Discrete-time min-max tracking

The problem of optimal state estimation of linear discrete-time systems with measured outputs that are corrupted by additive white noise is addressed. Such estimation is often encountered in problems of target tracking where the target dynamics is driven by finite energy signals, whereas the measurement noise is approximated by white noise. The relevant cost function for such tracking problems is the expected value of the standard H/sub /spl infin// performance index, with respect to the measurement noise statistics. The estimator, serving as a tracking filter, tries to minimize the mean-square estimation error, and the exogenous disturbance, which may represent the target maneuvers, tries to maximize this error while being penalized for its energy. The solution, which is obtained by completing the cost function to squares, is shown to satisfy also the matrix version of the maximum principle. The solution is derived in terms of two coupled Riccati difference equations from which the filter gains are derived. In the case where an infinite penalty is imposed on the energy of the exogenous disturbance, the celebrated discrete-time Kalman filter is recovered. A local iterations scheme which is based on linear matrix inequalities is proposed to solve these equations. An illustrative example is given where the velocity of a maneuvering target has to be estimated utilizing noisy measurements of the target position.     

Next generation digital GPS receiver

The architecture and technology features of the next-generation (NGR) digital GPS (Global Positioning System) receiver manufactured by Collin are described. The project's objective was to develop an advanced GPS receiver chipset with high antijam capabilities. The program, initiated in 1985, has provided the technology for miniature receiver products for both unmanned and manned vehicle applications. A two-channel version of the receiver is in full-scale development for tactical missile applications. A five-channel version is being tested and evaluated as a drop-in replacement for RCVR-3A, the US Department of Defense standard high dynamic receiver. The NGR design started with the digital signal processing architecture developed for the Defense Advanced Research Project Agency (DARPA) hand-held GPS receiver. Enhancements were made to improve the antijam and signal acquisition performance. Producible, qualifiable and cost-effective silicon monolithic microwave integrated circuits and semicustom digital technologies were used to develop the core GPS chipset. A system design approach was established to permit reuse of mature and validated GPS software     

Bodies in flowing plasmas: Laboratory studies

A brief review of early laboratory investigations of bodies in flowing, rarefied plasmas is given together with a discussion of more recent parametric studies carried out at NASA/Marshall Space Flight Center (MSFC), which include the effects of the ion acoustic Mach number and the normalized test body potential. Good agreement is found between the experimental results and theoretical calculations which omit ion thermal motion. The relation between laboratory investigations and the results of satellite-borne measurements is addressed. This relationship has led to an appreciation for the benefits of applying the methods and techniques of laboratory plasma physics to investigations in space, where several limitations inherent to the laboratory can be circumvented. These types of investigations, conducted in Earth orbit, can enhance our understanding of space plasma physics and have direct application to certain types of solar system phenomena.     

Discrete-time state estimation of analog double integrators

The problem of minimum variance discrete-time state estimation of a continuous-time double integrator via noisy continuous-time measurements is considered. The error covariance matrices of this estimation are calculated and analyzed. The relations between these covariance matrices and the error covariance matrix of the optimal continuous-time filter are obtained, and a way for determining the required sampling period is proposed. A commonly used approximated model is investigated; it is shown to be inappropriate unless a specific improvement is introduced in the model     

Measurements of LET spectra and comparison to models.

We present measurements of LET spectra for near earth orbits with various inclinations and altitudes. A comparison with calculated LET spectra shows that the contribution from direct ionizing galactic cosmic rays is well described by the models. An additional contribution to the spectra originates from stopping protons and from nuclear interactions of particles with material. In the case of an interaction a large amount of energy is deposited in a small volume by target recoils or target fragments. These events will be called short range (SR) events. For a low inclination orbit radiation belt protons are the main source of these events while galactic protons become more important when increasing the inclination to near polar orbits. We show that the contribution of SR events for orbits with low altitude (324 km) and 57 degrees inclination is comparable to that for an orbit with 28 degrees inclination at a high altitude (510 km).     

Synthesis of an Optimal Output Regulating System with a Reference Vector

The difficult problem of synthesizing an optimal regulation scheme for linear time-invariant systems, assuming an integral quadratic cost functional, when the desired output is a constant nonzero vector and when the upper time limit approaches infinity, is studied in this paper. The problem is solved by properly redefining the cost functional, so that consistence with the stability requirement is achieved. The problem of the optimal closed-loop regulation of a singleinput/single-output system has always been an interesting topic for control engineers. The most common assumption is that the cost functional to minimize be of quadratic type, with infinity as an upper limit. The classical frequency domain approach, assuming the absence of initial conditions, suggests an optimal compensator in forward loop. On the other hand, the modern time-domain approach, based on the direct application of the maximum principle, yields a closed-form solution for the case in which the desired output is zero. This paper presents results which allow one to consider from a unifying point of view the two above approaches and, in addition, suggests a stable optimal scheme which is valid for any initial state and for any desired constant output.