Harmonic Radar Systems for Near-Ground In-Foliage Nonlinear Scatterers

The radar transmission equation for a harmonic radar operating over a planar, finite dielectric Earth through foliage is derived for an interesting class of nonlinear scatterers. The received power can typically depend on range to the (-14) power for small objects near the ground. The maximum detection range of a ground-based system is related to all major system parameters: it is most sensitive to polarization, transmit antenna height, and transmit wavelength; moderately sensitive to transmit power and transmit antenna area; and least sensitive to receive antenna area, harmonic scattering cross section, and mode of data processing. For example, there is seen to be a best apportionment of total available aperture area into disjoint transmit and receive apertures which can be well approximated by the equal gain condition. Also, there is seen to be a critical path distance through foliage; at distances less than this, small wavelengths are desirable and, conversely, the upper transmit frequency limit may be set by nonlinear scatterer response. Airborne synthetic aperture radar systems are discussed and quantification of harmonic noise and effects of scatterer fluctuation are made. A useful phenomenological model of a nonlinear scatterer is given that is consistent with some observations and predicts a frequency dependence. Nonlinear scatterer effects on range resolution are discussed.     

Fuel Savings in the WHECON Attitude Control System Using an Asymmetric Controller

This paper investigates the possibility of generating an on-off limit cycle mode in the WHECON attitude control system. lt is shown that, by eliminating the off set angle ? in the roll-actuated control torquer, the WHECON system can be reduced to the structure considered earlier by Mendel and, therefore, an on-off limit cycle can be generated simply by introducing an appropriate amount of asymmetry into the deadzone characteristic of the WHECON controller. Because with this mode of on-off limit cycle the thruster jets will fire only once in each cycle of the oscillation, it is claimed that a considerable amount of fuel can be saved by the proposed modifications to the original WHECON design. Dynamic simulation study was conducted with the actual nonlinear model of the WHECON system considered and the results indicate that, for both zero and nonzero values of the off set angle ?, substantial fuel savings can be achieved by using the asymmetric controller.     

MTI Optimization in a Multiple-Clutter Environment

A means of optimizing a moving target indicator (MTI) filter for rejecting several types of clutter, which are generated by different mechanisms such as by rain or the ground, is formulated. lt is found that the optimal performance of such a filter depends on the spectral density functions, average radar cross sections, and the relative mean Doppler frequencies of each type of clutter. lt is shown that the optimal improvement factor of such a filter is bounded by the weighted average (weighted in accordance with the radar cross sections of the clutter types) of the improvement factor for the individual clutter type. lt is also shown that the improvement factor of such a filter is a function of the relative mean Doppler frequency f0 between the clutter types. As f0 increases, the performance of the MTI system degrades. The worst improvement factor occurs when f0 is equal to half of the radar pulse-repetition frequency (PRF).     

On the Design of Suboptimal Controllers for Minimal Time Feedback Regulators

A method for designing suboptimal minimal time controllers for linear, time-invariant regulator systems is presented. An economical closed-loop controller is obtained by using Lyapunov surfaces, corresponding to a quadratic Lyapunov function, as a continuous approximation for isochronal surfaces. Systems employing this controller are obtained by minimization, with respect to parameters of the quadratic Lyapunov function, of the worst case upper bound on the arrival time of an arbitrary initial state at the origin.     

Perspectives for Strategic Analysis in Advancing Ocean Technology

There is a growing need for the scientist, engineer, and administrator to better understand the kinds of overall, across-the-board planning problems involved in the development of off-shore coastal areas. A recommended approach is to use systems analysis techniques. Principles will be set forth on maintaining and improving the quality of the ocean environment, with considerations of cost in so doing. Each proponent of an aspect of ocean technology usually believes that his field must be greatly enlarged in the near future. Along with these advances there is the likelihood of unnecessary social costs emanating from undesirable modifications of the environment. In making the analysis, an early step is to identify specific needs and appropriate roles. A next step is to lay out alternatives for achieving objectives both within a given organization and external to it. The alternatives may be appraised through their costs and benefits. But how do you make choices between goals when resources are limited? How can we gather the data, accomplish the evaluation, and do the planning that will make rational choices possible? This paper elucidates the problems involved in these kinds of questions and suggests methodology that might lead toward solutions.     

A Functionally Redundant Altimeter

A scheme to provide redundant sensor data in an automatic control system using the principle of functional redundancy is described. Normally there are three redundant radar altimeters used in the terminal phase of automatic landing of jet transport airplanes. This scheme replaces one of these altimeters with a data processing scheme based on a Kalman filter. The filter is driven by altitude rate and acceleration signals from the air data computer and vertical accelerometer. A special initialization technique employs the two altimeter signals. The feasibility of this scheme is indicated by tests in which data obtained from these several sensors during flight tests are used to drive the functionally redundant altimeter.     

Signal to Noise Ratio Improvement by Frequency Division in the Presence of Bandpass Noise

The performance of a wide band frequency divider in the presence of additive Gaussian noise is presented. It is shown that the same theoretical signal to noise ratio improvement is attainable in this case as in the case where the spurious signal is a sine wave.     

Limiting Performance of Generalized Sign Test Detector

The limiting performance of the generalized sign test detector as the number M of target returns becomes infinite has been derived by Hansen and Olsen [1]. Simple expressions are derived herein for the limiting performance for finite M as K, the number of noise samples in the reference set, becomes infinite. Curves are presented which indicate how this limiting performance is approached as K is increased for both constant target returns and fluctuating target returns.     

Performance Degradation of MTI Circuits Due to Amplitude Limiting

The loss in output signal-to-noise ratio (SNR) due to amplitude limiting is obtained for a radar circuit consisting of a bandpass limiter, coherent demodulator, matched filter, and moving-target-indicator (MTI) filter. The circuit is used in scanning MTI radars. The tandem connection of the limiter and coherent demodulator is a model for the saturation of the intermediate-frequency (IF) demodulator of an MTI radar. Results on special functions are used to obtain simple formulas for the loss in output SNR relative to a linear IF demodulator when the input SNR is less than -15 dB and the number of hits per 3-dB beamwidth exceeds 15.     

Cross Correlation of Orthogonally Polarized Backscatter Components

The backscattering of an elliptically polarized plane wave from two kinds of complex radar objects is considered from the standpoint of coherency theory, and the antenna polarizations that give rise to maximum correlated and uncorrelated orthogonally polarized components in the receiving channels are discussed. The resulting observations may find application in radar systems which use the correlation between the received orthogonally polarized components for detecting targets in natural and man-made interference.