A major driving force behind the early implementation of ADS is the need to provide greater capacity to meet the growing demand for air transport services. Particular areas of the globe where ADS is perceived to offer capacity gains are over the North Atlantic and Pacific oceans. The principle capacity gain will come from a reduction in the non-radar separation standards used between aircraft in these areas. The extent to which ADS can help to reduce non-radar separation standards, and the manner in which it is evaluated need therefore to be considered.

The components of separation standards.

Separation standards are the result of an analysis and quantification of all of the sources of error which could contribute to the risk of collision and, for a given target level of safety, a value for the minimum separation produced. The sources of error may be divided into two areas:

• Technical factors:
  The accuracy and performance of surveillance and other ATC support systems such as communications facilities, and aircraft navigation accuracy’s.
• Operational factors:
  ATC procedures and human performance issues, both in the air and on the ground.

It should be evident from the foregoing that when using ADS any quantification becomes critically sensitive to navigational errors – be they derived form the avionics, human error in interpreting or inputting data, or from transmission errors. Because of the nature of the airspace and sectors likely to be flown in areas where dependent surveillance is of most benefit, aircraft will tend to operate for extended periods without access to ground based navigation aids. Thus unless aircraft are using accurate long distance aids such as GPS, these errors are likely to grow as a function of time. Additionally, for the ground system to detect errors it must rely upon the use of support systems, themselves potentially the source of errors. These operating areas may not be able to support VHF communications. Adequate regard must be given to delays in communications between air and ground to ensure that corrective action can be initiated and acknowledged by both controller and pilot in the event of potential loss of separation.

Radar separations recognise that when two aircraft are separated by the bare minima the ‘true’ separation may be less or greater than the minima. This is because errors can occur in the displayed positions of the aircraft. When separation standards are determined for airspace in which dependent surveillance is to be the primary means of surveillance, thorough consideration and examination of the potential effects and sources of errors must be carried out, and the minimum separation be set accordingly.

It should be recognised by those responsible for such studies that dependent surveillance is fundamentally different to independent surveillance and as such caution should be exercised in making comparisons with radar separation standards. However, it is quite evident that in order for aircraft to operate in airspace that is designated as that suitable for use with ADS and the appropriate separation standards, it will be necessary to validate the navigational accuracy independently prior to entry into ADS airspace. Presently this is envisaged as a flight deck task. Additionally, separation must be proved prior to entry into that airspace if aircraft are to enter ADS airspace under any reduced non-radar separation. In such cases the interface between ADS airspace and non ADS airspace will require procedures to be designed to enable the transfer of such aircraft.

Aircraft navigational fits

Even though it may be a requirement to conform to a particular RNP index to operate in a specific ADS area, differing navigational fits carried by aircraft, and the variation in therein, mean that in some circumstances the navigational performance may be less than the RNP minima. It is conceivable therefore that two aircraft operating in trail with the appropriate longitudinal separation for that particular RNP index may require controller intervention to restore standard separation in the event of equipment failure or degradation which leads to a change in the RNP index.

A ‘figure of merit’ will be provided in ADS reports so that navigational performance can be monitored. At such times, the systems supporting the control task should respond in a timely manner to alert the controller and pilot of the degradation of navigational accuracy and the need to regain the appropriate separation.

Collision risk modelling

Collision risk models can only account for a proportion of the technical and operational factors. Therefore it is important that separation standards should be set based upon the results of mathematical models and also sound operational judgement. Various states around the world are carrying out research and development into ADS operation. The operating characteristics of particular airspace domains, and the principles underlying the ICAO CNS/ATM concept are such that a flexible approach will be taken by differing states in planning the capabilities of an ADS-ATC system and therefore there will be variations. There is a risk that separations standards may be developed to suit a particular states requirements. Such a situation will lead to considerable confusion and complexity. Separation standards have been to date, with some exceptions, global standards to be applied globally. this principle should be retained, inviolate, when separation standards for use with ADS are set.

The use of ADS, and the systems to support an ADS-ATC service can be used to reduce non- radar separation standards. However, such systems are not radar systems, and are a completely new generation of surveillance systems which are susceptible to errors and failures beyond the control of the ATC service . Any such reductions must take this into account fully.

Any such reduction in separation must be proven by an appropriate method of collision risk modelling and a full consideration of all factors, both technical and operational, that are relevant and cannot be modelled. Separation standards for use with ADS should be global standards, any attempt to produce separation standards for use locally should be resisted.

Changes in navigational accuracy’s may compromise separation standards. Because ADS is entirely dependent upon aircraft derived data, ADS supporting systems must provide alerting functions to warn the controller and pilot of changes of navigational accuracy that affect separation standards in a timely manner. All such communication to enable restoration of standard separation should accord a high priority within the system, and ensure urgency in the cockpit.

Separation standards in areas using ADS shall only be less than the appropriate non-radar minima provided that an assessment of the effect of risks are quantified by an approved ICAO method. Such models should not be used alone, but in conjunction with operational judgement to account for those factors that cannot be modelled..

Separation standards for use with ADS shall be global standards, and applied globally.

ADS systems shall provide a warning to pilot and controller whenever navigational 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.

The number of separation standards set should be kept to the absolute minimum.