Research Publications/Outputs

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Analysis of high resolution land clutter using an X-band radar
(IEEE, 2015-10) Melebari, A; Abdul Gaffar, MY; Strydom, JJ
In modern radar systems with high range resolution, the statistical properties of clutter have a significant effect on the performance of the radar. Analyzing the radar returns from various clutter terrains is essential when aiming to optimize the detection performance of the radar within these terrains. This paper presents the statistical analysis of three types of land clutter terrains: Saudi Arabian urban, South African urban and Saudi Arabian date farms, all measured at low grazing angle. Measurements were performed with an X-band radar system with two instantaneous bandwidths of 40 MHz and 400 MHz. The clutter data was analyzed by fitting the amplitude Probability Distribution Function (PDF) to different distributions using the Method of Moments (MoM) and applying the Kullback-Leibler Divergence test to assess the accuracy of the PDF fittings. The analysis also included computing the Power Spectral Density (PSD) of the measurements. Finally, a comparison was made between the different land clutter terrains. The amplitude PDF of the urban clutter measurements had a longer tail compared to the amplitude PDF clutter measurements of the date farms terrain. The best fit for the urban clutter amplitude PDF measurements was found to be the Log-normal distribution whereas the date farms amplitude PDF measurements were best fitted by the K-distribution. Lastly, the Doppler bandwidth of the date farm clutter measurements was found to be larger than the Doppler bandwidth of the urban clutter measurements, however this could be explained by the difference in wind conditions on the measurement days.
Comparison of square law, linear and bessel detectors for CA and OS CFAR algorithms
(IEEE, 2015-10) Melebari, A; Mishra, AK; Abdul Gaffar, MY
A major challenge in radar detection is to optimize both the detection performance and computation cost. In Constant False Alarm Rate (CFAR) algorithms, different detectors can be implemented like linear, square law or Bessel detectors. These detectors have difference detection performances and computational costs. In this paper, the detection performances of these three detectors are investigated for CA-CFAR and Order Statistic CFAR (OS-CFAR) algorithms using simulated and measured data of semi-urban desert terrain. Monte Carlo simulation was used to compute the performance in both Gaussian noise and Weibull interference. Results obtained using both simulated and measured data show that the square law detector offers superior performance to the linear and Bessel detector for the OS-CFAR algorithm. In the case of the CA-CFAR algorithm, the square law detector offers the best performance for large values of the Weibull shape parameter. When the Weibull shape parameter is small, the linear detector offers the best performance.
Detecting 3-D rotational motion and extracting target information from the principal component analysis of scatterer range histories
(2009-10) Nel, Willem AJ; Stanton, D; Abdul Gaffar, MY
ISAR imagery of ships are complicated by the 3-D motion of the target, which causes blurring in the imagery. A technique is proposed which could help detect such motion and prove useful to both analyse the 3-D motion as well as possibly help to estimate the 3-D position of scatterers as a by-product of the analysis. The technique is based on principal component analysis of accurate scatterer range histories and is shown only in simulation. Future research should focus on practical application.
The effect of windowing on the performance of the CA-CFAR and OS-CFAR algorithms
(IEEE, 2015-10) Melebari, A; Melebari, A; Alomar, W; Abdul Gaffar, MY; De Wind, R; Cilliers, Jacques E
A major challenge in radar design is to not overflow the processor with false detections. This paper investigates the performance of Constant False Alarm Rate (CFAR) algorithms on maintaining a fixed false alarm rate when windowing is used to reduce the sidelobes in both the slant-range and the Doppler domain. A Monte Carlo simulation was used to investigate the performance of Cell Averaging CFAR (CA-CFAR) and Ordered Statistic CFAR (OS-CFAR) algorithms. Results show that the windowing operation induces correlation between the samples. This correlation causes a large error in the Probability of False Alarm (PFA) when the CFAR scale factor was calculated using closed form equations, which assume that the samples are uncorrelated. In order to maintain a fixed false alarm rate when the windowing operation is used, the scale factor value should be calculated by simulation.
Investigating the effect of a targets time-varying doppler generating axis of rotation on isar image distortion
(2007-10) Abdul Gaffar, MY; Nel, Willem AJ
ISAR imaging has potential in assisting with the classification of non-cooperative targets. Blurred ISAR imagery may however lead to misleading classification results. Much research has been done to understand some of the causes of distortion in ISAR imaging mostly under the limited assumption that a targets axis of rotation is constant over the CPI. This paper investigates how the targets time-varying Doppler generating axis of rotation, caused by the complex 3D motion of a target at sea, contributes to ISAR image blurring. Quaternion algebra is used to aid the characterisation of a time-varying Doppler generating axis of rotation on the migration through cross-range cells. Real motion data of a sailing yacht is used to examine the effects of 3D rotational motion on ISAR imagery of point scatterer like simulated targets and the associated blurring. Simulation results show that small yaw rate perturbation during side-view ISAR imaging intervals gives rise to significant changes in the direction of the Doppler generating axis which results in scatters migration through cross-range cells.
Investigation of 3-D RCS Image formation of ships using ISAR
(2006-05) Lord, RT; Nel, Willem AJ; Abdul Gaffar, MY
Conventional Inverse Synthetic Aperture Radar (ISAR) utilises the rotational motion of a target such as a ship or an aircraft to obtain a 2-D image of the target’s radar cross section (RCS) profile from a coherent radar system. This concept can be extended to obtain a 3-D RCS profile if the target's translational and rotational motion has the required attributes and is known with sufficient accuracy. This paper is a preliminary investigation to see whether 3-D ISAR images can be generated with a single antenna, using an approach that is different from inter-ferometric ISAR. Simulation results have verified that 3-D ISAR images can be obtained under certain circum-stances. A major problem experienced with ship ISAR data is multipath reflection off the sea surface. Simula-tions have been performed, which illustrate the degrading effect of specular multipath on the final 3-D image.
ISAR imaging using the instantaneous range instantaneous Doppler method
(IEEE, 2015-10) Wazna, TM; Balamesh, AS; Abdul Gaffar, MY
In Inverse Synthetic Aperture Radar (ISAR) imaging, the Range Instantaneous Doppler (RID) method is used to compensate for the nonuniform rotational motion of the target that degrades the Doppler resolution of the ISAR image. The Instantaneous Range Instantaneous Doppler method (IRID) is proposed in this paper as a tool that compensates for higher order phase terms that may degrade both, the slant-range resolution and the Doppler resolution of the ISAR image. In IRID, adaptive S-distribution was applied on both the slant-range and Doppler dimensions of the image, thus, producing a better focused ISAR image. The IRID method was applied to simulated and measured data sets, and the ISAR images show that the IRID results offer better visual ISAR images than the RID results.
Probability of helicopter blade flash interception in search radar
(2021-12) Berndt, Robert J; Nel, Willem AJ; Abdul Gaffar, MY; O’Hagan, D
Helicopter main rotor blade flashes in radar measurements are an oft-used feature for target classification. While focus is often placed on being able to detect them, little attention has been given to what is required to ensure that they are reliably and persistently intercepted. Using mathematical analysis and simulation, this paper explores the probability of helicopter blade flash interception in search radar. The effects of radar antenna azimuth beamwidth, scan rate, and waveform on the average probability of intercept for two-bladed helicopter blade flashes (a worst-case scenario) are examined. The results presented provide an upper bound on achievable flash detection performance and can be used to understand and optimize the performance of helicopter classification modes.
Quaternion-based transformation for extraction of image-generating Doppler for ISAR
(2008-10) Abdul Gaffar, MY; Nel, Willem AJ; Inggs, MR
Inverse synthetic aperture radar (ISAR) is an imaging technique that is dependent on an object’s rotational motion over a coherent processing interval. Maritime vessels and aircraft possess 3-D rotational motion, whereas it is only their ISAR contributing motion that is useful to the ISAR imaging process; the contributing motion consists of the Doppler generating axis and the effective angle of rotation. This letter presents a quaternion-based transformation that converts measured attitude and position data into an object’s Doppler generating axis and effective angular rotation rate. This transformation is significant since it isolates the component of the motion that directly influences the ISAR image. It provides an alternative approach that can be used to understand the causes of blurring of most ISAR images of sea vessels as well to identify good imaging intervals for applications such as cooperative ISAR for radar cross section measurement purposes
Selecting suitable coherent processing time window lengths for ground-based ISAR imaging of cooperative sea vessels
(IEEE, 2009-09) Abdul Gaffar, MY; Nel, Willem AJ; Inggs, MR
Inverse synthetic aperture radar (ISAR) imaging of sea vessels is a challenging task because their 3-D rotational motion over the coherent processing interval (CPI) often leads to blurred images. The selection of the duration of the CPI, also known as the coherent processing time window length (CPTWL), is critical because it should be short enough to limit the blurring caused by the 3-D rotational motion and long enough to ensure that the desired cross-range resolution is obtained. This paper proposes an algorithm, referred to as the motion-aided CPTWL selector (MACS) algorithm, which selects suitable CPTWLs for ISAR imaging of cooperative sea vessels. The suggested CPTWLs may be used to obtain motion-compensated ISAR images that have the desired medium cross-range resolution and limited blurring due to 3-D rotational motion. The proposed algorithm is applied to measured motion data of three different classes of sea vessels: a yacht, a fishing trawler, and a survey vessel. Results show that longer CPTWLs are needed for larger vessels in order to obtain ISAR images with the desired cross-range resolution. The effectiveness of the CPTWLs, suggested by the MACS algorithm, is shown using measured radar data. The suggested CPTWLs may also be used to select an effective initial CPTWL for Martorella/Berizzi’s optimum imaging selection algorithm when it is applied to measured radar data of small vessels. Lastly, the proposed technique offers significant computational savings for radar cross section measurement applications where a few high-quality ISAR images are desired from long radar recordings.
Synthetic range profiling, ISAR imaging of sea vessels and feature extraction, using a multimode radar to classify targets: initial results from field trials
(2011-04) Abdul Gaffar, MY; Nel, Willem AJ; Naicker, K; Steyn, J; Alanazi, T; Alzamil, A
This paper describes the design and working principles of an experimental multimode radar with a stepped-frequency Synthetic Range Profiling (SRP) and Inverse Synthetic Aperture Radar (ISAR) capability for the purpose of operator-based classification of small to medium sized sea vessels in littoral condition. The experimental multimode radar is based on an experimental tracking radar that was modified to generate SRP and ISAR images in both search and tracking modes. The architecture and functionality of the experimental system is described. Initial results from field experiments are presented to demonstrate the functionality of the system.
A time domain phase-gradient based ISAR autofocus algorithm
(IEEE, 2011-10) Nel, Willem AJ; Giusti, E; Martorella, M; Abdul Gaffar, MY
Autofocus is a well known required step in ISAR (and SAR) processing to compensate translational motion. This research proposes a time domain autofocus algorithm and discusses its relation to the well known phase gradient autofocus (PGA) technique. Results on simulated and measured data show that the algorithm performs well. Unlike many other ISAR autofocus techniques, the algorithm does not make use of several computationally intensive iterations between the data and image domains as part of the autofocus process. As such, the proposed algorithm could prove to be faster than other techniques. Observations are made regarding the type of phase errors that can be handled, and it is argued that the technique could be posed either as parametric or nonparametric depending on the type of phase errors expected.
Towards Inverse Synthetic Aperture Radar (ISAR) for small sea vessels
(2006-12) Abdul Gaffar, MY
Power point presentation on Inverse Synthetic Aperture Radar (ISAR)

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