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Rising SAR

The latest synthetic aperture radar (SAR) capabilities—including new frequencies, new platforms and techniques, and more-accessible interpretation tools--are increasing the technology's value for security needs while also greatly expanding its usefulness in a host of other areas.

"SAR was not a very fast growth market for 20 years, but in the past few years it has accelerated," said Claude Rousseau, research director at Northern Sky Research (NSR). Rousseau attributes the speed-up to more competition among SAR providers, better processing of SAR imagery and greater comfort with SAR among potential users.

SAR has been too complex to use, but smarter processing has helped a lot, he added, noting, "That enabled awareness of what it can do and can't do."

On the supply side, manufacturers are providing more high-resolution SAR imagers, high-res images are available at more frequent revisit rates, and different resolutions and coverage are available. SAR technology has advanced with enhanced digital beam-forming antennas enabling wide swaths and high resolution, reducing prices per image and increasing counts of image layers.

"Price points were horrible, but now they are coming down," reported Prateep Basu, an NSR analyst.
The World Radio Conference (WRC) has doubled bandwidth for SAR, from 600 megahertz to 1.2 gigahertz, which allows higher-quality images and better analysis of imaged surfaces. "They can now go for small areas, using SAR to detect small vessels for maritime safety and identifying suspicious activities," Basu explained.

Many ships have corner reflectors, allowing SAR to measure length. The technique can also measure wakes and ship direction, track oil leaks and illegal bilging of tanks.

"With more layers of data for the environment, we will see land mapping, and it will get more precise on surface movements and unstable layers," Basu predicted.

The NSR analyst pointed to SAR's cloud-piercing abilities, noting that some frequencies can even see through tree canopies. For agricultural users, SAR can assess humidity and determine whether crops are dry.

The multiple images enabled by SAR interferometry and polarimetry are powerful for measuring displacement and subsidence. Images of the same location taken at different times can quantify shifts of fault lines after earthquakes. Interferometry analyzes changes while polarimetry works with horizontal and vertical pulses sent by and returned to SAR imagers.

Active SAR imaging generates massive files, and transferring these to end-users had been a challenge, often met by physical distribution. But cloud computing now supports file storage, processing and distribution.

"Apps in the cloud will be a major support for greater use," Basu said. "People are more comfortable with SAR, and they get data in hours or minutes."

Rousseau said he sees SAR, once used almost exclusively for defense, getting into enterprise markets. High resolution, frequent revisit, data analytics and the cloud as platform make that possible. But prices, though declining, are still an issue. "Cost per square kilometer is not as low as optical, but getting lower."
Experts say SAR will be used most extensively in cloud-dense regions, enabling users to get SAR surveillance faster than by waiting for optical to get clear-day shots. So SAR may often complement rather than replace optical tools.

It's still tough to reduce SAR's footprint, as firms have tried to do for years. The active power array and other features present multiple challenges in minimizing SAR. Radars and lenses are now the major constraints on further reduction, Basu said, adding that four planned NovaSAR satellites, weighing 250 to 300 kilograms, will be the smallest satellites yet when they fly.

Bandwidth for Color
One company with high expectations for SAR is Airbus Defense and Space, which sees the WRC decision to expand bandwidth for SAR opening the door to an unprecedented image resolution quality in color of future SAR satellites.
Alexander Kaptein, head of future SAR programs at Airbus Defence and Space, said expansion of bandwidth for earth observation by X-Band SAR satellites to 1.2 GHz will give a clear boost to commercial SAR. "This will allow resolutions better than 25 centimeters and colorful imagery using multi-polarized data."
The new images will be 16 times better than standard 1-meter data used today, and closer to what users are accustomed to with optical images.
Airbus has been working for several years on its next generation of SAR satellites to follow TerraSAR-X and TanDEM-X. The next-gen constellation will be funded by Airbus as well as participating governments and private partners.
No date has been set yet for launch of next-gen SAR satellites, but the company will ensure business continuity, Kaptein said. Similarly, the number of satellites has not been fixed. As Airbus is planning on high-quality and high-resolution radar, power needs will be substantial, so satellite sizes will remain similar those previously launched.
While resolution will be better than 25 by 25 centimeters, Kaptein cautions that resolution is only one improvement. Image quality will also dramatically increase due to noise reduction. And the images will be in color, which he predicts will disrupt radar imagery.
The new constellation will operate between 500 and 800 km, likely in polar orbits in the same orbit tube Airbus has used before. "We want to maintain the constellation principle and allow interferometry," Kaptein said, while adding that Airbus will evaluate other orbits for special mid-latitude or equatorial monitoring.
High-resolution images with quadruple polarization will enable colorized images and better image interpretation, leading to improved recognition and classification of objects and infrastructure. "This will be possible even for larger areas of interest, such as military airbases, harbors and mining locations," he explained.
Better images also enable better monitoring of smaller vessels and suspicious activities at sea, especially piracy and smuggling. In addition, colorized images from polarimetry will improve environmental applications such as monitoring land cover, forestry and oil spills and mapping land use.
Airbus is targeting all these and other applications. The new satellites will measure geolocation in two dimensions, model elevation in three dimensions and detect surface movements in four dimensions.
"The new sensor generation will allow monitoring of single buildings or bridge dynamics and resulting stress thanks to unprecedented resolution and best-in-class geolocation accuracy," Kaptein said. "Satellite geodesy will become operational reality."
The company is also aiming for ground moving target identification with new processing approaches.
Still, no single sensor can solve all of the issues people are facing, Kaptein acknowledged. But he predicted that Airbus's smart combination of radar and optical satellites provides customers what they need, when they need it, in wide coverage, fine detail, intensive monitoring, reliable collection and extensive archives.
Airbus already offers the largest earth-observation satellite fleet commercially available. Its optical satellites comprise three twins: the very-high-resolution Pléiades and high-resolution SPOT 6 and SPOT 7, which share Pléiades' orbit, guaranteeing daily revisits worldwide. Weather-independent radar satellites TerraSAR-X and TanDEM-X will be complemented by Spain's PAZ in 2016. And Airbus has access to data from other firms' satellites such as DMC or KazEOSat-1.
Next-Generation Constellation
MacDonald, Dettwiler and Associates (MDA), meanwhile, is moving into next-generation SAR with a 2018 constellation for the Canadian government. Under contract with the Canadian Space Agency since January 2013, the RADARSAT Constellation Mission (RCM) will consist of three new RADARSAT satellites, orbiting at 586 to 615 km, well below their predecessor RADARSAT-2's 798 km.
"There's lots of activity in earth observation, not just SAR specifically," observed David Belton, MDA vice president of geospatial services. "There's a wealth of new satellite operators and new mission concepts, the bulk in optical, but also in SAR."
SAR satellites tend to be large, since SAR is power-hungry for projecting and receiving pulses, and this drives significant mass. There are ways to reduce weight, Belton said, but not to the minimum levels reached by optical satellites.
The new-generation RADARSATs will be significantly smaller than RADARSAT-2, with radar compacted into a smaller space to reduce launch costs without sacrificing image quality. At launch, RCM satellites will weigh 1,400 kg, more than a third less than RADARSAT-2. The radar alone will weigh 400 kg, down from 750 kg in RADARSAT-2, and antenna length at 6.75 meters is less than half that of previous antennae.
RCM mostly responds to Canada's desire for more frequent revisits and coverage of broader areas. Consistent and reliable matching of these goals is not possible with a single satellite. Certain applications require returning to locations in days or weeks. RCM will enable daily revisits to 90 percent of the earth's surface.
Defense, security, resource monitoring and maritime surveillance will be the primary missions of RCM. Although built for the Canadian government, the constellation will eventually serve commercial customers, many of which MDA already serves with RADARSAT-2.
A new feature will be compact polarimetry, which is a shortcut to obtain data similar to that of quad-polarimetry. This will yield radar images analogous to multi-spectral optical sensors that can see different elements of changes on the ground.
RCM resolution will be similar to RADARSAT-2, with the highest in spotlight mode at 1 by 3 meters. Broader coverage decreases resolution. RCM's largest coverage swathe will be 450 km. "There's a tradeoff of resolution versus coverage," Belton noted.
The new constellation has a mission life of seven years. MDA is already looking ahead of RCM, because it is important to plan for data continuity, as well as coverage, in SAR.
The obvious SAR advantage over optical is its ability to see through clouds, and some regions cannot be observed regularly without cloud cover. Another important driver is vessel detection in maritime surveillance, as optical cameras are best suited to look at local points, while SAR can observe large areas.
Apart from putting up satellites and providing images, MDA also offers value-added satellite services in geospatial markets, and this latter business is growing fast. The company is rolling out new applications to serve new markets.
MDA is now looking at monitoring illegal fisheries, an increasingly important function. It also wants to support ship navigation through ice as polar ice retreats and shipping lanes open up. RCM will offer four passes per day in Canada's far north and several passes per day over the Northwest Passage. MDA has already rolled out a service that detects deforestation in areas so consistently cloudy that optical sensors cannot do the job.
MDA has long supported the oil and gas industry, receiving three new oil-and gas contracts early this year. But the market is troubled by low oil prices, so MDA is looking at the mining industry, for which SAR can detect small changes in vertical elevation. Such detection can spot subsidence and ground heaves, which can represent significant risk factors for the environment and personnel safety in mining.
Belton stressed that MDA handles all SAR pieces, designing satellites, defining missions, building satellites, running ground stations, providing images and interpreting them.
RADARSAT satellites have the broadest areas of acquisition, and MDA has 30 direct-access stations around the globe that it downloads image data to. "We can process data in minutes and give near real-time data to customers," said Belton.
Low-Cost Approach
Surrey Satellite Technology Ltd. (SSTL) is working to bring the price point of space systems down to encourage wider use, according to Andrew Cawthorne, head of earth observation for the company. Gallium nitride amplifier technology has been a key enabler of low-cost SAR, and combining it with SSTL's low-cost construction of satellite platforms has led to development of a low-cost S-band SAR satellite.
NovaSAR, due for launch later this year, is the first application of the low-cost approach. SSTL is talking to several potential mission partners about the NovaSAR constellation, with both low costs and unparalleled revisit times.
Instead of an expensive satellite that does everything everybody wants, NovaSAR targets and is designed for a few applications. It will be perfectly tuned for maritime surveillance, forestry, flooding and a few other uses. It also has a distinctive maritime surveillance mode for very wide-area ship detection.
SAR images are not really colored like optical images, Cawthorne explained, but different polarizations can create pseudo-color images that are easier to interpret. The future will see more such colorized SAR images as people learn to exploit SAR more fully, he predicted.
Another player in this field is UrtheCast, which plans to launch a constellation of at least eight SAR and eight optical satellites by 2020. There will be two orbital planes, each with four satellite pairs. Dual-mode, high-resolution optical satellites will fly in tandem with dual-band high-resolution SAR satellites. Processing will be real-time onboard, satellites will cross-cue, and real-time cloud imaging by leading SAR satellites will enable cloud avoidance by trailing optical satellites. Two orbital planes will maximize revisit rates in mid-latitudes, while still yielding global coverage including poles.
SSTL will build the optical instruments and spacecraft, while UrtheCast provides SAR payloads. Flying in pairs, SAR will lead optical satellites by a minute or two. Having optical and SAR imaging at virtually the same time and look angle will enable an image with high-resolution optical, video, X-band and L-band SAR taken at the same time. This will revolutionize what can be done with the data, Cawthorne predicted.
Fast-Response Mapping
E-GEOS's COSMO-SkyMed constellation has four identical SAR satellites and provides revisit capabilities at least four times a day all over the world.
"SAR has day, night and all-weather imaging capability that guarantees use by customers," noted CEO Marcello Maranesi. "With such capability, services such as maritime surveillance or fast-response mapping are possible."
The future will see an increase in applications based on specific capabilities of SAR, Maranesi predicted. While SAR data can be regarded as images and photo-interpreted like optical images, other capabilities and requirements are also involved. "SAR data has much more information content that needs to be extracted by specific algorithms and application platforms."
Future applications will include monitoring of terrain displacement, transport and energy infrastructure, he said. SAR will also be used for detecting changes in human activity in both rural and urban areas. These capabilities will require not only next-gen SAR images but development of new applications.
Exploiting these possibilities requires not only understanding SAR, but also experience with the phenomena being observed and how they correlate with backscatter from SAR. "Education and training of many more people and more automated application platforms will be necessary to boost use of SAR data," Maranesi said.
E-GEOS itself has made substantial investments in research and development over the last decade to prepare for exploitation of its COSMO-SkyMed Constellation. Funds have come from e-GEOS and its European Union partners. The company has been working closely with potential end-users and learning about internal workflows at these users.
This collaboration with customers has helped developed chains for processing SAR data that are specialized for each product in each market segment. E-GEOS can now provide customized support for maritime surveillance, rapid mapping, change detection and monitoring agriculture, urban areas and infrastructure, many of which are part of the European Copernicus program.

Last modified on Thursday, 12 May 2016 14:10

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  • Issue: 7
  • Volume: 13
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