At NATS Prestwick Centre where controllers direct traffic across the North Atlantic almost every safety event reported to supervisors now starts with the words “Space-Based ADS-B just told me ….”
Space-Based ADS-B (SB ADS-B) provided by Aireon updates aircraft positions in the Shanwick Oceanic Control Area (OCA) – 700,000 square miles (2.2 million square kilometers) of airspace controlled from the Prestwick Centre in Scotland – about once every eight seconds. “The first indication that something is going wrong is now almost always space based ADS-B,” says Jacob Young, manager of operational performance for oceanic ATC at NATS.
Controllers can see the aircraft locations updated by SB ADS-B on the screens of the Canadian-designed Gander Automated Air Traffic System (GAATS). This is the same system used by Nav Canada at its Gander Control Centre. NATS handles air traffic on the North Atlantic from the midpoint 30 degrees West longitude to Europe while Gander handles traffic from 30 degrees West all the way to the coast of Canada.
Aireon achieves pole-to-pole SB ADS-B coverage with its payloads aboard 66 cross-linked Iridium satellites in low earth orbit at altitudes of 476 miles (766km). If an aircraft is equipped to transmit its position on ADS-B once every half second, no extra equipment is needed to reach the satellites. SB ADS-B is installed on most aircraft now thanks to mandates for domestic aircraft in the USA, Australia and Europe. Aireon can provide ATS surveillance for 70% of the world’s surface where this service was not previously available.
Faster solutions
Aireon, based in McLean, Virginia is a joint venture between Iridium, Nav Canada, NATS, Italy’s ENAV and the Irish Aviation Authority. In addition to ATS surveillance services, Aireon can provide gate-to-gate and performance data on flights and high fidelity, low latency position data on flights beyond the area of responsibility of an ANSP.
During a demonstration of the system, Aireon’s chief technology officer Vinny Capezzuto highlighted air traffic at Anchorage, Alaska to show how the system provides situational awareness. Satellite 117 was processing 343 aircraft out of approximately 12,650 targets being tracked around the world by Aireon at that moment. As many as seven satellites can be surveilling one target at a time.
Pilots flying the North Atlantic who experience an emergency and need to divert may act immediately using standard protocols. This entails flying a course offset by five miles (8km) from their designated track. The controller assumes immediately it is an emergency and that the pilot will soon be in touch. Deviations can occur due to weather such as pilots avoiding a hazardous thunderstorm dead ahead or an in-flight situation – if a passenger is seriously ill or when a mechanical problem develops.
“For a deviation or diversion it might have taken us ten minutes to come up with a solution in the past. Now we often have a solution, like a new route, almost as quickly as we have a request from a pilot,” Young says. “Controllers can see the ground speed data and the returns or position updates, so if an aircraft deviates from a route we’re seeing it almost instantaneously.”
Spotting the route or altitude deviation right away allows controllers to start formulating alternative plans for the aircraft even before the pilot communicates via controller-pilot data link communications (CPDLC) or by high frequency (HF) voice radio contact with a radio operator on the ground. About 50 radio operators work at an Irish Aviation Authority facility to relay information between pilots in Shanwick airspace and controllers at the NATS run Prestwick Control Centre.
It helps that about 90% of the aircraft over the ocean are now equipped with CPDLC, which works almost instantaneously. HF radio contact typically takes three to four minutes for a response.
Safety goals
While safety has always been maintained in the North Atlantic airspace, another benefit of SB ADS-B for both the Prestwick and Gander control centers is that they are meeting the target level of safety for air traffic on the North Atlantic for the first time since the 1990s. The target level refers to the overall risk estimate for the airspace. To a large extent, meeting the target depends on how quickly it can be determined that an aircraft is flying a profile that is incorrect. For example, ICAO’s North Atlantic Systems Planning Group determined the lateral collision risk in 2018 was 13.8 x 10−9 fatal accidents per flight hour, up 8% from the previous year. The biggest contribution to the lateral collision risk for the year was a total of 162 minutes spent by aircraft on the incorrect track.
Currently, the North Atlantic goal is to have the chance of a collision occurring to be 5 x 10 −9 fatal accidents per flight hour or less. If it takes a pilot ten minutes to report that an aircraft is off course, “that counts against your safety record,” says Noel Dwyer, national manager, ATS (air traffic services) standards delivery for Nav Canada. And it is not just a matter of the number of events but also the duration of time an aircraft spends in an unprotected profile. This means that if a single aircraft is off track for several hours, the safety goal might not be met for the entire year.
“It is always tricky to try to refer to safety improvements with that implicit suggestion that the airspace was unsafe,” says Dwyer. “Far from it. We have many tools in place to ensure that operations are safe. But meeting that target level of safety has not been achieved since reduced vertical separation minimum was introduced in the late 1990s.”
Aireion SB ADS-B alerts of deviations in seconds has reduced safety risk 76%.
Errors in flight level involve aircraft climbing or descending without a clearance or without executing emergency procedures correctly. “There are a number of reasons why aircraft are not in that protective profile 100% of the time,” Dwyer says. “That is the clearest indicator that meeting our target level of safety was not occurring.”
Pandemic boost
SB ADS-B combined with the pandemic lowering the level of traffic in the North Atlantic has resulted in achieving the target level of safety in the Gander portion of the airspace where air traffic services surveillance is in use full time. The current level of traffic in Gander’s airspace is around half of what it was in 2019. In Shanwick overall, the busiest oceanic airspace in the world, the number of flights dropped from 1,600 per day in 2019 to 300 a day after the pandemic hit and now is back up to around 800 a day.
ADS-B ALERT SAVES LIVESAireon Alert, a service established by Aireon and the Irish Aviation Authority is saving lives by locating aircraft that have ditched in the ocean or gone missing over land.Before the introduction of space based ADS-B, about 70% of the earth’s surface was not subject to air traffic service surveillance. Now that Aireon’s payloads are aboard 66 Iridum satellites in low earth orbit, the network coverage is pole to pole.The free Aireon Alert service provides position reports including latitude, longitude and altitude for the last 15 minutes of flight, including up to 12 updates in the last minute. So far 308 organizations in 119 nations, including ANSPs and rescue services have signed up for access to the reports, along with 145 airlines.Sean Patrick of the Irish Aviation Authority helped design the service. He says that when a Cessna Centurion ditched 14 miles (23km) off the Bahamas on December 23, 2019, rescue was dispatched to the waters below the last known radar fix at 1,300ft of altitude.Aireon data showed the FAA that the aircraft’s last position was 4km (2.5 miles) from that fix. A helicopter was directed straight there and found the pilot treading water and alive even though he lacked a life jacket.
“We are certainly achieving everything we said we would with the introduction of space based ADS-B,” says Young. The controllers are gaining confidence in the new system which greatly improves situational awareness by showing them where all of the other aircraft are located when there is an emergency or a request for deviation.
The introduction of SB ADS-B also allowed separation standards on the North Atlantic to be reduced from 40 nautical miles longitudinally to just 14 miles. Controllers issue clearances from oceanic entry points to oceanic exits and the entire clearance has to be conflict free.
When Young started as a controller a decade ago, the longitudinal clearance was 15 minutes or about 75 miles (220km). Now it is down to five minutes, the same as in domestic airspace. A shift was made to using distance instead of minutes as separation got tighter so there was a need to be more specific. Going to 14 miles (26km) allows NATS controllers to operate more aircraft at the same level in a huge portion of the Shanwick airspace. “It allows more crisscrossing of flight tracks and lets us to issue clearances according to pilot requests more often,” says Young.
SB ADS-B is also enabling a reduction in the use of the Organized Track System (OTS). This set of routes is published daily so that airliners and business jets can fly the precisely defined routes with the best tailwinds to Europe at night or the least headwinds to Canada during the day. There are about a dozen tracks, with the tracks parallel to the central one offset to the left and right. The OTS system has guided North Atlantic traffic since the 1960s.
After March 3 the tracks were only designed on days they were needed for air traffic management. There were nine days when there were no westbound tracks at all. “That proved that tactically and strategically we can do this,” Young says.
Then on July 1 the planners went back to having a minimum of one track every day. Now NATS and Nav Canada are trying to determine how the use of random rather than fixed tracks can provide benefits such as improved efficiency for aircraft operators. The ANSPs are also determining what procedural changes need to be made to coordination processes so that the approach can be used when traffic returns to pre-pandemic levels. “We may need to introduce new tools to change flight level allocations throughout the day to make the airspace more dynamic,” says Young.
Each flight has its own business case. “Quantifying benefits from the ground, as we do, can be a bit tricky,” says Dwyer. “What we are in control of in the Gander area is quantifying safety benefits associated with having air traffic services surveillance (SB ADS-B),” he says. “I’ll put it this way, errors are going to happen with this is kind of human and machine interface. No system works 100% perfectly.”
Trying to quantify the impact of errors has always been a challenge, particularly where there is no surveillance monitoring the progress of a flight profile in real time. Using SB ADS-B controllers can spot an error instantaneously as an aircraft starts to depart its profile. Controllers also know where every aircraft nearby is located. “This has been absolutely groundbreaking for us,” Dwyer says.
Additional improvements
It used to be that in the Shanwick area that pilots might not be heard from for up to 45 minutes between mandatory reporting points, where they call in their position over HF radio and estimate when they will arrive at the next check-in point. Ionospheric interference can make an HF frequency temporarily unusable, such as when solar storms are disturbing the atmosphere, causing additional delays.
“Anything could happen in between the check-in points and controllers on the ground might not know anything was wrong,” Dwyer says. “I don’t want to say no communication is required now because communications are paramount in air traffic control. But the waiting to see what is actually happening with a flight has been pretty much eliminated.”
The large airlines flying the North Atlantic are also working in parallel with the ANSPs, running tabletop exercises to simulate high North Atlantic traffic loadings with zero OTS tracks or a limited number of them. “We are simulating different scenarios, providing them to the airlines and they are giving us flight plans for each one and associated costs as well as fuel and time savings,” Young says. “We are working with our analytics team to determine what would be the ultimate benefit of a nil OTS environment for customers.”
If each flight saves around US$300, the financial benefits will quickly accumulate. This is the primary reason NATS and Nav Canada are putting effort into track removal and looking at options for the future in how the OTS can be managed.
Airliners can save fuel by climbing up to the optimum altitude for the lowest fuel consumption. Before SB ADS-B became available they could be blocked by an aircraft up ahead at a higher altitude that was too close. With SB ADS-B tracking the location of all aircraft every 8 seconds it is more likely that an aircraft will be cleared to climb up. “If the pilot wants a higher altitude because the aircraft is lighter and able to climb after fuel has been burned off, the probability of a clearance to climb is greatly increased,” Dwyer says.
The use of SB-ADS-B is allowing NATS to lean forward in developing new capabilities in North Atlantic air traffic control, helping airlines and other operators. Controllers have high confidence in the surveillance technology, which makes implementing changes to procedures much easier to make. NATS is also developing new features in its air traffic services in a study of what it will offer in the years from 2023 to 2027. “We’re looking at lots of new initiatives for the ocean and almost all of them rely heavily on SB ADS-B,” says Dwyer.