32 research outputs found
Statistical Analysis of Coherent Monostatic and Bistatic Radar Sea Clutter
Radar sea clutter analysis has been an important area of radar research for many years. Very limited research has been carried out on coherent monostatic sea clutter analysis and even less on bistatic sea clutter. This has left a significant gap in the global scientific knowledge within this area. This thesis describes research carried out to analyse, quantify and model coherent sea clutter statistics from multiple radar sources. The ultimate goal of the research is to improve maritime radars' ability to compensate for clutter and achieve effective detection of targets on or over the sea surface. The first analyses used monostatic data gathered during the fight trials of the Thales Searchwater 2000 AEW radar. A further sea clutter trials database from CSIR was then used to investigate the variation of clutter statistics with look angle and grazing angle. Finally simultaneous monostatic and bistatic sea clutter data recorded in South Africa using the S-band UCL radar system NetRAD were analysed. No simultaneous monostatic and bistatic coherent analysis has ever been reported before in the open literature. The datasets recorded included multiple bistatic angles at both horizontal and vertical polarisations. Throughout the analysis real data have been compared to accepted theoretic models of sea clutter. An additional metric of comparison was investigated relating to the area of information theoretic techniques. Information theory is a significant subject area, and some concepts from it have been applied in this research. In summary this research has produced quantifiable and novel results on the characteristics of sea clutter statistics as a function of Doppler. Analysis has been carried out on a wide range of monostatic and bistatic data. The results of this research will be extremely valuable in developing sea clutter suppression algorithms and thus improving detection performance in future maritime radar designs
Measurement and modelling of bistatic sea clutter
There is a growing interest in bistatic radars; however, such systems cannot reach their full potential
unless the designer has a proper understanding of the environment in which they operate. Rather little
information has been published on bistatic clutter and out-of-plane bistatic sea clutter in particular. This
is due to a number of factors including the inherent complexity of conducting bistatic radar trials and the
resulting lack of high quality bistatic data.
In this thesis the collection and analysis of a unique set of bistatic sea clutter data is described.
To achieve this objective a novel multistatic radar system was developed. The nodes do not need to be
physically connected. This system has a peak transmitted power of more than 500 W. Synchronisation in
time and frequency was achieved using GPS disciplined oscillators built and designed at the University
of Cape Town.
Using the above system simultaneous bistatic and monostatic sea clutter and target signatures were
recorded in the UK and South Africa at various geometries and weather conditions. Parts of this unique
data set related to out-of-plane bistatic sea clutter was analysed in this thesis. The data covered both
co- and cross-polarised sea clutter data at low grazing angles with bistatic angles between 30° and 120°.
Data sets covering a range of conditions with sea states from 2 â 5.
Using the recorded data it was shown that the ratio of the bistatic normalised radar cross section
to the monostatic normalised radar cross section dropped as the scattering angle was increased until
the scattering angle was around 90°. Furthermore, the cross-polarised bistatic normalised radar cross
section was found to be larger than the cross-polarised monostatic normalised radar cross section when
the scattering angle was around 90°.
A new empirical model for predicting bistatic normalised radar cross section has been developed.
The model is applicable to both in-plane and out-of-plane geometries. The model was able to provide a
good fit to both UCL and external data.
The temporal correlation properties of both monostatic and bistatic data were studied. It was found
that the speckle component of both bistatic and monostatic clutter decorrelated in tens of milliseconds,
with the decorrelation time longer for bistatic clutter. The texture of both bistatic and monostatic clutter
had similar autocorrelation functions and had similar decorrelation times. By comparing the texture and intensity autocorrelation functions it was concluded that the compound model still holds.
It was also found that bistatic clutter was less âspikyâ than monostatic clutter particularly at horizontal
polarisation. This was due to the reduction in the intensity of the spikes due to specular reflections.
By combing the effects of the reduction in reflectivity and spikiness it was shown that a bistatic radar
would require a smaller signal to interference ratio than a monostatic radar for the same probability of
detection and probability of false alarm. This was more evident at angles close to 90° and for horizontal
polarisation.
In summary this thesis reports the collection and analysis of novel simultaneous monostatic and
bistatic sea clutter and target data. This was achieved by the development of a unique multistatic radar
system. This work has resulted in significant advances in both netted radar technology and understanding
of bistatic sea clutter
Sea Surface Current Measurements Using Along-Track Interferometric SAR
Ocean currents affect the weather, the climate and the marine ecosystem. Observing ocean currents is important for understanding the upper-ocean layer dynamics and its interaction with the other components of the climate system. In-situ measurements are sparse and their deployment and maintenance is costly. Satellite remote sensing with large spatial coverage offers a good complement to the in-situ observations. In this work we have studied the spaceborne Along-Track Interferometric SAR (ATI-SAR) for measuring sea surface currents. The measurement principle is based on the fact that the phase difference between two SAR acquisitions is directly related to radial (line-of-sight) velocity of the illuminated surface. Previous studies based on similar systems were carried out in areas with well defined and strong tidal currents ( ~1 - 3 m/s). In this work we demonstrate thecapability of ATI-SAR, through several study cases, in areas with weak currents ( <0.5 m /s). This is challenging for the satellite measurements of surface currents because it requires very accurate processing and retrieval algorithms. In addition, it has been found that wave motion contribution, systematically dominates the measured ATI-SAR radial velocity in these weak current areas. Estimation of the wave motion contribution relies on high-resolution and accurate wind data. Thus, a wind speed retrieval algorithm from SAR is needed to support the ATI-SAR current retrieval. We have shown that with an appropriate processing of the ATI-SAR phase and with applying the necessary corrections to the measured velocity a good agreement with ocean circulation models is achieved (rmse =0.1 m /s). These corrections include phase calibration and wind compensation to correct for instrument and geophysical systematic errors, respectively. Finally, a novel method for removing the wind direction ambiguity, based on the ATI-SAR phase, is presented. In previous methods, the wind ambiguity removal was based on external information, e.g. an atmospheric model or on visual observation of wind shadows
Maritime forward scatter radar: data collection and clutter analysis
This thesis is the result of study into development, experimental testing and clutter analysis in a Forward Scatter Radar (FSR) designed to detect low reflectivity maritime targets at low grazing angles. The concept of such kind of maritime system is presented; its advantages for surveillance applications are described. Scattering of Electromagnetic (EM) Waves over the sea surface at different radar configurations is outlined with the focus made on forward scattering and appropriate sea clutter models. Phenomenology of the signals in FSR is examined and explained. The development of an experimental FSR hardware operating in X- and K- frequency bands for target detection and clutter analysis and its performance are described in details. It follows with the comprehensive analysis on the measured sea clutter which includes study of influence of a large number of parameters of the radar and sea conditions on the clutter spectral and statistical properties. Finally preliminary analysis of radio frequency (RF) target signatures made with the prototype radar is presented
Caractérisation de films d'huile de la bande X à la bande K, expérimentation en bassin à vagues
This paper depicts an experiment conducted in a wind-wave pool in Brest, France, to characterize oil films when observed at moderate incidence from a X-to-K-band radar. Simultaneous measurements of surface elevation and radar backscattered field were carried out for various sea water surface states and incident angles. From this meaningful dataset (mainly lying in simultaneous acquisitions in X-, Ku-and K-band), an inversion method is proposed to characterize some properties of the oil film: its origin (mineral or biogenic) and its fractional coverage indicator. This process is based on the minimization of the cost function correlating the values given by a physical model of the wave damping ratio and the measured ones. The resulting oil parameters are found in overall good agreement with the three different released oils (two mineral and one biogenic) and it is observed that the fractional filling indicator of the oil slick decreases with increasing the roughness surface state whatever the considered oil.Ce papier décrit une expérimentation menée dans un bassin à vagues à Brest, France. L'objectif est de caractériser les films d'huile observés au moyen d'un systÚme radar à des angles d'incidence modérée de la bande X à la bande K. Des mesures simultanées de l'élévation de la surface et du champ électromagnétique rétro-diffusé furent réalisées pour différents états de surface et divers angles d'incidence. A partir de cette base de données, une méthode d'inversion est proposée pour caractériser plusieurs propriétés du film d'huile: son origine (minérale ou biogénique) ainsi que sa fraction de couverture surfacique. Ce processus repose sur la minimisation d'une fonction coût liant la modélisation physique du coefficient d'atténuation des vagues avec la mesure. Les paramÚtres estimés sont en accord avec les trois huiles différentes déversées (deux minérales et une biogénique). Il est également observé que la fraction de couverture surfacique de la nappe d'huile décroßt avec l'augmentation de la rugosité de surface et ce, quelle que soit l'huile considérée
Maritime Radar Target Detection Using Time Frequency Analysis
Small target detection in sea clutter remains a challenging problem for radar operators as the backscatter from the sea-surface is complex, involving both time and range varying Doppler spectra with strong breaking waves which can last for seconds and resemble targets. The goal of this thesis is to investigate two different time frequency wavelet transforms to filter the sea clutter and improve target detection performance. The first technique looks at an application of stationary wavelet transforms (SWT) to improve target detection. The SWT decomposes a signal into different components (or sub-bands) which contain different characteristics of the interference (clutter + noise) and target. A method of selecting the sub-band with the most information about the target is then presented using an âentropyâ based metric. To validate the SWT detection scheme, real radar data recorded from both an airborne and a ground based radar systems are analysed. A Monte-Carlo simulation using a cell averaging constant false alarm rate detector is implemented to demonstrate and quantify the improvement of the new scheme against unfiltered data. The second technique utilises a sparse signal separation method known as basis pursuit denoising (BPD). Two main factors contribute to the quality of the separation between the target and sea-clutter: choice of dictionary that promotes sparsity, and the regularisation (or penalty) parameter in the BPD formulation. In this implementation, a tuned Q-factor wavelet transform (TQWT) is used for the dictionary with parameters chosen to match the desired target velocity. An adaptive method is then developed to improve the separation of targets from sea-clutter based on a smoothed estimate of the sea clutter standard deviation across range. A new detection scheme is then developed and the detection improvement is demonstrated using a Monte-Carlo simulation.Thesis (Ph.D.) -- University of Adelaide, School of Electrical & Electronic Engineering, 201
Remote Sensing of the Oceans
This book covers different topics in the framework of remote sensing of the oceans. Latest research advancements and brand-new studies are presented that address the exploitation of remote sensing instruments and simulation tools to improve the understanding of ocean processes and enable cutting-edge applications with the aim of preserving the ocean environment and supporting the blue economy. Hence, this book provides a reference framework for state-of-the-art remote sensing methods that deal with the generation of added-value products and the geophysical information retrieval in related fields, including: Oil spill detection and discrimination; Analysis of tropical cyclones and sea echoes; Shoreline and aquaculture area extraction; Monitoring coastal marine litter and moving vessels; Processing of SAR, HF radar and UAV measurements
Oil spill and ship detection using high resolution polarimetric X-band SAR data
Among illegal human activities, marine pollution and target detection are the key concern of Maritime Security and Safety. This thesis deals with oil spill and ship detection using high resolution X-band polarimetric SAR (PolSAR). Polarimetry aims at analysing the polarization state of a wave field, in order to obtain physical information from the observed object. In this dissertation PolSAR techniques are suggested as improvement of the current State-of-the-Art of SAR marine pollution and target detection, by examining in depth Near Real Time suitability
Polarimetric SAR for the monitoring of agricultural crops
The monitoring of agricultural crops is a matter of great importance. Remote
sensing has been unanimously recognized as one of the most important techniques for
agricultural crops monitoring. Within the framework of active remote sensing, the
capabilities of the Synthetic Aperture Radar (SAR) to provide fine spatial resolution
and a wide area coverage, both in day and night time and almost under all weather
conditions, make it a key tool for agricultural applications, including the monitoring
and the estimation of phenological stages of crops. The monitoring of crop phenology
is fundamental for the planning and the triggering of cultivation practices, since
they require timely information about the crop conditions along the cultivation
cycle. Due to the sensitivity of polarization of microwaves to crop structure and
dielectric properties of the canopy, which in turn depend on the crop type, retrieval
of phenology of agricultural crops by means of polarimetric SAR measurements is
a promising application of this technology, especially after the launch of a number
of polarimetric satellite sensors.
In this thesis C-band polarimetric SAR measurements are used to estimate pheno-
logical stages of agricultural crops. The behavior of polarimetric SAR observables
at different growth stages is analyzed and then estimation procedures, aimed at the
retrieval of such stages, are defined.
The second topic on which this thesis is focused on is the land cover types discrimi-
nation by means of X-band multi-polarization SAR data