S. Unnikrishna Pillai

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.

Chirp-like transmit waveforms with multiple frequency-notches

This paper describes the construction of chirp-like constant-modulus transmit waveforms designed so as to possess multiple notches in their frequency spectra at user-specified frequencies. We propose an iterative projection algorithm with low computational complexity. In wide-band radar systems, such frequency-notched transmit waveforms are needed so as to avoid transmitting into frequency bands that are used by other systems such as for communication, navigation, etc.

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.

An iterative algorithm for the construction of notched chirp signals

This paper describes an iterative algorithm for the design of a constant-modulus finite-duration chirp-like signal having a notch in its frequency spectrum. Frequency-notched signals with good pulse compression properties are needed in wide-band radar systems that must avoid radiating into frequency bands assigned to other systems (e.g. communication and navigation systems). The algorithm is computationally efficient and provides explicit control of the notch frequency and notch order.

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.

Extraction of Resonant Modes from Scattered Noisy Data

This paper presents a technique that combines Prony’s original approach together with the high resolution eigenstructure based methods to estimate resonant modes from scattered noisy data. The problem is equivalent to obtaining best rational approximations that fit the actual measurements in a least square sense.

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

Optimization of single transmit pulse shape to maximize detection and identification of ground mobile targets

This paper investigates the optimization of a single transmit pulse shape and the receiver response to maximize either target detection or identity discrimination between two structurally similar ground mobile targets: the T-72 and M1 main battle tanks. This theory incorporates effects due to the uncertainty in the prior knowledge of the target aspect relative to the sensor. The improvement in the signal-to-interference-plus-noise (SINR) resulting from the optimized transmit pulse shape over that of a standard chirped waveform typically lies between 4 dB and 9 dB. Similar improvements in target identification performance are also obtained.

A self inversive array processing scheme for angle-of-arrival estimation

Abstract This paper proposes an improved high-resolution technique that incorporates the forward (original) array together with its complex conjugated backward version to achieve superior performance. This is in turn realized by averaging the forward covariance matrix and the backward one, and using it in conjunction with the MUSIC technique. This paper also presents the asymptotic analysis in terms of first-order approximations of the mean and variance of the null spectrum estimator. It is shown here that in an uncorrelated scene, the bias corresponding to this scheme is exactly half of that associated with the conventional one while maintaining the same variance. Furthermore, the bias expression in the new scheme is used to obtain a resolution threshold for two uncorrelated, equipowered plane waves in white noise and the result is compared with that obtained by the conventional scheme for the same source scene.

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.