Having reviewed the essentials of ADS-B Out, Ken Elliott delves into the emerging world of Space-Based ADS-B. What is it, and why will it be important to Business Aviation?
Automatic Dependent Surveillance - Broadcast (ADS-B) is now an everyday term and a familiar technology across the world. As mandated equipage dates edge closer, operators are experiencing the advantages of ADS-B, via Air Traffic Control’s (ATC) ability to monitor and direct air traffic within areas previously not covered by traditional radar.
Because ATC now receives more flight profile and intention information from equipped aircraft, these aircraft are also better informed to enable reduced separation, which allows for more flexible flight planning.
This article uses several acronyms employed by those familiar with ADS-B. Table A provides an abbreviated list to assist the reader.
ADS-B relies on ground stations and rebroadcasting. It was not designed as a satellite service like the Global Positioning System (GPS). For oceanic and remote areas, the flying community has continued to rely on ADS-C, a less robust service utilizing Satcom.
This is less than optimal, because using ADS-C and traditional voice communications mean less frequent updates of the aircraft’s position and, as a result, greater required separation between aircraft both laterally and vertically.
How Does GPS Improve Aircraft Navigation?
It takes more than just surveillance to operate anywhere. Aircraft are equipped to communicate, navigate and have surveillance capability. These three elements of conducting flight fall under the heading ‘CNS’.
While surveillance and communication are also the domain of ATC, navigation is specific to the aircraft. So improved CNS is the key to closely separated oceanic and remote flight tracks.
The communications piece is via CPDLC, using reliable and consistent data, rather than voice. Under the CPDLC umbrella sit:
- VDL (using line of sight datalink to communicate intentions between aircraft and en-route air traffic), and
- FANS (using HFDL and SDL to communicate similar intentions over oceans and remote areas where VDL coverage is inadequate).
The navigation piece relies on highly accurate positioning and flight plan tracking, derived from Global Positioning Satellites (GPS). The WAAS version of GPS is a crucial component of the surveillance also, being the required position output of ADS-B Out (Version 2).
Aircraft are required to have different degrees of navigational capability, based on where they intend to fly. These variations are covered under RNP and Area Navigation (RNAV), where there are varying degrees of allowable flight track deviation.
Note that apart from CNS, an aircraft’s physical ability to maintain the flight path and its navigation system commands is also important. Maintaining altitude (RVSM) using fine-tuned air data systems is one method to achieve it. Another is the aerodynamic performance of the aircraft.
Yet another is the ability of the flight control system to maintain heading, roll and track angle.
What is Space-Based ADS-B, and How Does it Help?
Figure A introduces SB ADS-B, serving the same functionality and purpose over remote regions and oceans as ground-based ADS-B Out currently provides over terrestrial surfaces.
FIGURE A: Simplified Depiction of Land and Oceanic ATC Coverage
Figure B, meanwhile, brings together the CNS elements as they apply to four different types of worldwide airspace. The more advanced and ‘on-track’ the CNS is, the safer it is for everyone wanting to fly favorable tracks. (Tracks are favored because of variable winds, fuel burn rates, actual distance flown and several other factors.)
With everyone wanting to squeeze into these tracks, it is understandably difficult to safely maintain minimum separation.
Obviously, if everyone had FANS and RNP4 capability, it would also be easier to maintain RSVM between aircraft (above, below and to each side). Under an ADS-C service, where aircraft report at ~14-minute intervals, oceanic and remote air traffic controllers are forced to provide conservative separation.
Reducing the time interval to transfer surveillance data down from 14 minutes to less than 8 seconds using SB ADS-B will naturally provide the ability to reduce aircraft separation. In turn, SB ADS-B provides faster data between air and ground (and air-to-air), allowing controllers to reduce separation to less than 15nm in these regions.
In 2016 the FAA assigned to the RTCA NextGen Advisory Council the task of evaluating Enhanced Surveillance, including SB ADS-B. In summary, the Final Report captured perfectly how important it is to have a complete suite of NextGen technologies operating in space, in the aircraft and on the ground (ATC).
Space-based ADS-B technology, when coupled with FANS, CPDLC, ADS-C and RNP4 capabilities allows for significantly less separation than ADS-C.
Aircraft currently being equipped with ADS-B Out can expect to reap the benefits of SB ADS-B when using a service provider that offers it. Their existing on-board ADS-B Out equipment will work in just the same way as when flying across populated landscapes, relying on the same ADS-B transmission methods, but instead via satellites.
The make-up of the SB ADS-B system is as follows:
- Low Earth Orbit Satellite(s)
- Hosted ADS-B Payload(s)
What are Low Earth Orbit (LEO) Satellites & Hosted ADS-B Payloads?
Although there are several satellite contenders, including some using miniature Nano or cube satellites, Iridium has advanced to a leading position with its pole-to-pole coverage, and its ability to consistently and reliably track fast-moving aircraft. Iridium has just completed its final satellite launch, establishing its 66 Low Earth Orbit (LEO) NEXT, cross-linked, Thales-Alenia-built satellite constellation.
These include Harris ADS-B payloads hosted by Aireon, a joint venture of Nav Canada and Iridium.
Aireon has teamed with the UK’s ANSP, NATS, and Ireland’s IAA to embark on SB ADS-B trials across the North Atlantic Track network now that all the satellites are operational. These tests will commence imminently.
What is the ANSP?
The ANSP can be a government agency or a commercial company (e.g. ARINC, Satcom Direct). Several countries have signed agreements with Aireon signaling their intention to use SB ADS-B across their land surface, either avoiding or complimenting ground-based ADS-B infrastructure and service.
There are presently around eight ANSPs committed to offering service, including country agencies from Canada to South Africa, across Europe and from parts of Asia.
Note: There is a big difference between aircraft tracking and full ADS-B surveillance service. While there are several aircraft tracking companies that will continue to develop useful aircraft tracking resources, it’s likely to be some time before the Iridium-Aireon network has a true contender.
How Can Operators Access Space-Based ADS-B Services?
It is assumed operators will create or extend agreements with their service providers currently offering ADS-C, Satcom, high-speed data and other flight services. These agreements should provide SB ADS-B on an ‘as-used’ basis.
While some areas of the world will offer SB ADS-B as a free service to their airspace users, others will charge a user fee. The US, having a robust ground-based ADS-B service, plans to encourage operators to complement their existing ADS-B Out with SB ADS-B on an individual user basis.
From an aircraft or ATC operator perspective, the great thing about SB ADS-B is that it is essentially no different to its terrestrial relative, other than the important matter of accessing the service itself.
If you are equipped for ADS-B now, you will not be required to add to or change that equipment. Where you fly will dictate the options of Spaced-Based service available to you.
Initially, it’s likely that government and large commercial operators will embrace SB ADS-B services, perhaps negotiating bundled service rates for their fleets. Satellite hosts will further focus on Business Aviation through their existing service provider network.
However, as SB ADS-B migrates into common usage, it will not be without its issues and user concerns. Security of individual flight data is one fear, but a much broader case for deliberation is the fact that satellites are vulnerable to ‘attacks’ from those with ill intent.
Attacks can be either literal or cyber, both amounting to the same overall degradation or loss of ADS-B service.
Even worse, imagine losing GPS, SB ADS-B and Satcom all at the same time. Recently, India joined the elite group of nations that have targeted and blown up satellites (not currently an easy task).
A lot of nations are attempting to address the protection and back-up of their satellites. But you can bet some are seeking the opposite – disruption of normal service.
Whatever the outcome, the pressure for worldwide surveillance, for reasons of safety, economic convenience and real-time tracking, is strong. It’s certainly strong enough to ensure the wider aviation community will embrace this great resource.