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Automatic dependent surveillance


Nuances and benefits of ADS-B, C and R systems.

Broadcasts identification, position, altitude, and velocity to other aircraft, ground vehicles and ATC.
By Glenn Woodward
Contributing Writer

Much has been written about automatic dependent surveillance (ADS) and its various subsets – B (broadcast), C (contract), and R (rebroadcast), the latter of which receives ADS-B Out position reports between 1090 MHz ES and 978 MHz UAT. The extent of information available is extensive, and articles can be very specific, very vague, or both.

The challenge for many is absorbing and assimilating all of that data. I aspire to simplifying some of the facts and offer a framework which readers can build on if they want to unpack the various spokes of this alphabet wheelhouse.

Broadcast frequency 1090 MHz is mandated globally and in US Class A Airspace, and the ES part (Extended Squitter – mode S required for implementation) is a message or series of messages.

Broadcast frequency 978 MHz UAT (Universal Access Transceiver) is for use below 18,000 MSL.

An aircraft equipped with ADS-B Out transmits data about itself and its flight regime about once every second. Aircraft with ADS-B In capabilities receive data broadcast from other aircraft along with certain other information to include Traffic Information Service (TIS-B – available via 1090 MHz ES or 978 MHz UAT) and Flight Information Service (FIS-B – available only via 978 MHz UAT).

Other elements include SwiftBroadband-Safety (SB-S), a global, secure IP connection for operations and safe communications; North Atlantic Organized Track System (NAT-OTS), with 80% of transatlantic traffic passing through Shanwick OCA; GPS; and Air Navigation Service Provider (ANSP).

Receives ADS-B Out traffic information broadcast by other traffic as well as ATC.

Implications for ATC

ADS, with its peripheral and parallel participants, has a profound effect on both strategic and tactical applications of air traffic control. As an air traffic controller historically and exclusively immersed in the military/contract-ATC world, I can attest that ADS-B, ADS-C, Controller Pilot Data Link Communications (CPDLC) and Mode S have always been part of the infrastructure within which I serve the aviation community.

Several features and applications of ADS-B/C overlap, intertwine, and cross operational paths, making their definitions or the nature of their operations impossible to separate.

The bigger picture these technologies provide is a comprehensive monitoring and surveillance system designed and constructed to both radically increase efficiency and profoundly improve safety for flight ops – both on the ground and in the air.

These objectives are accomplished by an exponential increase in the accuracy of position verification of aircraft and airport vehicles (if equipped) in near real time. Just a few of the benefits of satellite-based ADS-B are reduction of departure separation, up-to-the-second monitoring of runway occupancy time (ROT) for arrival separation, more than 90% reduction of inflight separation, and a drastic reduction in unauthorized runway incursions by vehicles or aircraft.

ADS-B spikes your heart rate with either fear, anxiety, or glee. It all depends on whether you are the operator who has to integrate the mandated equipment along with all its implications (processes, procedures, etc), or the Part 145 station with a steady line of operators who need the system installed in their aircraft.

The people with the smallest angst are the front line air traffic control officers (ATCOs). Whereas air traffic controllers are touted as the primary beneficiaries of this new system, they are, and will continue to be, facilitators of the end result: more things being delivered faster and in greater quantities.

Traditional secondary surveillance radar (SSR) controllers must adjust to a new paradigm of precision paired with tighter separation minima. The technology of ADS locates and displays targets precisely and appropriately for control and management. For the controller on the scope, it means putting more aircraft within the same defined airspace parameters that previously could fit fewer airborne airframes, whether it’s class A, B, or C.

NATs could soon be free routing and self-separating with satellite-based ADS-B monitoring.

North Atlantic Tracks

For those controllers handling the North Atlantic Organized Track System (NAT-OTS), ADS-B/C and CPDLC, employing satellite communication technology, have reduced the minimum lateral separation of 60 nm to 23 nm, then to 19, with a projected decrease to 15. NATS (UK) will eventually transition to free routing with no defined track separation for operators.

This will allow individual operators to choose their optimal flight regime, saving millions of hours of flight time. For ATMs, this poses a tsunami of data for airspace management analysis.

This includes not just refining and tweaking the last few feet of airspace to exploit, but planning even further into the future to incorporate predictive analytics for increased safety and further savings.

For tower controllers, knowing the exact position of aircraft in the circuit/pattern can help avoid squeeze plays when a pilot may “fudge” his reported location to jump ahead in line to the runway. Also important is an exact distance figure for aircraft on final approach when deciding to launch, or not, another aircraft while visibility may be less than ideal, yet still above that magic threshold for VFR/IFR ops.

I did not find any conversations about a reduction of IFR departure separation, but that does not mean there aren’t any. ADS is also vital during periods of extremely reduced visibility on the airfield. Discerning the exact location of aircraft and/or airport vehicles equipped with ADS-B is crucial to preventing runway incursions and other accidents with aircraft, vehicles, or airfield obstacles.


Pilots operating in the NAT-OTS are familiar with ADS-C and CPDLC, as well as the SAT/HF communication requirements for position reporting. ADS-B could arguably be defined as the blue-collar sibling of ADS-C.

Whereas C creates digital contracts with aircraft and compiles information in various formats, quantities, and specificity with various broadcast-termination options, ADS-B merely transmits a pre-defined packet of data for consumption by controllers and other properly equipped aircraft.

Admittedly, very precise data that facilitates application of separation procedures and techniques and provides traffic and weather info is freely available to those aircraft equipped with a 978-MHz UAT transceiver (ADS-B In). ADS-B Out entities have no control over who receives the information and no interrogation is required.

The controllers are pivotal in the execution of the benefits of both C and B systems. They establish separation between aircraft with the information provided. Aside from the failsafe aspect, with ADS-C, only authorized agencies may receive certain data, if approved by the aircraft. In emergencies, however, if not deselected by the flightcrew, specific data can be broadcast irrespective and independent of internal aircraft power sources.

CLASS A/ADS-B 1090 ES Required

Possibilities for ATCOs

For all ATCOs using ADS systems, having the certainty of an exact location and relevant data immediately results in fewer radio calls to confirm position, meaning there’s more time available on the radio for actual control instructions and, by extension, a safer flying environment.

It is conceivable and regularly discussed that aircraft could self-separate in certain areas and under certain conditions. However, that is an enormous can of worms with regard to liability, training, and what would become a new definition of separation. As a concrete and viable objective in any published source material, I have yet to find it.

Truthfully, there is a real possibility of fewer controllers handling more aircraft. Yes, the system will assist greatly with the additional application of artificial intelligence and more automation. However, like all new systems, it could also be an impetus for a smaller ATCO cadre.

This would shift the workforce in the direction of system maintenance and monitoring. In other words, it would be far from actually controlling. This is not necessarily a bad thing – simply one to consider. The system could also be a wash with more aircraft able to use existing airspace and more controllers handling the increased volume.

Here is where the unions will want a large voice at the table. To me, this is starting to sound vaguely familiar to the argument made for cockpit automation.


Recently, FAA announced its decision to move forward with ADS-C along with Mode S, contrasting a rapidly expanding global client base happily entrenched in the satellite-based ADS-B corner.

Currently, the US is also heavily vested in ADS-B via multiple resources with an ambitious and optimistic radar-decommissioning goal of 50% by 2025. But there is one important nuance to the FAA statement regarding this decision. As it reads, ADS-C is “ops du jour” for US Oceanic airspace operations and separation applications, whereas for NAT-OTS, controlled by Canadian, Irish, and UK aviation authorities, satellite-based ADS-B will be de rigueur.

Aireon, a major player in the game that is 51% owned by Nav Canada, recently signed the Central American corporation for air nav services (COCESNA) into its satellite-based ADS-B system. They also integrated SAGITARIO, the air traffic control automation platform utilized by the Brazilian department of airspace control.

These are 2 additional authorities affiliated to a robust list of global regions already using, or planning to use, ADS-B. That said, ADS-B is still a very important part of FAA’s NextGen. FAA concedes that ADS-B, as another layer of safety, is not a negative. In controlled airspace, however, where multiple procedural standards are in play, the one with the larger separation minima will apply, essentially negating the benefits of the new technologies.

This invites the question: Will ADS-C separation be different from ADS-B, given that the possibility of self-separation exists for B? I can see that option in the Gulf of Mexico and other unique operating environments, but not over the distances required to cross an ocean.

It may be that an employment priority is assigned to one system over the other in the case of a degradation of those layers of safety, in the same way that radar procedures are used over non-radar procedures and shifted when the primary system fails. Had we had these technologies in 1937, ANSPs would have tracked, located, and responded to Amelia and Fred’s distress signals.

Emilia Earhart, while almost certainly not alive today, could have left a far greater legacy and arguably been a more powerful catalyst for women to participate at greater levels than we have today in aviation and space exploration. I would go so far as to say there would be at least a major international airport or aviation facility named after. But I digress.

Brass ring

ADS-B In/Out and ADS-C are game-changers when employed. Please note: ADS-B/C did not and do not need satellites to function. Previously, those same data signals were transmitted to ground-based stations (1300 in 25 European states) that re-transmitted the information to ATC or other aircraft. The limiting factor was, as in traditional communications, line of sight.

In other words, geography and terrain were the barriers to complete near-real-time positioning verification necessary for separation. This ground-based system was restricted in its application by severely constrained access to remote areas of the world. Such limitations have a direct effect on both installation and follow-up maintenance.

However, these were areas of the world over which flight operations most needed the coverage for safety and separation – precisely the benefits that satellite-based surveillance delivers. Initially, though, the same restrictions that prevented the installation and employment of traditional radar systems also prevented ADS-B via ground stations from providing the comprehensive coverage and safety it afforded.

What satellites did was free the signals from terrain-based interference. Now, those same ADS-B/C signals are picked up by a network of satellites that re-broadcast the data unfettered directly to ATC, other ground stations that could re-broadcast to other ATC facilities, or aircraft in the air and on the ground.

There are still many ground-based stations operating happily around the world, but going forward from this year, satellite-based surveillance should be the brass ring for ANSPs around the world.

ADS-B equipment helps prevent accidents in many situations, such as taxiing when visibility doesn’t allow pilots to see other aircraft or airport vehicles near by.

ADS-B Out/In and ADS-C

ADS-B Out provides information for the objective of separation. ADS-B In allows pilots to receive weather data capable of being displayed on a PFD or an EFB device, along with Mode S and traffic info to assist TCAS and ACAS II functions.

ADS-C is an aviation cornucopia of information available in 3 formats. I think FAA’s decision was stout and provides several security aspects that ADS-B does not.

Where various aviation ANSPs are in place around the world, ADS-B is a fantastic solution and a huge technological leap forward in terms of services provided.

In terms of cost, it is also much less expensive than a region/country/ocean-wide radar system for comprehensive coverage and sustainability, and here is where Aireon is capturing markets and clients.

Countries can easily buy or contract for a package that upgrades their existing services and brings them into compliance with ICAO’s new 15-minute reporting standards. All of this translates into more airplanes in the air, and, subsequently, fewer on the ground.

That perhaps means fewer gate-holds for airspace congestion and shorter lines to the runway, again reducing wasted fuel, which could, and should, translate into lower costs and lower ticket prices.

Moreover, FAA and private companies are spending billions of dollars to upgrade the ATC/ATM system domestically and globally. There are some cost-related questions to consider. Will an increase in flights pump up the coffers of airports, airlines, nations or states? Will the increased traffic generate elevated taxes sufficient to maintain the system and prevent higher costs to passengers?

Will the ROI to passengers mean that they realize a remarkable tangible benefit? Or will FAA introduce a new fee to offset the costs of implementing this new system, switching to a fee-based model that currently provides free TIS/FIS data? Will flights using ADS technology be more expensive than flights over areas where they do not employ satellite-based ADS-B ATC services?

An experience in Kabul

Even as a lowly contract-tower controller, I am aware of the enormous benefits available using satellite ADS technologies. Here are some personal real-world examples.

Our crew received a call from the tower supervisor. We were driving back to the main base after completing our shift, and he asked if we put a vehicle on the runway. “Of course not!” we answered. He asked because he had just received a report from a landing Boeing 747 pilot that he rolled out over a small truck under his right wing between #3 and #4 engines about halfway down the runway.

A week later, a departing Boeing 767 pilot also queried the tower about a small truck he saw under his left wing at V1. In both cases, which occurred at night, ADS-B would have alerted the tower to the unauthorized runway incursion. Fortunately, no one was injured and the operational environment was very laissez-faire.

Considering the other possibility, the consequences would have been devastating. They would have included loss of life, aircraft, and runway use in a strategically vital airport and region, and a negative impact on US global strategic policy.

For the approach and ACC controllers, during the initial push and build-up, ADS-B would have prevented at least 2 crashes, both CFIT. One commercial Boeing 737 airliner loaded with passengers (IFR), and one contract Lockheed L-100 (VFR) with 5 POB. Both aircraft flew into nearby mountains. A

DS-B would have given controllers exact position data to include altitude as well as climb/descend rates in time to warn both aircraft to prevent these tragedies. Even taking controllers out of the equation in these examples, the pilots, had they been flying aircraft equipped with ADS-B Out/In, could have ascertained via their own PFDs an accurate presentation of their position with respect to the local terrain and taken action to avoid disaster.

In a few years, I can envision the water-cooler talk at ATC training facilities as they try to imagine what controlling must have been like in the past (2015–2019) before global coverage of satellite-based ADS systems, and how we only had to handle half the volume of airplanes they do with new pilot-mandated self-separation protocols.

Glenn Woodward is an air traffic controller with 18 years of tower experience in the US, UK, and Afghanistan. He is an FAA-licensed flight dispatcher as well as a veteran flightcrew member on Boeing B-52s.