Imaging of a compact range using autoregressive spectral estimation[radiation patterns measurement]

The spurious scattering mechanisms of a compact range, which cause spurious signals as well as the desired plane wave to illuminate the test zone, are addressed. It is shown that an accurate estimation of the direction-of-arrival and amplitude of spurious signals in a compact range is beyond the resolution available from the Fourier transform. An estimation technique based on autoregressive (AR) modeling of field probe data is developed to locate and quantify the spurious signal sources. The probe aperture is divided into a number of overlapping subapertures so that the far-field criterion for each subaperture is satisfied. Then, the subaperture data are modeled as an AR process. Singular value decomposition and forward-backward linear prediction are used to estimate the model order and the AR parameters. Examples of probe measurements and analysis for an actual compact range are given     

The Toroidal Imaging Mass-Angle Spectrograph (TIMAS) for the polar mission

The science objectives of the Toroidal Imaging Mass-Angle Spectrograph (TIMAS) are to investigate the transfer of solar wind energy and momentum to the magnetosphere, the interaction between the magnetosphere and the ionosphere, the transport processes that distribute plasma and energy throughout the magnetosphere, and the interactions that occur as plasma of different origins and histories mix and interact. In order to meet these objectives the TIMAS instrument measures virtually the full three-dimensional velocity distribution functions of all major magnetospheric ion species with one-half spin period time resolution. The TIMAS is a first-order double focusing (angle and energy), imaging spectrograph that simultaneously measures all mass per charge components from 1 AMU e^–1 to greater than 32 AMU e^–1 over a nearly 360° by 10° instantaneous field-of-view. Mass per charge is dispersed radially on an annular microchannel plate detector and the azimuthal position on the detector is a map of the instantaneous 360° field of view. With the rotation of the spacecraft, the TIMAS sweeps out very nearly a 4 solid angle image in a half spin period. The energy per charge range from 15 eV e^–1 to 32 keV e^–1 is covered in 28 non-contiguous steps spaced approximately logarithmically with adjacent steps separated by about 30%. Each energy step is sampled for approximately 20 ms;14 step (odd or even) energy sweeps are completed 16 times per spin. In order to handle the large volume of data within the telemetry limitations the distributions are compressed to varying degrees in angle and energy, log-count compressed and then further compressed by a lossless technique. This data processing task is supported by two SA3300 microprocessors. The voltages (up to 5 kV) for the tandem toroidal electrostatic analyzers and preacceleration sections are supplied from fixed high voltage supplies using optically controlled series-shunt regulators.     

Radar target identification of aircraft using polarization-diversefeatures

The characteristics and target identification potential of a representation of the information from two polarization-diverse measurements of the radar backscatter of an unknown target are considered. The locus of these two polarization-diverse waveforms, termed the transient polarization response (TPR), has been shown to be closely related to the geometry of the scattering centers of the target. The polarization-related components of features derived from the TPR concur well with the shape and orientation of the major scattering centers distributed in the downrange profile of the object. This illustrates the intuitively appealing result that the polarization of the backscatter, as represented by the TPR and mapped onto the modified polarization chart, is determined by target geometry. It is shown that both polarization-related and amplitude-related features derived from the TPR are useful for target identification. By using distance measures that depend on various components, the elliptical parametrization information alone is sufficient to allow satisfactory target identification at SNRs of 0 dB and above. The significance of this result is that the absolute amplitudes of the received horizontally polarized and vertically polarized channels need not be known. However, if amplitude information is known, target identification percentages improve     

Multi-channel improvements to satellite-derived global sea surface temperatures

High-quality multispectral measurements from satellites, and the recent development of multiple-window techniques to correct infrared brightness temperatures for atmospheric attenuation, have enabled marked improvements in global mapping of sea surface temperatures. The 4-km resolution data are in two visual bands and three atmospheric windows in the thermal infrared from the advanced Very High Resolution Radiometer (AVHRR) on NOAA's operational polar satellites. Various threshold and/or spatial homogeniety tests are applied to small data arrays to discriminate nominally cloud-free samples for subsequent processing. Tests of the multi-channel equations against independent buoy data gave bias = 0.42C and scatter = 0.62C. Global statistical comparisons with ships indicate significant improvements in accuracy and coverage over previous satellite-derived surface temperatures.     

Orbiting Platform Communication and Data Management Systems

The communications and tracking (C&T) system on board the orbiting platform communicates with the ground facilities through the TDRS satellites. The C&T system operates on Ku and S-band. Using a high gain antenna the Ku-band channel can support a downlink data rate of 300 Mbps through the TDRS single axis channel. The S-band system communicates with the orbiter and with both multiple and single axis TDRS channels. The Data Management System (DMS) provides the following services to the orbiting platform: data distribution within and between core systems and payloads, data processing facilities for core systems, data base management, time and frequency standards, and overall platform management and control. The DMS is a distributed data processing network. The nodes are connected by a local area network (LAN). Each node is autonomous. Since the design is modular, nodes can be added or deleted without disturbing the system. Sensors and effectors communicate with the core system software via the network through multiplexers/demultiplexers.     

Modeling and performance of HF/OTH radar target classificationsystems

The effects of a class of multipath propagation channels on the performance of a over-the-horizon (OTH) radar target classification system are considered. A Rician frequency-selective fading channel model is employed to characterize the effects of the multipath propagation medium and evaluate the performance of radar target classification systems. The performance of classification algorithms that employ relative amplitude, relative phase, and absolute amplitude measurements as features is investigated. Performance estimates of the various classification algorithms for interesting sets of channel parameters are obtained by means of Monte-Carlo simulations     

Comparison of Two Target Classification Techniques

It has been shown that radar returns in the resonance region carry information regarding the overall dimensions and shape of targets. Two radar target classification techniques developed to utilize such returns are discussed. Both of these techniques utilize resonance region backscatter measurements of the radar cross section (RCS) and the intrinsic target backscattered phase. A target catalog used for testing the techniques was generated from measurements of the RCS of scale models of modern aircraft and naval ships using a radar range at The Ohio State University. To test the classification technique, targets had their RCS and phase taken from the data base and corrupted by errors to simulate full-scale propagation path and processing distortion. Several classification methods were then used to determine how well the corrupted measurements fit the measurement target signatures in the catalog. The first technique uses nearest neighbor (NN) algorithms on the RCS magnitude and (range corrected) phase at a number (e.g., 2, 4, or 8) of operating frequencies. The second technique uses an inverse Fourier transformation of the complex multifrequency radar returns to the time domain followed by cross correlation. Comparisons are made of the performance of the two techniques as a function of signal-to-error noise power ratio for various processing options.     

Detecting Instrument Malfunctions in Control Systems

A scheme for automatically detecting incipient failures in the feedback sensors (or instruments) of control systems is described. The feasibility of the scheme is investigated by applying it to a simplified version (fourth order) of the flight control system for a hydrofoil boat. A single set of inertial instruments is used to provide the feedback signals; the redundancy which is normally obtained by multiple instrument sets is obtained here, artificially, by a subsystem of multiple Luenberger observers and logic circuits. Tests indicate that scale factor errors, errors due to threshold effects, and bias errors in the instruments are detected as they occur. The tests also indicate further analytical work which should be done to explore the limitations of the basic scheme.