition is needed for the realization of the proposed method. The condition may be considered as a generic case and we may state that almost all periodic orbits can be stabilized by the proposed method. Besides, the idea of impulsive control makes the implementation of the proposed control method easy. The impulsive interval is embodied in the analytical expression of the stability condition, hence can be chosen qualitatively according to the real needs, which is an extension of the existed related results. The introduction of multi-delay enlarges the domain of the control parameters and makes the selection of the control parameters have many choices, and hence become flexible.

In this paper we provide a review of the state of the art for active vibration control of axially moving materials (string and beam, etc.) in the past ten years, with particular regard to the subjects of some important active control methods of axially moving strings. First, an introduction is given to explain the aims and scope of this paper. This is followed by a comprehensive discussion of the active wave control methods presented by many investigators in the past ten years, including feedback control methods (such as the modal control method, the direct velocity feedback control method and the control method via transfer function formulation), active wave control methods combining transfer function, boundary control methods using the Lyapunov function, the variable structure control method, the adaptive control method, the vibration control of an axially moving beam and the active control of a moving material with arbitrarily varying length, etc. Finally, we conclude with a discussion of several issues for future research in this area.

Aerodynamic drag analysis and reduction strategy for satellites in Very Low Earth Orbit

The first two Galileo In-Orbit Validation satellites were launched in October 2011 and started continuous signal transmission on all frequencies in early 2012. Both satellites are equipped with two different types of clocks, namely rubidium clocks and hydrogen masers. Based on two test periods, the quality of the Galileo orbit determination based on Global Navigation Satellite System (GNSS) and Satellite Laser Ranging (SLR) observations is assessed. The estimated satellite clock parameters are used as quality indicator for the orbits: A bump at orbital periods in the Allan deviation indicates systematic errors in the GNSS-only orbit determination. These errors almost vanish if SLR observations are considered in addition. As the internal consistency is degraded by the combination, the offset of the SLR reflector is shifted by +5 cm, resulting in an improved orbit consistency as well as accuracy. Another approach to reduce the systematic errors of the GNSS-only orbit determination employs constraints for the clock estimates with respect to a linear model. In general, one decimeter orbit accuracy could be achieved.

Satellite navigation for meteorological purposes: Inverse referencing for NOAA-N and ERS-1 imagers with a 1 km nadir pixel size

Many proposed formation flying missions seek to advance the state of the art in spacecraft science imaging by utilizing precision dual spacecraft formation flying to enable a “virtual” telescope (VT). Using precision alignment of two spacecraft very long focal lengths can be achieved by locating the optics on one spacecraft and the detector on the other. Furthermore, orders of magnitude improvement in science imaging over conventional single spacecraft instruments is possible with advances in precision formation flying. Precise astrometric alignment control is required for accurate inertial pointing of the VT. This work presents orbit design and control approaches for VT formation flying, with specific application to the recently proposed Virtual Telescope Demonstration Mission (VTDM) using cubesats. The characteristics of the mission orbit are derived from solutions to Hill’s equations for the relevant mission orbit. A relative orbit control technique pertaining to the astrometric alignment problem is developed by utilizing the Constrained Model Predictive Control (CMPC) approach. This method enables design of formation flying control laws for a thruster assembly with limited magnitude. Precision formation flying is achieved by use of separate control laws for formation maintenance and precision astrometric alignment. The proposed control scheme compensates for inertial astrometric alignment errors as well as differences in orbital perturbations between the VTDM satellites. Numerical simulations are shown to validate the performance of the proposed relative orbit and alignment control laws for the VTDM. The results satisfy mission requirements and indicate the general applicability of the control algorithm to the VT formation flying architecture. Consequently, the proposed methods of orbit design and control can be applied to inertial astrometric alignment and formation keeping for the many proposed dual spacecraft precision formation flying missions.

Since the beginning of satellite altimetry missions, the ocean surface topography community requires precise and accurate satellite orbits. With start of the satellite Sentinel-3A on February 16, 2016, two radio- and one altimeter onboard the satellite accomplish the Copernicus program with a ocean- and land monitoring mission, planned for a nominal mission lifetime of 7 years. The satellite is orbiting the Earth on a polar, Sun-synchronous trajectory at an altitude of 815 km. For the purpose of precise orbit determination, the satellite is equipped with a geodetic-grade dual-frequency Global Positioning System (GPS) receiver. The GPS measurements are employed together with a set of gravitational and non-gravitational models in a Reduced-Dynamic Orbit Determination (RDOD) approach, which combines the advantages of a dynamic and a kinematic positioning for deriving precise satellite orbits. However, especially the non-gravitational force models require sophisticated modeling techniques. Therefore, a satellite macro model is introduced, which allows a proper modeling of accelerations due to Solar Radiation Pressure (SRP), Earth Radiation Pressure (ERP), and atmospheric drag. Especially the Sun-synchronous orbit, and the huge solar array, which is loosely coupled to the satellite body, makes the precise orbit determination challenging. As members of the Copernicus Quality Working Group, the Astronomical Institute of the University of Bern (AIUB), and the German Aerospace Center (DLR) are, among others, responsible for the orbit validation of Sentinel-3A. Both groups make use of a satellite macro model within a reduced-dynamic approach but differ in the employed software solutions and the pseudo-stochastic modeling. Basically, the pseudo-stochastic parameters allow to compensate potential deficits in the employed force models. Within this poster, the modeling aspects are briefly introduced, followed by comparing the results of both groups. The results include the estimated empirical accelerations, and the estimated satellite orbits of Sentinel-3A. Furthermore, the orbit quality is assessed by Satellite Laser Ranging (SLR), an external and independent tool for orbit validation.

An impulsive feedback control is developed to establish specie c relative orbits for spacecraft formation e ying. The relative orbit tracking errors are expressed in terms of mean orbit elements. The feedback control, based on Gauss’ s variational equations of motion, allows specie c orbit elements or orbit element sets to be controlled with minimal impact on the remaining osculating orbit elements. This is advantageous when J2-invariant orbits are to be controlled, where only the argument of perigee and mean anomaly will drift apart at equal and opposite rates. The advantage of this impulsive feedback control, compared to optimal control solutions, is that it can operate with little computational effort and in a near-optimal manner, whilerequiring only a marginal penalty in fuel cost. When applied to the spacecraft formation e ying problem, this control could also be used to perform general orbit corrections. Formulas are developed providing accurate estimates of the sensitivities of the mean semimajor axis and mean eccentricity with respect to the osculating inclination angle. With these sensitivities, the tracking error in semimajor axis, eccentricity, and inclination angle can be canceled within one orbit.

An AMB is not widely used because of its high cost and because its dimensions are significantly larger than those of mechanical bearings. Since a 6 salient-pole type AMB has fewer poles than the usual 8 salient-pole type AMB, it is easier and less costly to miniaturize. However, because the magnetic circuit is not independent in the x and y directions, bearing forces for coil currents become complicated. To solve this problem, two control methods have been proposed. One is a minimum energy control method, which minimizes copper loss in coils. The other is a maximum bearing force control method, which generates a larger bearing force than that of the minimum energy control method. In this study, the two control methods are introduced and rotational tests are carried out for comparison. The rotor could rotate up to 53,000 rpm with the minimum energy control and maximum bearing force control methods. Moreover, rotational tests were carried out with attaching a weight to the rotor. The rotor could rotate up to 42,000 rpm and 38,000 rpm with the minimum energy control method and maximum bearing force control method, respectively. Experimental results show that the minimum energy control method provides less power consumption than the maximum bearing force control method. On the other hand, the maximum force control method provides smaller maximum coil current than that of the minimum energy control method.

Similar to many complex systems, the operation process of electric balanced forklifts has characteristics such as time-varying model parameters and nonlinearity. Establishing an accurate mathematical model becomes challenging, making it difficult to apply model-based control methods in engineering practice. Aiming at the longitudinal control system of electric forklifts containing external disturbances, this paper proposes an improved full-format dynamic linearization model-free adaptive PID control (iFFDL-MFA-PID) method. Firstly, the full-format dynamic linearization (FFDL) method is employed to transform the operating system of the electric balanced forklift into a virtual equivalent linear data model. Secondly, the nonlinear residual term and pseudo-gradient (PG) of the data model are estimated using the difference estimation algorithm and the optimal criterion function, respectively. Furthermore, in order to enhance the robustness of the system, the idea of intelligent PID (iPID) is introduced and the principle of equivalent feedback is utilized to derive the iFFDL-MFA-PID control scheme. The design process of this scheme only requires the use of the input and output data of the system, without relying on the mathematical model of the system. Finally, the iFFDL-MFA-PID method proposed in this paper is simulated and tested with the EFG-BC/320 counterbalanced forklift equipped in the Special Equipment Testing Center and compared with the model-free adaptive control method (FFDL-MFAC) and the PID control method. Simulation results show that the speed-tracking error of the electric forklift truck under the action of the iFFDL-MFA-PID algorithm is maintained within ±0.132 m/s throughout the process, achieving higher tracking accuracy and better robustness compared to the MFAC and PID methods.

Comparison of myo-electrical control methods: From muscle energy consumption to mechanical speed output

Electromyography (EMG) can be used as an input signal for Human-Computer Interfaces, which are used in the disabled community. Typically, both the user-condition and the end-goal application dictate the number and locations of EMG sensors. We developed two cursor control methods intended for high-level spinal cord injury patients, where available muscle sites are limited to the head and neck areas. For patient comfort, it is desirable to minimize the number of sensors and their intrusiveness while maximizing the functionality of the control method. Both our control methods use a single, noninvasive, surface, differential EMG sensor at the temporalis muscle that controls a cursor in two degrees of freedom (DOF). Fourteen inexperienced able-bodied subjects completed a Fitts’s law task-based cursor-to-target paradigm using both control methods (auto-rotate and rotate). Subjects evaluated and compared performance between the two control methods, which both enabled the cursor to move in 2-DOF. They differed in the level of user control, where one allowed direction manipulation in 1-DOF and 2-DOF in the other. Subjects also completed pre- and post-session surveys and the National Aeronautics and Space Administration Task Load Index for workload assessment. In general, subjects’ performance improved with subsequent sessions within each control method. Subjects achieved a higher throughput (better performance) in the auto-rotate method, had lower workload scores, and tended to prefer this control method. However, about half the subjects felt the manual rotate method allowed them more control over cursor behavior. Our results suggest that a viable cursor control method can be achieved with only a single muscle site.

At present, traffic signal systems almost exclusively use detection systems that provide input in the form of “contact closures,” or binary ON/OFF states representing vehicle presence in detection zones. This limited information constrains the potential performance of traffic signal control. Advanced control algorithms often make use of estimated vehicle arrival times but such data is challenging to estimate with existing detector data. Connected vehicles (CVs) are able to provide such data directly but it is unlikely that all or even most traffic will be connected within the foreseeable future. Vehicle trajectory data from sensors can fill this gap and facilitate a smooth transition from contact closure-based signal control to trajectory-based signal control. This dissertation seeks to improve this situation by exploring potential applications of vehicle trajectory data within existing traffic signal control infrastructure. One issue is the sensing range at which trajectory data can be acquired for intersection control. To test the sensitivity of advanced control methods to the sensing range, two advanced signal control methods, a self-organizing algorithm that adds to existing actuated control methods, and the Phase Allocation Algorithm proposed in recent research on CV-based signal control, were implemented and tested in simulation under varying volume configurations and sensing ranges. The results show that increasing sensing range is associated with improved performance, with the performance of the two advanced control methods converging at sensing ranges of 1000–1500 ft. Actuated control is an existing function in all modern traffic signal controllers and presents an opportunity for scalable improvement of signal control by replacing contact-closure detection with trajectory data. The second study described in this thesis explores the replacement of conventional passage timer logic, which seeks to measure gaps in traffic, with direct measurement of gaps with trajectory data. A highly customizable and robust signal controller was developed in a simulation framework to facilitate a proof-of-concept study. A trajectory-based control method called “gap seeking” was developed that uses trajectory data to identify gaps in the arriving traffic. A variation on this control method called “advanced gap seeking” searches for gaps at an upstream location. Results of a simulation study demonstrate that the methods are able to deliver significant improvements over conventional control methods. The third study of this dissertation proposes additional trajectory-based actuation methods for green extension and evaluates their performance individually and in combination when implemented under the previously developed gap seeking methods, under conventional control, and in a hybrid detection environment where trajectories are only available on the major street. The proposed methods include type II dilemma zone protection, a method called “queue clearance” that aims to eliminate split failures by extending green until vehicles that arrived in red have cleared the intersection, and two algorithms for accommodating platoons from previous studies, which carry a potential for self-organizing control. Simulation evaluations suggest that these methods individually meet their objectives, with some potential trade-offs in delay. Some combinations of the proposed methods show potential for substantial improvement over existing control methods.