Ke Yong Li

Angle-Doppler processing using sparse regularization

The detection of moving objects on the ground by airborne radar is one application of space-time adaptive processing (STAP). The goal is to estimate the position and velocity of objects. This paper considers the problem as a linear inverse problem and uses ℓ1-norm regularization to promote sparsity in the solution. It is proposed that the angle-Doppler plane be explicitly segmented into the clutter ridge component and a non-clutter-ridge component. We propose that the second component be modeled as sparse — as the moving objects are assumed to be well isolated in the angle-Doppler plane.

Reconstruction of constant envelope signals with given Fourier transform magnitude

In this paper, we consider the problem of reconstructing a signal with constant envelope property in the time domain from its Fourier transform magnitude. The constant envelope property makes the signal reconstruction problem difficult. In this context, several convex-set like signal properties are used to generate projection based iterative reconstruction algorithms possessing error reduction property.

Effect of earth's rotation and range foldover on space-based radar performance

Space-based radar (SBR) by virtue of its motion generates a Doppler frequency component to the clutter return from any point on the Earth as a function of the SBR-Earth geometry. The effect of the rotation of the Earth around its own axis also adds an additional component to this Doppler frequency. The overall effect of the rotation of the Earth on the Doppler turns out to be two correction factors in terms of a crab angle affecting the azimuth angle, and a crab magnitude scaling the Doppler magnitude of the clutter patch. Interestingly, both these quantities depend only on the SBR orbit inclination and its latitude and not on the location of the clutter patch of interest. Further, the crab angle has maximum effect for an SBR on a polar orbit that is above the equator. The crab magnitude, on the other hand, peaks for an SBR on an equatorial orbit. Together with the range foldover phenomenon, their overall effect is to generate Doppler spread/splitting resulting in wider clutter notches that degrade the clutter nulling performance of adaptive processing techniques. A detailed performance analysis and methods to minimize these effects are discussed here

Design of unimodular sequences using generalized receivers

This paper reviews recent advances in designing unimodular sequences with good auto/cross correlation properties along with a new approach that emphasizes on independent receiver design. The general problem is to design single or multiple sequences with constant modulus in the timedomain such that their respective matched filter outputs ideally resemble delta functions and the cross-matched filter outputs are zeros. In this context CAN (cyclic-algorithm new) and WeCAN (Weighted CAN) have been proposed for designing such sequences with good auto-cross correlation properties. In this paper, the equivalence of the CAN algorithms and the classic Gerchberg-Saxton (GS) algorithm involving the sequential magnitude substitution operations in the time and frequency domain is demonstrated. The design of unimodular sequences is further generalized here by considering the receiver design to be more general than the respective matched input sequences. The receiver design is carried out by taking care of the desired output requirements and the freedom present at the input can be used to further minimize the output side-lobe level.

Waveform design optimization using bandwidth and energy considerations

The problem of joint transmitter and receiver design for optimum output performance in the context of detection in the presence of clutter and noise is addressed here by maximizing the receiver output signal power to average interference plus noise ratio (SINR) at the specific decision instant. The receiver optimization leads to output SINR in terms of the transmit waveform, and transmitter optimization is further carried out subject to energy and bandwidth constraints. The new design proposed here leads to threshold constraints on transmit energy and the overall performance as a function of bandwidth is illustrated here for various target, clutter and noise scenes.

Orthogonal pulsing schemes for improved target detection in space based radar

Two phenomena that degrade the performance of ground moving target detection (GMTI) capabilities of space based radars (SBR) are (i) range foldover effects associated with multiple data points originating from different range bins due to the radar pulse stream and (ii) Earths rotational effect on clutter Doppler frequency. The degradation in performance due to these phenomena is quantified in this paper and methods to minimize their effect are discussed. In this context, transmit pulsing schemes involving waveform diversity is proposed for improved target detection capabilities

Constant envelope signals with prescribed discrete Fourier transform magnitude

This paper presents a new method for constant envelope signal generation using discrete values of the magnitude spectrum. The new approach uses the frequency magnitude values to generate a piece-wise linear chirp-like signal with excellent pulse compression properties.

Efficient wideband processing without subbanding

A new method for wideband spatio-temporal processing in the context of clutter mitigation and target detection is addressed in this paper. The frequency dependent, spatio-temporal steering vectors, are separated using a series expansion that depends on a sequence of Bessel function terms, and this is used to focus data vectors corresponding to various frequency components to a single frequency component. As a result, narrowband space-time adaptive processing (STAP) can be carried out on the focused data vector for clutter mitigation and target detection.

Effect of Bandwidth on Wideband-STAP Performance

A wideband signal occupies a finite bandwidth that is significant compared to its carrier frequency. As a result when transmitted, its returns cause bandwidth dispersion across the antenna. It is shown here that the effect of the finite bandwidth is to introduce a set of uncorrelated return signals for every physical scatter in the field. Further, each such uncorrelated return contains a set of coherent signals with different directional and Doppler components that result from a jittering effect both in angle and Doppler domain. As a result, adaptive clutter cancellation using traditional processing schemes does not work well. Although in principle it is possible to correct these decorrelating effects by 3D space-time adaptive processing (STAP), the present day methods are quite costly and difficult to implement. In addition to the new wideband signal modeling framework mentioned above, we outline a hierarchical processing scheme which has the potential for dramatically reducing both processing and sample support burdens.

Effect of wind on space-based radar performance

This paper addresses the effect of wind on target detection performance using a space based radar platform. Forests and lakes are constantly being modulated by wind, which then affects the radar pulse returns by suitably amplitude modulating the temporal returns, thus affecting the Doppler. Billingsley has modeled the windblown autocorrelations using a real symmetric function that is the sum of a constant term and a ldquoCauchy density function typerdquo term. Using a rational system approximation on the Billingsley spectrum, it is shown that a sixth order rational model containing two damped sinusoidal terms and two exponentially decaying terms can duplicate the wind spectrum up to about -80 dB in the case of low wind speed. Each sinusoidal component in the wind spectrum generates an additional clutter Doppler component and the effect of damping is shown to generate a bundle of uncorrelated returns for each such frequency, with each uncorrelated return containing a coherent sum of signals. As a result, the degradation in performance when wind is present can be attributed to the coherent returns that can not be nulled out using standard adaptive processing techniques.