AIR HAZARD: Open Source Flight Tracking Called threat to Military Aircraft.

• Author: Tegler, Jan

Publicly available flight tracking data is a growing problem for the U.S. military, a senior Pentagon official recently said.

Websites such as AD SBExchange.com, FlightRadar24.com and FlightAware.com aggregate flight data in the United States and abroad using a mix of commercial and citizen-owned sensors that capture the movements of commercial, civilian and military aircraft in real time, 24 hours a day.

"The Department of Defense considers open source flight tracking and data aggregation on our aircraft a direct threat to our ability to conduct military air operations around the world," the Air Force said in a statement responding to National Defense's questions. It cited a "senior Defense Department aviation policy expert," who it declined to name.

Thousands of aircraft in flight are visible on the websites, displayed as silhouettes overlaid on digital maps that cover the globe. Click on a silhouette and users see aircraft type, identity and ownership, altitude, speed and heading/track as well as origin and destination information.

Capturing the data is possible because of international regulations that require aircraft to broadcast position information via radio-based transmitters known as transponders. Transponders allow air traffic controllers to coordinate the movement of aircraft and enable aircraft equipped with avionics and software that receive transponder signals to see other aircraft around them on cockpit display screens as they fly.

The transponder signals can also be captured by inexpensive, ground-based receivers that hobbyists, aviation enthusiasts and others can build themselves for less than $100 using widely available hardware and software, some of which can be obtained from the flight tracking websites.

In the United States, almost every type of plane--from airliners and small privately owned aircraft to military fighters, helicopters, bombers, tankers, intelligence-gathering aircraft, transports, special operations planes, drones, and even the VIP aircraft that carry the president and members of Congress--are required to transpond in controlled airspace.

Most are now equipped with Federal Aviation Administration-mandated Automatic Dependent Surveillance Broadcast transponders, known as ADS-B.

The Pentagon has agreements in place with the FAA and other international air navigation service providers to turn off ADS-B when it is deemed necessary. But to help facilitate the safe and efficient movement of air traffic, U.S...