Error Analysis of Space-Stable Inertial Navigation Systems

The error equations for a space-stable inertial navigation system are derived. This is done by directly perturbing the mechanization equations in the inertial frame and then transforming in open-loop fashion to the local-level frame. A rotating inertial platform and velocity and altitude damping are considered. The relations between errors in space-stable and local-level systems are noted. Numerical results are presented for certain random error sources.     

A Closely Regulated TWT Converter

The design concept for the traveling wave tube amplifier converter for possible use in the Thermoelectric Outer Planet Spacecraft (TOPS) is presented. An unusual combination of semiconductors and magnetics were utilized to achieve very stable voltage regulation on a number of separate outputs to satisfy the requirements of a high-power traveling wave tube (TWT), and at the same time operate at an efficiency of better than 90 percent from a 30-volt source. The circuitry consists of an output filter, an auxiliary Jensen oscillator driving a high-reactance transformer to provide current limiting to the heater, a variable time delay, a main Jensen oscillator driving the power transformer with a maximum step-up ratio of 120 to 1, and series transistorized post regulators to provide precise voltage adjustment and low output impedance. This paper discusses the design of the high-reactance transformer and the high step-up ratio transformer, as well as the high-voltage series regulators that are limited in range and operate at the top of the unregulated output voltage. Test data is presented, and details of current transients caused by charging the filter circuits, input current ripple, and output voltage ripples are considered. The circuit provides better than 0.5 percent regulation against load change, input voltage change, and over-operating temperature range of from -20 to + 80°C, with output ripple voltage of less than 2 volts peak-to-peak on top of the 3600-Vdc output. The measured efficiency was typically 87 percent. and recommendations are included to improve this to in excess of 90 percent.     

Initial ISEE magnetometer results: magnetopause observations

The magnetic field profiles across the magnetopause obtained by the ISEE-1 and -2 spacecraft separated by only a few hundred kilometers are examined for four passes. During one of these passes the magnetosheath field was northward, during one it was slightly southward, and in two it was strongly southward. The velocity of the magnetopause is found to be highly irregular ranging from 4 to over 40 km s^-1 and varying in less time than it takes for a spacecraft to cross the boundary. Thicknesses ranged from 500 to over 1000 km.Clear evidence for reconnection is found in the data when the magnetosheath field is southward. However, this evidence is not in the form of classic rotational discontinuity signatures. Rather, it is in the form of flux transfer events, in which reconnection starts and stops in a matter of minutes or less, resulting in the ripping off of flux tubes from the magnetosphere. Evidence for flux transfer events can be found both in the magnetosheath and the outer magnetosphere due to their alteration of the boundary normal. In particular, their presence at the time of magnetopause crossings invalidates the usual 2-dimensional analysis of magnetopause structure. Not only are these flux transfer events probably the dominant means of reconnection on the magnetopause, but they may also serve as an important source of magnetopause oscillations, and hence of pulsations in the outer magnetosphere. On two days the flux transfer rate was estimated to be of the order of 2 × 10¹² Maxwells per second by the flux transfer events detected at ISEE. Events not detectable at ISEE and continued reconnection after passage of an FTE past ISEE could have resulted in an even greater reconnection rate at these times.     

Some early results of the keV plasma experiment on GEOS-1

Some preliminary new results are presented of the keV plasma experiment on GEOS-1. Electrons and ions have been observed to stream along the magnetic field lines in the dayside magnetosphere from the ionospheric side of the satellite towards the equatorial plane during magnetic storms, with streaming velocities corresponding to a kinetic energy of the order of a keV. The opposite streaming velocity has also been seen, but primarily in the ions only and with a smaller flux ratio for the two opposite directions along the field lines. The transition between the two opposite streaming directions, as seen by the satellite, has been found to occur even in a fraction of a second.In magnetic storm conditions azimuthal asymmetries in ion fluxes corresponding to electric field intensities of several tens of millivolts per meter have been observed.     

Adaptive Tracking Filter for Maneuvering Targets

A general method of continually restructuring an optimum Bayes-Kalman tracking filter is proposed by conceptualizing a growing tree of filters to maintain optimality on a target exhibiting maneuver variables. This tree concept is then constrained from growth by quantizing the continuously sensed maneuver variables and restricting these to a small value from which an average maneuver is calculated. Kalman filters are calculated and carried in parallel for each quantized variable. This constrained tree of several parallel Kalman filters demands only modest om; puter time, yet provides very good performance. This concept is implemented for a Doppler tracking system and the performance is compared to an extended Kalman filter. Simulation results are presented which show dramatic tracking improvement when using the adaptive tracking filter.     

An Upper Bound on Detection Probability for Fluctuating Signals

In the theory of signal detectability, the signal-to-noise ratio (SNR), defined as the quotient of the average received signal energy and the spectral density of the white Gaussian noise, is a fundamental parameter. For a signal which is exactly known, or known except for a random phase, this ratio uniquely defines the detection performance which can be achieved with a matched filter receiver. However, when the signal amplitude is a random parameter, the detection performance is changed and must be determined from the probability density function (pdf) of the amplitude. Relative to the case of a constant signal amplitude, such signal amplitude fluctuation usually degrades performance when a high probability of detection (Pd) is required, but improves performance at low values of Pd; the corresponding change in the required SNR is the so-called signal fluctuation loss Lf. Thus, since Lf in some cases represents an improvement in performance for low values of Pd, a question of at least theoretical interest is: how large might this improvement be, when the class of all signal amplitude pdf's is considered. The solution, presented here, results in a lower bound on the signal fluctuation loss Lf as a function of Pd, or equivalently an upper bound on Pd as a function of SNR. The corresponding most favorable pdf was determined using the Lagrange multiplier technique and results of a numerical maximization are included to provide insight into the general properties of the solution.     

A Unified Approach to Model-Reference Adaptive Systems Part I: Theory

A unified approach to model-reference adaptive control systems is presented. A linearized error equation for each adaptive system is derived about some operating point and converted into the standard form for plotting root loci. The contribution made here is to convert the selection of an adaptive technique and the choosing of adaptive coefficients for some desired system performance from a "mystical art" to standard control techniquess.