Featured,  in detail

The New Noisy Sky

Context

The surprise attack on Pearl Harbor shouldn’t have been a surprise. For starters, there was the small matter of a radar warning.

An hour before the planes arrived, two communications operators on the northern tip of Oahu noticed a sudden spike on their radar. This radar was not the giant sweeping kind seen in most war movies. Its five-inch screen could only display white spikes against a gray background. The farther the spike was to the right, the farther the airplane was from their radar. The taller that spike the more aircraft there were.

The spike these two operators saw that morning was tall and far to the right, indicating a cluster of planes roughly 130 miles out. And they were moving steadily toward them from the north.

Early oscilloscope-based radar (model A). The azimuthal angle was determined by sweeping the antenna until the spikes were maximized. On the morning of the attack these spikes were the only evidence of incoming planes.

 

After some debate about what to do next, the operators reported “a great number of planes from the north” to a phone operator at the local command center. Twenty minutes later a junior officer, new to the job and unfamiliar with radars, received that warning. His conclusion? The planes were just a squadron of six B-17s flying back from San Francisco. “Don’t worry about it,” he told them over the phone. He then wadded up the note and tossed it in the trash.

Had reconnaissance planes been deployed, they might have seen the big red circles painted on the sides of those aircraft. Or the shallow-water torpedos strapped to their underbellies. Or that there were 183 aircraft in that first wave. But most of all, they would have verified that this tall radar spike was most definitely not a squadron of six B-17s from San Francisco.

An early headline on the attack. The actual number killed was 2403 with an additional 1178 injured.

This fiasco was actually the last in a cascade of intelligence and tactical failures that extended far beyond the events of that morning. As a result, the Navy likely wouldn’t have been able to fend off the Japanese anyway. But there is a bright side. That final misstep accelerated engineering developments in air surveillance. Those developments can now provide context for the significant changes in surveillance that have occurred in the last decade.

 

Friend or Foe

The proper identification of distant aircraft was a priority among military engineers long before the sting of Pearl Harbor. In military jargon this is called “identification friend or foe” or IFF for short. In addition to enemy aircraft detection, IFF also addresses the complementary issue of fratricide, air defenses attacking their own. Both sides of the war effort were interested in this, of course, and both eventually devised workable solutions as the war progressed.

The solution that stuck was to outfit aircraft with devices called transponders. These are small radio units that respond to interrogation signals from ground-based stations called Secondary Surveillance Radar, or SSR. When pinged, transponders send back an aircraft’s identity and perhaps some additional information like altitude too. One way to think about it is that if transponders are the ID badges of the skies then the SSRs are the bouncers carding them. For military aircraft this process is encrypted to prevent enemy aircraft from faking a friendly identity.

After the war, this call-and-response system got adopted by civilian air surveillance too. Combining radar position with the additional data from a transponder allows an air traffic control tower to see both an aircraft’s position and its call sign. Six decades later this model has remained largely unchanged, even with the many evolutions in transponder and radar technology.

 

Aging Out, Looking Up

In the mid-1990s the FAA and the International Civil Aviation Organization (ICAO) started a campaign to migrate from radar to satellite air surveillance. They cited an ever-thickening airspace, aging radar equipment and the problem of radar blind spots—there just aren’t many places to put radars in open water and around mountain ranges, for example. Note that the commercial aviation industry already had a solution for these blind spots via long-range ACARS systems but the FAA and others were interested in a more general solution.

This change meant turning the transponders into something that could broadcast more detailed telemetry and do so without prompts from external beacons. Researchers at MIT’s Lincoln Laboratory were tasked with this problem, a good choice too since the lab also created the Mode S transponder found in over 100,000 aircraft worldwide.

When the dust cleared, the FAA and ICAO had a new wireless technology, Automatic Dependent Surveillance Broadcast or ADS-B. Designed to compatible with both Mode S and UAT, ADS-B can transmit nearly a dozen flight metrics and do so without prompts from remote beacons. Think of it as a flight data recorder blaring out the nose of every passing aircraft.

ADS-B’s ability to self-broadcast also means that it can transmit to low-orbit satellites. A secondary task would therefore be to deploy ADS-B receiver satellites in areas beyond the reach of radar—oceans, mountain ranges, etc. 

Diagram from Boeing.com

The acronym ADS-B stands for Automatic  Dependent Surveillance Broadcast. Automatic because it doesn’t need to be triggered, dependent because it relies on on-board GPS systems, and broadcast because it has no knowledge of who is receiving it. This differs from a traditional transponder which sends identification and altitude data only when an interrogation request is received (“friend or foe”). ADS-B can be used for in-flight surveillance of nearby air traffic too. These receivers are called ADS-B IN, as opposed to those that broadcast called ADS-B OUT. 

 

 

Approvals and Pushback

In 2007 the FAA unveiled the NextGen Air Transportation System, a sweeping air surveillance and safety program that relied heavily on this new technology. Congress allocated fifteen billion dollars to the FAA for this program with an understanding that the aviation industry would bear an additional fifteen billion in aircraft upgrades and other costs. This latter cost included the proposed satellite deployments. To that end, the FAA issued a 2010 mandate that all aircraft operating in the national airspace be equipped with an ADS-B transmitter by January 1st, 2020. Similar mandates were simultaneously being implemented by nineteen other air transportation authorities worldwide.

There was significant pushback in the general aviation community though:

“The high cost of the necessary avionics and the lack of direct benefits are the two greatest barriers to the adoption of ADS-B Out by a large segment of general aviation operators.” – Aircraft Owner and Pilots Association, 2014

The Aircraft Owner and Pilots Association elaborated that a basic ADS-B transmitter cost around six thousand dollars (plus installation) with the higher end systems costing upwards of $20K. Furthermore, these ADS-B OUT devices need to be connected to a separate GPS/GNSS system too, which means an additional install and purchase if missing. The pilots association further predicted the following:

AOPA expects many of its members to delay equipage as long as possible and anticipates some will be forced to ground their aircraft in 2020 unless a lower cost solution can be found. 

By 2019 only 40% of general aviators had equipped their aircraft with ADS-B. Then the deadline arrived and so did Covid-19. Those who hadn’t upgraded in time found themselves facing a significant backlog before they could fly again. On the commercial side, only one carrier missed the date, BahamasAir, who claimed their Boeing 737s were hit with a parts shortage. As promised the FAA immediately blocked those planes from flying into US airspace.

 

The Noisy Skies

Raspberry Pi ADS-B Ground Receiver (bear not included)

There are now 160,000 aircraft equipped with ADB-S in the US alone. Most of the other participating nations are similarly compliant. Supporting all of this, an armada of commercial satellites are now in fixed orbits over remote ocean and mountain terrain (note that communicating with these satellites only works with 1090ES models and requires an additional antenna on the aircraft, i.e., it’s geared more for commercial aircraft). The end result: our skies are now abuzz with ADS-B traffic.

Anyone with a suitable antenna and a single-board computer (like a Raspberry Pi) can tune into the flight data overhead — much like the ham radio operators of old. Think home weather station, except flight data instead of dew points. Aviation enthusiasts proved eager to share their data too, giving services like FlightAware a huge coverage boost for the super low cost of free. There’re even services like ADS-B Exchange that are based (almost) entirely on do-it-yourself stations like this.

Combining these civilian receivers with those already available has created extraordinary coverage worldwide—the majority of airborne aircraft are now trackable from take off to landing. Consequently, anyone with internet access can view realtime flight data for these aircraft including make and model, altitude and speed, flight history, even the owner’s name and a photo of the aircraft itself.

There’s much to learn clicking around on these apps. For example, did you know that Quest Labs does late night blood pick-ups in a Pilatus PC-12? Or that Texas law enforcement has a fleet of Cessna Caravans?

FlightRadar24.com view of the aircraft flying over the Indian Ocean with a close up on one of those flights.

 

ADS-B Exchange View of Western Europe showing all ADB-S equipped airplanes in flight

 

Privacy and Security

Elon Musk didn’t like all this publicly available flight data. In 2019 the controversial entrepreneur took action to try to stop his personal air travel from being published by the citizen-run ADS-B Exchange. Similar privacy concerns were raised by Saudi royals and the Chinese government. The FAA has been steadfast about this, saying, “There is no right to privacy when operating in the national airspace.”

Yet the demand for more flight privacy persisted and eventually the FAA and commercial flight tracking services began offering paid privacy options for those who didn’t want their movements disclosed. The community run ADS-B Exchange, however, has continued to resist such an offering.

A related sticking point is that this flight data lingers. ADS-B flight logs are automatically archived by the FAA making it a tool for NTSB and law enforcement. The FAA also uses it to investigate aviator infractions retroactively—at least one pilot has already lost her license because of such an investigation.

Questions have been raised about the reliability and security of these transmissions too. They’re not encrypted, which means they’re viewable by all. There’s no way to verify that a transmission is authentic (i.e, cryptographically unsigned). And a transmission can originate from anywhere (i.e., peer-to-peer) and not via a controlled environment like a web server or a telecom router. Collectively this means that ADS-B signals can be faked. And they can be faked in high volume, which could create all sorts of chaos and confusion.

The FAA responded to these concerns:

“The FAA concludes that ADS–B transmissions would be no more susceptible to spoofing (that is, intentionally broadcasting a false target) or intentional jamming than that experienced with SSR transmissions (Mode A, C, and S) today. Spoofing of false targets and intentional jamming very rarely occur with the surveillance systems in place today.” — FAA Rule 05/28/2010

But they also hedged against this assertion by requiring that air traffic control centers validate all ADS-B transmissions against radar. Airport surveillance has long been based on a composite of sources. If one is erroneous, the others will contradict that. If one source goes down, the others will still operate. One argument is that ADS-B was intended to replace radar and here it is depending on it. Another is that with ADS-B they’ve just added another information layer to an already redundant system. Whether that new data will prove more reliable is yet to be seen.

Many of these concerns will probably tail off over time. After all, we as a civilization have grown strangely numb the level of surveillance that exists in our world. When we walk our dogs or drive around town, our phones silently inform nearby cell towers of our movements. On that same dog walk, we also trip scores of front door cameras, which then stream our passing into the proverbial cloud. Not to mention how our online footprint gets bought and sold like so many eggs. If you want privacy—real privacy—turn off your phone and go live in a cave. But pilots don’t really have that luxury. Also, it’s really hard to fly in a cave.

 

Epilogue

It should be noted that the FAA does allow numerous exceptions on ADS-B usage. Aircraft operating outside the zone of a major airport and flying below 10,000 feet are exempt in some scenarios, though this severely limits travel options. A similar exemption applies to non-electrical aircraft, such as vintage planes.

Perhaps ironically this includes any World War II airplanes still in operation.


References

General

Wiki on ADS-B

FAA NextGen

ADS-B Exchange

Pearl Harbor

[1] Firsthand account by radar operator at Pearl Harbor

[2] Interview with that radar operator

Surveillance Overviews

[3] Wiki on Friend or Foe Systems

[4] More info on IFF Systems

ADS-B, ICAO and NextGen

[5] ADS-B Message Format

[5a] MIT’s Lincoln Lab and their role in ADS-B

[6] ICAO Guide to Global Surveillance

[7] ICAO Summary of ADS-B

[8] FAA Guidelines on Implementing ADS-B

[9] Data link formats: 978UAT vs 1090ES

[10] Satellite ADS-B Deployments

[10a] About Space-Based ADS-B

[11] Aircraft Owners and Pilots Association (AOPA) Study on Improved Safety with ADS-B IN

[12] AOPA: Rules and Requirements for ADS-B

Cost Issues

[13] AOPA: 2014 Statement on Costs to Small Aircraft Owners

[14] US Government Accountability Office: Cost Estimate on FAA NextGen

Security Concerns

[15] Journal Paper on ADS-B Security

[16] Journal Paper on ADS-B Security with recommendations

[17] Forum Discussion on LAX and ADS-B

ADS-B 2020 Deadline

[18] AOPA 2018 Article on Pending Deadline and its Impacts

[19] Bahamas Air Banned

[20] FAA § 91.225 on Deadline

ADSBExchange.com and FlightAware

[24] Wired: Elon Musk vs ADS-B Exchange

[25] Raspberry Pi ADS-B Receivers

[26] FlightAware DataSources

FAA Retroactive Investigations

[27] Pilot Loses License

Other Flight Services

[28] Quest Air Operations

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