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Satellite constellations and ADS-B


In perspective, global navigation satellite systems hold the potential to grow beyond aircraft positioning capabilities.

An overview of some satellite-based augmentation systems available today. The footprint coverage for WAAS and the European Geostationary Navigation Overlay System (EGNOS) are actually larger than shown.
By Bill Gunn
ATP/CFII Pro Pilot Regulations and Compliance Specialist

There are several worldwide global navigation satellite system (GNSS) constellations. GPS was the 1st to be deployed. The European Union’s Galileo is strictly for civil use and is available to all with more than 24 satellites to be fully deployed by 2020. Galileo is compatible with US satellite air navigation systems, including LPV arrivals.

The Chinese BeiDou system is expanding to become a global network with 35 satellites in place by 2020. The Russian GLONASS constellation is deployed worldwide but not available for air navigation for US-equipped aircraft.

Although BeiDou and GLONASS may not support certified navigation systems, they are available for non-certified aids to navigation and non-certified electronic flight bags (EFBs).

The more satellites an on-board system can choose from, the better the accuracy – even if there is no differential correction for the system. While the major satellite systems are worldwide, without differential correction none of these systems can meet the accuracy requirements for vertically guided terminal arrival procedures.

Each such differential correction system is defined to a specific region. Called satellite-based augmentation systems (SBAS), the US wide area augmentation system (WAAS) was the 1st deployed.

Ground receivers with a highly accurate known position across the US, Canada, and Mexico relay a correction factor to airborne navigation systems using geostationary communication satellites. Similar differential correction systems are available for other areas in the world.

Typical wide area multilateration transceiver. These are in use in multiple locations worldwide. One benefit is that aircraft only need a transponder to provide positioning information to ATC.

Satellite navigation and ATC

Air traffic control (ATC) in the US is ready for the Jan 2, 2020 transition to Automatic Dependent Surveillance–Broadcast (ADS-B). The final systems put in place as of late 2019 were for the last of 155 terminal arrival locations.

Differentially-corrected certified satellite navigators on board aircraft supply the position information to ATC via ground relay sites. Position, speed, height, and ID information is transmitted every second by the aircraft.

While ADS-B will be primary for surveillance, it is not the only means available to controllers. The decision was made not to require the controller to know specifically how each position update is derived.

Secondary radar, wide area multilateration (WAM), and other ground-based navaids can also contribute to position updates. The ATC system simply manages inputs from the various systems and presents the most valid position to the controller. The number of radar sites is being reduced, mainly in the enroute structure.

However, radar coverage will be maintained, albeit with fewer backup sites. WAM is a ground-based cluster of transceivers that interrogate an aircraft transponder simultaneously.

The slightly different time of arrival of the reply to each ground transceiver defines the aircraft’s position. The 1st US WAM clusters were installed in eastern Colorado to provide coverage in the Rockies for Denver Center, where radar coverage is limited.

Additional WAM sites are in use in the National Airspace System (NAS) to augment radar and ADS-B if needed. Although ADS-B is primary for surveillance, radar, WAM, and systems such as ground-based augmentation systems (GBAS), digital DME, and pseudolites (ground-based systems that provide positioning signals to on-board satellite navigation systems) will be part of the mix for the NAS.

GBAS is similar to satellite-based augmentation, but the correction signal is transmitted directly to the aircraft line of sight. In the US, only IAH (Houston TX) and EWR (Newark NJ) have public GBAS systems for arrival.

One looming consideration in all of this is the real possibility that satellite signals might be denied for a length of time because of extreme solar flare activity or intentional jamming, so ATC must maintain adequate alternate position systems to manage traffic in such a case. The integration of these multiple systems will be handled automatically so the controllers sees the best solution to manage traffic.

Visual depiction of coverage for Iridium NEXT satellites. There are 6 plains of rotation with 11 satellites in each plane in near pole-to-pole orbits. Each satellite can track ADS-B Out during its 100-minute orbit. Data is relayed from satellite to satellite, then to the appropriate ground site.

Iridium NEXT satellites

Iridium NEXT are low-orbit satellites which have the capability to receive ADS-B Out signals on 1090 Extended Squitter (ES). While the US has 1090ES in all of the NAS, and 978 UAT below Class A airspace, it should be noted that 1090ES is the world standard for ADS-B Out signals.

Considering only about 30% of the world has radar coverage, a means to track aircraft via satellite vs ground-based receiver sites and provide the potential for worldwide tracking, is tantalizing.

Iridium, Aireon, FlightAware, and Nav Canada were primary partners in this space-based ADS-B reception technology. Iridium NEXT satellites orbit at 485 miles up, as opposed to GNSS systems, which are in the 10,000 to 11,000-mile range.

There are at least 66 Iridium satellites in orbit to ensure full-time coverage. The partners in this system can provide turn-key surveillance for aircraft equipped with ADS-B Out with no additional instruments required other than the possibility of a top-mounted ADS-B Out antenna.

While Nav Canada will require this install, FAA has not declared yet, but will run a test using NEXT satellite tracking in the Caribbean region starting in March 2020.

This is just a test to determine if antenna diversity is required for NEXT reception. Several other organizations are contracting with FlightAware for NEXT surveillance information, including Italy’s ENAV, the UK NATS, Ireland’s IAA, and NavAir in Denmark. All of this will take some time to implement, coordinate, and ensure continuous coverage contingency.

Aireon ALERT

Beyond surveillance, Aireon offers a public service to the world’s aviation industry for emergency locating and tracking of ADS-B-equipped aircraft. Aireon’s aircraft locating and emergency response tracking (ALERT) is a global emergency aircraft location service that comes at no cost to the user, and does not require the customer be a client of Aireon or IAA.

Users of the ALERT system only have to register with the Irish Aviation Authority North Atlantic Communications Center in Ballygirreen. Online registration is available at www.aireonalert.com. In emergency situations where an aircraft cannot otherwise be located, a pre-registered aviation stakeholder can call the Aireon ALERT 24/7 phone number and provide the missing aircraft’s flight ID or unique ICAO 24-bit address, which is shown in the FAA database as “Mode S Code (base 16/hex).”

ADS-B tracking and positioning is required in transponder airspace in the USA starting in 2020. How will aviation change in the USA and what are the prospects for the future?

The Aireon ALERT operator will then locate the last known position of the aircraft and, if found, will provide that location in WGS 84 coordinates to the aviation stakeholder over the phone. The operator will also e-mail a report of the location of the missing aircraft to the aviation stakeholder. For routine fleet management, Aireon and FlightAware offer Global Beacon – a fleet tracking service using NEXT satellites.

A position and altitude fix can be observed on a 1-minute interval for aircraft equipped with ADS-B Out. Customers can expect positioning information reported for their fleet anywhere in the world via Global Beacon at least every 15 minutes with this, and the service will increase to every minute by 2021 for any aircraft in distress.

All that is required is for the aircraft to be ADS-B Out capable on 1090ES, and have antenna diversity with top-mounted antennas.

The ICAO requirement under the global aeronautical distress safety system (GADSS) is met by Global Beacon. GADSS is made up of standards and operating procedures for airlines and operators for normal tracking, as well as tracking for aircraft in distress.

ADS-B and protocol

ATC has been using satellite positioning for some time as part of the mix for surveillance – radar being primary until the transition date. A few lessons have been learned and some clarification has already come up. FAA stated in the Federal Register on July 3, 2019 (FAA 2019 0539 0001) guidelines for use of a Service Availability Prediction Tool (SAPT), which is required for some operators and available to all. SAPT estimates probability of loss of GPS coverage along the filed route of flight.

Stated in this final rule is, “After January 1, 2020, unless otherwise authorized by ATC, all aircraft operating in the airspace identified in § 91.225 must comply with the ADS-B Out performance requirements in § 91.227.

There are circumstances outside of an operator’s control that may result in a temporary degradation of GPS performance and an apparent violation of § 91.227. An operator may exercise due diligence in performing a preflight availability prediction for its intended route of flight but experience rerouting by ATC after obtaining an initial ATC route clearance, which may cause an unanticipated degradation of performance.

Additionally, an operator may encounter actual GPS interference on its intended path of flight, which would affect the ability of an aircraft to meet the performance requirements of § 91.227. Lastly, an operator may not be able to complete a preflight availability prediction for its intended route of flight due to the FAA’s SAPT being out of service.

As previously explained, the FAA recognizes that these situations are outside of the operator’s control. Therefore, the FAA will not consider these events to constitute noncompliance with § 91.227 due to the circumstances discussed in this document to the extent such an application would impose a standard of conduct wholly outside the operator’s control.”

Essentially, an operator required to use SAPT and doing so would not be in violation of FAR 91.227 in the explained circumstances. The most recent SAPT user guide is available at https://sapt.faa.gov/default.php.

FAA issued Exemption Nº 12555, a time-limited grant of exemption from § 91.227(c)(1)(i) and (iii) ADS-B Out performance requirements for the period from January 1, 2020 through December 31, 2024 for some operators. Those under this exemption must follow specific guidelines for GPS predictability and routing.

A sample SAPT reply. Note the prediction that GPS may not be available over ACY (Atlantic City NJ) but that alternate surveillance is predicted to be available. The system can recommend, as above, offsetting departure time by 15 minutes for possible improved coverage.


If 2 or more aircraft are operating in close formation, the air traffic controller may now request all aircraft in the flight (except the leader) to place ADS-B Out in standby. In such a case, ATC treats the formation as a single aircraft for the purposes of separation and sequencing.

Other issues

FAA is finding numerous errors with ADS-B system installations, one of which is a flight ID associated with the incorrect ICAO 24-bit address. Aircraft with a fixed N registration can have this corrected by their avionics shop, and those with random changing callsigns will require ADS-B Out transmitters that can be configured for call sign.

The rules require the bit address and call sign match what is filed as well as what the “Out” system transmits. Finally, revised AC 91-85B (1/29/19) provides relief from a letter of authorization (LoA) from an FSDO for some Part 91 US operators seeking reduced vertical separation minimums.

Only the LoA requirement is dropped if the aircraft is equipped with ADS-B Out. All other requirements for installation, inspection, maintenance, and training remain.

A beginning

Mandatory ADS-B Out in the NAS is a milestone. But there are other developing systems in the cockpit. Controller pilot data link communications (CPDLC), for example, is growing to replace voice control with text over a staged period.

Departure or pre-departure clearances have been available at 55 US hub airports, and CPDLC is in use for transoceanic operations, as well as in the Pacific Rim and Europe. FAA has sector handoffs via CPDLC for those so equipped. Voice communication will remain and CPDLC is not mandatory for most operators. Traffic via ADS-B In is advisory for most of the fleet and, unlike TCAS II, it is not a stand-alone traffic separation device.

Regardless, the new domestic ICAO flightplan format has a block to advise ATC of any aircraft’s ability to observe traffic via ADS-B, so the value of this tool is understood. Minimum performance standards for ADS-B In traffic are defined in TSO C195b and discussed in AC 20-172B, Airworthiness Approval for ADS-B In Systems and Applications.

Aircraft that must have ground traffic “In” capability as well as certain traffic to follow and In trail capabilities via In traffic are a hint to the long-range possibilities for traffic in the cockpit. Today we must concentrate on the shift from radar to satellite-based positioning, and the increase in capabilities this provides for ATC. The only certainty is that, once this is better understood, the capacity for technology growth is there.

Bill Gunn is former manager of airport compliance and training for the Texas DOT, Aviation Division. Gunn retired from the USAF flying the RF-4C Phantom, including 18 months in Southeast Asia during the Vietnam War, flew corporate for 10 years and managed federal contracts for aviation services. He lectures throughout the US for an international aviation advocacy group.