Saturday, September 24, 2016

Aircraft Tracking using ADS-B - We've come a long ways!

Automatic Dependent Surveillance - Broadcast, or ADS-B, is revolutionizing Air Traffic Control.  With some minor enhancements to the aircraft transponder, space-based ADS-B can become the cornerstone of an ICAO Autonomous Distress Tracking service, mandated for new aircraft starting in 2021.  Such an installation may also allow the removal of one of the required Emergency Locator Transmitters.   The use of satellite data link and navigation together with ADS-B are powering emerging Performance Based Navigation initiatives as well.
Aireon Space-based ADS-B
Air Traffic Control
In 1929, fifteen air carriers pooled $100,000 to set up the not-for-profit organization, Aeronautical Radio, Inc. (ARINC), to serve as the single coordinator of aeronautical communications for the air transport industry, using a common network of ground stations.

The Bureau of Air Commerce established an air traffic control system in 1936, built largely off of pioneering airline networks.   A civil process was set forth to assure aircraft separation while flying in instrument conditions, including flight plans and position reporting.  The transition to civil control from the airline control opened the airways to include military/government and general aviation pilots with instrument flight rules (IFR).  The military was expected to provide equivalent certifications, whilst the general aviation pilots joined the commercial pilots.  Stated objectives for Air Traffic Control included:
·       Flight in instrument flight conditions demand separation horizontally and vertically.
·       Orderly sequencing for arrival.
·       Air-traffic control operates continually even in favorable weather.
·       Airplane reports departure, position, arrival
·       Position report when crossing over radio beacon
·       Filed flight plan including private and state aircraft
·       Departure clearance to first fix
·       15 minute position reporting
·       Using vertical separation, speed limits, and holding patterns to manage traffic

Inflight position reporting is the cornerstone of separation, allowing the ground controller situational awareness of the traffic in each air corridor.  Every fifteen minutes, the controller would move “shrimp boats” on a map to designate the position of each aircraft inflight.  The airline dispatch office would arbitrate between communications with the aircraft and the controller; the controller had no way to talk to the aircraft directly. 

HF, SSR, ACAS and Procedures

In many ways, oceanic and remote airspace remains the last vestige of the pioneering days of air traffic control.  Today, HF voice is still the only long range communications approved for use in all regions. Thankfully satellite navigation and satellite data link are powering much greater performance out of the remote airways.  

Required Navigation Performance (RNP), Required Surveillance Performance (RSP) and Required Communications Performance (RCP) are the cornerstones of Performance Based Navigation (PBN).

Global Navigation Satellite Systems (GNSS), such as GPS, offer the ability to measure position uncertainty and validity.  This assurance allows confidence to ensure safe separation, while operating beyond line of sight (LOS) of any independent position surveillance. 

Traditionally, LOS surveillance is through secondary surveillance radar (SSR). Primary surveillance radar is normally relegated to intrusion detection.  SSR relies on a cooperative transponder (Mode A, C, or S) to respond with specific information regarding that airplane.  SSR independently judges bearing and distance to determine position, but also relies on reported altitude for vertical position.   Each airplane is equipped with a transponder that listens on 1030 MHz for an interrogation, and responds with a coded message on 1090 MHz.

Air Traffic Control Radar Beacons System (ATCRBS) 1950    

Transponders also respond to Aircraft (or Traffic) Collision and Alerting System (ACAS or TCAS) probes by proximate equipped aircraft.  The following figures (from Eurocontrol) show a typical installation including the directional antenna.  

Vertical and Horizontal areas distinguish alerting and recommended avoidance based on proximity.  Figures from Eurocontrol.


Automatic Dependent Surveillance (ADS) is a concept where the airplane transmits its position regularly based on a contract (ADS-C) or broadcast regularly on a time interval (ADS-B).

ADS-C was first promoted as a feature in the Future Air Navigation System (FANS), starting in 1995.  ADS-C relies on an ACARS data link to communicate position reports to requesting air navigation service providers (ANSP) at a rate of their choosing.   ADS-C has provisions for reports to be triggered if the airplane deviates from the approved flight plan and for other reasons.  ADS-C reporting rate has a commercial consideration; each message has a cost both in currency and in capacity.  ADS-C is typically communicated using L-band satcom (Inmarsat, Iridium).  With Inmarsat SwiftBroadBand Safety, L-band satcom is capable of high reporting rates (once per minute) and with much lower cost than with a classic channel. 

ADS-B was first promoted in the FAA Alaska Capstone project, starting in 1999.  The aircraft used GPS to determine position once per second and a special Universal Asynchronous Transceiver (UAT) operating on 978 MHz would broadcast the position and other related information twice per second.

Alaska Capstone - The Impact of Capstone Phase 1 Program Final Report, Sep 2005

Notably, the UAT could receive traffic information (TIS) and other flight information (FIS) transmitted by other aircraft and by the FAA.   These receiver features are "IN" functions.  The transmit broadcast is an "OUT" function.  ADS-B IN holds the promise for considerable enhancements to allow aircraft maneuvering in closer proximity than normally allowed procedurally, or with better situational awareness.

ADS-B is normally associated with transmissions on 1090 MHz (1090ES, Extended Squitter).  The extended squitter is a 56 bit field that communicates a variety of information on a rotating schedule.  UAT 978 MHz operates differently, and for the remainder of this discussion will be "left out", in preference to 1090ES operations.  It is noted that space-based ADS-B receivers are typically oriented to receive 1090 MHz and have no facility to interwork with UAT 978 MHz equipped aircraft.

Given 1090ES operations, ADS-B IN applications may depend on receiving 1090ES ADS-B from nearby aircraft, typically using the ACAS receiver.   This is a distinctly different process as professed in ACAS itself.  In ADS-B, the receiver reads the position from the report.  In ACAS, the receiver determines range and bearing based on the transponder responding to its probe.  Furthermore, ACAS and ADS-B operating together offer a powerful combination for self-checking and enhanced awareness.  Crowd-sourcing ACAS correlation to ADS-B, at least on an exceptional basis (as in, where they disagree significantly), could offer extraordinary benefits in overall position assurance.
I had a chance to fly in the jump seat of 747-400 aircraft many times.  I took note of the "other aircraft" display when an aircraft crossed head-on over us once somewhere in Asia, yet was displayed below us.  This aircraft probably never flew in SSR airspace and had no knowledge its altitude broadcast was seriously in error (yet shown valid).
The In-Trail Procedure (ITP) is an example of an ADS-B IN application, allowing an airplane to climb between two other aircraft to reach its desired altitude while operating closer than normally permitted using procedural separation.

ITP Climb from FL340 to FL360 (FAA AC 90-114A)    
The update interval for surveillance and the quality of the information relate to aircraft separation.

ADS-B is currently received through a network of ADS-B ground stations deployed strategically by each ANSP.  Space-based ADS-B offers an overlay of receivers that can offer truly global coverage.   Aireon professes its network, once operational, will match SSR updates, if not exceed them, as required for separation as close as 5 nm.

Aireon Performance Goals

Another form of surveillance uses Time-of-Arrival from any airborne transponder as measured by a network of ground stations.  These Wide-Area Multi-lateration (WAM) networks can locate a transmitter by its radiated energy.

Recognizing the benefits of ADS-B over SSR (cost, coverage), ANSP's around the world have mandates that all aircraft should install compliant ADS-B systems, broadly by 2020,  but required already in some cases.  It should be noted that ADS-B mandates do not require the installation of top mounted transmitting antennas.  These top mounted antennas are utilized where ACAS is mandated, so most large airplanes will be equipped.  Top mounted ADS-B antennas offer better coupling to space-based ADS-B receivers - while the required belly antennas are still possibly received.

Airservices Australia has complete enroute continental ADS-B coverage 

Aireon highlights areas with ADS-B coverage and gaps that can be filled with space-based receivers
If one assumes all aircraft are equipped and operating with ADS-B, then one can assume any ADS-B receiver (ground or space-based) will receive the broadcasted information when in LOS.  Thus many applications can be built upon the ADS-B broadcast without any incremental cost to the airplane.

ADS-C offers the potential for update intervals under 60 seconds, but each transmission introduces an  incremental cost.  ADS-B networks have incremental costs as well in communicating their receivers  to their back-end servers and correlating the information.  Service providers for ADS-C and ADS-B networks are free to choose how to charge for their services.

ADS-C depends on a data link system.  While it is likely to be installed, specifically to facilitate preferred oceanic routings, data link systems are not universal.  One can argue that ADS-B will be universal.

Aircraft Tracking

ADS-C and ADS-B can both deliver position reporting of one minute or less.  

Normal aircraft tracking is mandated by ICAO starting in 2018.
On 10 November 2015, the ICAO Council adopted Amendment 39 to Annex 6 — Operation of Aircraft, Part I — International Commercial Air Transport — Aeroplanes which included the normal aircraft tracking Standards and Recommended Practices (SARPs). These SARPs became effective on 20 March 2016 and will be applicable on 8 November 2018. Amendment 39 will be issued in April 2016. 
The normal aircraft tracking SARPs establish the air operator’s responsibility to track its aircraft throughout its area of operations. It establishes an aircraft-tracking time interval of 15 minutes whenever air traffic services obtain an aircraft’s position information at greater than 15-minute intervals for aeroplanes with a seating capacity greater than nineteen. This aircraft-tracking time interval further applies as a recommendation to all operations of aircraft with a take-off mass of 27 000 kg and as a requirement to all operations of aircraft with a take-off mass of 45 500 kg when flying over oceanic areas.
ADS-C and ADS-B easily satisfy ICAO normal aircraft tracking standards assuming their networks offer suitable coverage.

Autonomous Distress Tracking (ADT) is a new concept mandated by ICAO starting in 2021.
On 2 March 2016, the ICAO Council adopted Amendment 40 to Annex 6, Part I which included, among other elements, SARPs relating to the location of an aeroplane in distress. These SARPs address the Global Aeronautical Distress Safety System (GADSS) autonomous distress tracking (ADT) concept. The SARPs will become effective on 11 July 2016 and will be applicable on 1 January 2021. Amendment 40 will be issued in July 2016. 
The SARPs relating to the location of an aeroplane in distress establish the requirement for an aeroplane to autonomously transmit information from which a position can be determined at least once every minute when in a distress condition. An aircraft is in a distress condition when it is in a state that, if the aircraft behaviour event is left uncorrected, could result in an accident. The SARPs are applicable to new aeroplanes with take-off mass greater than 27 000 kg from 1 January 2021. The requirement also recommends that it applies to new aeroplanes with take-off mass greater than 5 700 kg from the same date. 
The SARPs specify that autonomous transmission of position information needs to be active when an aircraft is in a distress condition. This will provide a high probability of locating an accident site to within a 6 NM radius. It also specifies that the transmission can be activated manually. The SARP is not technology-specific and will allow for various solutions, including a triggered transmission system. It specifies performance criteria such as that the autonomous transmission of position information needs to be capable of transmitting the information in the event of aircraft electrical power loss, at least for the expected duration of the entire flight. 
Finally, although these SARPs apply only to newly manufactured aircraft, there is an incentive to retrofit aeroplanes with ADT systems since they can replace one of two required emergency locator transmitters (ELT).
While there is an expectation to utilize the ELT for ADT, and that the ADT system should be independent from the normal tracking system, ICAO states performance based criteria, not prescriptive.  Aviation regulators will weigh the investment needed and the benefits gained against minimum performance.   Particularly, investment into ADS-B Versus investment in ELT creates different challenges.  I generally find that dollars spent on a system that operates normally are more easily justified than dollars spent on a system that hopefully is never used.

ADS-C and ADS-B are capable of meeting the one minute position update performance requirements.  However, neither of these systems currently satisfies the full scope of the ADT SARPs which require tamper-proof features and independent sources of power and position.  An ADS-B installation can be upgraded to meet these criteria, and if so, the operator is free to remove one of the required Emergency Locating Transmitters (ELT) as a result.  These benefits would be granted after a suitable time period demonstrating the performance of the end-end system. Enhancing ADS-B to include autonomous features will offer more utility than the removed ELT.

ADS-C offers full integration with the flight plan utilizing L-band satellite spectrum set aside for Route services.  ADS-C is tied closely to each ANSP as a basic feature.  ADS-B affords a complementary service.  It operates on an entirely different spectrum as ADS-C.

ADS-B transponder installations typically are already dual redundant, with one channel operating under degraded aircraft conditions.  An autonomous installation would upgrade the power and source of position to an internal source (battery and GNSS receiver).  No single failure would cause loss of the ADS-B transmissions by at least one of the two transponders.

My view of the aircraft installation end state we are moving towards (noting ICAO doc 9750 block point updates too much to quote here):
  • Fail-Op Autonomous ADS-B OUT with space-based ADS-B receivers
  • ADS-B IN Airborne Surveillance Applications
  • ACAS
  • Single 406 MHz ELT
  • Single Satellite ADS-C, CPDLC, AOC
  • Dual, Dissimilar Satellite Voice (Direct Pilot/Controller)
  • RNP-based procedures
  • Fail-Op RNP-based navigation
  • Single Satellite  SWIM, AAC

Peter Lemme
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Follow me on twitter: @Satcom_Guru

Copyright 2016     All Rights Reserved

Peter Lemme has been a leader in avionics engineering for 35 years. He offers independent consulting services largely focused on avionics and L, Ku, and Ka band satellite communications to aircraft. Peter chairs the SAE-ITC AEEC Ku/Ka-band satcom subcommittee developing PP848, ARINC 791, and PP792 standards and characteristics. 

Peter was Boeing avionics supervisor for 767 and 747-400 data link recording, data link reporting, and satellite communications. He was an FAA designated engineering representative (DER) for ACARS, satellite communications, DFDAU, DFDR, ACMS and printers. Peter was lead engineer for Thrust Management System (757, 767, 747-400), also supervisor for satellite communications for 777, and was manager of terminal-area projects (GLS, MLS, enhanced vision).

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