Radar waveform diversity has received considerable attention in recent years due to increasing spectral congestion and the burgeoning capabilities of digital waveform generation. The promise of waveform diversity is far greater utilization of available degrees of freedom to enhance sensing performance and to even facilitate new operating modes. This tutorial provides an overview of this very broad topic, from the basic principles upon which it is founded to the myriad different areas being explored in research for practical sensing applications.

Overview of radar waveform diversity

The effect of soil moisture on the radar backscattering coefficient was investigated by measuring the 4-8 GHz spectral response from two types of bare-soil fields: slightly rough and very rough, in terms of the wavelength. An FM-CW radar system mounted atop a 75-ft truck-mounted boom was used to measure the return at 10 frequency points across the 4-8 GHz band, at 8 different look angles ( 0\deg through 70\deg ), and for all polarization combinations. A total of 17 sets of data were collected covering the range 4-36 percent soil moisture content by weight. The results indicate that the radar response to soil moisture content is highly dependent on the surface roughness, microwave frequency, and look angle. The response seems to be linear, however, over the range 15-30 percent moisture content for all angles, frequencies, polarizations, and surface conditions.

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Radar measurement of soil moisture content

Theoretical aspects of estimating vegetation parameters from SAR interferometry are presented. In conventional applications of interferometric SAR (INSAR), the phase of the interferogram is used to retrieve the location of the scattering phase center of the target. Although the location of scattering phase center for point targets can be determined very accurately, for a distributed target such as a forest canopy this is not the case. For distributed targets the phase of the interferogram is a random variable which in general is a function of the system and target attributes. To relate the statistics of the interferogram phase to the target attributes, first an equivalence relationship between the two-antenna interferometer system and an equivalent /spl Delta/k radar system is established. This equivalence relationship provides a general tool to related the frequency correlation function (FCF) of distributed targets, which can conveniently be obtained experimentally, analytically, or numerically, to the phase statistics of the interferogram. An analytical form for the p.d.f. of the interferogram phase is obtained in terms of two independent parameters: 1) /spl zeta/: mean phase and 2) /spl alpha/: degree of correlation. /spl zeta/ is proportional to the scattering phase center and n is inversely proportional to the uncertainty with which /spl zeta/ can be estimated. It is shown that /spl alpha/ is directly related to the FCF of the distributed target which in turn is a function of scattering mechanisms and system parameters. It is also shown that for a uniform closed canopy the extinction and the physical height of the canopy top can be estimated very accurately. Some analytical and numerical simulations are demonstrated.

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/spl Delta/k-radar equivalent of interferometric SAR's: a theoretical study for determination of vegetation height

The applicability of the Rayleigh fading model for characterizing radar scattering from terrain is examined at 35 GHz for both backscattering and bistatic scattering. The model is found to be in excellent agreement with experimental observations for single-frequency observations of uniform targets such as asphalt and snow-covered ground. The use of frequency averaging to reduce signal fading variations was examined experimentally by sweeping the radar signal from 34-36 GHz in 401 steps. The results show that the formulation based on the Rayleigh model relating the reduction in signal fluctuation to the bandwidth used provides a reasonable estimate for the improvement provided by frequency averaging. >

Fluctuation statistics of millimeter-wave scattering from distributed targets

The performance of adaptive multiple-input multiple-output (MIMO) radar waveforms is examined. We begin by defining a metric, called "MISO cancellation ratio", for numerically screening MIMO waveforms so as to assess their clutter cancellation performance. Next, we evaluate the MISO cancellation ratio levels provided by a number of MIMO waveform classes. Finally, we describe a new MIMO waveform approach, called "dithered DDMA" (Doppler division multiple access), which enables high performance clutter cancellation over a large range-Doppler region without introducing ambiguous ranges or blind speeds, and without increasing the computational load on the MIMO processor.

MIMO Radar Waveforms and Cancellation Ratio

Angular scintillation tracking noise for simplified radar target models has been analyzed by Delano, Muchmore, Freeman, and others for monofrequency pulse radars. This paper extends their work to the case of pulse-to-pulse frequency-modulated radars and demonstrates that such radars offer a significant reduction in angle tracking error. Tracking improvement is derived as a function of tracking servo bandwidth, scintillation spectrum, and FM (frequency modulation) regimen. A quantity, ΔLfc, is defined as the minimum pulse-to-pulse frequency shift required to achieve maximum tracking improvement, and is derived as a function of target model and radar parameters.

W. P. Birkemeier

Papers

Radar tracking accuracy improvement by means of pulse-to-pulse frequency modulation