An Overview of ADS-B Airspace & Technology

Rules and regulations differ slightly from region-to-region with regard to how aircraft should be equipped to have ADS-B functionality. Following is a brief overview.

Europe: All aircraft require elementary Mode S Transponders, while fixed-wing aircraft certified with a maximum take-off mass exceeding 5,700kg, or having a true airspeed at maximum cruise greater than 250 knots, must have enhanced Mode S Transponders and ADS-B Out capability.

Non-fixed-wing aircraft need only have the elementary Mode S Transponders installed. The current final compliance date is June 3, 2023, with exemptions available for aircraft being flown to partake in maintenance for export, or if the aircraft will cease operations prior to October 31, 2025.

United States: The US allows aircraft to operate with different ‘transponding/broadcasting’ frequencies (978/1090MHz) that by default provides for greater variation in equipage options and requirements.

For aircraft operating at or above 18,000ft (FL180) and for operations outside the US, approved Mode S transponders with ADS-B capability must be used. Within US ADS-B airspace, below FL180 owners/operators have the alternative option to operate with 978 UAT equipage. All ADS-B equipment in the United States must be Version 2.

International Civil Aviation Organization (ICAO): The ICAO guidelines used by most aviation authorities around the world are like those of Europe, in that they require Mode S Transponders. ICAO ‘ADS-B Implementation and Operations Guidance Document’, Edition 14 (August 2021) discusses all aspects of ADS-B including harmonization across different regions.

Oceanic: ADS-Contract (ADS-C) is an integral part of Oceanic operations, and is used as an automated means of monitoring and connecting with aircraft in Oceanic airspace, using onboard satellite data communications and an associated service provider.

The Technology Behind ADS-B

Equipment: The design and manufacture of avionics equipment for ADS-B is required to comply with a Technical Standing Order (TSO) that defines the minimum requirements and provides a standard for everyone to follow. For ADS-B, the necessary equipment and respective TSOs are:

• 1090 Extended Squitter Transponders under TSO C166c.
• 978 UAT ADS-B and Diplexers under TSO C154d.

Note the equipment does not include existing avionics systems that the ADS-B needs to integrate with, and which may need upgrading. One example of such systems would be the Global Positioning System (GPS).

The equipment performance and testing requirements are designed under RTCA/EUROCAE developed guidance, specifically:

• DO 260C/ED102 for 1090ES Transponder based ADS-B and TIS-B.
• DO 282C for UAT based ADS-B.

The performance of ADS-B Out in the aircraft, once integrated to accommodate all its parameters, is ruled by FAA FAR 91.227 and corresponding international requirements.

Installation: The integration of ADS-B avionics equipment into a new aircraft is completed under a Type Certificate (TC) and aftermarket aircraft under a Supplemental Type Certificate (STC). An Approved Model List (AML) STC, where the same basic installation can be applied to several different models of aircraft, also became a popular and economical path for compliance.

For delivered aircraft that were already provisioned, or required only minor changes, manufacturers issued a Service Bulletin (SB). This also implies that any future changes to your aircraft from ADS-B advancements may require an amendment to the original TC/STC.

Airworthiness Circulars provide the guidance for avionics integration and include:

• ADS-B Out – AC 20-165b
• ADS-B In – AC 20-172b.

Figure 2 shows a typical business aircraft ADS-B Out layout, using ES1090 MHz transponders. Also, TIS B ADS-B In information can be displayed on the aircraft flight displays (which is not shown in this ADS-B broadcast-only version). There are many layout versions of 978 MHz UAT integration, mostly employed by General Aviation aircraft operating below 18,000ft.

Operation: FAA FAR 9.225, and corresponding international rules, dictate the use of ADS-B Out in airspace. These rules provide airspace operating requirements, including the need to be in continuous transmit mode throughout the flight.

ICAO Address and Flight ID (Identification)

The Flight ID should match the aircraft call sign, as used for each flight leg. This ID is used by ATC at an operational level.

The 24-bit ICAO Mode S address, strapped at the time of transponder installation, is unique to each aircraft. This address is used technically by onboard Mode S Transponders and TCAS to prevent confusion during multiple operations and busy ground radar environments.

In-Flight Testing and Verification

Usually within one hour of flight completion, a Public ADS-B Performance Report (PAPR) can be obtained from the FAA or local verification website. A corresponding User Guide is normally available to download and assist in analyzing the report. Where ADS-B is active, 978 and ES1090 MHz data is collected for both ground and flight operations.

Because the printable report contains detailed information resulting from the aircraft’s configuration and transponder strapping, it is particularly useful to maintenance and installation facilities as an aid when troubleshooting.

Common ADS-B Out Problems

Initial installations should be thoroughly ground-checked and verified during a test flight to prevent the broadcast of erroneous ADS-B data. However, mistakes occur and are potentially overlooked prior to the aircraft entering service.

Common errors, usually shown on a PAPR download, include:

• Missing barometric pressure altitude.
• Air/Ground status.
• Flight ID error.
• Wrong ICAO aircraft address.
• Incorrect Mode A code.
• Incorrect emitter category.
• Position error.

The Future of ADS-B

ADS-B usefulness to aviation operations is just beginning. With the adoption of space-based ADS-B and advancements in ADS-C, this technology will supplement, and eventually replace TCAS as we know it.

Aviation authorities are conducting trials and introducing programs that permit closer aircraft maneuvering around airport approaches and taxiways. With ADS-B In displayed in cockpits pilots will have the ability to make reliable decisions on visual separation and sophisticated operations in congested airspace.

Three informative documents and programs provide insight into some of this future. Specifically:

• FAA TSO C195b/RTCA DO 317C and equivalent international standards, covering Aircraft Surveillance Applications (ASA).
• ADS-B Version 3 Extended Squitter, adding more applications and accommodating Advanced Air Mobility (AAM).
• Eurocontrol CASCADE Program (ADS-B and multi-lateration), under Surveillance Performance and Interoperability (SPI-IR), advancing Performance Based Surveillance (PBS).

In Summary

ICAO and others are working hard to ensure aviation technology is performance-based. Under the umbrella of Performance-Based Communication & Surveillance (PBCS), ICAO is aligning both communication and surveillance with the existing Performance-Based Navigation (PBN) requirements.

Each has their unique operating standard for flight departments to meet, based on the routes being flown. ADS-B is a key operational component of Required Surveillance Performance (RSP). Provided the operator can fly a route or procedure to the prescribed PBCS/PBN parameters, the equipage may vary to suit the aircraft design.

For ADS-B Out there have been many creative solutions. Particularly popular on pre-flown aircraft has been the AML-STC method of certifying the integration of these solutions.

As with most modern avionics, ADS-B cannot be understood in isolation. Just as it relies on several key inputs from different avionic sources, it serves to assist pilots in situational awareness alongside TAWS and TCAS, and with operating around and on airports alongside traditional navigation equipment.

Advanced Air Mobility (AAM) consisting of Urban Air Mobility (UAM) and Uncrewed Aircraft Systems (UAS) will require minimized flight separation with little margin of error.

The ADS-B in service today meets the needs of the current classifications of airspace and must develop to meet the combined needs of both traditional and AAM airspace.

This, along with ADS-B’s contribution to national Wide Area Multilateration (WAM) programs, provides for tremendous possibilities in the long-term.