Synthetic Aperture Radar: Seeing Through The Clouds
By Mark Ellsworth, Media Manager, IMSAR LLC, and Curtis Thomas, External Communications, IMSAR LLC
It’s tomorrow. The rain has been falling for hours, showing no signs of relenting. The town’s river, already full from a wetter-than-usual spring, has begun to overtop its banks and the preventative levees. Water flows into the floodplain, works its way into the streets, and approaches homes and nearby structures. In the face of this apparent disaster, the town’s residents are surprisingly well-informed.
Individuals are receiving real-time updates on their computers, tablets, and smartphones. The local government, emergency response personnel, crisis managers, and residents know where the water is. They know the forecast for the rain. They know the river’s boundaries: where it was, where it is, at what rate it is changing, and where it is forecasted to be. They know the status of the levees: where they are strong, where they have been compromised, and where bulges and soil saturation levels indicate near-collapse. First responders also know the status and location of the people who have been caught in the flood and the best, most unobstructed path to take to provide assistance to them. The information available has allowed the residents to be more prepared, better protect themselves, and work better with local and state officials.
Behind all of the updates provided to residents and emergency personnel is synthetic aperture radar (SAR) technology flown on small unmanned aerial vehicles (UAVs) owned by the local municipality, the state, or commercial services. SAR, which utilizes radio frequencies to generate imagery, obtains high-resolution imagery of an area regardless of weather conditions. By comparing imagery of the same area obtained from multiple passes, a SAR system provides information about how an area has changed over time. SAR systems isolate and track moving objects, such as people and vehicles. All of this information is processed in the air, sent to the ground via a communication link, and disseminated via the Internet in virtually real time.
Understanding SAR Technology
Although the situation described above is somewhat hypothetical, SAR technology has advanced so far in the past few decades that this idealistic use of SAR is a realistic possibility in the very near future. In general, SAR technology is able to produce high-resolution images of an area by leveraging a moving platform to synthesize an aperture that is much larger than the antenna’s physical size. Most antennas are only able to produce an angular resolution that is no better than the wavelength divided by the instrument’s aperture. SAR technology synthesizes an aperture that is much broader than the physical antenna by combining measurements obtained as the equipment is flown over a scene. This allows a SAR to produce high-resolution images that are not dependent on the distance to the target or the conditions under which the image was obtained. In other words, a SAR is able to produce the same high-resolution images on a clear day or in darkness, in thick fog, or in smoke.
SAR technology has existed in some form since the 1950s. Early versions of the technology were so large that images could only be produced from large aircraft or orbiting satellites. In addition to being large, early SAR technology either used optical image formation methods or required so much data processing that images could only be produced long after the data was acquired using on-the-ground, post-processing techniques.
Recent advancements in engineering, manufacturing, and in the way SAR data is collected and processed have decreased the size, weight, power consumption, and cost of SAR systems and enabled data to be collected and processed in real time. Take, for instance, IMSAR’s NanoSAR, which weighs less than 2.6 pounds when combined with antennas and an inertial navigation system, has a volume of less than 40 in3, and consumes less than 30 Watts of power in most modes. SAR systems with similar specifications are now small enough to be integrated onto sub-20-pound UAVs and onto smaller manned aircraft. In testing exercises and demonstrations, SAR systems have obtained imagery from UAVs as small as a ScanEagle and Puma.
At the same time that the size of SAR systems has been decreasing, the capabilities of those systems have been increasing. SAR systems are now able to perform multiple modes, including stripmap SAR imaging, circSAR, spotlight SAR, coherent and non-coherent change detection (CCD/NCCD), multi-pass change detection (MCD), maritime search, and moving target indication (MTI). SAR systems have also been successfully used with other sensors to create multi-mode systems. For example, SAR systems can cross-cue electro-optical/infrared (EO/IR) sensors based on detections obtained in SAR imagery or in data from other modes. In virtually all of these modes, simplified data processing techniques allow data to be processed in the air and sent to the ground in virtually real time.
Potential Uses: Military and Beyond
The decrease in size and increase in capabilities of SAR systems have significantly broadened their potential applications. Like most radar technology, SAR systems have been primarily used for military applications. As currently used by armed forces, SAR systems provide all-weather intelligence, surveillance, and reconnaissance (ISR) information during the day and at night. The systems perform widearea surveillance, detect change, and moving targets, and complement (instead of replace) other sensors. These features make SAR systems a valuable military ISR asset for monitoring patterns of life, detecting otherwise hidden objects, and tracking targets of interest without the need for large numbers of operators.
Although militaries are likely to remain significant users of SAR systems, the applications of the technology as well as the smaller size and lower cost of these systems makes them increasingly attractive and available for use in research and commercial applications. The move of SAR systems into the research and commercial space has already begun with interesting applications coming from many industrial sectors to solve existing challenges. For instance, the ability of SAR systems to penetrate snow and reveal ice ridges in large sheets of ice could locate openings in pack ice and save significant amounts of time and money for navigation of ice breaking ships. In agriculture, SAR systems can identify water on land or in soil, allowing farmers to identify areas of over- and under-watering in their fields. In the oil industry, SAR can detect oil on water, empowering oil companies to effectively and efficiently locate and track oil spills. SAR systems can also be used by search-and-rescue operations to locate lost individuals at night, in low-visibility conditions, in water, or other difficult environments and conditions.
The advantages of radar-based solutions, demonstrated in current industries, have led researchers to ascertain how radar can solve emerging problems such as those born out of the popular interest in UAVs. For instance, recent research has sought to determine the use of SAR-like radar systems to perform collision avoidance. This would allow UAVs to fly within the National Airspace System (NAS). Like other collision-avoidance sensors, radar systems are able to sense other airborne objects within the surrounding airspace with detection ranges long enough to provide early warnings of potential threats. These systems can be designed to cover forward, peripheral, and postern threats. One advantage of radar systems over other sensors in collision avoidance is that radar systems can sense other airborne objects during the day, at night, in inclement weather, and in other low-visibility conditions. Radar can also detect non-cooperative potential threats, those that may not have a similar system. Furthermore, today’s radar systems can be designed small enough to perform collision avoidance functions from small UAVs while maintaining other sensors on the UAVs for other purposes.
In addition to applications of SAR technology which are already being utilized and explored, it is difficult to overemphasize the impact that the small size, weight, and power of SAR systems have on many other potential applications, yet unexplored. Many areas of industry and commerce are expanding, and the need to maintain a clear idea of space, place, and activity demands solutions that are quick, versatile, efficient, and available. The greatest challenge to the adaptation of SAR systems in non-military spaces is the lack of knowledge concerning the increasingly available technology. Indeed, much of the current and future challenges that face contemporary entities—whether law enforcement, archaeology, cartography, raising livestock, fishing, or many others—can be helped by the information available by modern radar systems.
SAR technology has come a long way since the large, cumbersome versions that existed since the 1950s. Decreases in size, weight, power, and cost, in conjunction with increases in capabilities, have significantly expanded the potential uses of and markets for SAR technology. The future is likely to see increasing use of SAR technology in commercial applications, and the near-future possibilities for the technology are impressive. Before too long, residents of a town may be using their tablets or smartphones to receive real-time updates on the status of a nearby flood. SAR technology could make it happen.
Lead art: A SAR image of a golf course shows water, land, and structure detection.
This article originally appeared in the November 2014 issue of Unmanned Tech Solutions.