MANASSAS, Va. — Officials in Manassas said they are getting phone calls from concerned residents about helicopters flying overhead.

In response, police and operators at the Manassas 911 call center have been instructed to provide information from the National Nuclear Security Administration. According to that agency, the helicopters are measuring naturally occurring radiation in the Washington area.

More in a press statement from the NNSA:

The National Nuclear Security Administration (NNSA) will be flying a helicopter over portions of Washington, D.C., between Dec. 27, 2012, and Jan. 11, 2013. The purpose of these flights is to measure naturally occurring radiation in the Washington, D.C., area. The radiation assessment will cover approximately 70 square miles and NNSA will complete the assessment using a helicopter equipped with remote gamma radiation sensing technology. The helicopter will fly in a grid pattern over the areas, 150 feet or higher above the ground surface, at a speed of approximately 80 miles per hour. Flyovers will occur only during daylight hours and it is estimated to take about two weeks to complete the assessment. The measurement of naturally occurring radiation to establish baseline levels is a normal part of security and emergency preparedness. NNSA is making the public aware of the upcoming flights so that citizens who see the low-flying aircraft are not alarmed. The NNSA’s Remote Sensing Laboratory (RSL) out of Joint Base Andrews will be performing the assessment for local law enforcement of Washington, D.C.

The agency has several social media channels including Facebook and Twitter but none warned about the overhead helicopter flights.