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Lasers Detect Targets From the Sky

December 2006
By Henry S. Kenyon

The Synthetic Aperture Ladar for Tactical Imaging (SALTI) program is studying practical applications for synthetic aperture ladar (SAL) technology. Unlike synthetic aperture radar, which requires trained personnel to interpret data, SAL can produce photo-realistic images.
Airborne light-based system delivers more detail, high data rates.

An experimental sensor technology may one day permit reconnaissance and combat aircraft to detect and identify ground targets more rapidly and efficiently than with radar. The prototype equipment uses a laser to create a high-resolution image of an object from an aircraft in flight, something that only radar had been able to achieve.

The goal of the U.S. Defense Advanced Research Projects Agency’s (DARPA’s) Synthetic Aperture Ladar (laser detection and ranging) for Tactical Imaging (SALTI) program is to demonstrate the practicality of synthetic aperture ladar (SAL) and to refine the technology. Synthetic aperture systems use digital signal processing and aircraft motion to generate images from small physical apertures in the aircraft’s body.

In early 2006, researchers proved that SAL images could be taken successfully from an aircraft, explains SALTI program manager Dr. Jennifer C. Ricklin, DARPA, Arlington, Virginia. The program’s second phase, which ends this month, focuses on overcoming engineering challenges. Ricklin describes SALTI as a classic DARPA program that develops a revolutionary capability ahead of a perceived need or application.

Synthetic aperture radar (SAR) is a well-established technology. SALTI program researchers seek to develop a system that uses some of the same basic principles of SAR but that can produce higher resolution images more rapidly. Because SAL scans at optical frequencies, it has several significant advantages over SAR, the most important being enhanced resolution. SAL also has some disadvantages; for instance, the short wavelength of the laser-based beam presents several complex engineering problems. 

The original goal of the program was to demonstrate a proof of concept that SAL could be conducted at optical frequencies from an aircraft. Ricklin notes that this capability already had been tested successfully in the laboratory, but DARPA researchers wanted results from an airborne platform that were close to the technology’s theoretical limits. The effort is now in a six-month period devoted to expanding the technology’s imagery database. This period also will feature an investigation of the performance limits of the system, studying how it responds to atmospheric conditions and improving the equipment overall.

Under the program, DARPA awarded contracts to the Raytheon Company and Northrop Grumman Corporation to develop and test two SAL applications. Ricklin explains that the current phase of the program involves a series of flight tests for both systems, adding that the companies are using very different technologies.

The DARPA flight tests took place at Edwards Air Force Base (AFB), California. Each participant conducted a week’s worth of test flights followed by six weeks of processing imagery data and analyzing system performance. A second series of flights in 2007 will focus on more optimized system performance to determine what SAL technology is capable of, she says.

The major advantage of laser-based technology is its resolution, which is similar to an electro-optical or optical system. But SAL is not limited by range, only by the system’s bandwidth. “You could have very high resolution at an extended range. That’s the real charm of this program,” Ricklin shares.

Compared to SAR systems, SAL produces images that are easier to interpret. Ricklin notes that SAR images require interpretation by specialists familiar with SAR imagery, while SAL images are photo-realistic. Another advantage of SAL technology is fast image production. SAR systems take several seconds or minutes to form an image, but SAL images can be formed in milliseconds. “You would be able to capture an image faster than the human visual system would be able to update,” she explains.

Because SALTI is solely a research program, the Raytheon and Northrop Grumman teams are not competing for a contract, Ricklin shares. “It’s not a competition. It’s developing two completely different systems, each of which has its own unique advantages and disadvantages. There are reasons and applications for both technologies,” she says. 

Raytheon is using a fiber optic laser for its imagery. Ricklin explains that this technology originally was developed for the telecommunications industry in the late 1990s. She adds that much of the development work for the technology already is complete, which allows Raytheon engineers to focus on applying it to an airborne environment.

Raytheon Company and Northrop Grumman Corporation are testing
their SAL systems from airborne platforms as part of the SALTI program. This British Aircraft Corporation 1-11 is Northrop Grumman’s test aircraft.
Northrop Grumman designed a specialized carbon dioxide laser system to meet power and bandwidth requirements for its application. Ricklin states that the key difference between the systems is that they operate at different optical frequencies. Each technology has its strengths. The Raytheon system has a much higher resolution than the Northrop Grumman version but is very sensitive to aircraft motion and atmospheric interference. The Northrop Grumman system is more resistant to these disturbances, but its technology is not as mature, she says.

The U.S. Air Force Research Laboratory’s Sensors Directorate at Wright-Patterson AFB, Ohio, is managing the SALTI program’s flight tests. The directorate has a team at Edwards AFB managing operations. Ricklin notes that flight tests there require careful coordination because the base is located next to the busy flight corridor to Los AngelesInternationalAirport.

Northrop Grumman is using a British Aircraft Corporation 1-11 aircraft as a test platform, while Raytheon is using a Boeing 707-based platform. Ricklin observes that it takes a significant engineering effort to move the SAL systems from the laboratory to their respective aircraft. Both systems are still experimental prototypes that are very large and awkward, filling nearly a third of each test craft. However, she adds that as the technology is refined, more compact systems will appear. 

Several target-rich fields were set up at Edwards AFB for the tests. In addition to the target fields on the ground, the teams have established global positioning system coordinates and are constantly monitoring atmospheric conditions for turbulence and meteorological events that might impact the SAL systems. Ricklin shares that the Air Force also helps maintain the atmospheric monitoring for the program.

Aircraft vibration is a major challenge for SAL technology because the platform’s motion combined with atmospheric turbulence affects the system’s fidelity. Ricklin notes that each of the contractors has developed motion compensation systems. However, atmospheric turbulence remains an impediment. One potential solution is autofocusing technology, but Ricklin explains that currently there is no way to compensate effectively for turbulence. She also observes that adaptive correction systems developed for astronomy do not completely apply to SAL.

Although the question of adaptive control must be addressed at some point in the program, scientists are still determining the extent to which turbulence affects the technology. Ricklin notes that atmospheric distortions can be on the order of a wavelength, which greatly affects the beam. The core engineering problem is that the beam goes through a series of alterations as it leaves and returns to the aircraft. The laser first is transmitted through a window in the craft, which creates some small aberrations. Further alteration is produced by the atmospheric sheer layer around the aircraft. Ricklin explains that both companies had to develop methods to minimize this effect. These factors contribute to some distortion in the beam before it travels through the atmosphere, where it encounters turbulence effects before it scatters off of the target. This scattering process causes additional distortion before the beam again encounters turbulence returning to the aircraft. Beam data then must be captured out of this atmospheric noise and mixed with a reference laser to produce a signal. Ricklin admits this is a significant technical challenge.

Signal processing is another critical area for SAL technology. SAR systems develop a single image, but SAL scans in small patches that must be arranged in a mosaic pattern. “How do you do that? It’s a very complicated and intricate signal processing issue. It was a much more difficult problem than anybody realized early on,” Ricklin says.

Both firms will examine the mosaicing and signal processing issues in the program’s next phase. Ricklin notes that at the beginning of the program, it was thought that SAR techniques would transfer to SAL technology. “That just didn’t prove to be the case,” she states.

Ricklin admits that initially not much thought was given to how the data would be put together as the work was more focused on building the equipment. She notes that at the beginning of the program, the first SAL data collected by one of the companies took almost a week to transfer from the aircraft to the system processing the data.

The goal of the current six-month phase is to create compelling tactical imagery and to understand better the performance limits of the systems. Depending on the results of this phase, a decision will be made about whether to progress to the next stage, which would seek to develop the system into a form factor for a specific platform with required range and operational parameters.


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