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ADS-b - Do we need it and what can we expect?

By Rob Russell

Much has been spoken about Automatic Dependent Surveillance-Broadcast (ADS-b), what it does and how it works, but how will it impact on aviation and how will it affect your flights?

ADS-B is a Surveillance technique that relies on aircraft, or airport vehicles, broadcasting their identity, position and other information derived from onboard systems (Global Navigation Satellite System etc.). This signal (ADS-B Out) can be captured for surveillance purposes on the ground (ADS-B Out) or onboard other aircraft in order to facilitate airborne traffic situational awareness, spacing, separation and self-separation (ADS-B In).

ADS-B is automatic because no external stimulus is required; it is dependent because it relies on on-board systems to provide surveillance information to other parties. Finally, the data is broadcast, the originating source has no knowledge of who receives the data and there is no interrogation or two-way contract.

Since man first started to fly, there has been a need to track and follow aircraft. The first means were by flying over specific clearly identifiable points and someone on the ground kept a log. With the development of radio and Morse code, it was possible for aircraft to fly along predefined routes and make position reports. Efficient at the time, but not today! With modern technology came certain developments. The first was radar. With time the old primary radar evolved – secondary surveillance radar was developed, making it possible to see an aircraft’s call signs, altitudes and speeds – and then came mode S – a refinement and advance and more information was available to ATS. With the introduction of satellite navigation and GPS systems, more efficient and better systems have been developed.

The main surveillance technologies used today are:

• Primary surveillance radar (PSR) transmits a high-power signal, some of which is reflected by the aircraft back to the radar. The radar determines the aircraft’s position based on the elapsed time between signal transmission and reception of the signal’s reflection (range) and the antenna position (bearing). Primary radar is slowly dying and seldom found anywhere in the world now.

• Secondary surveillance radar (SSR) consists of two main elements, a ground-based interrogator/receiver and an aircraft transponder. The transponder responds to interrogations from the ground station, enabling the aircraft’s identity, range and bearing from the ground station to be determined.

• Mode S SSR is an improvement of the SSR. It contains all the functions of SSR, and also allows selective addressing of targets by the use of unique 24 bit aircraft addresses, and a two-way data link between the ground station and aircraft for the exchange of information. The transponder is coded to allow for discreet transmissions from the aircraft and remains allocated to that particular aircraft for its entire use

• Combined PSR/SSR makes use of the advantages of the two radar types in one installation, i.e. one radar head with the two aerials on it

• Multilateration relies on signals from an aircraft’s transponder being detected at a number of receiving stations to locate the aircraft. It uses a technique known as Time Difference of Arrival (TDOA) aka logarithmic, to determine the position of the aircraft. For Multilateration to work, it needs a Mode C or S transponder on the aircraft

• ADS-Broadcast (ADS-B) uses GPS technology to determine an aircraft's location, airspeed and other data, and broadcasts that information to a network of transceivers, which relays the data to air traffic control displays.

• ADS-Contract (ADS-C) uses an automatic position-reporting system to provide a commercial service to operators and others. It has been in wide use for over 30 years, particularly over oceanic airspace. It requires that a contract be established between the aircraft operator and the ground-based service provider.

The days of secondary radar systems are slowly coming to an end. They have serious limitations, like their predecessor, primary systems – the most obvious being they are limited in their ranges, as well as the costs of maintaining them. New systems have been developed and are taking over from radars. Multilateration – a system, which is far cheaper than a radar system, was developed. It requires the aircraft to have a transponder and also, like radar, was limited to line of sight. ADS-B is a newer development that will replace radar systems, eventually.

With the advent of satellite technology, it was possible to track aircraft over the horizon and it was possible to display these tracks on a display in front of a controller. Modern day ATCC’s do not have radar displays any more – they have ASDs – air situation displays. They are essentially a digital screen and have replaced the old radar displays. They are able to integrate and display information from many systems – radars, Multilateration and ADS-B. This has vastly enhanced the information available to controllers, who can now see a far bigger picture, more accurately than a radar display. As a result, safety has been dramatically enhanced.

What does this all mean?

It means that the surveillance of aircraft has vastly changed – from one of the service providers just using radar to track your flight, in a limited area, to aircraft operators having to spend money installing equipment, transponders, GPS's and TCAS – traffic collision avoidance systems, for example, and linking these to other equipment in the aircraft, to enable the aircraft to transmit this information to the air traffic service provider, making it possible for an ATS provider to see far more aircraft, over a far greater range. This in turn, has vastly improved the monitoring of flights and also to provide a better service to aircraft operators. It is now possible for an ATCC to provide information of possible airspace incursions, (into danger or prohibited areas), altitude alerts – i.e. if an aircraft deviates from its assigned flight level. Then with the advent of satellite systems, it was possible to transmit this information. And so, ADS-b was developed.

It has been recognised that ADS-B will eventually become the preferred surveillance technology worldwide, although this will take time. ICAO, at Air Navigation Commission 11 (ANC11) in 2007 it resolved, “ICAO and States recognize ADS-B as an enabler of the global ATM operational concept bringing substantial safety and capacity benefits.” Further development of the ADS-B technology is using satellites to relay the signals from aircraft in areas where ground equipment installation is either impossible or is not feasible. This extension is called space-based ADS-B. The key features of ADS-B (and space-based ADS-B) are:

• ADS-B is an air to air, air to ground autonomous surveillance application;

• ADS-B has been in use since the mid to late 1990’s;

• Reception of existing ADS-B transmissions at satellites is planned;

• Satellite reception does not change or impact on existing RF environment;

• No changes required to aircraft avionics or certification with the introduction of space-based ADS-B, as opposed to ground-based ADS-B

• Space-based ADS-B is mainly intended for oceanic, polar and remote regions.

The obvious benefits are:

• A single global surveillance system. The implementation of space-based ADS-B will be the first to provide global surveillance from a single system. The system is immediately capable of providing surveillance in airspace where traditional systems are impracticable and/or cost-prohibitive to deploy.

• Reduced oceanic separation standards. The current minimum separation standard for locations where traditional surveillance and direct VHF radio communications are not available over the ocean is 30 nm. At the present stage of research and analysis, the ICAO Separation and Airspace Safety Panel (SASP) view is that a 15 nm standard is anticipated, using space-based ADS-B combined with existing communications systems.

• Enhanced Situational Awareness. ATC experience, where ADS-B has been introduced into a non-surveyed region, has greatly enhanced the controllers’ ability to safely manage complex traffic scenarios caused by adverse weather conditions such as thunderstorms, i.e. in mountainous and hilly areas, where radars have serious limitations. Prior to the introduction of ADS-B, complex traffic scenarios combined with bad weather would require highly restrictive traffic management by controllers, (often time-wasting and airspace wasting procedural control – the precursor to radar) resulting in aircraft operating at non-preferred/inefficient levels, and flight crews being forced to conduct extensive diversions to ensure separation.

• Enhanced global flight tracking. The aircraft tracking SARPs at present establish the air operator’s responsibility to track its aircraft position at a time interval of 15 minutes whenever ATS obtain it at greater intervals. Future SARPs relating to the location of an aircraft in distress establish the requirement for an aircraft to autonomously transmit information from which a position can be determined at least once every minute when in a distress condition. This SARPs became effective in July 2016 and will be applicable as from 1 January 2021. Space-based ADS-B would be expected to form a key part of many airline solutions to comply with this requirement.

(All airlines operating in Europe are now required by the European Union Aviation Safety Agency (EASA) to track their aircraft at a frequency of one position every 15 minutes during normal operations. It is expected that by 2021, they will need to receive one position every minute if an aircraft is in distress. In the event of an incident, this will improve response efficiency by inherently confining the search and rescue radius to an area of 11 kilometres. The new EASA requirements are based on the Global Aeronautical Distress & Safety System (GADSS), which was created by the International Civil Aviation Organization (ICAO) largely in response to the loss of flights AF447, over the Atlantic, and MH370.)

• Enhanced Search and Rescue. Recent world events have refocused the aviation industry on its capability to identify the location of an aircraft lost, in distress or involved in an accident. Space-based ADS-B will be able to support SAR services globally in retaining position data. Close to real-time position will be available for all equipped aircraft regardless of where they are in the world.

• Reduction in Pilot and ATC workload. ADS-B enables the display of an accurate and near real-time traffic picture to ATC. It would also facilitate far more efficient ATC planning and use of a wider range of traffic control and management tools. Together with the use of CPDLC, radio communications would be cut down and safety enhanced as instructions would be relayed by data and not voice, reducing the chances of any misunderstandings, especially when HF radio communications are used.

• Improved cross–flight information boundary error detection. Aircraft position errors that occur near the boundaries of two FIRs are still relatively common, creating increased ATC and pilot workload and a negative influence on overall safety, particularly in oceanic airspace, where high-accuracy surveillance is currently limited or not available. The accuracy of space-based ADS-B should allow cross-FIR boundary position errors to be detected more regularly, and the handover between ATC to be more precise.

• Improved and earlier detection of off-track errors. Current monitoring of flight trajectory conformance in oceanic and continental airspace without surveillance coverage is generally limited to position reports received at 30 minute intervals. Space-based ADS-B (with an anticipated data-update rate of once every 8 seconds) will introduce an almost real-time detection of an aircraft that is not conforming with its expected flight path.

• Enhanced safety alerting. Space-based ADS-B enables a range of automated safety alerts to ATC. Some of the alerts that can be included in a global safety net are:

o Danger area infringement warning (DAIW).

o Short-term conflict alert (STCA).

o Minimum safe altitude warning (MSAW).

o Route adherence monitoring (RAM).

o Cleared level adherence monitoring (CLAM)

Interestingly, the ATNS TopSky project has these installed and ATCs are already able to use these systems – they vastly enhance safety. STCA is, essentially, the ATS version of an aircraft’s TCAS. CLAM is a vital tool in RVSM airspace, as it alerts the ATC should any aircraft commence an (often unauthorised) deviation from its assigned flight level and in turn reduce the possibility of a reduction.

• Improved weather avoidance. ADS-B has the potential to enhance the ability of ATC to provide the most efficient off-track routing in order to minimize additional track miles, manage multiple track deviation requests simultaneously and safely, and use the most effective avoidance options because of the reduced separation minima enabled by ADS-B.

• Enhanced Height Monitoring in RVSM airspace. ICAO requires states to have an acceptable method for monitoring aircraft height-keeping performance in Reduced Vertical Separation Minima (RVSM)airspace. Traditionally, this has required data collection devices to be fitted temporarily to the aircraft, which is usually costly and time-consuming. ADS-B, because of the reliability and accuracy, has become an alternative to these legacy systems.

• Surveillance system augmentation and elimination of surveillance gaps. For some existing ground-based surveillance systems, space-based ADS-B should be a suitable augmentation to achieve improved coverage and to fill gaps caused by e.g. terrain. ADS-B is considered a cost-effective means of service enhancement, availability and reliability.

• Enhanced safety for offshore helicopter operations. Offshore helicopter operations are a niche, but an essential type of aircraft operation, and with them comes some unique safety challenges. The advent of space-based ADS-B may bring significant safety benefits because such flights can be fully monitored and operations around adverse weather and SAR response can be better coordinated, especially in areas where ground-based ADS-B is not available.

• More efficient flight trajectory and availability of preferred levels. The optimum flight trajectory involves typically a combination of factors such as operating at the most fuel-efficient altitude and route, seeking the most favourable winds, consideration of passenger comfort and the most efficient flight time. With the anticipated reduced separation minima ATC should be able to clear more aircraft on preferred trajectories. Such systems will vastly improve and enhance trans-Atlantic and pacific operations, leading to greater cost savings for all operators.

• Enhanced incident and accident investigations. Incident and accident investigations often rely on accessing the recorded data by the aircraft FDR and CVR. Space-based ADS-B may offer an important benefit by supporting aircraft position tracking the timely location of an accident aircraft.

Where are we now?

EASA has decided that all aircraft that wish to fly in controlled airspace, in Europe, are to be ADS-B equipped from the 7th June 2020, i.e. in a year’s time! However, there are many hurdles to be mounted and overcome. The biggest one is to ensure that the equipment on aircraft is compatible with the ground equipment.

In American airspace, the FAA has decreed that it will be a requirement from Jan 2020.

New aircraft being delivered now have ADS-B as standard, but older aircraft need to be retrofitted and the equipment updated. It has been estimated that this could be from $60000 upwards to as much as $500000! Airlines say that with large fleets it is not possible to update all their aircraft, and of course, the “bean counters” are still not convinced there are savings involved and advantageous to the airlines. So, it looks as if the target date might have to be moved out.

As to South Africa, the CAA has indicated that it is the way to go and are encouraging aircraft operators to equip their aircraft with ADS-B equipment. For those operators flying in American and European airspace, they will have to get it fitted.

ATNS has spent vast amounts of money developing and upgrading their systems and has signed a contract with Aireon to provide ADS-B surveillance within South African airspaces. ATNS has decided to use a satellite-based system, as opposed to a ground-based system. They hope eventually, to be able to develop and provide this service to other countries on the continent.

The CAA published an AIC – 23/2017 about ADS-b and the proposal has not been taken any further. An AIC is just that – it’s an information circular. To date, the CAA appears not to have decided on any definite date of introduction. ATNS is busy upgrading their equipment to be ADS-b compatible but attempts to get a definite date of introduction has proved fruitless. Confusion still exists exactly where you need to have an ADS-b receiver. There is no reference in the AIP as to what sort of receivers are required and what training is needed on them and who will be certified to use them.

As to airspace, will they be compulsory in all airspace, or just all controlled airspace or just certain types of controlled airspace? If one reads and interprets that AIC, it is in class A,B,C and E airspaces. Many people seem to read this circular as legislation, but a circular is not that – it’s an information document, but confusion exists about the timeline. It has not been upgraded, amended or changed since its publication.

But with the large costs involved and integration into the aircraft and its avionics, is it financially feasible for the average general aviation pilot? Is the average pilot going to be able to afford to pay for the upgrade and is it necessary for the average general aviation flight that is conducted largely in uncontrolled airspace? Many questions and not many answers.

In an effort to provide a better and safer service to all pilots in South African airspace, ATNS investigated in the use of WAM – Wide Area Multilateration.

This was first initiated by ATNS in 2008 and the trial area was around Cape Town. This trial was necessary to get CAA certification, which has now been obtained. What this means is that ATNS plans to roll out the project from December this year beginning in the Lowveld area. It is envisaged that the Northern Cape will also be high on the roll-out list and could happen next year.

What is wide area multilateration and what does it mean to you as a pilot?

WAM is a technique where several ground-receiving stations listen to signals transmitted from an aircraft; then the aircraft's location is mathematically calculated -- typically in two dimensions, with the aircraft providing its altitude. Aircraft position, altitude and other data are ultimately transmitted, through an Air Traffic Control automation system, to screens viewed by air traffic controllers for separation of aircraft. It can and has been interfaced to terminal or en-route automation systems. WAM provides performance that is comparable to (SSR) in terms of accuracy, probability of detection, update rate and availability/ reliability. The primary advantage of WAM is that it can be installed in mountainous terrain, where the line-of-sight propagation paths required for SSRs would be blocked. A second advantage is that, in many situations, its cost is considerably lower than that of SSR systems.

The costs advantage over SSR is massive. It is a simpler system, in essence just requiring a regular power source and good reliable data sending connection. No maintenance is necessary, by the service provider

It all sounds very complicated, but as a pilot all you need is a Mode C transponder and ATNS will be able to track you, even though there is no radar coverage in that area. As can be seen from above, radar is not necessary in these areas. So, a better service will be available to all operators.

From a cost point of view, for a general aviation pilot, rather spend some money and get a good mode C or S transponder fitted to your aircraft. One that has the option to upgrade to ADS-B at a later stage.

There is no doubt that ADS-B is the way to go and will eventually become the primary source of aircraft tracking and monitoring, but for many pilots and operators the costs are still prohibitive and is it a necessary system for the average general aviation pilot that just wants to fly for fun.

From a safety perspective, ADS-B will enhance safety and performance in controlled airspace as well as a better search and rescue service, should it be needed, but once again it is necessary for the general aviation fraternity, who spend the majority of their time flying around and having fun in uncontrolled airspace? But at least put in a transponder with altitude readout, so ATNS will have the ability of providing you with a flight information service, in areas where WAM is available. That will enhance safety as well as better information services will be available to pilots that wish to communicate with ATS.

So, until ADS-B becomes mandatory for all aircraft in all airspace, spend some money and install a good Mode C, upgradable to mode S, if you going to be flying in South African airspace.

Just an indication of some countries and what they have proposed or are implementing, to date.

China: ADS-B OUT Mandate eff. 01JUL19

U.A.E.: ADS-B OUT Mandate eff. 01JAN20.

U.S. Airspace: ADS-B OUT Mandate eff. 01JAN20.

India: ADS-B Mandate for all aircraft flying on PBN Routes in Indian continental airspace with Designators L, M, N, P, Q, T and Routes A201, A347, A465, A474, A791, B211, B466. G450, R457, R460, R461, W15, W19, W20, W29, W41, W43, W45, W47, W56S/N, W67, W111, W112, W114, W115, W118, W153, at or above FL290 must carry serviceable 1090 MHz ES ADS-B transmitting equipment postponed to 01JAN20.

NAT: NAT DataLink Mandate Phase 2C, further expands CPCLC / ADS-C mandated airspace to include FL 290 and above throughout the ICAO NAT region eff. 30JAN20.

EASA airspaces: ADS-B Out Mandate eff 07JUN20.


CAA Standards and Procedures Manual



EASA – European Union Aviation Safety Agency


SA CAA AIC 23/2017



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