Canada's RADARSAT: More Than Just a Cure For Rainy Day Blues
By Kevin P. Corbley

The good news for remote sensing customers is that the days of waiting for clear skies to acquire imagery will end in September. That's when Canada will launch radarsat, the first commercial Earth observation satellite equipped with synthetic aperture radar (SAR). And even better news is that users will be pleasantly surprised to discover that radarsat is more than just a rainy-day substitute for Landsat and SPOT.
      "Guaranteed image acquisition regardless of cloud cover is definitely radar's primary selling point, but that is only a small part of what radarsat will do for users," said Robert Tack, president of radarsat International (RSI), the Vancouver firm responsible for commercializing Canada's remote sensing industry. Tack explained that although SAR imagery makes an excellent complement to optical image data, it is also a valuable stand-alone data source because the information it contains is substantially different from Landsat or SPOT. "Customers will choose to purchase radarsat data based on their application requirements, not just the weather."
      More than 20 years of airborne SAR research and the recent practical uses of imagery collected by the experimental Japanese, Russian, and European radar satellites have demonstrated many advantages of SAR data in a variety of commercial disciplines, including ocean monitoring, natural resource management, and mineral exploration.
      As a program developed jointly by Canadian industry and federal and provincial governments, radarsat was conceived primarily for applications in Canada's high-latitude regions, but utility of the data is not limited to any geographic area. Imagery acquired at the equator will be every bit as valuable as that acquired over the poles.
      Nor will its availability be confined geographically. radarsat data will be commercially available to remote sensing customers around the world through many of the same distributors and ground stations that now market Landsat and SPOT. Marketing and distribution of the data will be directed by RSI, a partnership of Spar Aerospace, MacDonald Dettwiler & Associates, and COM DEV Ltd. In 1994, Lockheed Martin Astronautics joined as an investor and operational partner.
      Despite years of experimental radar studies, participants in the radarsat program are keenly aware that education and training in commercial radar applications have been lacking. For the past three years, RSI, Canada Centre for Remote Sensing (CCRS) and the Canadian Space Agency (CSA) have conducted application projects throughout the world using simulated radar data to learn - and teach - exactly what radar can do for end users.
      These projects have provided a greater understanding of those applications for which radar has long been known to offer significant benefits: coastal monitoring, sea ice detection, hydrology, soil analysis, and geologic mapping. In addition to these mainstay radar applications, demonstration projects are revealing the advantages of using radar in disciplines traditionally dominated by optical sensors, such as topographic mapping, agriculture, forestry, and land use planning.

The SAR Advantage
Day-or-night, all-weather imaging is the feature most often associated with radar systems. Unlike the passive optical systems that rely on sunlight to measure energy reflection from the ground, the radarsat SAR is an active system that transmits its own energy signal to the ground. Day or night, this microwave energy bounces off the Earth's surface, a portion of which returns to the sensor. The strength of the returned signal is recorded and later processed into imagery.
      Night-time imaging is critical in polar regions where optical sensor acquisitions for sea-ice navigation, mineral exploration, and environmental monitoring must cease during several months of darkness. radarsat's SAR, however, will allow for year-round polar imaging.
      Another of radarsat's most anticipated benefits is all-weather imaging. The long wavelength of its radar signal permits image acquisition regardless of clouds, fog haze, dust or rain. SAR lets geologists and foresters gaze into the cloudy tropical regions near the Earth's equator to search for minerals and track deforestation. For the first time, oceanographers will have unlimited access to the many coastal regions often shrouded in fog.
      Geologists, civil engineers and other users who map underground structures will benefit from radarsat's ability to acquire imagery on both its ascending and descending orbits. This means the satellite can acquire images of a land feature from two different look directions, either east or west. Three-dimensional terrain information - a necessity in many civil engineering and geologic applications - can be extracted from stereo images acquired in this manner or using same-side images with different incident angles, said Nazarenko.
      The C-Band wavelength of radarsat's SAR will permit limited penetration of some land cover features such as tree canopies and sand. In test projects, C-band microwave has been successfully used to analyze geologic structure in vegetated areas and is helping resource managers find underground streams in the desert.
      Another prominent radar characteristic, which users may not be familiar with, is its black & white imagery, said Dennis Nazarenko, RSI's manager of applications solutions. Image interpretation with radar differs from that of optical imagery, but the theory behind it is basically the same.
      In optical imaging, the characteristics of surface features affect the reflection of energy in many wavelengths, which are displayed in the image as a series of colors. Radar signals interact with ground features based on their size, shape, surface roughness, moisture content and other physical conditions. However, radar operates in only one wavelength, and instead, the interaction that occurs between the signal and surface conditions is represented by the tone, texture and density of the black & white image created from the return signal.
      "Users will learn to interpret tone, texture and density in radar just as they now interpret red, green and blue in optical images," said Nazarenko.
       Color can be used in radar imagery analysis. When images acquired on different dates are combined and compared for change detection, each image can be assigned a different color so that change areas are easily identified.

The Advantages of RADARSAT
A feature unique to radarsat is its adjustable beam modes. At the request of customers, ground controllers can program the satellite to operate in one of seven beam modes, which means the signal can be focused narrowly to acquire high-resolution imagery of a small area, or diffused for broad area coverage at lower resolution. The modes vary from 10 meter resolution in a 50 x 50 kilometer (km) swath to 100 meter resolution in a 500 x 500 km scene.
      "Users have the option to customize the parameters of their imagery based on their application needs," said Nazarenko. "Ocean monitoring usually requires wide-area coverage with relatively low resolution, while many agricultural and forestry applications need to see detail in small areas."
      Scene size affects the repeat cycle. For wide-area imaging, radarsat will provide repeat coverage every four days at the equator and less than 24 hours at the poles - ideal for monitoring rapidly changing situations such as the build-up and drifting of sea ice in arctic shipping channels.
      Another significant feature is radarsat's adjustable incidence angle. The SAR beam, which always transmits at a right angle to the satellite path, can be adjusted by controllers to provide a variety of incidence angles between 10 and 60 degrees.
      Users will select an incidence angle based on their application requirements and ground topography, said Nazarenko. In general, shallow angles (i.e. 10 degrees) are best at detecting subtle land features in areas of low topographic relief, while a steep angle (i.e. 60 degrees) is needed to image in the valleys of very mountainous terrain.
      An interesting application of the adjustable incidence angle will be a process called 'radargrammetry.' Similar to photogrammetry, this technique will be used to extract elevation information from two images taken over the same area from different angles. Geologists often use this elevation data to interpret underground structures.
      Another feature of radarsat is its HH polarization; it has been designed to transmit and receive a horizontally polarized signal. This will benefit ocean application users. The HH polarization is less sensitive to interference from slight wind disturbances on the ocean surface.
      "The HH polarization will allow radarsat to monitor sea ice even in very rough seas," said Nazarenko, adding that radarsat will also very accurately delineate land-water boundaries.
      Radarsat's designers also chose to equip the satellite with two in-flight data recorders. Consequently, users in all parts of the world, regardless of their proximity to a ground receiving station, will be able to obtain images of their area.
      Users should be equally pleased with the radarsat ground segment. Due to the satellite's role in detecting rapidly changing sea ice conditions for ship navigation, quick data turnaround is a priority. radarsat facilities at the Gatineau and Prince Albert stations have been designed to process data in as little as four hours after reception. And orders for critical acquisitions will require only about seven day's lead time. Genuine emergency acquisitions can be handled with even less lead time.

Simulations and Pilots
RSI, CSA and CCRS have spearheaded several SAR education and training programs around the world to prepare markets for radarsat data. The most high-profile radarsat education program is the four-year, $7 million GlobeSAR program that involves more than 60 projects in 13 Asian, African, and Middle Eastern countries. During the GlobeSAR mission, SAR data was acquired by aircraft and used to simulate radarsat imagery.
      GlobeSAR data has been used for a variety of natural resource management projects: detecting land use changes in Malaysia, tracking rice growth in China, harvesting water in Jordan, exploring for minerals in Vietnam, and protecting mangrove forests in Thailand.
      In China, GlobeSAR researchers took advantage of radar's sensitivity to moisture content and frequent revisit cycle to track the stages of rice growth in flooded paddies. Based on the crop's appearance in the airborne SAR imagery, Chinese researchers were able to identify the various stages of crop growth.
      A Jordanian GlobeSAR project involved harvesting of a different kind - water. Using the radar, project researchers detected wadis, or seasonal river beds, buried beneath desert sands. By placing dams across these wadis, they expect to inhibit water runoff after unexpected rains and hold the water long enough for it to recharge the aquifer.
      Coastal and ocean monitoring is one of radarsat's key application areas. RSI has teamed with Seaconsult Marine Research of Vancouver, B.C., Canada, to study the uses of SAR for ship detection, oil spill identification, and ocean current monitoring. Imagery from the European ERS-1 radar satellite and the CCRS Convair aircraft was acquired over the Georgia Strait off the coast of Vancouver.
      "With a steep incidence angle, we were able to detect ships in the harbor by their wake turbulence in the water," explained Nazarenko. "When the water was choppy, we could still find ships by changing to a shallow incidence angle, which illuminated the ship itself."
      Ship detection will be a major feature of radarsat's arctic sea ice monitoring program. It will also serve Canada in its efforts to protect fishing rights in its coastal waters.
      Success in ship detection and oil slick monitoring had been expected in the pilot, but the researchers were surprised at SAR's ability to detect internal waves, thermal fronts and current shear zones beneath the water's surface. Identification of these features will have application to oil rig safety and underwater construction projects.
      CCRS geologists have conducted exhaustive studies with airborne radar for structural mapping in the mineral-rich Sudbury Basin in north-central Ontario and other parts of the Canadian shield. Extensive geologic mapping was completed there during more than a century of mineral exploration and served as the baseline data for the experiment.
      The Sudbury project, conducted with several image data sets, confirmed the ability of SAR to penetrate vegetative cover and reveal underlying rock. The imagery successfully identified all known dikes, lithologic contacts and major structures in the area. Structures running perpendicular to the SAR's east or west look direction were easier to detect in the radar imagery than in Landsat Thematic Mapper data.
      Dr. Vern Singhroy, a senior research scientist at CCRS, said the most interesting results from the Sudbury projects were in merging SAR with Landsat and magnetic survey data. Together, these three data sets could provide geologists with nearly all the information necessary for a mineral exploration project.
      The Landsat data provided the vegetation and land cover information required for environmental protection, and the magnetic data helped the scientists to interpret the subsurface geology expressed in the radar imagery, explained Singhroy.

Ordering Radarsat Data
Commercial customers may order radarsat data either directly through RSI's Client Services office in Vancouver, or from the worldwide network of radarsat distributors and ground stations. RSI Client Services will assist clients in scheduling acquisitions and searching the data archive. Applications Solutions specialists will help clients determine which beam modes, incidence angles and scene size best meet the requirements of specific applications.
      RSI will offer numerous radarsat products based on combinations of the beam modes, positions, and six processing levels, which range from raw to geocoded. RSI also plans to offer radarsat data merged with Landsat or SPOT.
      Clients have a choice of how quickly they would like their data processed. Standard processing time is two weeks or less after acquisition, and with an additional surcharge, clients can have their data processed within two days after acquisition. Digital products will be available on 9-track tapes, 8mm data cartridges and CD-ROM. Film and print products are available by request. Standard delivery is by courier, but RSI will distribute data electronically on high-speed data lines. Electronic delivery must be arranged in advance with Client Services.

About the Author:
Kevin Corbley is the principal in Corbley Communications, which specializes in providing PR and marketing services to remote sensing, GIS and GPS companies. He is located in Lakewood, Colo., and may be reached at 303-987-3979.

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