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Study of synthetic aperture radar and automatic identification system for ship target detection

Courtesy: ScienceDirect
Sudhir Kumar Chaturvedi

Highlight
• In the past, the main goals of Synthetic Aperture Radar (SAR) systems were the study of the interaction of electromagnetic waves with the earth surface.

• Recently, the development of multi-channel SAR systems has enabled the development of more sophisticated techniques for the surveillance activities.

• SAR is the most efficient instrument, which provides high-resolution data for wide ocean area surveillance under all weather conditions. The intrinsic capability of this instrument is to provide a quick view of the oceanic surface features such as vessels, waves and currents, oil spills, laver facilities and wind fields.

• The ship detection/recognition is achieved in two steps: the first step is to identify the ship in SAR images of a busy traffic, which corresponds to Automatic Identification System (AIS) signals by the “dead-reckoning (DR) position”, and the second step is to estimate the position, size and speed of the ship from SAR images and compare these results with the AIS “true” data.

Abstract
In the past, the main goals of Synthetic Aperture Radar (SAR) systems were the study of the interaction of electromagnetic waves with the earth surface. Recently, the development of multi-channel SAR systems has enabled the development of more sophisticated techniques for the surveillance activities. SAR is the most efficient instrument, which provides high-resolution data for wide ocean area surveillance under all weather conditions. The intrinsic capability of this instrument is to provide a quick view of the oceanic surface features such as vessels, waves and currents, oil spills, laver facilities and wind fields. The ship detection or recognition is achieved in two steps: the first step is to identify the target in SAR images of a busy traffic, which corresponds to Automatic Identification System (AIS) signals by the “dead-reckoning (DR) position”, and the second step is to estimate the position, size and speed of the ship from SAR images and compare these results with the AIS “true” data. This paper presents the fundamentals of SAR and its integration with the AIS data for the ship target detection.

Keywords: SARAIS, Dead reckoning, Target detection

sar-ais
Figure 1. Left: Side looking SAR geometry and flight path, Right: SAR image consists of the hard targets (brighter pixels due to high backscattered response) and ocean background (dark pixels due to low backscattering response) [36].

1. Introduction
Ocean Environment Monitoring (OEM) has become one of the most important tasks now a day, which involves tracking and monitoring of illegal vessel activities, oil spills, retrieval of wave parameters, wind and current observations etc. Mapping and monitoring of these systems, features and activities require the wide area of imaging with high enough resolution. Many times the imagery must be acquired under various weather conditions or day as well as night. Space-borne remote sensing techniques have become the most cost effective research tools today for rapidly expanding ocean observation and surveillance tasks and requirements. The backscattering response of surface materials to illumination by microwave energy, also referred to a “backscattering signal”, is very different from spectral reflectance of the visible sunlight of the same material [36]. Synthetic Aperture Radar (SAR), an active microwave sensor onboard either space-borne or air-borne, illuminates the target with a focused, directional beam of energy, producing unique scattering effect depending on the orientation of the sensed objects [10].

High resolution SAR is able to provide images of the two dimensional information objects on to the earth surface. A good radar image depends upon a smooth variation in phase history over the data-gathering interval. The high resolution and large spatial coverage of SAR imaging systems offers a unique opportunity to derive the various oceanic features [3]. SAR systems take an advantage of the long-range propagation characteristics of radar signals and the complex information processing capability of modern digital electronics to provide high-resolution imagery [35]. SAR images are a relatively new data type requiring special interpretation techniques in order for users to utilize the data in most operational and reliable applications. These data have been widely used for fishing vessel detection, ship traffic monitoring, immigration control and other physical oceanographic applications 8, 47.

With increasing worldwide world travel and transport of goods, vessel traffic services, ship routing and monitoring of ship movements on sea and along coastlines have become the most important tasks of the coastal authorities. Most applications need the vessels to be detected and categorized as much detail as possible and afterwards the extracted information are transferred to the existing systems. Hence, fusion of information from different information systems is of great importance. For example, modern ships beyond a certain size are mostly equipped with so-called Automatic Identification Systems (AIS) and Voyage Data Recorder (VDR) systems. They identify themselves before leaving or entering a harbor and after having passed technical inspections by authorized entities [17]. The system is useful to make comparisons among different processing schemes and sensors, and to initialize the input parameters of data association and tracking algorithms, which fuses information from different sources to provide an integrated maritime surveillance picture of an area of interest [40]. The Vessel Detection System (VDS) relies on the polar-orbiting satellites carrying SAR which can be used for the detection of vessels at sea under most conditions-day and night and through clouds.

For the full article, visit ScienceDirect

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