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  • Aircraft Detection System Ensures Free-Space Laser Safety

Photonics Spectra
Dec 2004
Brian Smithgall and Dr. Keith E. Wilson

As scientists continue to explore our solar system, there are increasing demands to return greater volumes of data from smaller deep-space probes. Accordingly, NASA is studying advanced strategies based on free-space laser transmissions, which offer secure, high-bandwidth communications using smaller subsystems of much lower power and mass than existing ones. These approaches, however, can pose a danger to pilots in the beam path because the lasers may illuminate aircraft and blind them. Researchers thus are investigating systems that will monitor the surrounding airspace for aircraft that could be affected.

Aircraft Detection System Ensures Free-Space Laser Safety
An end view of the assembly at the Optical Communications Telescope Laboratory reveals the main telescope, pointing telescope, radar and Sky Sentry IR aircraft detection system housing. The assembly is designed to detect aircraft that may be affected by free-space laser transmissions.

As early as 1992, the Galileo Optical Experiment, conducted by the Jet Propulsion Laboratory at California Institute of Technology in Pasadena, demonstrated the feasibility of free-space optical communication with space probes. Using frequency-doubled Nd:YAG lasers at ground facilities in California and New Mexico, scientists transmitted optical signals to the Jupiter-bound Galileo spacecraft at a range of 1 million to 6 million km. During the experiment, observers remained outside the telescope facilities, where they could communicate with the laser operators to alert them of aircraft at risk.

Current methods for safe free-space laser propagation through the atmosphere are much the same. Human spotters search the skies for planes that may encounter laser transmissions. But the human eye is limited, reaction times can be slow, and attention spans can wane -- all of which affect safety.

As a result, the Jet Propulsion Laboratory developed an integrated strategy for the safe transmission of uplink laser beams through the atmosphere consisting of three overlapping "tiers" of protection. Tier 1 is a region regulated by the Federal Aviation Administration (FAA) that extends to a little more than 3 km from the source. Tier 2 is another FAA-controlled region that extends to 20 km at zenith and to 58 km at 20° elevation. Tier 3, the region beyond FAA airspace, is controlled by the Laser Clearinghouse of the US Strategic Command at Cheyenne Mountain in Colorado. By knowing the presence of craft at these levels, laser stations can transmit safely.

In conjunction with the lab, Image Labs International of Bozeman, Mont., developed the Tier 1 safety system used at the Optical Communications Telescope Laboratory test facility on Table Mountain in Wrightwood, Calif. Called Sky Sentry, it uses a pair of long-wave infrared cameras to provide different fields of view, and it features built-in object-detection algorithms.

Sky Sentry's cameras are bore-sighted to the telescope component of the transmitter, so they constantly monitor the airspace envelope around the beam for the thermal signature of aircraft. Although clouds have significant energy in the long-wave band, the laser transmissions are performed only when clouds are not in the beam path. The cloudless sky, whether black at night or blue by day, appears as a cold background, providing high contrast for the detection of the relatively warm aircraft.

The telescope may be set to track quickly moving satellites as they cross the sky, so objects appear to have motion vectors relative to the beam. The object detection algorithms use this motion vector information to discriminate objects that may intersect the beam path. The system ignores objects in the field of view, such as clouds, trees and the moon, and it detects very closely crossing planes as well as distant, small aircraft that move relatively slowly through the field of view. If calculations show that an object will interrupt the beam path, two independent laser gates immediately block the laser.

The Optical Communications Telescope Laboratory also will serve as a test bed for the Jet Propulsion Laboratory's full system for safe laser beam propagation.

The system will incorporate Sky Sentry, boresighted radar and aircraft coordinate feeds from local FAA radar, and will operate in coordination with Cheyenne Mountain. Aircraft positions overlapping a topographic map will be displayed on a screen for the operator.

This advanced system will eliminate the need to station aircraft observers and will support the unattended and autonomous operation of future optical communications ground stations.

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