MARITIME RADAR WITH ELECTRONICALLY CONTROLLED ARRAY ANTENNA
Due to the higher production and development costs, radar systems with electronically controlled antenna arrays are normally only used in a military environment. Thanks to a change in the legal requirements and technical progress, active antenna arrays can now also be introduced in the civilian navigation area. With the aim of keeping production costs as low as possible, Fraunhofer FHR recently completed the first active S-band antenna demonstrator for this application area.
Radar devices make an important contribution to safety in maritime transport. They support the crew in navigational tasks and warn of collisions with obstacles in dense traffic or while navigating in difficult waters. Due to the utilization of microwaves, they can also operate at night or in poor visibility. The continuous increase in the number of transports and ships has, however, led to a great increase in the demands placed on radar systems. In some situations, the higher traffic density requires a better resolution, particularly in the close range area. Most of the navigation and surveillance systems currently being used today do, however, work with an obsolete RF technology: they use a mechanically rotating antenna and signals are generated by magnetron tubes which do not permit the utilization of coherent signal processing techniques.
Two new developments indicate the emergence of a new trend: an amendment of the regulations for maritime navigation provides for the operation of S-band radar systems with reduced transmit power. This will pave the way for the utilization of semiconductor amplifiers and coherent signal processing techniques in the future. Ideally, the mechanically rotating antennas can be replaced by array antennas that offer electronic beam scanning. Up to now, the use of array systems in civil fields proved to be uneconomical due to the high costs for electronic components. However, thanks to ongoing technical development and the increased integration of components in application-specific integrated circuits (ASICs), transmit-receive modules can now be produced in a more cost-effective manner. The principle of phased array antennas will therefore become more attractive for civil radar applications. Coherent signal processing techniques and highly agile beam scanning enable the identification of a much larger number of objects with smaller dimensions and tracking with higher precision. This feature is not only suitable for standard navigation tasks but can also be used in other application areas, e.g. in the surveillance of port facilities, coastal areas and river sections, in the search for shipwrecked persons or to warn of floating obstacles that are difficult to identify, such as icebergs or lost containers. Since the active array antennas also remain operational with a certain number a defect antenna elements (graceful degradation) and the well-established magnetron tube is replaced by distributed RF power generation, significantly reduced maintenance is to be expected compared to conventional systems. Against the backdrop of the current and steadily increasing threat from pirates in waters throughout the world, this new technology can also give ships' crews a decisive head start.
A new concept for a maritime radar system with acceptable production costs is presented in a study commissioned by a German-based company. This prompted Fraunhofer FHR to build and test an active antenna demonstrator for the new concept. Innovations within the framework of this project include a patented serial RF feeding network, which operates without any measures for reducing the mutual coupling of the antenna elements, a special calibration strategy for the active antenna array, and last but not least the system’s modularity, which makes the installation of a single antenna at an exposed position with 360° field-of-view redundant. The Institute for Integrated Analogue Circuits (IAS) at RWTH Aachen also played an important role during the implementation of the study. IAS is responsible for the development of the highly integrated mixed-signal circuits on silicon-germanium (SiGe) basis that are used in the transmit/receive modules of the active antenna (Fig. 1).
The antenna demonstrator developed at Fraunhofer FHR is only equipped with a portion of the active antenna elements that would be necessary for an operational system. The reduced number of antenna elements is, however, fully adequate to demonstrate the most important properties. These include electronic beam scanning with low side lobes, and proof of stability with regard to the serial feeding network. Other points in the central focus of the investigations are the impact of interferences and the development of a cost and time efficient calibration strategy. The next steps will focus on the operation of the antenna front end with a radar system and testing under realistic conditions. The system will be developed into a commercial product in cooperation with industrial partners.