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FAA Chooses Enhanced ADS-C Over Space-Based ADS-B for Oceanic Airspace

The FAA is targeting 2022 for reducing the minimum separation standards between aircraft from 30 to 23 nautical miles laterally, affecting aircraft flying within the oceanic regions for which it provides air navigation services.

A July report published by the U.S. Government Accountability Office (GAO) found it would be significantly more cost-effective for the FAA to use a faster version of the existing automatic dependent surveillance-contract (ADS-C) technology within its various oceanic regions instead of investing in Aireon’s space-based automatic dependent surveillance broadcast network (ADS-B).

While both technologies can increase the frequency in reporting of an aircraft’s location, the near-term flight operational efficiency benefits of enhanced ADS-C outweigh the costs 2-to-1, according to the report. Conversely, the same independent research first that completed the FAA’s financial analysis also determined that the cost of investing in space-based ADS-B outweighs its operational benefits by a factor of 6-to-1.

At the core of the report is the FAA’s position — along with several other air navigation service providers — on how to improve their existing air traffic surveillance system’s ability to monitor growth in air traffic. The FAA uses internationally accepted minimum separation standards to manage air traffic within its various oceanic regions, which require 30 nautical miles lateral separation.

An overview of the oceanic airspace regions managed by FAA air traffic controllers. GAO

By 2022, the FAA plans to introduce a new 23 nm separation standard using a version of ADS-C that receives more position, altitude, speed, intent and meteorological data every 3.2 minutes.

“In April 2019, FAA executives approved a schedule and funding for the implementation of these new minimum separation standards (i.e., 23 nautical miles lateral and 20 nautical miles longitudinal) in U.S. oceanic airspace using enhanced ADS-C. To implement these new standards, FAA officials are upgrading ATOP and working through a review process required to change minimum separation standards in U.S. oceanic airspace,” the report says.

FAA has set a timeline that includes publishing a 20 nautical mile longitudinal standard for operational use by 2020 while simultaneously developing a 23 nautical mile lateral separation standard for use by pilots and controllers. By 2021, the agency will upgrade the flight and radar data processing system it uses to provide satellite data link, communication and aircraft conflict resolution management for aircraft flying in oceanic airspace, known as the Advanced Technologies and Oceanic Procedures (ATOP) system. That upgrade will allow the implementation of a 23 nm lateral separation standard in oceanic airspace, followed by a 20 nm longitudinal separation standard by 2022.

However, FAA officials have not completely given up on space-based ADS-B as a long-term solution for managing oceanic air traffic. Over the next five years, FAA will conduct a study of the use of space-based ADS-B in oceanic airspace to see what benefits it could actually enable and whether or not a final investment in the technology should be made. Based on the timeline cited by GAO in the report, the FAA will not be ready to make a decision on that investment until 2025 at the earliest.

Avionics equipage of aircraft flying in U.S. oceanic airspace, the volume of flights in certain areas and disruptive weather patterns were all major aspects of the FAA’s decision-making and business case analysis when comparing potential benefits derived from enhanced ADS-C versus space-based ADS-B.

ADS-C uses onboard aircraft systems to automatically provide position, altitude, speed, intent and meteorological data sent in a report to an Air Traffic Service Unit (ATSU) or Airline Operational Center (AOC) ground system for surveillance and route conformance monitoring.

An overview of how the two different surveillance technologies work in managing flights within oceanic airspace controlled by the FAA. GAO

Controllers working at the oceanic air traffic facilities told GAO that they rarely actually use the existing 30 nm standard due to a number of different reasons.

One reason is the air traffic in oceanic airspace managed by FAA is not dense enough to require such close spacing. The highest volume of flights, 821 per day, occur in the Oakland Oceanic region where controllers are responsible for aircraft flying between the western coast of the United States and Hawaii, as well as Pacific island destinations and routes from the Asia-Pacific region including Australia, China and Japan.

FAA estimates that only 75 percent of aircraft flying in airspace managed by Oakland Center will be equipped with FANS 1/A avionics by 2020, which enables the enhanced used of ADS-C and the faster refresh rate for ATOP that the new separation standard would require.

Aircraft need to be equipped with the following avionics to enable enhanced ADS-C:

  • A flight management system and multi-purpose control display unit capable of serving as an interface for controller to pilot data link communication (CPDLC) messages.
  • ARINC 741 Level D compliant satellite communications system. Operators using Inmarsat service need the satcom system to meet Technical Standard Order (TSO) C-132. The use of Iridium requires TSO C-159A.
  • A Cockpit Voice Recorder (CVR) capable of recording data link communications.
  • A Communications Management Unit (CMU)
  • Very High Frequency (VHF) transceiver compatible with VDL Mode 2

Ultimately, whether or not the FAA can introduce the new minimum separation standard under their current enhanced ADS-C will be determined by how many aircraft flying in U.S. oceanic airspace become equipped with FANS avionics.

The future of space-based ADS-B relies just as much on avionics equipage rates as the use of enhanced ADS-C does. FAA data as of May 2019 show that over 83,000 U.S. aircraft have equipped with ADS-B equipment out of an estimated 241,000 aircraft that will need to equip by the 2020 deadline. The majority of aircraft flying internationally into U.S. oceanic airspace face similar deadlines or have already gone through ADS-B mandates, meaning that over the next few years the majority of aircraft flying in oceanic airspace will be equipped with ADS-B.

Further, the decision to use ADS-C was driven by analysis provided by the Joint Resources Council (JRC), which provides corporate-level acquisition decision-making for the FAA. Between January 2014 and April 2019, JRC tasked the FAA’s Advanced Surveillance Enhanced Procedural Separation (ASEPS) program with comparing the costs and benefits of using enhanced ADS-C versus space-based ADS-B.

While the report did not include the specific amount of the cost of investment for each service, the business case analysis focused on the costs the FAA would incur from using the data provided by space-based ADS-B versus the cost of updating the ATOP software. Costs to airspace users were also considered, although the FAA assumed that operators would continue to pay for ADS-C services and estimated that those operators would see an increase in messaging costs per flight hour for the faster version of ADS-C. The same analysis was applied to space-based ADS-B, with the FAA assuming how much a subscription fee for space-based ADS-B would cost, even though that official amount is unknown.

“As a new service that FAA has not yet contracted for, the actual cost of space-based ADS-B subscription fees are not known. However, initial estimates of the cost per flight hour for space-based ADS-B are much greater than the estimated cost per flight hour of additional ADS-C messages, according to FAA,” the GAO report said.

Among the major benefits FAA expects to see from implementing the new separation standards with enhanced ADS-C include improved accommodation of altitude requests. On routes such as those between Hawaii and the West Coast, for example, there is competition for access to the most fuel-efficient altitudes at certain times of the day where multiple aircraft are departing west coast airports at the same time. In the West Atlantic Route System, the current minimum separation standard of 50 nautical miles laterally only permits the existence of 10 total routes for airlines such as JetBlue flying in that airspace. One of the airlines interviewed by GAO for its report noted that reducing the lateral separation required to 30 nautical miles could increase the number of routes there from 10 to 19.

There was also consideration of some of the benefits that could be derived from implementing space-based ADS-B. Among those benefits, the FAA found that space-based ADS-B could be a viable backup to rely on in the rare cases when ground-based surveillance systems become unavailable, such as in the event of a hurricane that damages infrastructure.

The agency is also going to use its five-year study of space-based ADS-B to assess the technology’s ability to provide surveillance for aircraft flying between the east coast and Caribbean islands. On those routes, controllers currency use a radar located on Grand Turk Island in the Turks and Caicos Islands to use domestic airspace separation standards of just 5 nautical miles. Space-based ADS-B could help with those routes as well though, as that radar frequently becomes unavailable due to severe weather and service disruptions.

FAA’s decision to use enhanced ADS-C would mean more reliance on Inmarsat’s SwiftBroadband Safety and Classic Aero services section of its satellite network that provide the technological backbone for the technology. It will also mean the agency is still undecided on an investment decision in Aireon’s space-based ADS-B network, which went live in April. Although Aireon now has 12 total ANSPs, including a recent contract with India, FAA officials pointed out to GAO officials that several other ANSPs including Japan also currently have no plans to implement space-based ADS-B any time soon.

It remains to be seen how this decision will impact the use of space-based ADS-B in flight information regions closest to U.S. oceanic airspace. The three other ANSPs responsible for managing air traffic in the North Atlantic Track System — Irish Aviation Authority, NATS UK and Nav Canada — are all now using space-based ADS-B surveillance, enabling new minimum separation standards in their respective flight information regions. According to the report, as air traffic grows and controllers in those regions apply reduced separation standards, they may have to space out aircraft more as they cross into the FAA’s flight region boundaries.