B.H. Cantrell

A spectrally clean transmitting system for solid-state phased-array radars

Navy radar operations are being curtailed in a littoral environment. This is due to two factors: the encroachment of cell phone systems into the naval radar bands; in-band interference from other radars. The spectral width of most pulsed radars is significantly wider than necessary with present modulation schemes. Most radars utilize some form of constant envelope pulse with phase or frequency modulation. This causes the spectrum to broaden to several times the information bandwidth. If both the amplitude and phase of the transmitted signal are allowed to change, a significantly narrower bandwidth can be achieved. The paper presents a method to create waveforms with instantaneous bandwidths of 20 MHz confined within -100 dB. The theoretical spectral results of three popular phase modulation schemes (phase shift keying, minimum phase shift keying and derivative phase shift keying) are compared with the spectrally clean results. In addition, the Chireix out-phasing method is presented as an alternative to generating amplitude and phase modulated waveforms. The Chireix method provides a way of improving the efficiency compared to the conventional class A power amplifier. Preliminary results are shown for a spectrally clean waveform.

Highly bandlimited radar signals

This paper describes a method for generating highly bandlimited or spectrally clean signals and investigates an amplification scheme for physically realizing such signals. The method for generating spectrally clean signals involves interpolating discrete-time signals with Gaussian-windowed sinc functions to obtain highly-bandlimited continuous-time signals. Using this technique, spectrally clean continuous-time signals were obtained which are reasonably efficient and have desirable autocorrelation functions as well as being bandlimited. The modulation technique is illustrated with several examples using the thirteen bit Barker code. The amplifier configuration known as LINC (linear amplification using nonlinear components) is proposed as a means of generating spectrally clean signals.

Development of a Digital Array Radar (DAR)

Twenty-first century littoral and open-sea missions present US Navy (USN) shipboard-radar systems with the challenge of detecting small targets in severe clutter and against multiple sources of interference. In Fiscal Year 2000 (FY00), the Office of Naval Research (ONR) sponsored a program to develop an active array radar that includes a digital beamforming (DBF) architecture. The DBF radar system has the potential for improved time-energy management, improved signal-to-clutter (S/C) ratios, improved reliability and reduced life-cycle costs. This paper summarizes the latest developments of the program during FY00.

Maximum Likelihood Elevation Angle Estimates of Radar Targets Using Subapertures

The maximum likelihood estimates of the elevation angles of two closely spaced targets within the beamwidth is considered. For an array divided into three subapertures, a simple, closed form solution is found whose accuracy compares favorably to the maximum likelihood estimate which uses all the individual elements. Simulation results are presented for the case of a radar target located over a smooth reflecting surface.

Modified Generalized Sign Test Processor for 2-D Radar

The modified generalized sign test processor is a nonparametric, adaptive detector for 2-D search radars. The detector ranks a sample under test with its neighboring samples and integrates (on a pulse-to-pulse basis) the ranks with a two-pole filter. A target is declared when the integrated output exceeds two thresholds. The first threshold is fixed and yields a 10-6 probability of false alarm when the neighboring samples are independent and identically distributed. The second threshold is adaptive and maintains a low false-alarm rate when the integrated neighboring samples are correlated and when there are nonhomogeneities, such as extraneous targets, in the neighboring cells. Using Monte Carlo techniques, probability of false-alarm results, probability of detection curves, and angular accuracy curves have been generated for this detector. The detector was built and PPI photographs are used to indicate the detectors performance when the radar is operated over land clutter.

Development of a digital array radar (DAR)

Twenty-first century littoral and open-sea missions present USA Navy (USN) shipboard-radar systems with the challenge of detecting small targets in severe clutter and against multiple sources of interference. In fiscal year 2000 (FY00), the Office of Naval Research (ONR) sponsored a program to develop an active array radar that includes a digital beamforming (DBF) architecture. The DBF radar system has the potential for improved time-energy management, improved signal-to-clutter (S/C) ratios, improved reliability and reduced life-cycle costs. This paper summarizes the latest developments of the program during FY00.

Bounds on Elevation Error Estimates of a Target in Multipath

The Cramer-Rao bound for an unbiased estimate of the elevation angle of a target in the presence of multipath is calculated for the symmetric (target and image symmetric about the elevation symmetry plane of antenna) and nonsymmetric cases for an antenna consisting of 21 elements. These bounds are compared to the maximum likelihood estimates and it is found that the rms error of the maximum likelihood estimate (which has a bias) is below the Cramer-Rao bound for unbiased estimates.

False alarm control using ambiguous velocity

A novel technique for rejecting clutter residues is proposed. The technique uses maximum-likelihood estimates of the target Doppler and target amplitude generated from consecutive pulses out of an MTI (moving target indicator). Multiple estimates are made and consistency checks are applied to the estimates. Simulation results indicate that for large clutter-to-noise ratios (>or=55 dB) the probability of false alarm from clutter residues is reduced from 1.0 to below 0.01.<<ETX>>

Delta-sigma waveform generation for digital radars

We detail a delta-sigma based system built to produce high-resolution analog waveforms at UHF. With bandwidths as large as 80 MHz and noise floors as low as -140 dBc/Hz, the generated waveforms show potential for use as LO and RF transmission waveforms in a UHF radar system. Finally, the waveform generator system is described as an ideal technology for use in a digital phased array radar.

Probability Density of the Maximum Likelihood Elevation Estimate of Radar Targets

The probability density of the maximum likelihood estimate of elevation angle of a radar target in the presence of multipath is calculated. For detectable signals that have low signal-to-noise ratios, the density is a mixture of a Gaussian density and a delta function at the horizon.