John J. Dougherty

A Design Approach for a GPS User Segment for Aerospace Vehicles

As new applications for the use of the Global Positioning System (GPS) on aerospace vehicles emerge, more attention is being paid to the design of the user segment, which comprises the hardware and software employed by the user to obtain navigation information from GPS. The complexity of the design of the user segment, as well as the performance demanded of the components (such as the antenna), depends on user requirements such as total navigation accuracy. Other factors, for instance the expected satellite/vehicle geometry or the accuracy of an accompanying inertial navigation system, can also affect the user segment design. The interaction between these effects, the user requirements, and the user segment design is studied. Design curves are developed which allow quick trade studies to be performed.

Modeling and identification of a triaxial shaker control system

Computer modeling and simulation has been a successful tool in the design of complex control systems. This paper discusses a computer simulation that is being developed to evaluate a controller for a triaxial shaker system. The shaker system has eight inputs and six outputs. Frequency response data from testing of the actual system has been used to identify parameters for the model.

GPS modeling for designing aerospace vehicle navigation systems

The complexity of the design of a Global Positioning System (GPS) user segment, as well as the performance demanded of the components, depends on user requirements such as total navigation accuracy. Other factors, for instance the expected satellite/vehicle geometry or the accuracy of an accompanying inertial navigation system can also affect the user segment design. Models of GPS measurements are used to predict user segment performance at various levels. Design curves are developed which illustrate the relationship between user requirements, the user segment design, and component performance. >

Application of GPS for Missile Post Flight Guidance Accuracy Analysis

The high quality of a GPS-based trajectory reference makes it useful for evaluating the performance of a missiles Inertial Navigation System (INS) during a missile flight test. The complexity of the design of the GPS user segment depends on flight test objectives and factors such as the expected satellite/vehicle geometry. Design curves are developed which illustrate the relationship between flight test objectives, the GPS user segment design, and such factors as antenna performance.

An ARMA approach for modeling a triaxial shaker system

A simulation of a triaxial shaker system can be used to predict system capabilities and limitations. The model used in the simulation is considered. A model based on a finite element model of the shaker table and on models for the actuators does not correspond closely to measured response data. An alternate model, in the form of an autoregressive moving average transfer function, can be used instead. A method for identifying the function parameters is discussed. Parameters for the triaxial shaker system have been obtained. The new model based on those parameters results in good agreement with measured response data from the actual shaker system.

Improved approximations for near-optimal interceptor guidance

A perturbation procedure is applied to the problem of finding an optimal control for a ballistic missile interceptor. Certain forces, such as thrust and gravity, are assnmetl to tlorninate the equations of motion. The optima.1 cont,rol problem is integrable if the remaining forces are neglected; the approximate effects of the neglected forces can be calculat,ed noniteratively and added to the solution. For certain trajectories, however, the aerodynamic forces are not negligible. Including the aerodynamics direct,ly in the dominant tlynarnics destroys the analytical solution upon which the procedure depends. Instead, approximations of the aerodynamic forces are included through narrow pulse functions. This t,echnique produces a good approximation to the optimal control and is computationally more efficient than previous methotls. Extensions to previous work are also mule to account for the interceptors coast phase and ter~nirial const~ra.irits. The nearoptimal guidance law is used to produce intercept t,raject,ories against a number of target trajectories. The approximate trajectories compare well with ntlmerically-ge~ierat~ecl optiri~al trajectories.

Parameter identification for a triaxial shaker system

Different approaches to parameter identification for a triaxial shaker system are examined. One approach uses an output error formulation of a cost function, which is then minimized using a standard algorithm. Two other approaches use a cost function defined through an input error formulation. The cost function is minimized through, alternately, a standard function minimization algorithm and a linearized least-squares algorithm. For the case of fewer measurements than degrees of freedom, a generalized technique for reconstructing the states is proposed. This technique does not make any assumptions regarding the absence of external forces on unmeasured degrees of freedom. The three approaches to parameter identification are compared for two different systems.