Continuing his series, Ken Elliott addresses Automatic Dependent Surveillance, Broadcast mode, less than three years before the equipage requirement comes due.Back to Articles
Continuing his series, Ken Elliott addresses Automatic Dependent Surveillance, Broadcast mode, less than three years before the equipage requirement comes due.
Previously, we addressed equipage rates, summarized the technology, data sources and operational requirements of ADS-B (Out) compliance. This month, we’ll explore the technical aspects of ADS-B of possible interest to flight departments.
We’ll then conclude our series with a closer look at the worldwide implementation and operational differences of ADS-B.
Flight history can be a useful yardstick, recording the incremental steps of aviation and with respect to this article, the evolution to ADS-B. With dates aside, the rationale for ADS-B began with Primary Surveillance Radar (PSR), portraying each aircraft as a single, unidentified dot on a controller’s screen.
On-board aircraft transponders responding to interrogations then added the ability to ‘see’ specific aircraft by providing a matched response to an ATC-assigned four digit Mode A code, entered by the pilot. To confirm that a specific radar return on their display was the actual aircraft being tracked, the controller also requested the pilot push-select an ID button on the transponder cockpit control (known as Squawk ID).
This action created an eyebrow feature above the radar return, turning it into an identified target. By then adding Mode C (Mode B was not used), the aircraft’s altitude also displayed in 100ft increments, alongside the identified radar return. Transponders, and what they provided, became known as Secondary Surveillance Radar (SSR).
Evolving further, to meet Europe’s condensed airspace requirements, transponders were adapted to provide data-based information, such as FLT ID. Being an early form of Mode S, it combined the existing Modes A & C with additional identification data. Pilots entered their specific Flight ID code into the transponder control unit or flight management system.
Each step in the evolution of surveillance provided greater position accuracy. ADS-B is the logical progression in the quest for identifying a specific aircraft in an exact location.
Flight ID has since expanded into Elementary Surveillance (ELS), using a personalized 24 bit ICAO address, based on its registration, and then strapped specifically into each aircraft. If an aircraft changes its registration, a new Mode S address must be added as a maintenance action item. ELS also provides the aircraft’s altitude information in 25ft increments.
ELS has further evolved into Enhanced Surveillance (EHS), by adding more data elements to the transponder transmission, using Downlink Aircraft Parameters (DAP) services. These data elements consist of:
Integration with existing AHRS and Air Data systems is required for some of these data elements while for older aircraft, additional wiring may be required. Also, if aircraft are TCAS II equipped they will have full Mode S capability.
For ADS-B Out using transponders operating at 1090 MHz, the EHS version becomes Extended Squitter, or 1090ES.
For the US, Specifically…
Aside from 1090ES transponders, Universal Access Transmitter (UAT) versions may also be deployed for ADS-B Out, operating at a lower 978 MHz frequency. This version is popular with General Aviation light aircraft, especially if they do not need to fly in Class A airspace or internationally, where 1090ES is always required.
Essentially, UAT services are only provided below 18,000 feet and only available in the US. Having 978 UAT also allows ‘ADS-B In FIS-B’, a no cost traffic and flight information service. This FIS-B service is provided in addition to TIS-B, available to all with ADS-B In.
Traffic Information Services – Broadcast (TIS-B): a transitional technology that provides aircraft, equipped with ADS-B Out and utilizing ADS-B In capability, with client-based surveillance information. This relates to aircraft that are not ADS-B equipped and over the next three years should be less of a need. To qualify as a TIS-B target, an aircraft must be equipped with a transponder and be within radar coverage.
Flight Information Services – Broadcast (FIS-B): provides meteorological and aeronautical data to the cockpit. This service is not client-based and is always broadcast on the 978 MHz UAT frequency. FIS-B is not available on the 1090 MHz frequency (1090ES).
Owners and operators of most jet and turboprop aircraft must determine if they have 1090ES version transponders that are integrated to other aircraft systems for the additional ADS-B data required. It is further essential to make sure that Mode S strapping is current for the ICAO address, specifically assigned to your aircraft registration.
Another available client-based service is Automatic Dependent Surveillance ‐ Rebroadcast (ADS-R), which relays ADS-B information transmitted by an aircraft broadcasting on one link (for example UAT) to other aircraft equipped with ADS-B In operating on a different link (for example 1090ES), and vice-versa. Ground stations convert and relay the respective frequency data matching the aircraft receiving frequency.
Aircraft utilizing ADS-R will have a ‘hockey puck’ shaped area created around them, for sharing the ADS-B information with other aircraft within the same area but not on the same datalink (frequency).
Aircraft Position Information
ADS-B not only relies on the communication provided via the Transponders but also on the aircraft’s position, available from the onboard Global Positioning Systems (GPS). The existing GPS sends a highly accurate position to the transponders that, in turn, broadcast it as part of the extended squitter.
GPS receivers and antennas are found in different configurations, with some manufacturers embedding GPS sensors within Flight Management Systems (such as Universal Avionics) and others electing to provide stand-alone modules or cards (such as Honeywell).
When updating or adding the GPS to the latest version required for ADS-B, it is possible to go down two paths. One is the update of the existing GPS, via either hardware or software (or both), and the other is to add a third dedicated Flight Management System or GPS sensor that will provide the necessary qualified GPS signal.
Sometimes aftermarket solutions are provided. These may be an alternate to the aircraft OEM solution, or available because the OEM has not yet provided a solution.
The older the aircraft type, the more likely there is not yet a factory solution available - although, in the case of ADS-B Out, OEMs have made impressive strides to serve older aircraft with solutions.
The GPS position source does not need to be the same source as that used for navigation. Because of this, an independent (or third source) may instead be embedded in a dedicated ADS-B system. Some manufacturers are producing single box solutions, housing a 1090ES or UAT transponder and a compatible GPS sensor.
If the GPS antenna needs to be changed, or a third one added, its location may incur additional cost impact. GPS antennas are located on top of the aircraft fuselage, requiring access to equipment inside the aircraft’s pressured cabin. This installation can range from minor overhead access panel removal to the complete take-down of a one-piece headliner. Of course, the same panel or headliner will need to be reinstalled, once the antenna has been tested for normal operation and any skin penetration pressure sealed.
Adding a third GPS solely to satisfy an ADS-B Out requirement may be fully justified in some situations. However, it may not be best served for most operators to go down this route. There are two concerns with it:
The qualification of GPS sensors for ADS-B Out is based on the requirements of a Technical Standard Order (TSO), and there are three of them. Table A shows, for example, the different likelihood of signal availability between them.
ADS-B Out Versions
One characteristic of ADS-B Out that may be confusing to operators while ensuring the compliance of their specific installation is whether the system they have is ADS-B Out - Version 0, 1 or 2. (Version 0, 1 and 2 equates to RTCA DO 260, 260(A) and DO 260(B) respectively.)
To make matters more difficult, there are regional variations in their requirement. For example, the US, Europe and China mandate Version 2 equipage, so even though some other countries/regions have ADS-B requirements today, they do not require Version 2. Most, however, will require Version 1. If an aircraft is Version 2-equipped it will still operate satisfactorily in areas requiring a lower version number.
These ADS-B versions generally indicate different accuracy parameters and designations of data provided. Good advice for those equipping is to cut no corners and make sure you are Version 2 equipped, especially if you operate across oceans or borders and most certainly if operating where Version 2 is required.
Not all aircraft noted as equipped are considered to have good installs. This is because when each aircraft is field monitored, the data provided may not be accurate or fully compliant. The FAA, via their ADS-B website, allows the flying public to complete a ‘Public ADS-B Performance Report’ (PAPR), submit it electronically, and within 30 minutes receive a summary of the ADS-B Out performance monitored from a previous flight.
These are considered verification tests for operators. Of course, the data of the aircraft, equipment and flight must match the FAA database. It is also recommended to only request a report on flights known to have flown in airspace that has good ADS-B coverage. There is a coverage map provided on the PAPR site.
For a PAPR request, you will need to know aircraft equipment information regarding type of transmit datalink (UAT, 1090ES, or both), along with make and model of the transponders and GPS sensors.
For the installing agency, the FAA provides an Operational Flight Evaluation (OFE) service. Each flight is conducted to guidelines under FAR 91.407 (b), and the installer then requests a report from an FAA AFS website. The report results must match the FAR 91.227 (or 225) requirement, a very important rule covering ADS-B Out performance.
For itself, and to feed data into equipage rates, the FAA also conducts ADS-B Performance Monitoring (APM), an ongoing procedure for operational ADS-B systems.
Considered as trend monitoring, data are collected from the last 10-flights per aircraft. Unusual findings are notified to both operators and installing companies held on record.
The FAA claims, however, to work with customers in resolving their discrepancies. Equipment found to be non-compliant is termed Non-Performing Equipment (NPE) by the FAA. Aside from operator manual entry errors, some of the ADS-B, installation related, non-compliance issues are covered in Table B.
Approval Basis for ADS-B Installations
In March of 2016 the FAA issued a Policy Memorandum titled ‘Installation Approval for ADS-B Out Systems’, which outlined very clearly the different means to compliance of ADS-B Out systems in an aircraft.
Interestingly for business aircraft customers, the path for certification is easier if systems have already been approved elsewhere (including the grouping of components within a total system). For example, different GPS sensors combined with different Transponders.
Applicants for approvals need to ensure the different ADS-B Technical Standard Orders (TSOs) have been met by manufacturers, by verifying the equipment. Deviations for installations approved under Amended Type Certificates (TCs) or Supplemental Type Certificates (STCs) are permitted, as well as the lesser hurdle of utilizing Field Approvals.
Because many STCs have been completed under the basis of an Approved Model List, owners and installers can check for their aircraft type against a list of different aircraft models, all approved under the umbrella of one STC number.
The FAA has been very understanding in qualifying the same groupings of equipment into multiple different aircraft models, lessening the burden of certifications for many installing agencies.
Aircraft that include the addition of a Cockpit Display of Traffic Information (CDTI) will enjoy the features of ADS-B In, turning the cockpit environment into a mini version of air traffic control, enabling crews to view similar traffic positioning and intention as controllers.
Specifically, ADS-B In traffic data include aircraft identification, position, altitude, velocity, and other information. The actual targets displayed are predicated upon the limits of TIS-B and ADS-R, as well as broadcasts directly received from other aircraft.
ADS-B In data can be surface or airborne traffic. TSO C159b calls out for four ADS-B In equipment classifications, including:
The actual ADS-B In applications are:
ADS-C (Contract) temporarily fills the void where there is no ADS-B ground station coverage. This includes Oceanic regions and remote continental areas. For meeting Oceanic Separation standards, the world’s Satcom providers enable contracts between individual aircraft and en route air traffic control centers.
For aircraft that are FANS-equipped, or planning on it, ADS-C is part of the service offered. Given the Performance Based Navigation requirements of FANS compliant tracks, ADS-C functionality is certainly integral to it.
In essence, FANS (including ADS-C), ‘communicates’ critical data via Satcom, including aircraft position, track, trend and its intention to en route ATC.
The temporary use of ADS-C should transition into Space-Based ADS-B, where satellites will perform the function of ground stations, allowing both air traffic control and aircraft to enjoy the same benefits of ADS-B over oceans and remote continental regions.
The use of Space-Based ADS-B is currently being evaluated and will be addressed in greater detail in our next article.
While ADS-C communicates data via the aircraft’s Satcom system, Spaced-Based ADS-B will broadcast data in the same way it does over populated areas, but utilizing satellites as part of the infrastructure.
Somewhat surprisingly, both ADS-B Out and In have many layers involved in implementation and integration. Sorting through the options, in selecting both the type and extent of your ADS-B solution, is not an easy task and is best supported by a trusted service center.
The more one scratches the surface of ADS-B technically, the more information is revealed to share. This article has attempted to focus on the more relevant and interesting aspects of the subject.
With only three years to go until the ADS-B equipage deadline, pilot- and technician-friendly resources are now prolific. Many aircraft and equipment OEMs have informative and dedicated web pages for their customers. Equally MROs and Avionics Shops are providing useful links to all sorts of solution-based ADS-B information.
The FAA, NBAA, AOPA and AEA have outshone themselves in ADS-B communications.
With all the information available, owners and operators should be aware of ADS-B requirements. They should also understand the options and realize they have until December 31, 2019 to comply.
Irrespective of an anticipated status of aircraft ownership and ignoring false rumors of implementation delay, there is every reason to upgrade in a timely manner. To place that in graphical terms, if two identical aircraft are parked side by side on the ramp, one with ADS-B and the other without, it would not be difficult to figure out which aircraft maintains the selling edge.
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