Investigate the Differences Between Dependent and Independent Surveillance in Determining Position Reports

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Investigate the Differences Between Dependent and Independent Surveillance in Determining Position Reports

43RD ANNUAL CONFERENCE, Hong Kong, China (SAR), 22-26 March 2004

WP No. 102

Investigate the Differences Between Dependent and Independent Surveillance in Determining Position Reports

Presented by EVP Technical


1.1. Dependent surveillance is not new, however there is a planned rapid increase in the use of Automatic Dependent Surveillance (ADS), and this requires a review of the IFATCA position in relation to dependent surveillance.

1.2. In addition, for the development of new separation standards, the International Civil Aviation Organisation (ICAO) permits a comparison with an existing equivalent separation standard – and automatic dependent surveillance is being compared to independent radar surveillance. This working paper investigates whether dependant and independent surveillance are similar enough to permit this comparison.


2.1 For this paper, independence refers to the independence of the position data and does not negate the need for primary surveillance as a means for detecting airspace intrusions. In other words, this paper considers surveillance in respect to aircraft position reports.

2.2 IFATCA does not have specific definitions for dependent and independent surveillance however IFATCA WP 93/1994 “The Application of Air Ground Datalink”, which is policy material, discusses dependent surveillance. WP93/1994 states: “an independent (surveillance) system is one that does not rely upon aircraft based sensors for surveillance data. A dependant surveillance sensor meant that the data used comes from the other sensors or sources (sic) and is therefore not the subject of independent corroboration.” Primary radar is therefore independent surveillance as no aircraft-based sensors are required. Automatic Dependant Surveillance transmits a position report determined by aircraft equipment and so is dependant surveillance. Secondary Surveillance Radar (SSR) requires the use of transponder on the aircraft, which can transmit sensor information (for example SSR Mode C that transmits level information). WP 93/1994 considered SSR as independent surveillance, however there is a need to review this material and to have clear definitions of the different types of surveillance. For this paper, Mode C SSR should be considered as dependant surveillance.

2.3 Previous IFATCA work has made a clear distinction between radar and ADS. WP 93/1994 states that “independent surveillance is fundamentally different from dependent surveillance, hence such displays are categorically not radar displays and should be considered as a new tool requiring new procedural rules and operating methods.” It is IFATCA Policy that “control of aircraft via ADS and Controller/Pilot Data Link Communication (CPDLC) is sufficiently different to other forms of ATC rating to warrant comprehensive training and a separate rating”. The main form of ADS used at the time of formulation of this policy was ADSContract (ADS-C) which has reporting rates less than radar updates.

2.4 ADS implementation continues to evolve. ADS–Broadcast (ADS-B) has high reporting rates similar to radar and is starting to be used for Air Traffic Control surveillance. ICAO Doc 4444 is being amended with ADS in mind. Despite the distinction in paragraph 2.3 above, IFATCA policy does allow for combined surveillance displays with the following policy. “Control of traffic using position data derived from ADS and radar surveillance can only be used where the control system supports both types of surveillance. Whenever a controller interface derives data from a combination of surveillance systems, the source and derivation of position data in use must be clearly and continuously evident to the controller.”

2.5 In line with ICAO requirements for new separation standards, Australia is conducting a comparison between radar and ADS-B surveillance for Air Traffic Control (ATC) use, with the intention of applying an ADS-B separation standard of 5 nautical miles. That is, Australia’s study is to demonstrate that ADS-B is more accurate than the “equivalent” radar standard. Australia has announced its intention for a network of ADS-B receivers to enable high-level surveillance over 3 million square miles of continental airspace where there is currently almost no radar surveillance. In some fora there is even discussion of ADS-B as the “new radar” that is more accurate than existing radar equipment and is much cheaper and easier to implement and maintain than existing radar equipment.

2.6 IFATCA already has some policy on ADS, although as it is implemented a process of refinement, including addition of more material, will be required. Consider the following existing policy. ”The ADS system shall provide a warning to pilot and controller whenever navigation accuracy is degraded below that required to operate in the airspace, and that this will affect separation standards. Procedures must be in place to restore any loss of separation in a timely manner.”

2.7 Consider how this “automatic dependant surveillance” applies to SSR Mode C. The pilot is not aware of what data is transmitted; instead the pilot navigates via an independent altimeter. The controller is unaware of what pressure level has been transmitted, or of the exact details of the software translation of this pressure level into level information as displayed on the controller’s screen. Despite vertical separation being the smallest separation standard applied between aircraft, it has not been until the advent of Reduced Vertical Separation Minima (RVSM) that Height Measurement Units (HMUs) have been used to monitor aircraft height keeping performance. Even with HMUs, pilots and controllers are not advised when Mode C information becomes unusable. This has been, as is still, done procedurally in many areas of the world. The height readout is checked for each flight, and controllers are trained to detect anomalies, such as variance to cleared level or other inconsistencies. In these cases, procedures require the controller (after other attempts are unsuccessful) to instruct the aircraft to switch off the altitude squawk. This not only removes the incorrect information from the controllers display, but also prevents Airborne Collision Avoidance Systems (ACAS) giving false Resolution Advisories (RAs) based on incorrect level information.

2.8 A significant change is that it is now an ICAO requirement to transmit both Mode A and C – or none – and this is being reflected in new aircraft avionics. However in many areas there is only SSR electronic surveillance and so in order to inhibit the incorrect Mode C, the controller loses radar surveillance. Many states have filed a variance with ICAO on this, but this does not affect the aircraft manufacturers. In addition some states have procedures that allow the incorrect Mode C information to be transmitted so that Mode A surveillance can continue, however other pilots are not informed of the incorrect level information being transmitted. It is essential that all the interrelationships of surveillance must be considered included effects on ACAS RAs.

2.9 Drawing from the experience with Mode C automatic dependant surveillance, a number of questions arise. How does the current practice with Mode C compare to horizontal position data from an aircraft? Should the pilot’s navigation display be independent from the ADS transmitted data? Does the pilot need to be aware of the transmitted data? Should the pilot be able to switch off ADS transmissions? What are the accuracy requirements of dependant surveillance and how shall these be monitored? Should Mode C be changed to reflect new ADS procedures, or should ADS procedures seek to reflect the Mode C experience and procedures?

2.10 Australia intends to install a few “static” ADS-B sites. These would function similar to the radar site monitors used to automatically check radar accuracy. It would also be used to report loss of accuracy using the Receiver Autonomous Integrity Monitoring (RAIM) function. However there are no guidelines as to how many such stations are required for system integrity in any particular region.

2.11 In considering ADS, it is important to realise the wide range in reporting rates. For example, some ADS (ADS-C) is being used by Aeronautical Radio Incorporated (ARINC) to generate the equivalent of an “automatic HF position report”. Other ADS (like Mode C or ADS-B via Mode S Extended Squitter) is at a high update rate.

2.12 It is important to note that non-automatic dependant surveillance has been used for many years, and will continue to be. Pilot-reported position is a form of dependant surveillance. However the pilot is involved in reviewing and modifying the position information, whereas in ADS often the pilot is not aware of either the data or when it is transmitted. What elements of IFATCA policy should relate to dependant surveillance – and what are issues that only relate to automatic dependant surveillance?

2.13 Another issue is end-to-end integrity. There will be different ways of processing the data received, however the controller is concerned that the final display on the controller’s display matches the “reported” position.

2.14 Are there acceptable additional measures that can be taken to ensure that highrate update automatic dependant surveillance is equivalent to independent radar surveillance? How far does this equivalence go, for example is vectoring of ADSB tracks acceptable?


3.1 Dependant Surveillance is when unverified position information is used from the aircraft. Controllers should be trained and equipped to detect inconsistencies and verify the aircraft position and accuracy of that position information.

3.2 Automatic dependant surveillance is when position information is transmitted without pilot initiation (other than enabling the transmission). Procedures must be in place for the controller to verify that the ATC displayed position matches an independently verified position. This independent report may be a pilot-reported position – for example, a pilot-report on the ground at a known location that matches the controller’s display. Other cases of acceptable pilot reported position need to be determined.

3.3 To ensure integrity of system surveillance data (not just ATC surveillance) it is essential there is the option of disabling automatic transmission of dependant position data if it is known to be inaccurate


4.1 That IFATCA adopt the following as ADS policy, IFATCA Manual page 3213 paragraph 1.2:

a) Global standards and procedures must address requirements of what independent verification of position data is required before dependent position data is used for separation.

b) To ensure integrity of system surveillance data (not just ATC surveillance) it is essential that the automatic transmission of erroneous dependent position data can be disabled or marked as inaccurate during all stages of flight.

4.2 That SC1 be tasked with reviewing IFATCA policy in relation to surveillance, and in particular to:

a) Develop definitions for dependent, automatic dependent and independent surveillance, and,

b) Classify SSR modes as to the type of surveillance

Last Update: September 29, 2020  

March 24, 2020   251   Jean-Francois Lepage    2004    

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