Lately, I’ve been using some of the very inexpensive Realtek Software Defined Radio (RTL-SDR) receivers to passively listen to different radio bands.  The RTL-SDR chipset effectively lets any personal computer receive radio broadcasts: from commercial radio stations, to ham radio repeaters, and shortwave radio just to name a few.  Even local emergency services like police and fire rescue were receivable thanks to free and open-source software.

The RTL-SDR chipset is often based on 2 important integrated circuits: the Realtek RTL2832U demodulator and the R820T2 tuner.  Paired with a proper antenna, the device can receive radio emissions ranging from about 24 MHz to well over 1 GHz.  One of the more intriguing signals available near this range is the Automatic Dependent Surveillance-Broadcast (ADS-B) waveform at 1,090 MHz.

ADS-B Background Information

The ADS-B system was envisioned as a way to transition air traffic control and air traffic surveillance from being ground-based to being ground and air-based.  This was motivated in part by increasing congestion in air routes around the late 1990s.

Air traffic control radar systems throughout much of the 20^th century located aircraft using revolving ground-based antennas.  These systems are generalized as:

• Primary surveillance radars (PSRs), which operate by “pinging” the sky with radio waveforms and listening for returned echoes;
• Secondary surveillance radars (SSRs), which interrogate a transponder aboard the aircraft for a reply.  Unlike primary surveillance radar, this requires cooperation with the aircraft operator; and it won’t detect vehicles if their transponder is defective, switched off, or non-existent.

In short, ADS-B was made to operate alongside primary and secondary radar networks.  This would provide updates on aircraft positions and headings with reduced error and latency, which were major figures of merit during ADS-B’s implementation.

The move is also helpful for airports without control towers, which constitute a majority of the nation’s airports.  The United States Federal Aviation Administration (FAA) would achieve this as part of a long-term rollout plan called NextGen.

Today, many aircraft in the United States are broadcasting ADS-B messages constantly.  This is a function of ADS-B out, a standard for having the aircraft transponder periodically broadcast its 3-dimensional position and velocity.  It is still up to the aircraft to determine its own position using Global Navigation Satellite Systems (GNSS) and an altitude encoder.  The ADS-B data format itself is just part of a larger data standard called Mode-S Extended.  All of these data are transmitted at 1090 MHz as squitter, meaning it happens in an unsolicited manner.  The radio broadcasts can then be decoded on the ground using open-source utilities such as dump1090, rtl_adsb, or pyModeS.

One interesting piece of information is the International Civil Aviation Organization (ICAO) field.  Using public information, knowing the ICAO field yields search results about the vehicle owner or operator. The ICAO search may also reveal further details such as the aircraft model or flight plan.

Changes Coming to ADS-B Privacy

This ability to check the identity of any ADS-B out-equipped vehicle brings us to the privacy controversy behind the system.  The NextGen program includes an ADS-B equipage mandate.  According to the FAA, air vehicles flying in ADS-B airspace need to be ADS-B equipped by January 1^st, 2020.  After the mandate goes into effect, non-equipped planes must obtain authorization to operate in these locations prior to each flight.

Because of the extensive flight tracking possibilities afforded by low-cost equipment, the general aviation community has requested new privacy mechanisms to protect the anonymity of operators and owners.  To date, one way to mitigate privacy concerns was through the optional Blocked Aircraft Registration Request (BARR) program.  However, the BARR program is easily bypassed by ground-based receivers and by correlating data between broadcasts.

In response, the FAA announced upcoming changes to the way it processes ADS-B information about aircraft.  According to the FAA, this includes a Privacy ICAO Address (PIA) program.  Registering in the PIA program lets operators use a third-party call sign.  This allows flights to be tracked for air traffic control purposes while anonymizing the vehicle to the general public.

Conclusions

The latest privacy programs to affect ADS-B are part of a two-phase implementation by the FAA.  This is an example of a growing challenge in electronic communications, where there is a clear desire to start protecting user privacy in broadcast communications without resorting to cost-prohibitive mandates or methods.

References

[1]	G. Donohue, “Vision on aviation surveillance systems,” in Proceedings International Radar Conference, Alexandria, VA, USA, 1995.
[2]	C. Rekkas and M. Rees, “Towards ADS-B implementation in Europe,” in Tyrrhenian International Workshop on Digital Communications – Enhanced Surveillance of Aircraft and Vehicles, Capri, Italy, 2008.
[3]	U.S. Federal Aviation Administration, “What is NextGen?,” U.S. Department of Transportation, [Online]. Available: https://www.faa.gov/nextgen/what_is_nextgen/. [Accessed 7 Dec 2019].
[4]	U.S. Office of the Federal Register, “U.S. Electronic Code of Federal Regulations: 14 CFR 91.227(d)(8),” U.S. Government Publishing Office, 5 Dec 2019. [Online]. Available: https://www.ecfr.gov/cgi-bin/text-idx?SID=8137158693744ba666e318c1f474d81b&node=se14.2.91_1227&rgn=div8. [Accessed 7 Dec 2019].
[5]	M. Strohmeier, “Large-scale analysis of aircraft transponder data,” IEEE Aerospace and Electronic Systems Magazine, vol. 32, no. 1, pp. 42-44, Jan. 2017.
[6]	M. McNamara, J. Mott and D. Bullock, “Leveraging Aircraft Avionics for Fleet and Airport Management,” Transportation Research Record, vol. 1, no. 10, pp. 32-41, 2016.
[7]	J. Sun, “Aircraft Identification,” mode-s.org, [Online]. Available: https://mode-s.org/decode/adsb/identification.html. [Accessed 7 Dec 2019].
[8]	General Aviation News, “FAA issues policy for flying without ADS-B after 2020,” General Aviation News, 9 Apr. 2019.
[9]	U.S. Federal Aviation Administration, “ADS-B Privacy,” U.S. Department of Transportation, [Online]. Available: https://www.faa.gov/nextgen/equipadsb/privacy/. [Accessed 8 Dec. 2019].
[10]	W. Bellamy, “What FAA’s new ADS-B privacy policy means for business aviation operators,” Avionics International, 8 Nov 2019.
[11]	Aircraft Owners and Pilots Association, “Petition for exemption from 14 CFR § 91.227(d)(8) and (11) to allow general aviation pilots to operate in ADS-B anonymous mode while on a VFR flight plan,” 2 Nov. 2018. [Online]. Available: https://download.aopa.org/advocacy/2019/11.02.18_AOPA_Petition_for_Exemption_91.227.pdf. [Accessed 8 Dec. 2019].
[12]	K. Sampigethaya, S. Taylor and R. Poovendran, “Flight privacy in the NextGen: Challenges and opportunities,” in Integrated Communications, Navigation and Surveillance Conference, Herndon, VA, USA, 2013.

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