Airborne or Spaceborne Synthetic aperture ladar (SAL) for centimeter-class resolution battlefield awareness

Rajesh Uppal October 8, 2018 Photonics Comments Off on Airborne or Spaceborne Synthetic aperture ladar (SAL) for centimeter-class resolution battlefield awareness 717 Views

Conventional optical imagers are limited in spatial resolution by the diffraction limit of the telescope aperture, as well as by the practical difficulties in increasing the telescope aperture. Synthetic aperture (SA) techniques can increase resolution beyond the diffraction limit of the receiving aperture.

Just as synthetic aperture radar(SAR) working in radio frequency, makes use of the movement between the platform and the target plus the signal processing techniques to obtain high resolution image. SAL in the optical laser domain, can enable fine-resolution, two-dimensional, active imaging at long range with small diameter optics.

A synthetic aperture ladar (SAL) could provide dramatic improvements in either resolution or, compared to synthetic aperture radar (SAR), the time needed to record an image, or both. The reduced imaging time results from the shorter time needed by the platform to traverse the synthetic aperture (SA) that produces the same resolution with a shorter wavelength.

When the observation range reaches a thousand kilometers or more, no other method of imaging can offer centimeter-class resolution with a real aperture size no larger than a few meters. Additionally, because SAL is an active sensing method, it is not restricted to daylight operation.

The first airborne synthetic aperture ladar image data was collected at Edwards AFB, Calif., February 17, 2006, by SALTI contractor Raytheon Space and Airborne Systems using fiber lasers developed for the telecommunications industry and employing customized commercial fiber oscillators, amplifiers and detectors.

“Synthetic aperture laser radar technology satisfies the critical need for reliable, long-range battlefield awareness. An image that takes radar tens of seconds to produce can be produced in a few thousandths of a second at optical frequencies,” explained Dr. Jennifer Ricklin, DARPA program manager for SALTI. “While radar waves respond to macroscopic features such as corners, edges, and facets, laser waves interact with microscopic surface characteristics, which results in imagery that appears more familiar and is more easily interpreted.”

Northrop Grumman Corporation has successfully designed, built and flight demonstrated one of the world’s first synthetic aperture laser radar systems.

Under Defense Advanced Research Projects Agency (DARPA) funding, the Synthetic Aperture Ladar for Tactical Imaging (SALTI) program transfers synthetic aperture radar (SAR) technologies, proven in numerous radio frequency radars, into the laser radar domain, providing unprecedented resolution at significantly improved standoff ranges.

“This is an historic milestone: an airborne laser radar has successfully demonstrated the SAR technique,” said Chris Patrick, Northrop Grumman’s SALTI program manager. “Until now, all optical systems and laser radars were limited in resolution by the size of their aperture. Now, much higher resolutions are possible. The diffraction limit has been surpassed.”

The tests, held March 31 – April 3 at Edwards Air Force Base, proved that SAR works at optical wavelengths, enabling high-resolution, nearly photographic quality imagery at distances much further than current electro-optical systems can provide.

The tests included a series of flights on board a Northrop Grumman BAC 1-11 test aircraft, which were supported by the Air Force Research Laboratory and the 412th Test Wing.

“Synthetic aperture laser radar technology is aimed at satisfying the critical need for reliable long-range Battlefield Awareness,” said Dr. Jennifer Ricklin, DARPA’s program manager for the SALTI program. “SALTI combines the long-range day/night access afforded by conventional SAR with the interpretability of high-resolution optical imaging, along with the exploitability of three-dimensional imagery.”

Synthetic aperture ladar can enhance mission effectiveness across the spectrum of air assets, from fighters and bombers to surveillance aircraft and unmanned aerial vehicles. To help explore these potential applications, Northrop Grumman has been asked by DARPA to continue testing this new capability at additional ranges and under various environmental conditions.

Synthetic aperture ladar can enhance mission effectiveness across the spectrum of air assets, from fighters and bombers to surveillance aircraft and unmanned aerial vehicles.

Challenges

In SAL imaging, many physical processes which could be ignored in microwave imaging become noticeable because of the shortness of laser wavelength. For microwave radiation with typical wavelength of centimeters, it is straightforward to keep phase errors within a fraction of a wavelength; however, for optical wavelength this is not a trivial task. According to the experiment of the coherent ladar and the national military standard for environment, the vibration of the airborne platform is the one with its maximum amplitude on the order of millimeter, which can be ignored in SAR but a big problem in SAL.

“The Achilles heel of synthetic aperture laser radar has been its resolution. If we can improve this then we can improve our critical mission combat ID performance.” Addressing potential research and development providers who might be in the audience, Stone added that it would also be necessary also to implement any new resolution capabilities into manufacturable technology.