Transponder Based Aircraft Detection

When you shine a powerful laser into the sky, someone is likely to notice.

That someone is likely to be the Federal Aviation Administration, who, for some reason, seem to be concerned about the possibility of our illuminating a passenger airliner with an AO laser.

Both Keck lasers in operation
Both the Keck 1 and Keck 2 lasers in operation under a nearly full Moon
We currently use laser spotters to insure this does not happen. Yes, some poor soul must sit outside all night long and watch the skies for aircraft near the beams. When the weather is nice this is not a problem. It is seldom that nice, a bitterly cold wind is the usual condition. I have done this duty, for about an hour, and really do not need to do it again. After a night in the cold, is a person really an alert observer? An automated system that removes the human element from the equation is really a better solution.

Enter TBAD, the Transponder Based Aircraft Detector. All commercial and most civil aircraft carry a 1090MHz ADS-B transponder that identifies the aircraft and provides basic data. The transponder is part of an aircraft tracking system now used by air traffic control centers around the world to supplement, or in some cases replace, radar systems. An idea… Create a directional antenna that can determine if a 1090MHz transmitter is in the beam of the antenna and mount that antenna to the telescope. With such a system we can detect an aircraft approaching our beam and shutter the laser. The idea was conceived by Tom Murphy and Bill Coles at the University of California San Diego. Thus TBAD can alternately mean Tom and Bill’s Aircraft Detector.

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Lasers and Aircraft

Those of us who use green lasers for astronomy outreach are always worried about law enforcement cracking down on these devices. As the lasers get cheaper and more available they inevitably get into the hands of those who do not use them responsibly. Worse, the lasers are easily available at power levels that are truly dangerous.

Laser and Stars
Deb pointing out the star ╬▓Phoenicis to VIS volunteer Joe McDonough
The problem has continued to escalate, each year there are more reported incidents of aircraft being illuminated by the laser of some idiot (yes, the correct term) who thinks it might be cool to tempt fate and the law. In 2010 there were 2836 incidents reported to the FAA, up from only a few hundred a few years before. With this sort of trend it seems inevitable there will be some sort of official reaction.

Illuminating an aircraft with a laser can be prosecuted under federal law. Not because there is any specific statute addressing lasers, but as it is deemed “Interference with a Crewmember” using an interpretation of a pre-existing 1961 federal law, specifically 14 CFR 91.11.

The FAA has put together a new webpage on lasers and aircraft safety. The page organizes and links some informative resources. This includes a couple reports on the possible effects of laser illumination on aircraft crew, as well as the legal and regulatory recommendations of the FAA. I urge anyone who uses these devices to follow the link and do a little reading.

Used responsibly these lasers are extraordinarily useful in astronomy education. Nothing grabs the crowd’s attention so quickly as that brilliant green beam. Everyone can follow along without confusion as objects are pointed to across the sky. From the constellations to the Milky Way, satellites, planets and zodiacal light, on to star clusters and galaxies, everyone knows right where to look. I do prefer lasers in the 20-30mW range, bright enough to be seen by a crowd, even under moonlight. Not powerful enough to easily injure in the case of a brief exposure to the beam.