Coming Off the Production Line: Is Your Aircraft’s GPS Receiver Accurate Enough for ADS-B?

ADS-B will be a cornerstone of future air traffic services provisions, globally. But to fuel the new generation of GPS avionics, new equipment still in production will need to provide a more reliable, robust output, than systems currently on the market enable.

Among the many initiatives of its NextGen air traffic modernization program, the FAA is transitioning away from the traditional reliance on radar surveillance in restricted airspace toward primary Automatic Dependent Surveillance-Broadcast (ADS-B) functionality. ADS-B uses GPS positioning, as well as the ADS-B ground infrastructure that has already been deployed and is being used by operators. But new FAA rulings call for improved, third generation GPS equipment to deliver sufficiently robust data for use in primary air traffic management.

The FAA’s 2020 mandate, which calls for increased reliability to enable 2 nm ADS-B separation, applies only to ADS-B Out equipment, which broadcasts horizontal and vertical aircraft velocity relative to the Earth, barometric and geometric altitude, and position data, slightly less than once per second — compared to surveillance radar, which typically updates an aircraft’s position every 5 to 12 seconds. This capability enables controllers to manage airspace more effectively, while the technology also delivers a controller’s situational awareness into the cockpit.

Performance Requirements

NextGen’s ADS-B Out performance requirements are described in rule 14 CFR § 91.227(c). At an FAA ‘Call to Action’ meeting in October 2014, agency and industry stakeholders identified barriers delaying operators from equipping with third-generation GPS receivers, or those with satellite-based Augmentation System (SBAS) compatibility, to support the new standard in ADS-B Out before the Jan. 1, 2020 mandated compliance deadline. The Equip 2020 initiative arose from this meeting, enforced as Exemption 12555 and enabling operators to continue until 2025 with first- and second-generation GPS receivers, considered as Selective Availability (SA) unaware/on and SA aware, respectively. However all other equipage requirements under 14 CFR § 91.227(c), are still applicable starting Jan. 1, 2020, and operators whose equipment qualifies for the exemption are subject to certain operational conditions and limitations established by FAA under Exemption 12555.

The rule lays out the following specifications for the ADS-B Out equipment for Navigation Accuracy Category for Position (NACp), Navigation Accuracy Category for Velocity (NACv) and Navigation Integrity Category (NIC), as:

The aircraft’s NACp must be less than 0.05nm (NACp ≥ 8)

The aircraft’s NACv must be less than 10ms-1 (NACv ≥1)

The aircraft’s NIC must be less than 0.2nm (NIC≥7)

The rule also describes ADS-B performance requirements, with actual performance depending on the GPS receiver used as the ADS-B position source. The rule does not dictate a particular receiver type and operators, therefore, use different equipment to satisfy the ADS-B Out performance requirements. The operator is responsible for determining that a given sensor and ADS-B pairing is adequate to meet the rule.

Exemption 12555 provides for one-time exemption from 14 CFR § 91.227(c)’s NACp and NIC requirements for aircraft equipped with GPS receivers unable to match the specification for NextGen ADS-B. The operator must request the exemption by the operator between August 2018 and January 2020, the exemption runs for five years, from the Jan.1, 2020 mandate until December 31, 2024.

“Operators holding the exemption will be allowed in the airspace, under certain limitations, allowing them more time to operate with existing GPS,” an FAA spokesperson confirmed.

Furthermore, in an April 2016 ICAO presentation during the 15th meeting of the ADS-B and implementation task force, Airbus’ Surveillance Systems Manager Laurent Vidal laid down the impact of 14 CFR § 91.227 upon the GPS systems fitted as standard in Airbus aircraft. Critically, the regulation calls for aircraft position availability of 99.9 percent, a stipulation currently only met by the Rockwell Collins GLU-925 Multi-Mode Receiver (MMR) certified in the Airbus A350.

The Honeywell RMA-55B, Rockwell Collins GLU-920/925 and Thales TLS755 (discontinued in 2015) MMRs installed as line fit across the Airbus range are all compliant with the future EASA Certification Specification-Airborne Communication, Navigation and Surveillance (CS-ACNS) ruling, which does not demand the 99.9 percent aircraft position availability of the FAA regulation.

Airbus reports that Honeywell and Rockwell Collins are working on new MMRs with SBAS, to satisfy the U.S. requirement. Honeywell’s equipment is scheduled for certification in the last quarter of 2018 and Rockwell Collins’ is aiming for certification in the first quarter 2019.

European Future

“Current GPS accuracy is more than enough for aviation applications and is expected to continue offering outstanding performance. The primary requirement for future Global Navigation Satellite Systems [GNSS] is to increase robustness against identified vulnerabilities,” Christos Rekkas, Eurocontrol’s head of surveillance modernization activities explains.

Europe is also looking to future air traffic requirements, within which, Rekkas says, Eurocontrol is leading the development of an ICAO concept of operations for next generation GNSS. “An important element is to identify the operational benefits that the promised additional robustness will bring to operators and air navigation service providers — for example the ability to continue performance-based navigation [PBN] or ADS-B applications in case of L1 interference,” Rekkas adds.

He notes that the so-called legacy L1 GPS frequency and GPS L5 will be essential elements of a next-generation multi-constellation, dual-frequency system that marks a major milestone in European GNSS strategy. It follows a decision, not unlike that made by the FAA, to make additional continent-wide use of ADS-B. Operators will need to install enhanced avionics for full-system compatibility, as defined under the Surveillance Performance and Interoperability EU Regulation No. 1207/2011, which is currently under amendment. Nonetheless, the reduced separation demanded by European regulations renders all generations of GPS avionics suitable for ADS-B Out.

Inevitably, however, operators flying from Europe into the U.S. will have no option but to upgrade to the latest equipment standard to fully benefit from the next generation service available on both sides of the Atlantic. Still, Rekkas notes: “Regardless of Exemption 12555, first- and second-generation GPS receivers will be permitted for operation in the U.S. beyond 2025, but with increased dispatch risk, especially for first-generation receivers, and assessment burden on operators.”

Two Automatic Dependent Surveillance-Broadcast (ADS-B) antennas (short, white with three guide wires) atop a communications tower located on the Nullabor Plain in Australia.Photo courtesy of Airservices Australia.

Australia Today

ADS-B already contributes to air traffic management over Europe and the United States, and authorities are looking to expand its utility in the immediate future, but it has been the dominant, continent-wide surveillance system in Australian airspace since 2009.

“Australian ADS-B is different to that in the United States The initial objective of ADS-B in Australia has been to provide an [Air Traffic Control] ATC surveillance service as good as radar, across the continent, then to transition to more demanding requirements as the applications become ready and the world’s fleet adopts ADS-B Out,” explains Greg Dunstone, Airservices Australia’s surveillance strategic planning and architect manager. “Airservices has been providing a full ADS-B-only separation service to all equipped aircraft across the continent for more than a decade, without any significant issue. It has changed the nature of air traffic control from one based on voice position reports to a full surveillance environment using 5 nm separation standards. Air traffic controllers, pilots and all significant stakeholders recognize the safety and efficiency benefits provided.”

Australian regulations have required ADS-B for operations above FL290 since Dec. 12, 2013, but this is being revised as a requirement for all Instrument Flight Rule (IFR) operations at all flight levels. Thus operators falling into this category must have their fleets properly equipped before Feb. 2, 2017. According to Dunstone, all the nation’s commercial jetliners are already equipped, while more than 83 percent of current IFR aircraft are flying with ADS-B equipment onboard.

Dunstone notes that Australia’s approach to ADS-B and supporting GPS receivers has differed considerably to that evolved by the FAA. “We allow DO260, DO260A and DO260B minimum operational performance standards, whereas the FAA allows only DO260B and also requires more demanding reported integrity and accuracy from ADS-B.

“Initially, almost all of our customers were using DO260 and a large percentage use SA On GPS. Now, 44 percent of airframes have DO260B, although a much lower percentage of flights use it. Denying service to these customers would deny the safety advantages of surveillance across the continent where we have no radars,” says Dunstone.

He notes that like much of the world, Australia does not have a satellite-based augmentation system, such as a Wide Area Augmentation System (WAAS). Instead, the Australian Civil Aviation Safety Authority (CASA) allows the use of older generations of GPS to support ADS-B, including TSO129-compliant systems that output horizontal protection limit integrity data and have fault detection and exclusion functionality excluding potentially faulty satellites from their navigation solution.

It follows that the regulations published in Australia in March 2009 allow operators to use first- and second-generation GPS receivers, but the regulatory authorities to recognize the benefits of DO260B and SA-aware. Their use is encouraged rather than mandated, although aircraft manufactured after December 2016 must employ SA-aware GPS for ADS-B.

Primary airspace regulators and service providers in Australia, Europe and North America are clear on the advantages of ADS-B and the requirement for more robust GPS systems to meet future requirements. Yet, insufficient guidance appears to have been made available for precise definition of what ‘third-generation’ GPS receivers will offer and the industry remains reticent on the subject.

Europe’s ADS-B Expansion

Europe is substantially modernizing its surveillance systems, with surveillance rapidly moving away from radar-only to multiple sources. More than 750 ADS-B ground stations are being installed within multilateration systems or as stand-alone systems, in 25 European states.

“ADS-B operation has already begun in a few regions of low-density airspace. ADS-B operation in medium- and high-density airspace depends on the progress of aircraft equipage in compliance with European Regulation 1207/2011, which carries a mandate of 2020,” notes Christos Rekkas, head of Eurocontrol’s surveillance modernization activities.

As it prepares Europe’s skies for the future demands of air travel, Eurocontrol continues its tradition of close cooperation and joint standard development with stakeholders in Europe, the U.S. and other countries. Rekkas says that as a result of the close collaboration, European and U.S. ADS-B Out requirements are closely aligned, but there are a few crucial differences.

“The key variation relates to requirements on position source and is driven by the fact that the European ADS-B Out regulations target 3 nm separation as the most demanding ATC surveillance use case, compared to the U.S. target of 2 nm separation for dependent parallel approaches. This leads to a European NIC requirement of 0.6 nm, as specified by CS-ACNS/ED-161, compared to the U.S. NIC requirement of 0.2 nm, as specified by 14 CFR § 91.227,” Rekkas explains. “The European requirements for GPS systems supporting ADS-B Out under 1207/2011, are detailed in the EASA CS-ACNS. In principle, however, for approval the horizontal position and velocity data source should hold an EASA equipment authorization in accordance with ETSO-C129a, ETSO-C196 or ETSO-C145/ETSO-C146, with additional qualification requirements specified in EASA CS-ACNS.”

In addition, Eurocontrol has been leading development of ICAO’s GNSS interference mitigation plan — including counter-jamming strategies.

“We’ve assessed and simulated the operational impact of GPS interference and developed mitigations as part of our contribution to the SESAR [Single European Sky ATM Research] program,” says Rekkas. “We expect next-generation GNSS receivers to be more resilient to interference and we’ll continue our participation in related discussions with RTCA/EUROCAE, FAA and the ICAO.”

Europe’s ADS-B Expansion

Europe is substantially modernizing its surveillance systems, with surveillance rapidly moving away from radar-only to multiple sources. More than 750 ADS-B ground stations are being installed within multilateration systems or as stand-alone systems, in 25 European states.

“ADS-B operation has already begun in a few regions of low-density airspace. ADS-B operation in medium- and high-density airspace depends on the progress of aircraft equipage in compliance with European Regulation 1207/2011, which carries a mandate of 2020,” notes Christos Rekkas, head of Eurocontrol’s surveillance modernization activities.

As it prepares Europe’s skies for the future demands of air travel, Eurocontrol continues its tradition of close cooperation and joint standard development with stakeholders in Europe, the U.S. and other countries. Rekkas says that as a result of the close collaboration, European and U.S. ADS-B Out requirements are closely aligned, but there are a few crucial differences.

“The key variation relates to requirements on position source and is driven by the fact that the European ADS-B Out regulations target 3 nm separation as the most demanding ATC surveillance use case, compared to the U.S. target of 2 nm separation for dependent parallel approaches. This leads to a European NIC requirement of 0.6 nm, as specified by CS-ACNS/ED-161, compared to the U.S. NIC requirement of 0.2 nm, as specified by 14 CFR § 91.227,” Rekkas explains. “The European requirements for GPS systems supporting ADS-B Out under 1207/2011, are detailed in the EASA CS-ACNS. In principle, however, for approval the horizontal position and velocity data source should hold an EASA equipment authorization in accordance with ETSO-C129a, ETSO-C196 or ETSO-C145/ETSO-C146, with additional qualification requirements specified in EASA CS-ACNS.”

In addition, Eurocontrol has been leading development of ICAO’s GNSS interference mitigation plan — including counter-jamming strategies.

“We’ve assessed and simulated the operational impact of GPS interference and developed mitigations as part of our contribution to the SESAR [Single European Sky ATM Research] program,” says Rekkas. “We expect next-generation GNSS receivers to be more resilient to interference and we’ll continue our participation in related discussions with RTCA/EUROCAE, FAA and the ICAO.” AVS

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