Maybe you've just seen The Bourne Legacy and have nagging feeling that you're being watched at all times. Maybe you related a little too much to Close Encounters of the Third Kind. So if you see low-flying helicopters crisscrossing the skies this week, you might think the government is after you, or that the aliens have finally arrived. But the more likely explanation is that you've caught a glimpse of the federal effort to measure "naturally occurring background radiation."
Flying at a height of approximately 300 feet, the helicopters will be gathering information for a research project carried out by the U.S. Department of Homeland Security's Domestic Nuclear Detention Office and the National Nuclear Security Administration (NNSA). According to a statement released by the NNSA, mapping radiation levels will help set baseline safety levels for radiation. The flights will be limited to daytime hours, so if you see one after the sun has set, then yeah, you might have something to worry about.
You can find more information at these links:
http://www.sfexaminer.com/local/2012/08/helicopter-testing-radiation-levels-san-francisco-low-altitude
