Chopper watchers: Get ready for the annual pre-Marathon background-radiation check

In what has become an annual tradition, the National Nuclear Security Administration will fly a helicopter low - and loud - along the Boston Marathon route Friday and over the weekend.

Devices onboard will record background radiation readings that would aid in response to any nuclear incident during the Marathon. In describing last year's flyover, the agency described the mission:

The helicopter will fly in a grid pattern over the areas at 150 feet (or higher) above the ground at a speed of approximately 80 mph. Flyovers will occur only during daylight hours and are estimated to take approximately two hours to complete per area.