The classical radar range equation, in its various forms, is the most widely-used analytical tool employed by radar systems designers to determine the relationships among various radar system parameters. Implicit in the usual expressions of this equation is a narrow-band assumption, which permits the simple real-valued multiplication of frequency-dependent terms. The lack of a recognized canonical representation of the radar equation in a wide-band or time-domain form has hindered the development of an understanding of impulse and other wide-band radars. Presented here is a wide-band radar range equation which has been rigorously derived and exercised. A comparison is made to the narrow-band equation and it is shown that, in the narrow-band situation, the two are equivalent. Several examples demonstrating the utility of the time-domain equation are presented. A discussion provides guidance on when the wide-band or time-domain forms are necessary or when the narrow-band form or some compromise are adequate.

A variety of novel solutions of the homogeneous free space wave equation and Maxwell's equations that predict a much slower energy decay than the observed classical 1/R squared far-field behavior of an electromagnetic wave from a finite source have appeared in the literature. Some of the more well publicized solutions are: electromagnetic missiles, electromagnetic directed energy pulse trains, Bessel beams, and electromagnetic bullets. We examine some of these issues via a tutorial examination of the underlying physics behind the slowly diffracting behavior of these localized propagating electromagnetic waveforms using electromagnetic engineering concepts that are familiar to radar engineers. We also investigate concepts that would be relevant to developing a radar which employed any of these localized electromagnetic energy forms such as the 'near-field' and 'far-field' behavior of these solutions, and any unique source requirements.

For narrowband arrays, given the one way patterns (the transmit beam pattern and the receive beam pattern), the composite two way (transmit / receive) beam pattern may be found by beam pattern multiplication. However, as is shown, beam pattern multiplication does not generally hold for ultra-wideband arrays. Transmit beam patterns from ultra-wideband arrays have been described previously, and receive beam patterns are similar. Here examples (three dimensional plots) of two way, transmit / receive, beam patterns are given in which beam pattern multiplication does not hold. A theory and equations are given for mathematically deriving the two way patterns from the individual one way patterns. This theory uses the active imaging Green's function. The theory covers beam pattern multiplication as a special case, and the conditions required for beam pattern multiplication to hold are derived. For the far field two way linear array pattern, in the case of monochromatic transmission, the theory produces the familiar sinc squared function.

A computer simulation of the measurement and data processing operations is used to demonstrate important factors influencing image formation in an ultrawideband (UWB) synthetic aperture radar (SAR). We first summarize the basic operations involved in UWB SAR measurements and processing. Then we present example calculations based on a two point target scene illustrating the effects of sampling, detector approach, noise and propagation loss due to foliage. The output consists of a computer generated scene illustrating the fidelity, contrast and resolution. Results indicate that UWB imagery is feasible under a wide range of operating conditions.

Abstract not available.

Since ultrawideband (UWB) radar views the sea scattering surface in minute detail, the prediction of UWB sea clutter entails knowledge of the signal's physical origin, or at least a 'plausibility model' that describes likely sea clutter behavior under a given set of operational circumstances. Calculations are presently conducted for the rate at which the 'islands' of scattering that are generated by surface illumination of an evaporative layer above the sea surface can be expected to be encountered.

Abstract not available.

Preliminary results are presented from evaluations of ultrawideband (UWB) radar echo signal data, using both bispectral processing and the affine approach of kernel analysis. It appears possible to describe the backscatter properties of radar-absorbing materials and clutter-type materials, raising the prospect of a characterization of material types on the basis of UWB echo signal processing. Because these techniques are effective under low S/N operating conditions, they may be of great practical significance.

An account is given of the basic principles of radar polarimetry, and various optimization procedures for the propagation (scattering) range operator equation and the received-power expressions are presented and compared. On the basis of a complete description of isolated and distributed scatterers, polarimetric target classification, target vs clutter discrimination, and optimal polarimetric contrast enhancement algorithms are derived; these should be useful in the interpretation of wideband polarimetric data sets obtained with wideband coherent, dual (orthogonal) polarization channel radar systems.

ultra wide band (UWB) time domain radar signals and presents experimental measurements used to illustrate the advantages and disadvantages of such methods. The techniques are developed in three stages; early time processing based on time domain inverse scattering identities for the design of matched filter detectors, radar cross section modelling using finite difference time domain techniques for investigating the effects of change of target geometry and incident pulse shape and finally, the use of late time information on target resonance and damping for the identification of important target features. Two targets are considered in detail, backscatter from a metallic sphere and scattering from a rectangular box with variable aperture sizes.

This paper is a transcript of the Ultra Wide Band Microwave Source technology demonstration progress report video prepared for the DoD Balanced Technology Initiative Office.

An ultrawideband (UWB) adaptive transmitter has been developed with a view to the feasibility of a UWB radar design able to transmit a train of 200 picosec pulses; the generation of such a pulsetrain is via the transmission of all its frequency spectral components. A tradeoff study is conducted to ascertain the design parameters and the requisite transmitter components. A complete demonstration model of the UWB transmitter has been built and tested in an outdoor far-field range; its short-pulse waveform is in good agreement with calculated results.

The requirements for antennas for ultrawideband radar are extremely stringent. The design process for such an antenna must be carried out in close conjunction with the systems design, and take into account the available pulse shape from the transmitter, the pulse shape required on the target and the signal processing available. The antenna transfer functions on transmit and receive must not distort the pulse to an unacceptable level. This requirement and the wide bandwidth limit the choice of antenna particularly for enhanced gain and may require separate antennas for receive and transmit. Computations of the pulse shape at various positions in the near and far field of the antenna show that the angular zone of 'good' performance is limited in a manner analogous to that of a narrowband antenna. The methods of achieving enhanced gain are limited to the use of reflectors and arrays. The choice of subsidiary elements of these antennas is crucial to good performance and computations have shown that the achievable gain is limited.

This paper describes the performance of a broadband (0.3-10 GHz) cavity backed TEM horn with an exponential flare in two planes. The antenna pattern was measured using a pulsed source and found to have a 3 dB width of 50 deg. The two-way transfer function of a pair of these horns was determined using pulsed measurements. The two way gain of the horns was found to be 6 dB. Diffraction of the low frequency components of the radiated pulse was observed and the effects on waveform shape noted. In particular, we have observed that the back and side lobes of this antenna are dominated by the low frequency components, and the corresponding temporal waveform has a different shape than the waveform found in the main lobe.

A 300-900 MHz ultrawideband (UWB) bistatic test range has been developed in order to analyze echo signals that are based on a transient stimulus. By means of a 2 gigasample/sec digital oscilloscopy allowed a characterization of the true wideband response of materials; two low time-dispersion, quasi-TEM wave-supporting antennas were developed for the bistatic range. Attention is given to the responses of this UWB test range and its antennas to a transient stimulus.

Diffraction plays an important role in determining the RCS of a body. At some frequencies, typically low frequencies, diffraction can dominate the scattering process. This work examines the effects of diffraction on ultrawide band (UWB) radar pulses. We measured the RCS of three different size square aluminum plates using a UWB pulse and broadband sampling oscilloscope. We used a bistatic measurement configuration with a bistatic angle of 10.58 deg. The plates were rotated from -90 deg to +90 deg in 2 deg increments (angles relative to the center line of the bistatic configuration). Diffraction patterns for each plate are plotted and the time domain wave forms are analyze. We find that the smallest plate has a significant amount of diffracted energy even in its specular reflection lobe. The largest plate is relatively free from diffraction. Travelling waves and late time resonances are observed for the plates.

Two receiver systems are under development which elaborate a ultrawideband (UWB) receiver architecture based on a 0-1 GHz time delay-channelized receiver. With the appropriate processing hardware, this receiver may be able to operate at much higher data rates than the transient digitizers currently in use. While one receiver system version employs 200 MHz A/D converters, the other uses 500 MHz A/D converters.

Ultrawideband (UWB) radar pulse width at low grazing angles determines whether dominant scattering features of the sea surface, such as whitecaps, are resolved, and therefore has a major influence on the time-varying character of clutter-return signals in individual range cells. Attention is presently given to the design and performance of two clutter-measurement radar systems with variable resolution, as well as to initial sea-clutter data thus obtained at a nominal radar pulse center frequency of 10 GHz.

An ultrawideband (UWB) radar facility has been developed to experimentally and analytically explore the potential of technology based on such radar waveforms and to characterize the sea echo environment as sensed by an ultrawide fractional bandwidth radar waveform. Initial UWB results are presented for sea clutter, a small boat with and without a corner reflector, a helicopter-suspended calibration standard, the helicopter in question, a calibration tower standard, and ships of various sizes.

The present bistatic radar system is polarimetric and operates coherently over the 30-1300 MHz band; it is able to scan foliage in angle to ascertain spatial variations in the transmission properties of the foliage that can degrade the ability of the radar to focus a foliage-obscured object. The system is ground-based, with one antenna attached to a carriage that can move 10 m horizontally along a rigid track and a second, fixed-tripod antenna. Angular and frequency-correlation measurements are presented and discussed.

A ray-tracing model was used to derive the light scattering Mueller matrix element curves for a dipole near a perfect surface as a function of incident angle, scattering angle, and surface refractive index. This system represents a fundamental system composed of a perfect plane surface and a perfect (Rayleigh) scatterer.

A spatial modulation display is described that permits the observation of a phantom or transparent image by several persons simultaneously and is suitable for medical imaging. The display uses spatial light modulators and large format convex lenses within a Schlieren optical system. The number of sectional images in a three-dimensional image is limited by the number of spatial light modulators. The display is electro-optical and requires no moving parts.

High voltage pulse generators make possible the radiation of a Gaussian-like electromagnetic field of very short duration, which can illuminate both simple and complex structures; the field thus scattered is detected by ultrawideband sensors, and then analyzed in both the time and frequency domains. Attention is presently given to the experimental equipment used in such a system, the results obtained for representative structures, and the significance of optical signal processing and time-domain cancellation techniques for ultrawideband radar systems.

This paper summarizes experiments conducted for the purpose of developing compact, lightweight, nanosecond modulators. Several well known technologies were combined to fabricate pulsers capable of generating multi-megawatt peak power pulses at kilovolt voltages and kilohertz repetition rates with nanosecond risetimes. Two modulator designs were developed which met these objectives. One approach used a hydrogen thyratron to switch a coaxial cable pulse forming line (PFL). This design required a sharpening gap assist to achieve the nanosecond risetime. The second approach used a selfbreaking spark gap to switch the PFL. This design led to the development of a battery operated 'briefcase' pulser that weighed less than 10 pounds. The 'briefcase' pulser demonstrated outputs of 2 megawatts of peak power at repetition rates up to 1000 hertz. Pulse risetimes of a nanosecond were achieved with a 10 nanosecond pulse width. A pulse of this description is known as a 'video' pulse.

A monolithic, photoconductive impulse generating device, in which the functions of energy storage and switching are combined into a single semi-insulating GaAs wafer substrate, was investigated under various operating conditions. Depending on the load impedance, this new type of device produced nanosecond impulses with various pulse shapes. The operation of this device with a positively mismatched load impedance (50 ohm) resulted in output voltage enhancement (voltage gain factor of 2.0) as well as fast pulse risetime (260 ps). Further, an ultra-wideband horn antenna was excited by a output pulse from this device and its radiative waveform was measured using a B-dot sensor.

High-power ultra-wideband pulses with equivalent center frequences of 10 GHz require picosecond switching times of high electric fields. We are developing a high-pressure gas switch that is designed to store several joules of energy. The switch has an overvoltaged gap that is pulse charged to operating voltages before the electron avalanche causes the voltage to collapse. Depending on the gas pressure (designed for up to 100 atm) and the electric field, the voltage collapse time can be hundreds of picoseconds down to a few picoseconds. Uniform development of the avalanche is initiated by photoionization from a bare spark or an ultraviolet laser. The pulse-power charging system is designed to charge the transmission line in a few nanoseconds. The stored energy that would flow as a post-pulse component is to be minimized. Tests of the repetition rate for the switch are part of the development. Data in this paper indicate the achieved gap electric field, the voltage collapse time, and the launched pulse power and energy down a transmission line.

Wideband, high-power microwave pulses are expected to have important applications in the future. One of these applications is ultra-wideband radar. The wide bandwidth should generate increased information for targetcharacterization and identification. The high power should result in increased target detection range for conventional targets and targets with reduced signatures. A way to generate wideband, high-power microwave pulses with relatively conventional technology is to tail erode high-power pulses by passage through a low-pressure air cell. In this process, the tails ofshort(3 — 10 ns), high-amplitude (<1 MV/rn) pulses are removed. This erosion shortens thepulses and generates transmitted pulses with broadenedbandwidths. Thepressure mustbe matched to several incident pulse characteristics to create enough electron density to cause strong tail erosion. The important pulse characteristics are amplitude, frequency, pulse length, and pulse shape. Tail erosion of microwave pulses in the earth's atmosphere has previously been examined with one-dimensional, finite difference computer calculations.13 Experiments on tail erosion in a rectangular waveguide have verified two-dimensional (2D), finite difference computer calculations.4'5 We have shown experimentally that tail erosion from air breakdown broadens the 3 dB bandwidths of 2.8608 GHz incident pulses in a rectangular waveguide at 3.5 ton. The incident pulse amplitude varied from 0.67 —1.16 MV/rn. The pulse bandwidth increased from 0. 147 GHz by 0.0097 — 0.039 GHz or 0.34 — 1.4% relative. The incident bandwidth was 5. 12% relative to the incident carrierfrequency. This experimentalbroadening was simulated with a2D, electromagnetic, electron fluid computercode foravalanche ionization. The simulation predicted bandwidth broadening by 0.029 —0.13 GHz or 1.0 —4.4% relative for a peak initial electron density of 10 electrons/cm3. Although the measured and calculated transmitted electric field envelopes were in close agreement, the calculated bandwidths exceeded those measured by 13 —47%. Because the detectors were not fast enough to resolve individual cycles and therefore determine thelocalfrequency across thepulses, wepresently conclude that the simulation gives betterestimates of reality than do the measurements. The computer code gives predictions of the bandwidth broadening of 3.5 ns incident pulses at 3.5 torr for a Gaussian spatial background electron distribution with a 3 cm fullwidth at halfmaximum (FWHM)in the axialdirection. The peak valueofthe electron density distribution and its transverse FWHM are variable. Incident amplitudes of 1 —18 MV/rn and peak electron densities of 10 — 1011 electrons/cm3 are considered. The transmittedbandwidth varies from 0.352— 3.21 GHz or 12.3 —112% relative. The transmitted spectralcenter frequency varies from 2.87 —4.72 GHz. The transmitted amplitude at 54.6 cm from the input to thelow-pressure section varies from 0.263 — 12.7 MV/rn on the waveguide center line.

The gas avalanche switch is a recently conceived, laser-activated, high-voltage, picosecond-speed switch. It basically consists of a set of pulse-charged electrodes immersed in a gas at high-pressure (2-800 atm). A picosecond-scale laser pulse initiates an avalanche discharge in the gas between the electrodes. With the proper configuration, the avalanche, which is fueled by the immense number of electrons available in the gas, causes the applied voltage to collapse in picoseconds and generates electromagnetic waves. A two-dimensional (2D) electron fluid computer code solves Maxwell's curl equations and a set of electron fluid equations for transverse magnetic (TM) modes in air between parallel plane conductors. Collision frequencies for ionization and momentum and energy exchange to neutral molecules are taken to scale directly with neutral pressure. Electrode charging and initial electron deposition are considered to be instarnaneous. Calculations are performed for a Blumlein pulse generator geometry, featuring a charged rectangular center electrode between grounded parallel plates spaced 2 mm apart. In one mode of operation, initial electrons are induced in the lower air gap, the gap between the center electrode and the lower plate. At 292 kV on the center electrode, 27 atm pressure, and uniform ionization under the full width of the center electrode, induced voltage pulses of 300 kV, 2.4 ps rise time, 9.1 p5 full width at half maximum (FWHM), 38 ps duration, and 22 GHz bandwidth at 3 dB occur at the ends ofthe parallel plates. Reduction ofthe initial electronnumber over eleven orders ofmagnitude has very little gross effect on these pulses. Concentration ofthe initial electrons into a narrow, centered column generally results in reductions in rise time, FWHM, and pulse duration, as well as increases in peak voltage. Movement of the narrow column to the extreme left of the center electrode causes the voltage pulses at the left and right sides of the parallel plates to differ somewhat in their characteristics. In a second mode of operation, slightly delayed laser pulses initiate avalanches in both the upper and lower air gaps. Voltage pulses of reduced widths (1 .1—1.8 Ps FWHM) and amplitudes (7—51 kV) result from the interference of the opposite polarity electromagnetic waves generated in the two gaps.

The development of ultrawideband radar systems has led to the production of echo waveforms with high resolution in range. The Valley Forge Research Center has studied the problem of extending this capability by adding the angular dimension to produce a 2-D image of the radar target. The system currently in use combines adaptive beamforming (ABF) phased array processing with a UWB radar to provide high resolution images of a variety of target configurations. The images were produced using the dominant scatterer ABF algorithm developed at VFRC over the past two decades. The tests were conducted at three frequencies (S, X and Ku bands) in order to allow diversity techniques to enhance the image quality. The formation of a monostatic synthetic aperture array with a 1 GHz bandwidth radar set allows the system to achieve a resolution of 15 cm. This permits fine detail to be observed on images of targets such as cars and trucks. Several examples are presented.