The technology of synthetic aperture radar, which has been used to map the Earth from space, will play an integral role in the U.S. Department of Defense’s space-based radar programs.
By John McHale
Visitors to Windsor Castle in Windsor, England, often marvel at the detailed world maps created by 15th and 16th century cartographers for the 400-year-old books on display at the castle’s cathedral. The maps are detailed and uncannily close in accuracy to the satellite map imagery easily downloadable from the Internet today.
Despite all that skill and hard work, those talented mapmakers would probably kill to get their hands on satellite payloads and synthetic aperture radar (SAR).
SAR has been used by the military, NASA, and other government agencies for years for mission-critical applications ranging from mapping planets to battlefield intelligence, surveillance and reconnaissance (ISR) applications.
According to Sandia National Labs environmental monitoring, Earth-resource mapping, and military systems require broad-area imaging at high resolutions. Often the imagery must be acquired in inclement weather or during night as well as day.
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,î states the Sandia web site. Synthetic aperture radar complements photographic and other optical imaging capabilities because of the minimum constraints on time-of-day and atmospheric conditions and because of the unique responses of terrain and cultural targets to radar frequencies.
Space-based radar
The U.S. Air Force’s Space Based Radar (SBR) program’s mission will be to provide worldwide, persistent coverage in SAR for radar-based imagery, as well as surface moving-target indication for detection, identification, geolocation, and tracking of moving objects near or on the surface of land or water. It will also provide high-resolution terrain information to a broad array of customers.
SBR will have a bottoms-up consideration of linkages to other space, air, and ground programs, which is envisioned to provide previously unattainable tasking, delivery, and assured response from space.
The Air Force Space and Missile Systems Center awarded teams led by Lockheed Martin and Northrop Grumman Corp. the 24-month study contract to continue the definition of SBR system requirements and evaluate alternative design concepts for the SBR system architecture. Final selection of the prime contractor for the multibillion-dollar program will be made this year, with initial launch scheduled for 2012.
Northrop Grumman, as one of two competing contractors, and systems integrator, will provide the concept development of ground-segment, user-segment, and operations and support elements. Boeing is responsible for the design, development, and production of the SBR space segment, including launch-vehicle integration. Raytheon will provide global mission and system management, and support to other ground-segment elements. General Dynamics Corp. will provide mission and ground-system analysis and mission data-processing elements. BAE Systems will supply large-scale, information storage and retrieval technologies.
The Air Force, in partnership with the National Reconnaissance Office, National Geo-Spatial Agency and Electronic Systems Center will provide program leadership at the Joint Program Office, based at the Air Force Space and Missile Systems Center at Los Angeles Air Force Base.
The first phase of the program calls for Northrop Grumman and Lockheed Martin teams to develop system and software architectures; conduct studies that balance performance, affordability and risk and schedule; evolve a modeling and simulation capability; demonstrate SBR technology maturity; and develop and manage life cycle costs, among others.
Lockheed chose Harris Corp. in Melbourne, Fla., to develop the Flight Demonstration System (FDS) radar payload for the Innovative Space-Based Radar Antenna Technology (ISAT) program. The ISAT program, under the direction of the Defense Advanced Research Projects Agency (DARPA) Special Projects Office and the U.S. Air Force Research Lab’s (AFRL) Vehicle Systems Space Vehicles Directorate, includes the development of technologies required to deploy extremely large antennas in space for tactical sensing of moving targets on the ground. Harris is responsible for the design of the radar sensor payload, which will provide Moving Target Indicator (MTI) surveillance of areas previously obscured from view by airborne assets.
The contract will enable Harris engineers to further enhance the technologies previously developed for the ISAT payload, according to Russ Haney, president of the National Programs business unit of Harris Corp.’s Government Communications Systems Division (GCSD). .
Harris will complete the design of a scaled demonstration unit of the large, phased-array radar sensor that will be used to validate the technology and structural design concept for the full-size objective system. At nearly the height of the Empire State Building, the 300-meter objective system structure must be capable of being compressed to the size of an SUV for launch.
Watchkeeper
Northrop Grumman officials use SAR technology for applications for the United Kingdom’s WatchKeeper unmanned aerial vehicle (UAV) program.
The company’s Integrated Systems sector developed the integration and end-to-end air vehicle/payload testing of an all-weather, high-resolution, tactical synthetic aperture radar/moving target indicator (SAR/MTI) on the company’s production-configured Fire Scout tactical unmanned air vehicle (UAV) at its Fire Scout System Center, in San Diego, Calif.
How SAR works
According to Sandia National Labs SAR typically produces a two-dimensional (2-D) image. One dimension in the image is called range (or cross track) and is a measure of the “line-of-sight” distance from the radar to the target. Range measurement and resolution are achieved in synthetic aperture radar in the same manner as most other radars: range is determined by precisely measuring the time from transmission of a pulse to receiving the echo from a target and, in the simplest SAR, range resolution is determined by the transmitted pulse width; for example, narrow pulses yield fine range resolution.
The second dimension is azimuth, or along track, and is perpendicular to range. It is the ability of SAR to produce relatively fine azimuth resolution that differentiates it from other radars.
“To obtain fine azimuth resolution, a physically large antenna is needed to focus the transmitted and received energy into a sharp beam,” Sandia’s Web site notes. The sharpness of the beam defines the azimuth resolution. Similarly, optical systems, such as telescopes, require large apertures (mirrors or lenses which are analogous to the radar antenna) to obtain fine imaging resolution. Since SARs are much lower in frequency than optical systems, even moderate SAR resolutions require an antenna physically larger than can be practically carried by an airborne platform: antenna lengths several hundred meters long are often required. However, airborne radar could collect data while flying this distance and then process the data as if it came from a physically long antenna. The distance the aircraft flies in synthesizing the antenna is known as the synthetic aperture. A narrow synthetic beamwidth results from the relatively long synthetic aperture, which yields finer resolution than is possible from a smaller physical antenna.
Sandia experts caution that SARs are not simple and that transmitting short pulses to provide range resolution is generally not practical. Typically, longer pulses with wide-bandwidth modulation are transmitted which complicates the range processing but decreases the peak power requirements on the transmitter. For even moderate azimuth resolutions, a target’s range to each location on the synthetic aperture changes along the synthetic aperture. The energy reflected from the target must be “mathematically focused” to compensate for the range dependence across the aperture prior to image formation. Additionally, for fine-resolution systems, the range and azimuth processing is coupled (dependent on each other), which greatly increases the computational processing.
Foliage penetration
Lockheed Martin, under the sponsorship of DARPA and the U.S. Army and Air Force, developed Foliage penetration radar (FOPEN), the airborne VHF/UHF dual-band synthetic aperture radar for imaging concealed targets. Since its inception in 1997, the FOPEN program has advanced to a mature system that has completed more than 370 flights.
The FOPEN radar was designed for operation from low to very high altitudes in a variety of manned and unmanned platforms. It has collected images at ranges greater than 40 kilometers. Image formation and subsequent sophisticated target detection processing are performed in real-time onboard the aircraft, Lockheed officials say. The system design supports a data link, which allows for processed results to be down-linked immediately.
Northrop Grumman to provide Hellenic air force with AN/APG-68(V)9 radars
Northrop Grumman Electronic Systems sector in Baltimore will provide AN/APG-68(V)9 airborne radars for F-16 Block 52+ aircraft for the Hellenic air force.
Northrop Grumman has a direct contract with the U.S. Air Force for this procurement, which is being managed by the Air Force Materiel Command, located at Wright-Patterson Air Force Base, Ohio. Delivery of the first of 33 radar systems, which includes three spares, will commence in 2007. The contract also includes an option to provide radars for an additional 10 aircraft.
The AN/APG-68(V)9 radar is an advanced capability system that seven foreign countries have purchased as standard equipment on new F-16s or as upgrade kits for their existing F-16 fleets. To date, Northrop Grumman has delivered more than 250 AN/APG-68(V)9 radars.
“This system delivers greatly improved operational capability-essentially providing for all-weather, day and night, air-to-ground attack,” says Katie A. Gray, vice president of F-16 sensor systems at Northrop Grumman’s Electronic Systems sector. “The Hellenic Air Force was one of the first customers for this system, and this is their second major order for the AN/APG-68(V)9 radar. They have been operating Block 52+ F-16s for three years and have developed enhanced mission profiles that are enabled by the improved performance of the radar.”
The AN/APG-68(V)9 also offers a 33 percent increase in air-to-air detection range over earlier versions of the radar and introduces synthetic aperture radar, which provides high-resolution ground mapping. When combined with other weapon-system improvements, it enables F-16s with autonomous 24-hour, all-weather precision-strike capability, Northrop Grumman officials say. The AN/APG-68(V)9 is also available as an easily installed upgrade kit for existing F-16 aircraft, above.
Mercury Computer Systems contributes to VistaNav 3-D synthetic vision system
The VistaNav MFD, a multifunction flight display system with synthetic vision technology, uses the XB computing platform from Mercury Computer Systems, in Chelmsford, Mass., to build scalable UAV ground stations that use synthetic vision to reconstruct terrain independent of weather.
VistaNav helps pilots to better visualize surrounding terrain while in flight. It uses several sensor data sources and integrates several databases to ensure complete ground-to-air support and 3-D full-terrain views in real time. The VistaNav software can be utilized as a UAV ground station subsystem when combined with Mercury XB computing platforms, or as a powerful, portable navigation system when combined with a mobile computer and a Mercury-developed Inertial Navigation Unit (INU).
“Three-dimensional synthetic vision substantially improves situation awareness and is a key technology for the future of general aviation and unmanned aerial reconnaissance,” says Philippe Roy, director of the Visualization and Simulation Group at Mercury.
Mercury technology is also part of the VistaNav-GA system, which includes an INU featuring 3-D solid-state inertial sensors, a WAAS-enabled (Wide Area Augmentation System) GPS receiver, and a Bluetooth wireless interface. The unit can be mounted in a number of places inside an aircraft and communicates through a wireless interface using a tablet PC mobile computing platform (MCP). The MCP has a high-resolution 5-by-8-inch LCD display with a full navigation user interface that allows pilots to manage all phases of flight, from preparation to parking.
The entire unit is powered by the aircraft power supply and includes rechargeable batteries that will operate for up to one hour in the event of an aircraft electrical failure. Both the INU and MCP are designed to be removed or installed in an aircraft in less than three minutes.
For more information or to order the VistaNav system, visit www.mc.com/vistanav.
Raytheon designs UAV radar with DARPA grant
Engineers at Raytheon Space and Airborne Systems (SAS) in El Segundo, Calif., will develop airborne radar that can search broadly for ground targets while transmitting data about them at Ka-band, according to a program award from the Defense Advanced Research Projects Agency (DARPA) in Arlington, Va.
The Affordable Adaptive Conformal ESA Radar (AACER) program is being administered by the U.S. Army Research Laboratory in Adelphi, Md.
Intended for use on rotary unmanned aerial vehicles in development by DARPA and the Army, the AACER system will feature ground moving target detection and track, dismount detection, synthetic aperture radar imaging, and high-data-rate communications capability at Ka-band. The technology for electronic processing combines elements of Raytheon’s APG-79 electronically scanned array radar for the F/A-18 and seeker technology from the company’s Advanced Medium Range Air-to-Air Missile with new low-cost millimeter-wave hardware designs.
AACER is based on the design and experience gained on DARPA’s A-160 Ka- band Radar Flight Demonstration and Ka-band Electronically Scanned Array programs, the Air Force Research Laboratory’s Varactor CTS ESA DUST program, and an investment by Raytheon during the past 10 years.
“This is truly a multifunctional and multifrequency system,” says Nick Uros, vice president for the Advanced Concepts and Technology unit of the company’s Space and Airborne Systems business. “It will scan electronically. No mechanical parts are involved. Therefore, it will operate at the speed of light.”
Raytheon was selected to proceed with Phase II of a planned three-phase, four-year program after a competitive down-select with Northrop-Grumman Electronic Systems at the end of Phase I.
Raytheon Space and Airborne Systems is a provider of sensor systems giving warfighters the accurate and timely information available for the network-centric battlefield. SAS has additional facilities in Goleta, Calif.; Forest, Miss.; Dallas, McKinney and Plano, Texas; and several international locations.
Global Hawk uses Raytheon optics
Pilots of the Global Hawk unmanned aerial vehicle (UAV) monitor enemy targets with an electro-optical/infrared (EO/IR) high-resolution imaging system built by Raytheon, Corp.’s Space and Airborne Systems division in El Segundo, Calif.
Combined with other sensors, the package is called the Integrated Sensor Suite (ISS), and is slated to fly aboard the Northrop Grumman RQ-4A Global Hawk. Raytheon delivered the first production shipment of Lot-3 units in November.
ISS combines a cloud-penetrating synthetic aperture radar (SAR) antenna with a ground moving target indicator (GMTI), a high-resolution electro-optical (EO) digital camera, and an infrared (IR) sensor. Because those components are integrated, battlefield commanders can select radar, infrared, or visible wavelength modes to suit their mission.
Current ISS units provide near-real-time imagery for missions in Iraq. Raytheon engineers are also building a next-generation enhanced ISS, which will extend the range capabilities of both the SAR and EO sensors by 50 percent over the basic ISS. For more information, see www.raytheon.com.
Global Hawk, above, is a high-altitude, long-endurance craft that can linger at 60,000 feet for 36 hours, providing intelligence, surveillance, and reconnaissance (ISR) data to military field commanders. Used to scan large areas, they can cover an area of 40,000 square nautical miles with 1-meter resolution within 24 hours, or to spot specific targets, they can search 1,900 2-by-2-km spots with 0.33-meter resolution.
