Kevin Rokosh Canada’s world-leading spaceborne radars give The Engineer’s Copywriter Maria Rey Space Strategies Consulting Ltd. planners and decision makers a unique capability Director General of Defence Research and Development Canada, Ottawa Lab. (retired) to monitor our ocean territories, aid in resource David G. Michelson University of British Columbia development and assess humanitarian needs. nytime, ground, the quality and utility of the azimuthal or such images suffer greatly. “along-track” res- anywhere – Radars provide their own olution of SAR illumination and operate at images is indepen- Earth Imaging using wavelengths that easily penetrate dent of range, yield- Spaceborne Radar clouds, fog and smoke, but con- ing the possibility of Since the dawn of the space age, ventional side looking radars creating photograph- orbiting satellites have provided lack the resolution required to like images with high Azimuth planners and decision makers generate useful radar images of resolution. with a unique vantage point the Earth’s surface from orbit. Range from which to routinely observe Canada has made the Earth and gather informa- The advent of Synthetic Aperture significant contribu- tion for applications as diverse Radars mounted on fast moving tions to SAR research, In Synthetic Aperture Radar (SAR), resolution in the as weather forecasting, crop airborne platforms introduced development and azimuth direction is independent of range. SAR was assessment, strategic surveil- the first practical way to generate application during the first developed for airborne application. lance, and geology. Until the high resolution two-dimensional past 40 years includ- late 1970s, however, virtually radar images of the land below.1 ing development of fundamen- international customers. Since all Earth observation from As in conventional radar images, tal SAR image processing tech- the advent of Canada’s RADARSAT space was based on optical the intensity of individual pixels niques and providing SAR program in the mid 1990s, images produced from reflected corresponds to the radar reflec- image processors and systems Canada has played a leading role light in the visible and infrared. tivity of the scene. Unlike con- engineering expertise to major in demonstrating the routine use ventional radar images, however, U.S., European, Asian and other of SAR imagery in both public While optical imagery from and private sector applications. space has proven to be extremely 1 Polarimetric and interferometric SARs go a step beyond conventional SARs and allow special types useful, producing it requires of multi-channel or multi-image SAR data to be processed to yield image products useful in the Today, Canadian decision mak- both adequate illumination and identification of targets, interpretation of target structure, observation of fine ground movement and ers benefit from a wide range of clear skies. At night, or when production of terrain elevation maps. In this manner, the utility of advanced SAR imagery can Canadian-produced SAR image clouds, fog or smoke obscure the fundamentally surpass (and complement) conventional optical imagery. products. Marine surveillance and 10 Spring / Printemps 2016 ice-mapping have become routine width using simple beam- SAR MAKES SPACEBORNE RADAR POSSIBLE applications. Geotechnical map- forming techniques is not suffi- ping and environmental monitor- cient. A simple geometric con- ing are becoming increasingly struction reveals that the width important. Humanitarian disaster of the pixels in the radar image relief is a less common applica- will increase linearly with the A long tion — thankfully — but is of range to the target. Satellite antenna is Effective immense benefit when needed. orbits increase that range to synthesized by recording the antenna hundreds of kilometres and reflected pulses lengths of fatally degrade the azimuth received by the SAR several kilo- The “Magic” of SAR as it moves along its metres are resolution. A new approach is possible in this required. orbital track, and (Synthetic Aperture Radar) combining the result using way, required due appropriate techniques. to satellite altitudes. The linear path traversed by the Synthetic aperture radar is simplest form of SAR as it moves based upon the observation that along the scene is called the the energy from a given point track. Resolution in range or the target is spread in both range cross track direction – the direc- and azimuth. In SAR image SAR creates photograph-like images tion in which the radar antenna reconstruction, this dispersed points – is determined by the energy is collected or focused ake Superior is easily rec- ered. The SAR was operating effective duration of the radar into a single pixel in the output Lognized in this RADARSAT-1 in ScanSAR Wide mode, giving pulse. In this respect, SAR func- image using advanced signal image. Taken in late winter, the a swath width of 500 km. tions in the same manner as con- processing techniques. The lake is almost entirely ice cov- ventional radar and conventional underlying principle also forms pulse compression techniques can the basis for the well known be used to improve resolution. A medical diagnostic tool com- monostatic SAR uses the same monly called CAT Scan, short antenna for transmission and for Computer Aided reception and is sensitive to the Tomography, that yields high diffuse waves reflected back resolution images of the body from the target (backscatter using low power X-rays. In radar response). In bistatic SARs, the application, the result is also a transmitting and receiving anten- high-resolution image. But RADARSAT Data © RADARSAT Canadian Space Agency (CSA) 1997 nas are separate.2 unlike conventional side-look- n enlargement of the north ing radar, the azimuth resolu- shore near the east end of Resolution in the azimuthal or tion is independent of range. A along track direction is determined the lake. RADARSAT-1 operated by the radar antenna’s length. It is Such fine and uniform resolu- at an altitude of about 800 km. In well known that an antenna’s tion in both the azimuth and ScanSAR wide mode, the reso- beamwidth becomes finer as the range directions means that a lution of pixels in the azimuth antenna lengthens. There are limits SAR can create photograph-like direction was about 100 m. to how long a physical antenna can images of the landscape, as seen satellite equipped with be realized given the many in the RADARSAT-1 image in Aconventional real aperature constraints imposed upon satellite the sidebar to the right. This is radar operating at the same alti- payloads. However, it is possible the magic of SAR. However, tude would yield a resolution in to synthesize an arbitrarily long because the location of a point the azimuth direction in excess antenna by recording the signal target in a SAR image is based of 5 km. Very little useful infor- received by the SAR as it moves upon the time of flight, not the mation would be obtainable. along its orbital track and line of sight as in an optical combining the result using image, SAR images suffer from rocessing of the appropriate techniques. a characteristic distortion in PRADARSAT-1 image using which higher terrain appears to calibration software and a However, a side-looking radar be folded towards the observer. library of backscatter signatures that simply synthesizes a large This is most apparent in SAR allows classification of ice antenna with a very fine beam- images of mountainous terrain. coverage. Image created by George Leshkevich and Son 2 They may even be carried by different platforms, either to permit characterization of forward scatter Nghiem, 2007 [REF: 1] from the target, to allow a secondary receiver to take advantage of existing transmitter in a parasitic Consolidated ice flows Brash ice Stratified ice Calm water mode of operation, or to make it difficult for a third party to detect the receiver. Spring / Printemps 2016 11 RADARSAT Operating Modes Stripmap Mode ScanSAR Mode Spotlight Mode he basic SAR configuration modes in order to enhance certain than is possible in conventional In Spotlight SAR, the radar antenna in which the radar antenna is aspects of SAR performance. SAR systems. Canada’s is electronically steered in order to T pointed broadside to the RADARSAT-2 ScanSAR uses two keep the target in view for a longer direction of travel permits formation In ScanSAR, as orginally developed, or more physical beams operated period. Because this extends the of a continuous image of the ground. the radar antenna is electronically sequentially, each producing size of the synthetic aperture, it This is called Stripmap SAR. steered to different angles in order separate image blocks that are increases the azimuthal resolution Synthetic aperture radars can be to illuminate multiple swaths. This then electronically merged of the image but at the expense of configured to operate in other permits a wider area to be imaged together. spatial coverage. A Brief History of SAR Communications Research early 1990s. The total develop- Centre (CRC) and The Defence RADARSAT ment contract was about $620 The synthetic antenna concept Research Establishment Ottawa In 1990 Canada created the million (almost $1 billion in was first successfully applied to (now DRDC Ottawa). These Canadian Space Agency (CSA) today’s dollars), and the CSA radar by Carl Wiley at the algorithms were shared with within the Department of owned and operated the final Goodyear Aircraft Corp. in 1953. Canadian industry in keeping Industry. The RADARSAT-1 system. The major Canadian He used an aircraft for the mov- with the commercialization man- project was not only its first industrial team members who ing antenna platform and a stor- date of the CRC dating back to major project, but also the designed and built RADARSAT-1 age tube display that was config- its creation in 1969.3 world’s first operational civil- for the CSA included ured to mimic a conventional ian SAR satellite mission.