Simon Watts

Radar Spectrum Engineering and Management: Technical and Regulatory Issues

The radio-frequency (RF) electromagnetic spectrum, extending from below 1 MHz to above 100 GHz, represents a precious resource. It is used for a wide range of purposes, including communications, radio and television broadcasting, radionavigation, and sensing. Radar represents a fundamentally important use of the electromagnetic (EM) spectrum, in applications which include air traffic control, geophysical monitoring of Earth resources from space, automotive safety, severe weather tracking, and surveillance for defense and security. Nearly all services have a need for greater bandwidth, which means that there will be ever-greater competition for this finite resource. The paper explains the nature of the spectrum congestion problem from a radar perspective, and describes a number of possible approaches to its solution both from technical and regulatory points of view. These include improved transmitter spectral purity, passive radar, and intelligent, cognitive approaches that dynamically optimize spectrum use.

Modeling and Simulation of Coherent Sea Clutter

This paper describes a new technique for modeling and simulating the coherent returns from radar sea clutter, based on the compound K-distribution model for clutter amplitude statistics. Using observations of recorded Doppler spectra, a simple method is proposed for characterizing the temporal variations of the Doppler spectrum observed in a single-range cell. It is shown that simulations based on this model can reproduce the main statistical features observed in real measurements.

The performance of cell-averaging CFAR systems in sea clutter

This paper explores the performance of cell-averaging CFAR (CA-CFAR) detectors in sea clutter, using the compound K-distribution clutter model. The first section provides a brief introduction to the compound K-distribution model. This is followed by a description of the CA-CFAR configuration being investigated and a definition of CFAR loss. The definition of ideal CFAR performance is given to provide the basis against which CFAR gain can be assessed. Then, the CFAR loss in noise is developed analytically and compared with equivalent simulation results for clutter with no spatial correlation and for clutter with spatial correlation in range. Finally, the paper investigates methods for determining the threshold multiplier value, /spl alpha/, in different conditions, covering the estimation of the K-distribution shape parameter in clutter-plus-noise and analysis of the clutter distribution normalised by the cell-averager estimate of the mean level.

The physics and modelling of discrete spikes in radar sea clutter

This paper first examines the physical source of sea clutter spikes from considerations of oceanographic principles and electromagnetic scattering; it is shown how electromagnetic scattering theory can be used to predict the characteristics of discrete spikes in backscatter from the sea. The spikes can also be modelled statistically as extensions to the standard K-distribution model for sea clutter, as proposed by K.D. Ward et al. (1990). In this paper, the work in K.D. Ward et al. (1990) is extended to include the addition of thermal noise and it is also shown how the parameters of this model can be matched to real data. Results are presented for recorded airborne radar data, showing how the model can accurately match the characteristics of real clutter-plus-spikes-plus-noise, including the effects of pulse-to-pulse integration with frequency agility.

Passive bistatic radar experiments from an airborne platform

The initial analysis of data collected during the airborne experiments has demonstrated airborne passive radar operation, using commercial FM radio signals as illuminators of opportunity, and these are believed to be the first results of this kind. The results have highlighted several practical techniques for improving the system performance: using multiple perspectives, optimal illuminator modulation bandwidth, and multiple looks over time.

Application of the K+Rayleigh distribution to high grazing angle sea-clutter

The probability distribution of the radar backscatter is commonly used to determine the threshold for separating targets from clutter. Analysis of sea-clutter data collected at high grazing angles, between 15° and 45°, by the Defence Science Technology Organisation (DSTO) Ingara fully polarimetric X-band radar has been used extensively to test distribution models given a large number of samples. The focus of this paper is to determine the most appropriate sea-clutter model for high grazing angle sea-clutter given the smaller number of samples in a typical target detection scenario. For this purpose, a recently proposed K+Rayleigh distribution is introduced to account for the extra Rayleigh scattering observed in the radar backscatter.

Radar sea clutter: Recent progress and future challenges

This paper reviews current developments in sea clutter modelling, covering both statistical models, mainly based on empirical observations, and modelling of electromagnetic scattering from simulated sea surfaces. The latest developments in detection processing are also discussed, within the framework provided by these models. The emphasis throughout the paper is on the modelling of low grazing angle microwave radar backscatter in relation to airborne and surface maritime surveillance radar. As well as reviewing current knowledge in these areas the paper also highlights areas where further research is needed.

A new method for the simulation of coherent sea clutter

This paper describes a new technique for modeling and simulating the coherent returns from radar sea clutter, based on the compound K-distribution model for clutter amplitude statistics. Using observations of recorded Doppler spectra, a simple method is proposed for characterizing the temporal variations of the Doppler spectrum observed in a single range cell. It is shown that simulations based on this model can reproduce the main features observed in real measurements. These features include the asymmetry of the average power spectral density (PSD), the temporal variation of the local spectra and the increasingly spiky characteristics of the amplitude statistics observed in the extremes of the Doppler spectrum.

Characterising the Doppler spectra of high grazing angle sea clutter

This paper characterises the Doppler spectra of radar sea clutter using data collected by the Ingara radar at grazing angles in the range 29° to 38°. It is shown that recently proposed modelling and simulation methods for lower grazing angles are still applicable for this data. The modelling method is also extended to capture the bimodal behaviour observed with high intensity returns from breaking waves looking up or downwind.

Simulation of coherent sea clutter with inverse gamma texture

A method for simulating coherent sea clutter has been adapted for Pareto distributed clutter by using an inverse gamma distribution for the local clutter intensity. The model is applied to the range variation of Doppler spectra from Pareto distributed clutter collected at grazing angles of 31°-37° with the DSTO Ingara radar. The model produces Doppler spectra with a Gaussian shape that vary with range in a similar way to those obtained from the data. The simulations contain clutter spikes that replicate the properties of real sea spikes in respect of their intensity statistics, Doppler spectra and position near the wave peaks.