Remote sensing is the science of acquiring information about the Earth’s surface without actually being in contact with it. This is done by sensing and recording reflected or emitted energy and processing, analyzing, and applying that information. Unlike optical satellites that capture reflected sunlight to produce detailed photos of Earth, synthetic aperture radar (SAR) satellites bounce radar signals off the ground and record the reflections to create images. This allows radar satellites to collect imagery day or night, regardless of cloud cover.
A number of earth-observing radar satellites, such as RADARSAT, have employed synthetic aperture radar (SAR) to obtain terrain and land-cover information about the Earth. Synthetic Aperture Radar (SAR) systems take advantage of the long-range propagation characteristics of radar signals and the complex information processing capability of modern digital electronics to provide high resolution imagery.
Synthetic Aperture Radar (SAR) complements photographic and other optical imaging capabilities because it is not limited by the time of day or atmospheric conditions and because of the unique responses of terrain and cultural targets to radar frequencies. Compared with traditional optical imaging, SAR imaging provides more details about the surface of the Earth because of the way in which SAR signals interact with particular surfaces (e.g. buildings, trees, mountains, lakes, etc.). Optical imaging is similar to taking a picture of the Earth, whereas SAR imaging is more similar to measuring the topography of the Earth. Moreover, satellite as a platform facilitates observation over wide swathes and global coverage of remote areas, which cannot be achieved with aircrafts or unmanned aerial vehicles (UAVs).
SAR can also “see” through other types of cover such as smoke, vegetation, snow, or sand, depending on the satellite’s designated operating band (which indicates the sensor’s associated frequency and wavelength). SAR bands are helpful in categorizing the penetration strength and thus the potential applications of a satellite, such as Germany’s TanDEM-X (low-penetration X-band), Canada’s RCM (moderate-penetration C-band), and Japan’s ALOS-2 (high-penetration L-band).
Satellites with Synthetic Apeture Radar (SAR) orbit the Earth in a sun-synchronous LEO polar orbit and data acquisitions can be made at any time of day or night and independent of cloud coverage, collecting both amplitude and phase data. Most SAR satellites operate in a sun-synchronous orbit (SSO), a type of near-polar orbit. The orbit’s SSO geometry is kept nearly fixed with respect to the sun, so satellites in SSO are synchronized to consistently remain in the same position relative to the sun. That is, the satellite visits the same spot at almost the same time twice daily, for example, at 12:00/00:00 every day. Thanks to these features, SSO provides global coverage at all latitudes (except for just a few degrees from the poles), and enables constant observation at the same local time. Each SSO can be identified by its mean local time (MLT).
The SAR satellites have repeating paths which, using two-phase datasets for the same location at different times, allows for interferometric SAR (InSAR) showing relative ground displacements between the two datasets along the direction of the radar beam.
Radar satellites have traditionally been big, expensive beasts, but technology developments are shrinking the size and cost of these platforms. A new trend in SAR technology arrived in 2018, however, with multiple launches of inexpensive small satellites (under 500 kg) carrying fully functional SAR sensors. Micro- and nano-satellites are attractive due to the low development and launching costs. Carried by micro- and nano-satellites, synthetic aperture radars (SARs) have great potential for urban, oceanography, land use, and agriculture usages.
However, the observation opportunities offered by current SAR satellite systems are insufficient from the perspective of rapid flood monitoring in the context of disaster management. In the context of flood monitoring, the disaster management community requires flood extent information with little latency and frequent updating, but inadequate satellite revisit time is one issue preventing the collection of this information. The dynamic flood process, which may both expand and shrink, is difficult to track within the limited time available.
Micro- and nano-satellite SAR constellations
Constellations of small satellites equipped with synthetic aperture radar (SAR) payloads can realize observations in short time intervals independently from daylight and weather conditions and this technology is now in the early stages of development.
Small satellites have attracted considerable attention due to their wide-ranging applicability, supported by technological advancements in space engineering over the last decade. These satellites cost significantly less than conventional large satellites, and this enables ‘constellation’ operation involving several satellites; by working in concert, groups of satellites have an excellent capacity for high revisit rates and short revisit time.
However, the miniaturization of SAR satellites has been slow compared to that of optical satellites due to the challenging design requirements of small SAR satellites, such as larger antennae and higher power throughputs. In recent years, however, the miniaturization of electronic components and recent technological advances have finally ensured the compatibility of SAR with small platforms.
Satellite Constellation growth
Since Finland’s Iceye proved in early 2018 that a small satellite can gather radar data and imagery, SAR startups have raised hundreds of millions of dollars for constellations. San-Francisco-based Capella Space, Iceye, Japan’s iQPS, PredaSAR of Boca Raton, Florida, Japan’s Synspective and Umbra Lab of Santa Barbara, California, have different business models and target audiences but all anticipate growing demand for SAR data.
The 2018 list of SAR launches includes NovaSAR-1 by Surrey Satellite Technology Ltd. of the U.K. in September, ICEYE-X2 by ICEYE Oy of Finland and Capella 2 by Capella Space of the U.S., both in December. At 450 kg, NovaSAR-1 is considered a smallsat, while the Finnish and American missions fall into the microsatellite class, at less than 100 kg each.
In 2020, Finnish New Space company ICEYE unveiled its latest capability of 25 cm resolution imaging with synthetic-aperture radar (SAR) small satellites (smallsats), utilizing the company’s current commercial SAR satellite constellation in-orbit. With this high-resolution imaging capability, ICEYE SAR data achieve the same resolution class provided by larger, conventional commercial SAR satellites operating at their highest performance. Following standard industry definitions, the native slant plane resolution of the SAR data is 25 cm in the azimuth direction, and 50 cm in the range direction, before ground-plane adjustments are applied.
Current SAR systems are capable of operating in different imaging modes by controlling the antenna radiation pattern, which practically results in different combinations of the swath width and resolution. The most fundamental mode is the Stripmap mode, where the ground swath is illuminated with a continuous sequence of pulses while the antenna beam is fixed in its orientation, thus imaging a long strip parallel to the flight direction.
For a better azimuth resolution, the Spotlight mode is utilized, but this operation is usually at the expense of spatial coverage (1-m resolution and 5 × 5 km2 scene in the reference SAR-equipped small satellite). For a wider swath, the system can be operated in the ScanSAR mode, but the azimuth resolution is degraded when compared to the Stripmap mode (this mode is currently under development in the reference SAR-equipped small satellite, but with 100 m resolution and 350 km swath in a traditional SAR satellite ALOS-2).
Iceye began offering customers access to imagery with a resolution of 25 centimeters, which it acquires from a single satellite staring at a location for 10 seconds. The finest resolution data will be provided to customers in ICEYE’s standard product formats that are accessible with standard Geographic Information System (GIS) tools. The company also has demonstrated interferometric imaging as well as a product nicknamed SAR video that includes multiple images of a single location captured in a single satellite pass.
Capella launched Sequoia, its first operational SAR satellite, in Aug. 202 on a Rocket Lab Electron. The company plans to establish a constellation of 36 satellites to obtain imagery with a resolution of 50 centimeters updated within an hour.
IQPS, part of Japan’s QPS Research Institute, launched its first 100-kilogram SAR satellite in November 2019 on an Indian Polar Satellite Launch Vehicle. The firm’s second satellite, QPS-SAR 2, is booked on Spaceflight’s SXRS-3 rideshare mission launching with SpaceX as soon as December. By 2025, iQPS Inc. plans to operate 36 SAR satellites to “observe almost any point in the world in approximately 10 minutes and conduct fixed-point observations of particular areas once every 10 minutes,” the company said in a July news release.
PredaSAR, founded in 2019, plans to launch its first satellite in 2021 on a SpaceX rideshare mission. The firm plans to operate an initial constellation of 48 SAR satellites, which it will expand “to provide as close as possible to persistent coverage over areas of interest,” said Marc Bell, PredaSAR co-founder and chairman. PredaSAR’s constellation size will be dictated by the needs of government and industry customers, Bell said.
Space SAR satellites applications
SARs applications are many: they range from geology to crop monitoring, from measurement of sea ice to disaster monitoring to vessel traffic surveillance, not to forget the military applications (many civilian SAR satellites are, in fact, dual-use systems). SAR imaging offer the great advantage, over its optical counterparts, of not being affected by meteorological conditions such as clouds, fog, etc., making it the sensor of choice when continuity of data must be ensured. This all-weather seeing feature is what makes them special for security forces and disaster relief agencies. Additionally, SAR interferometry (both dual-pass or single-pass, as used in the SRTM mission) allows accurate 3-D Reconstruction.
To date, defense and intelligence agencies around the world have been the primary consumers of SAR imagery and data. Companies like Synspective and Umbra Lab are betting on robust commercial demand in the not-too-distant future. Synspective plans to become a onestop shop for geospatial data solutions with its planned constellation of about 30 satellites. “We are trying to extend the SAR market from government use to industry,” said Motoyuki Arai, Synspective founder and CEO. “We are focusing on infrastructure development, disaster response and relevant financial sectors.”
SAR satellites capable of collecting all-weather imagery like this color-enhanced Iceye image are projected to comprise a larger percentage of the world’s commercial Earth-observation spacecraft as constellations of smallsats continue to be deployed. Unlike many of its competitors, Umbra Lab plans to sell SAR imagery without offering geospatial analytics. “We are a bent pipe,” said Gabe Dominocielo, Umbra Lab co-founder. “We are a data provider.” Umbra Lab expects the market for SAR to expand in response to its plan to offer inexpensive imagery with a resolution of 25 centimeters by 25 centimeters per pixel. “As more sensors are launched and prices come down, we’re going to see the commercial market expand,” Dominocielo said.
Rapidly changing natural disasters, such as floods, wildfires and volcanic eruptions, require ongoing, frequent temporal monitoring, and this is an inherent strength of SAR technology. Before the clouds and smoke have cleared, SAR data acquisitions can occur that enable users to produce preliminary products and information thus enabling first responders to assess the disaster situation.
Flood detection / monitoring is a great example of this. Determining the extent of flood damage quickly, accurately and at scale is a major problem for both disaster relief and insurance providers. As Iceye has recently shown in its work on Storm Christoph in the UK, data from its constellation combined with the company’s cutting edge analytics can be used to provide real-time analysis of the impact of flooding.
In desert environments, SAR can distinguish surface deformations as small as tire marks left behind by vehicles crossing the sand.
Coherent Change Detection workflows using SAR data provide the ability to measure terrain elevations to an accuracy of a few millimeters. Phase differences in SAR data can measure terrain elevations to an accuracy of a few millimeters. Continuous monitoring over time may reveal subtle ground changes that may indicate clandestine underground tunneling and excavation.
• SAR signal intensity can be interpreted to differentiate water, snow and ice. More importantly, ice thickness can be calculated to determine if a waterway is safe for ship navigation, as Canada and other high-latitude nations have done with airborne and space-borne SAR for decades.
• SAR’s sensitivity to moisture and guaranteed monitoring capabilities can track agricultural growth to assess crop health and forecast harvest yields – vital aspects of food security monitoring.
GEOINT applications of satellite-based SAR data have been around for years, and commercial radar systems operated by MDA Corp, Airbus Defence & Space, ESA, and e-GEOS have supplied the data. In the past few years, several government organizations have launched, or are planning, SAR systems specifically developed for Defence & Intelligence (D&I) purposes. The operation of SAR sensors aboard relatively inexpensive constellations of small orbiting platforms is new, and it ensures the volume, frequency, and timeliness of the data will increase dramatically.New SAR smallsats can inexpensively complement national D&I assets.
“Satellite-based SAR technology is a tremendous asset for GEOINT applications because it captures data at any time of day or night in most weather conditions,” said Paolo Pasquali, Sarmap president. “But the challenges of archiving, processing, and analyzing the data are significant.”
New high-definition satellite radar can detect bridges at risk of collapse from space
Researchers from the NASA Jet Propulsion Laboratory (JPL) and the University of Bath have developed a satellite-based early warning system that could spot tiny movements in bridges that indicate they could collapse. Combining data from a new generation of satellites with a sophisticated algorithm, the monitoring system could be used by governments or developers to act as a warning system ensuring large-scale infrastructure projects are safe.
The team of experts led by NASA’s JPL and engineers from Bath verified the technique by reviewing 15 years of satellite imagery of the Morandi Bridge in Genoa, Italy, a section of which collapsed in August 2018, killing 43 people. The review, published in the journal Remote Sensing, showed that the bridge did show signs of warping in the months before the tragedy.
Dr Giorgia Giardina, Lecturer in the University’s Department of Architecture and Civil Engineering, said: “The state of the bridge has been reported on before, but using the satellite information we can see for the first time the deformation that preceded the collapse. “We have proved that it is possible to use this tool, specifically the combination of different data from satellites, with a mathematical model, to detect the early signs of collapse or deformation.”
While current structural monitoring techniques can detect signs of movement in a bridge or building, they focus only on specific points where sensors are placed. The new technique can be used for near-real time monitoring of an entire structure. Jet Propulsion Laboratory Lead author Dr Pietro Milillo said: “The technique marks an improvement over traditional methods because it allows scientists to gauge changes in ground deformation across a single infrastructure with unprecedented frequency and accuracy.
“This is about developing a new technique that can assist in the characterisation of the health of bridges and other infrastructure. We couldn’t have forecasted this particular collapse because standard assessment techniques available at the time couldn’t detect what we can see now. But going forward, this technique, combined with techniques already in use, has the potential to do a lot of good.”
This is made possible by advances in satellite technology, specifically on the combined use of the Italian Space Agency’s (ASI) COSMO-SkyMed constellation and the European Space Agency’s (ESA’s) Sentinel-1a and 1b satellites, which allows for more accurate data to be gathered. Precise Synthetic Aperture Radar (SAR) data, when gathered from multiple satellites pointed at different angles, can be used to build a 3D picture of a building, bridge or city street.
Dr Giardina added: “Previously the satellites we tried to use for this research could create radar imagery accurate to within about a centimetre. Now we can use data that is accurate to within a millimetre — and possibly even better, if the conditions are right. The difference is like switching to an Ultra-HD TV — we now have the level of detail needed to monitor structures effectively.
“There is clearly the potential for this to be applied continuously on large structures. The tools for this are cheap compared to traditional monitoring and can be more extensive. Normally you need to install sensors at specific points within a building, but this method can monitor many points at one time.”The technique can also be used to monitor movement of structures when underground excavations, such as tunnel boring, are taking place.
“We monitored the displacement of buildings in London above the Crossrail route,” said Dr Giardina. “During underground projects there is often a lot of data captured at the ground level, while fewer measurements of structures are available. Our technique could provide an extra layer of information and confirm whether everything is going to plan.” Dr Giardina has already been approached by infrastructure organisations in the UK with a view to setting up monitoring of roads and rail networks.
Military, and Intelligence Applications
The Space Radar system is intended to carry out four missions for members of the military and the intelligence community:
Synthetic aperture radar imaging—using transmitted microwaves to produce images of the Earth’s surface (somewhat akin to photographs produced by optical imaging). By providing their own illumination, radars can produce images day or night, and microwaves have the advantage of being able to penetrate obscuring layers of clouds (although heavy rain or snow can reduce the quality of the images). However, radar images can be more difficult to interpret than pictures produced with visible light.
Ground moving-target indication—detecting moving targets on the surface of the Earth using special radar techniques. Radar signals that reflect off objects in motion have a different Doppler shift (the change in the frequency of a signal caused by the relative motion of the source and receiver) than do signals that reflect off the surface around them. Through careful signal processing, that Doppler shift can be detected and used to highlight the locations of moving targets. GMTI is used to conduct surveillance of large areas. It typically provides the operator with an indication of moving targets, which can be superimposed on a map or image.
Provision of high-resolution terrain information—making precise measurements of surface elevation. If two observations of the same piece of terrain are collected from slightly different angles, small differences in timing between the returning radar signals used to form the two images can be used to estimate the terrain’s height (through a technique known as interferometric SAR).
Open-ocean surveillance—observing wide areas of the oceans to monitor the movement of ships. Satellite-based SAR is also ideal for tracking ships in open seas. Radar signals are sensitive to the roughness of water surfaces, allowing the wakes of vessels to be detected easily.
Synthetic Aperture Radar (SAR) satellites are very useful in maritime surveillance, thanks to their wide swath, which can reach several hundred kilometers. This enables them to find ships, given a very rough idea of where they might be, in any weather. However, the wide swaths modes of such a system generally have a low resolution, measured in the tens of meters. This makes ship identification difficult. Consequently, a higher-resolution system, or another pass of the same satellite but in high-resolution mode, are needed. Ship motion can severely limit the image quality in high-resolution modes. They could have the capability for ship detection using special algorithms for detecting the ships themselves as well as their wakes.
Race to deploy radar satellites for intelligence and military
Since the late 1970s, various nations have operated SAR imaging satellites for national security or scientific purposes.
The Lacrosse system of the United States includes six spacecraft capable of “seeing” objects less than one meter in size in all weathers, at any time of the day. The monitoring is conducted in a stripe, which can be up to 100 kilometers wide. US Space-based radar (SBR) is a proposed constellation of active radar satellites for the United States Department of Defense. The SBR system would allow detection and tracking of aircraft, ocean-going vessels (similar to the Soviet US-A program), and potentially land vehicles from space. This information would then be relayed to regional and national command centers, as well as E-10 MC2A airborne command posts.
Russian Ministry of Defense announced the imminent launch of all elements of the new system of radar intelligence system code-named as “Liana.” Four space vehicles – two “Lotos-S” and two “Pion NKC” – were deployed in near space at an altitude of about 100 kilometers. They will allow the Russian armed forces to monitor objects on the surface of the Earth that can be as small as a car. The first satellite went into orbit back in 2009, while the second one was deployed in space in 2018. It is believed that the new system will gain the full potential after the launch of the remaining two units in the coming months. Thus, the complex will be fully deployed in early 2020.
Presumably, Liana is a system that has global coverage. Like its American analogue, the Russian system can monitor the earth’s surface through a layer of clouds. The spacecraft of the system have a synthetic aperture, which increases the angular resolution of the radar station. The use of such a technology makes it possible to receive detailed images of an object on the surface, and even in the subsoil layer.
According to Russia these satellites played critical part in the network-centric organization of command of a military conflict. Instead of the rigid vertical arrangement of the transfer of decisions from top to bottom, the principle of simultaneous collection and processing of information is used while the centers of responsibility for headquarters at various levels were preserved.
In a nutshell, each war unit (from soldier to general), each element of the tracking system and artificial intelligence of each combat vehicle were united into joint information space. The command could thus monitor the situation on the battlefield in real time. Radar reconnaissance satellites play a crucial role in the network-centric control system. Based on their experiences received in Iraq, Libya, Syria and other hotspots on the planet, the Americans developed new Lacrosse spacecraft (they were developed by Martin Marietta company). The launch of the Liana system will enable Russia to arrange large-scale military actions on the basis of principles of network-centric warfare.
SpaceX in June 2019 launched three next-generation radar satellites for the Canadian government from California’s Vandenberg Air Force Base. The satellites, which will settle into an orbit about 370 miles above Earth, will use synthetic aperture radar, or SAR, which, unlike visual imaging satellites, uses radio energy to see through clouds, Quartz reported. The three Earth observation satellites launched Wednesday will gather data for a variety of uses, primarily for maritime surveillance, disaster management and ecosystem monitoring, according to the Canadian Space Agency.
“Built by MDA, a Maxar company, the three-satellite configuration of the RCM will provide daily revisits of Canada’s vast territory and maritime approaches, including the Arctic up to 4 times a day, as well as daily access to any point of 90% of the world’s surface,” SpaceX reps wrote in a mission description, according to Space.com. “The RCM will support the Government of Canada in delivering responsive and cost-effective services to meet Canadian needs in areas like maritime surveillance, ecosystem and climate change monitoring and helping disaster relief efforts,” they added.
The UK government is pushing ahead with its plans for a cluster of military radar satellites, placing a design study with Airbus. Project Oberon, as it’s known, has been in discussion for a while. It envisages a network of small spacecraft capable of seeing the Earth’s surface in all weathers and at night, and at very high resolution. The satellites would also have sensors to locate the use of radio transmissions. This is information that can be used in tandem with the radar pictures to better identify targets on the ground and interpret their behaviour.
A Chinese Long March 11 rocket successfully launched five new remote-sensing satellites in Sept. 2019. Those satellites will join a commercial satellite constellation built and operated by the Chinese company Zhuhai Orbita Aerospace Science and Technology Co., Ltd. That constellation, called Zhuhai-1, will ultimately consist of 34 small satellites, including video, hyperspectral, and high-resolution optical satellites, as well as radar and infrared satellites, according to China’s state-run Xinhuanet news agency.
RISAT-2B, the satellite launched in May 2019 from Sriharikota, will mark the resumption of a vital ring of Indian all-seeing radar imaging satellites after seven years. The RISAT, which was first deployed in orbit on April 20, 2009 as the RISAT-2, uses synthetic aperture radars (SAR) to provide Indian forces with all-weather surveillance and observation, which are crucial to notice any potential threat or malicious activity around the nation’s borders. While RISAT-1 was expected to be released first, the incident of the 2008 terror attacks in Mumbai meant that the deployment of the satellite needed to be hastened. With the C-band SAR, being built by Inda, not being ready in time, India deployed the RISAT-2, which was based on the X-band SAR — technology built by the Israel Aerospace Industries.
At least a half-dozen could be foreseen in the near future, mainly to add to the reconnaissance capability from about 500 km in space. In a recent conversation with The Hindu, Indian Space Research Organisation Chairman K. Sivan had said many RISATs were planned. RISAT-2B is to be followed by RISAT-2BR1, 2BR2, RISAT-1A, 1B, 2A and so on. If ISRO orbited its first two radar satellites in 2009 and 2012, it plans to deploy four or five of them in 2019 alone. A constellation of such space-based radars means a comprehensive vigil over the country.
India has almost kept pace with the world with radar sats, said Arup Das Gupta, former Deputy Director, ISRO’s Space Applications Centre, Ahmedabad, currently managing editor of Geospatial World. Radar imaging satellites pick up structures, new bunkers very well, and sometimes help to count them, too.
“In India we also use radar imaging for crop estimation because our main crop growing season of kharif is in May-September when it rains and gets cloudy. We have used this data extensively for forestry, soil, land use, geology and during floods and cyclone.”
Challenges to space based SAR
A key design consideration for any satellite is the orbital altitude. On one hand, higher orbits provide better coverage of the Earth’s surface; they also have lower velocities, which helps with ground moving-target indication. On the other hand, the wide-area surveillance rate for GMTI is proportional to the radar’s transmit power multiplied by its aperture size (the “power-aperture product”) and is inversely proportional to the square of the distance to the target. Thus, a radar in medium earth orbit at 10,000 kilometers would require 100 times the power or 100 times the antenna area of an equivalent radar in low earth orbit at 1,000 kilometers.
With the range of options extended, it is becoming increasingly challenging to balance radar performance characteristics against other parameters of a SAR launch mission. Some of the variables involved are the available orbits, radar and satellite models — with their physical dimensions and a host of characteristics, such as data rate and power consumption. This complexity calls for a computational approach to support the design of future SAR-based Earth observation missions.
Skoltech researchers Alessandro Golkar and Ksenia Osipova, and former Massachusetts Institute of Technology (MIT) student Giuseppe Cataldo (now working at NASA’s Goddard Space Flight Center) have developed, within the framework of a Skoltech-MIT collaboration, a model to help engineers create and select the most promising conceptual designs of satellite radar systems. By optimizing the design of these rapidly evolving instruments, the model promotes their faster and more cost-efficient introduction, leading to better maps and storm, flood, and landslide monitoring.
The researchers conclude that small satellites are a feasible platform for the higher-frequency 8-12 GHz and 4-8 GHz radars, but not for the 1-2 GHz band. Conditions for making the latter type of SARs feasible are discussed, along with the feasibility bounds and technical constraints on the associated instrument and spacecraft requirements. Pulse repetition frequency emerges as the main limiting constraint on the SAR trade space. In other words, this characteristic is the most powerful factor — ahead of power consumption, antenna size, data rate, etc. — for narrowing down radar configurations to a limited set of feasible designs.
The model presented in the study applies to radar systems mounted on a single satellite. It could, however, be extended in the future to account for ways of combining SAR satellites into constellations. Researchers have employed Walker Constellation (WC) method to design satellite configuration with desired revisit times which consists of three integer parameters T/P/F, where T denotes the total number of satellites, P is the number of orbit planes, and F is the relative phase difference between satellites in adjacent planes.
Production of the Space Radar satellites would also require that various hardware challenges be met. Those challenges would include developing a large phased-array radar antenna that could survive launch and deployment in space; improving the efficiency of batteries that operate in space; and developing signal-processing systems, satellite-to-ground communications, and intelligence exploitation systems that could handle the large flow of raw data and resulting intelligence products that a Space Radar satellite would generate.
Different than conventional satellites, the payload of micro- and nano-satellites is limited. This imposes great challenges on the SAR system design. Traditional SARs adopt thousands of millimeter-wave integrated circuit (MMIC) components; they are bulky and power hungry.New technologies are needed to shrink the volume, reduce the cost, and improve the power efficiency of the SAR system.
Arguably, the most critical technical challenge facing the Space Radar program is the development of dataprocessing algorithms that can distinguish between moving targets and the background clutter around them. Identifying such targets from space is especially hard: from the point of view of an orbiting satellite, the ground is moving at about 15,000 miles per hour, so distinguishing a vehicle that is moving only a few miles per hour faster than that is a difficult task.
With SAR data, the challenge is magnified because the information contained within the data requires complex processing methodologies. Individual SAR data files can be quite large. And some analysis workflows require several multi-temporal datasets to achieve highly accurate results. Much of the valuable information in radar data is contained in the intensity and phase of the radar signal that reflects off the surface and returns to the sensor. Interpreting the signal intensity and phase difference is vital to exploiting SAR data, but it requires complex processing techniques.
A larger number of remote sensing satellites means more rapid revisit and global coverage. On the downside, however, the enormous volumes of data soon to come from dozens of satellites pose immense challenges to archiving, processing and analysis. Organizations with image processing capabilities developed primarily to exploit optical imagery are not prepared for the SAR onslaught, according to Pasquali.
Some of the desired technologies for micro- and nano-satellites missions for compact SARs are Highly integrated radar transceiver integrated circuits (ICs) or SAR transceiver ICs, low-noise amplifiers (LNAs), high-power amplifiers (HPAs) and Gallium Nitride (GaN) technology. A novel microelectromechanical system-based delay line is also proposed for satellite SAR to reduce the system size. Existing issues and expected improvements of these technologies are also elaborated.
Sarmap is one of just a handful of companies that has focused on this challenge by developing automated SARscape software using tools in the ENVI geospatial analysis package. The API architecture allows users to process enormous SAR files at scale and even integrate them with optical image data sets, which leverages the strengths of both types of valuable remote sensing data.
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