Engineering topics
Publications and source records attributed to Hung, J. C..
Individual source errors and their effects on the accuracy of the Gravity Probe B (GP-B) experiment were investigated. Emphasis was placed on: (1) the refinement of source error identification and classifications of error according to their physical nature; (2) error analysis for the GP-B data processing; and (3) measurement geometry for the experiment.
The pointing control of a microwave antenna of the Satellite Power System was investigated emphasizing: (1) the SPS antenna pointing error sensing method; (2) a rigid body pointing control design; and (3) approaches for modeling the flexible body characteristics of the solar collector. Accuracy requirements for the antenna pointing control consist of a mechanical pointing control accuracy of three arc-minutes and an electronic phased array pointing accuracy of three arc-seconds. Results based on the factors considered in current analysis, show that the three arc-minute overall pointing control accuracy can be achieved in practice.
The first-phase effort concentrated on defining the minimum basic functions that the retrodirective array must perform, identifying circuits that are capable of satisfying the basic functions, and looking at some of the error sources in the system and how they affect accuracy. The initial effort also examined three methods for generating torques for mechanical antenna control, performed a rough analysis of the flexible body characteristics of the solar collector, and defined a control system configuration for mechanical pointing control of the array.
The instrument pointing system concept discussed in the present paper was developed for Spacelab missions. The system is mounted on soft shockmounts to minimize Shuttle disturbances (such as man motion, thruster firings, etc.) and to reduce the effect of large center of mass offsets between the instrument and the pointing system gimbal axes. The instrument pointing system is soft mounted to the Spacelab pallet located in the Orbiter payload bay. The nonlinear model, incorporating the dynamic equations of motion, the controller equations and the definition of sensor dynamics and gimbal bearing friction, is given, along with a system of linearized equations of motion of the Space Shuttle Orbiter and instrument pointing system (linearized model).
This paper presents the design of digital rebalance loops for a tuned-rotor gyro which is intended to be an attitude sensor in a strapdown inertial measurement unit. A tuned-rotor gyro is a two-degree-of-freedom gyro which has two input axes, with cross-coupling between them. The rebalance loop of the gyro serves for two purposes. First, it restores the position of the rotor to its null position after experiencing an attitude input. Second, the rebalancing signal, which is proportional to the attitude change, is calibrated to give the desired attitude information. Since the gyro has more than one input and one output, a multivariable control technique is needed for designing the rebalance loop. The analytic model for the gyro is described and the available design methods are examined. A new root-locus design technique specially developed for the present problem is outlined. The design of the rebalance loop using the new technique is given. The computer simulation result of the designed system is presented and discussed.
This article reports the results of an investigation on time-sharing of a rebalance electronics among several sensors used in a strapdown navigation platform. Various concepts are given. The merits and demerits of the time-sharing for a typical situation are discussed in detail. A possible hardware realization is proposed. In addition, this study develops a general approach, applicable to other systems, for evaluating the use of time-sharing concept.
Two different methods of target detection when the return signal is contaminated with noise are discussed and compared. The first method uses Neyman-Pearson detection philosophy and selects the threshold level to give a desired false alarm probability. The maximum probability of false alarm is constrained by the target cross scan velocity component. The second method (minimum acquisition time detection), which is similar to the ideal observer, selects the threshold level to minimize the expected target acquisition time. The probabilities of false alarm and missed detection are selected so that the errors produced by these effects produce the minimum acquisition time. Three different scan techniques - linear, spiral and two-mode scan - are studied and compared.
Target acquisition is often needed in a deep space mission where the detector on board the space vehicle must be able to perform the decision making process in acquiring the target. That is, the system for target acquisition must be autonomous. This paper presents several autonomous target acquisition techniques, applicable to deep space mission, for detecting stationary and moving targets. These techniques are useful for sensors such as radar, star tracker and television since the target must be found before it can be tracked. A minimum signal-to-noise ratio can be specified for successful acquisition of target.
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The design of a free-flying teleoperator (FFTO) intended to assist the Space Shuttle in performing its various missions is discussed, where FFTO is to be carried into earth orbit and returned to earth by the Shuttle. The FFTO system is described as to configuration, major subsystems, and mechanization concept. The kinematics of the manipulator and methods of sedating a satellite are discussed. Satellite equations of motion are provided, and cases of zero tumbling rate and non-zero tumbling rate are examined. Cases for which use of FFTO is compulsory are indicated, and the FFTO contributions to the Shuttle missions are analyzed.
New development in high reliability strapdown navigation platform using two-degree-of-freedom sensors and redundancy concept is presented in this paper. The development is based on the assumption that each axis of the sensor can fail without affecting the remaining axis. The systems reliability is investigated, the optimum redundancy configurations are proposed, a technique of sensor performance management is developed for the proposed systems.
The present work outlines eight candidate navigation systems for autonomous target relative navigation near a comet, asteroid, or planet. An analytic model is developed for each system, showing how the solution of the relative state vector is obtained from measurement data.
This paper presents a new concept of high-reliability strapdown attitude sensing systems for space vehicles. Each system utilizes a set of redundant two-degree-of-freedom gyros. An optimum system configuration is obtained for maximum system reliability and the best measurement accuracy. Improved accuracy of the final data is obtained by using the least-square data reduction technique. Each system possesses a 'sensor performance management' feature which is capable of failure detection, faulty gyro identification, system reconfiguration, and, possibly, sensor recalibration. Improvement in reliability, as compared to other types of strapdown systems, is demonstrated. Details of the development are described in terms of a system containing four gyros.
An overview is presented of typical inertial grade instrumentation available to mechanize precision strapdown attitude reference systems as well as a novel scheme of redundancy management, if two degree of freedom instruments are used. The instrumentation is divided between conventional and unconventional sensors with some assessment of their readiness included.
The study of comets is of important scientific value as part of the exploration of the solar system. This paper proposes a method, called 'stationary target identification,' to detect the nucleus within the coma. The method makes use of the fact that the coma is ever expanding about the nucleus. Therefore, when a radar beam is used for detection, the coma particles will produce a return signal that is Doppler shifted. The effects of the spatial extent of the coma and Doppler bandwidth of the coma returns upon the required data processing are considered. Also different methods of implementation are considered.-
Two new methods of landmark navigation were recently presented. The landmarks are assumed to be within sight of the navigator but with unknown positions. Both methods require computations which are time consuming when a computer is not available. This correspondence presents the concept of a new navigation device called 'landmark navigation rule' which eliminates all the computation effort required in both new methods. The device is simple in construction, lightweight, and consumes no power.
Proceedings of conference on principles of strapdown inertial navigation platforms
An analysis of the principles of strapdown inertial platforms is presented, and the advantages of the strapdown system as compared to other stabilized systems are discussed. The application of the rebalance loop to continuously restore the position of sensor output axes is described. The attitude logarithms and the rebalance electronics which are required for accurate navigation are examined.