This year marks the 100th anniversary of the invention of the radar principle. In 1904, C. Hülsmeyer demonstrated the first experimental radar in Cologne, Germany. Later, in 1920, G. Marconi also observed the radio detection of targets in his experiments, but it was not until World War II that dynamic development of radar began. It has since evolved into an indispensable all-weather, long-range sensor. Therefore, this year provides a good opportunity to devote a special issue of this journal to research in the still exciting and rapidly developing field of radar. Military and security applications have always been the main drivers of radar development; however, since the 1950s radar has become a key sensor for civil applications including air, maritime and ground traffic control and the guidance of aircraft and vehicles on airport surfaces. Recently, radar sensors have also been developed for automobile traffic control systems beyond the well-known speed control. The classical radar task, according to its acronym RAdio Detection And Ranging, is to detect and locate (point) objects. With the advent of coherent pulse radar, velocity measurements have become possible by exploiting the Doppler effect. Today specialized radars measure elevation, aid in weather monitoring and help with target classification. Radar frequencies vary from HF/VHF/UHF, used for radars with ranges up to thousands of kilometres, to millimetre waves at frequencies up to View the MathML source. Optical radar, otherwise known as laser/light detection and ranging (LADAR or LIDAR), also uses the same echo principle. One of the main problems in radar technology has always been the high energy necessary for long-range detection due to the 1/R4 decay of the reflected power. An additional problem is the effective mitigation of echoes reverberating from the environment surrounding the radar (clutter). A major advance, leading to a new quality of radar, arrived with the invention of array of antennas. Firstly, this concept enabled distributed generation of RF power, over many transmit-receive (TR) modules, instead of centralised high-power RF generation. However, cost-effective TR modules remain a key challenge to radar array technology today. Secondly, the electronically steered array allows much faster redirection of radar energy than mechanical reflectors. Fast electronic switching of the radar beam enables optimal spatial distribution of the radar energy. Thus, longer dwell times in regions with strong interference can be realised, especially with adapted or optimal waveforms, leading to an approximately constant probability of detection. In addition, different radar tasks (e.g. search, target acquisition, target tracking, target feature extraction) can be performed in time multiplex. An important characteristic of a modern radar is multi-functional operation that comprises all of these features. A third advantage of array of antennas is that they provide, in principle, spatial samples of the received wavefront. This enables the use of different spatial processing techniques according to the application, in particular pattern shaping to suppress interference in certain directions or super-resolution methods for special target configurations. Finally, spatial and temporal processing can be varied data dependent resulting in space–time adaptive processing (STAP). STAP is one of the key topics in modern radar and most of the contributions to this issue are devoted thereto. The practical relevance of STAP lies in its ability to suppress clutter from airborne and space-based radar systems, where, depending on angle and range, clutter is spread in Doppler frequency. A number of papers in this special issue tackle the problem of clutter mitigation in the realistic situations where clutter is non-homogeneous and non-stationary, and when the training data contain outliers. By addressing this important problem, the radar community is making significant advances in adaptive signal processing. We believe that the special issue sheds further light on this intriguing problem. The question, however, still remains open as to whether an “ultimate adaptive filtering algorithm” can be conceived; perhaps, it will be a fusion of already existing algorithms including the ones presented in this issue. However, the exploitation of a priori knowledge (such as terrain maps, weather condition, etc.) will greatly help. Another issue is how to manage the new processing options when applied to multi-functional systems. Given the inherent lack of full a priori knowledge no method is uniformly optimal. New decision criteria must be formulated as to when to switch between adaptive and non-adaptive, classical and sophisticated techniques. In addition, standard detection and angle estimation procedures have to be modified for adaptive processing; papers addressing these problems are also included in this issue. The array antenna principle is also applicable in cases where the spatial samples are taken sequentially in time, e.g. during the flight of an aircraft. This leads to synthetically generated antenna apertures of very large dimension, resulting in extremely high resolution. Imaging radars that use such a synthetic aperture (SAR) differ greatly from real aperture radars. They have been developed since the 1970s as a new all-weather sensor for ground observation. The main goal of SAR is to achieve high resolution. Challenges to focus the synthetic array include mitigation of sensor position errors (e.g. by designing and applying auto-focusing techniques), and handling of large bandwidth, among others. Combining both types of arrays, i.e. using an antenna array as a sensor of the synthetic array (multi-channel SAR), results in new techniques to solve the focusing problem. In addition, multi-channel SAR offers new features, particularly the ability to detect moving targets (GMTI, ground moving target indication), and interferometric SAR, which can provide 3D images of the terrain. Recent interest in multi-channel SAR for airborne or space-based remote sensing, both for military and civilian applications, is the most dynamic developing radar technology. At the beginning of the new millennium the Space Shuttle Radar Topography Mission (SRTM) surveyed almost the entire inhabited landmass of Earth to generate a consistent and highly accurate topographic map of the Earth's surface in a single 11-day flight. The newest generation of remote sensing satellites, such as ENVISAT, RADARSAT-2 and TerraSAR-X will see the migration of active array technology into space. Various special conferences are devoted to the SAR topic, the most prominent being perhaps the series of EUSAR conferences. One paper in this special issue by Sikaneta and Chouinard gives a flavour of the techniques and problems arising in SAR signal processing.
|Autori:||A. FARINA; C.H. GIERULL; GINI F; U.R.O. NICKEL|
|Titolo:||Foreword. Special Issue on New trends and findings in antenna array processing for radar|
|Anno del prodotto:||2004|
|Appare nelle tipologie:||1.1 Articolo in rivista|