For decades aviation hobbyists have enjoyed observing, tracking, and photographing aircraft. "Spotters" flock to airport fences and parking garage roofs when unusual and exotic aircraft transition through airports nationwide. At home, the same hobbyists can now enjoy tracking interesting aircraft using internet flight data vendors such as FlightAware, FlightRadar24, and other publicly available websites. Unfortunately, technology-savvy criminals have also learned how to provide their own counter-surveillance using publicly available aircraft tracking websites that can watch the operations of law enforcement aircraft.

Transmitting Data

Generally speaking, aircraft traverse the world receiving radar separation instructions from air traffic control in part due to an aviation electronic transmitter onboard the aircraft called a transponder.

I don't want to get too far into the technicalities of the different transponders, but most aircraft operate using one of two main types of transponder systems: the Mode S and the Mode S-ES. The Mode S transponder transmits an aircraft's position, altitude, and unique aircraft registration when interrogated by radar. The Mode S Extended Squitter (Mode S-ES) transmits the same information as the Mode S continuously without interrogation by radar. Another difference is that the Mode S-ES transmits and receives more precise location and altitude information based on satellite data.

A critical thing for you to know about both Mode S and Mode S-ES transmissions is they are not encrypted and can be received, stored, and analyzed by open source receiver systems and software. Consequently, internet flight data vendors can currently track

Mode S equipped aircraft independent of any FAA provided flight data.

And soon internet vendors will be able to track aircraft more precisely. The Federal Aviation Administration (FAA) has mandated that all aircraft operating in most controlled airspace be equipped with Mode S (ES) ADS-B (Automatic Dependent Surveillance-Broadcast) Out technology no later than Jan. 1, 2020.

The highly precise GPS accuracy of ADS-B information provides for a significant improvement in safety and efficiency. While radar systems feed air traffic controllers an aircraft's position every five to 12 seconds, ADS-B transmits the aircraft's position and information every second. Additional features of ADS-B include satellite weather and airspace restriction information being fed directly to pilots to improve situational awareness.

That's the good part of the change to this technology. The bad, or potentially bad, is that it makes it easier to track an aircraft on the internet.

Flight tracking websites such as FlightAware and FlightRadar24 offer incentives to subscribers who are willing to place an ADS-B receiver system in their house and feed the data it collects to their website. Other websites such as ADSBExchange provide instructions for how to build an ADS-B receiver system using easily sourced components for around $100.

Tracking Police Aircraft

What does this mean for law enforcement agencies?

Although internet flight data vendors can currently track all military and law enforcement aircraft that use Mode S transponders, the use of Mode S-ES ADS-B technology presents additional risks.

In January 2018, the United States Government Accountability Office released a report outlining the specific risks related to aircraft tracking using ADS-B Out technology. The report warns: "Individuals—including adversaries—could track military aircraft equipped with ADS-B Out technology, posing risks to physical security and operations."

Of course, this concern of compromising physical and mission security also applies to law enforcement operations.

A series of articles by BuzzFeed during a one-year period from 2016 to 2017 revealed how law enforcement aircraft can be tracked over the United States. The article "Spies in the Skies" highlights the FBI and Department of Homeland Security utilization of Mode S equipped airborne surveillance platforms following high-profile incidents such as the 2016 terror attack in San Bernardino, CA, and the protests in Ferguson, MO. Taking the issue a step further, BuzzFeed provided links to various documents that listed the registration information of aircraft the online news service believed to be linked to government agencies.

The Columbia Journalism Review conducted independent research into the tracking of government aircraft by creating a computer algorithm that mirrored the efforts of BuzzFeed. In both of these cases, the computer algorithms were developed using data gleaned from the publicly available information found on websites such as FlightRadar24, FlightAware, and ADSBExchange.

For the aviation hobbyist, these systems are designed to be operated in the background of the individual's home or office. Additional software packages translate the information received by the antenna into mapping software that displays live data for the user while simultaneously feeding the device's associated website. However, the ability exists for the device to be turned into a portable ADS-B monitoring system.

The advertised range of devices such as FlightAware's PiAware is between 50 and 200 miles, depending on normal reception factors such as antenna placement and structural obscurations. By connecting the device to an inexpensive power inverter found at any large retail store, the user can operate the ADS-B monitoring system in a land-based vehicle or boat and have an instant mobile monitoring platform.

The Solution

The FAA and other applicable federal agencies are evaluating mitigation techniques. However, a limited number of viable options are already available.

Legal rulemaking changes are being evaluated by the federal government regarding the mandatory broadcast of an aircraft's ADS-B Out transmission while the aircraft is in flight. Potential changes include regulatory relief to government agencies conducting national security and law enforcement-related missions.

Law enforcement agencies that utilize an aviation support unit can receive a briefing on options available to them by contacting the Federal Aviation Administration System Operations Security at 9-ATOR-HQ-IFOS@faa.gov (please put LEA ADS-B Support in the subject line). Agencies can also get in touch with their regional FAA Law Enforcement Assistance Program (LEAP) Special Agent by emailing LEAP@faa.gov .

Michael Hamann is a certified air traffic control specialist with the Federal Aviation Administration. He holds a master of arts degree in emergency management and homeland security from Arizona State University and a Bachelor of Science Degree in aeronautical studies from Embry-Riddle Aeronautical University. His views do not necessarily represent those of the FAA.