34TH ANNUAL CONFERENCE, Jerusalem, Israel, 27-31 March 1995
WP No. 105
Mixed and Segregated Modes of ADS Operations
Within an area of airspace designated for ADS services it is perfectly feasible to provide both a procedural style non-radar ATC service and an ADS-ATC service The desire to obtain the benefits that an ADS-ATC service can provide early on, will also add to aircraft of differing capabilities operating coincident in the same airspace.
Mixed mode versus segregated mode
An important issue in the planning for an ADS-ATC service is operating in mixed mode, i.e. within a defined airspace having aircraft suitably equipped to use ADS and other aircraft that are not so equipped, operating concurrently – ‘mixed mode’ operation.
To the controller this poses several problems The cognitive issue of dealing for some of the time with datalink only communications and some of the time with voice communications for one, whilst the problem of low arousal rates for pilots using the voice medium can potentially slow down response to requests made by voice producing increases controller workload.
There exists the potential for confusion – do I respond to that aircraft by voice or datalink? Working with the different operating systems that each type of ATC service requires can become complicated and cumbersome. If it is necessary to update the ATC operating system for voice only equipped aircraft, extra tasks may add to the workload. Mixed mode operations will induce complexity in determining separation standards between aircraft by virtue of the combinations of traffic that may be presented to the controller. It is fundamentally an issue of controller workload.
Capacity of ATC systems is inextricably and demonstrably linked to controller workload. It can be envisaged that there will be an increase in controller workload attendant in mixed mode operating regimes over and above the same density of traffic in an entirely segregated ADS mode. This increase stems partly from working with complicated and varied separation rules that must apply over long sector lengths, which are characteristics of the typical ADS airspace, and that cover all combination of traffic mixes. But also stems from the changes in the way that aircraft will be controlled in an ADS environment. The route structure of ADS airspace may not be founded on the classic route structure that today’s non-radar environment requires. To enable the controller to visualise the traffic situation greater use of computer tools to assist the controller will make flexible routes more acceptable operationally. ADS equipped aircraft can of course update the ground system automatically, but non-ADS equipped aircraft will still require the controller to issue clearances by RT, to receive position reports etc. In effect, t may be the case that the controller will have to keep his data display up to date for the system to provide support. In a segregated ADS environment, much of the routine workload is envisaged to become automatic, freeing the controller for other tasks. In a mixed environment, the controller will still retain some of this routine workload. It should be clear just how intensive mixed mode could become in the event of a pilot requests for example. In high density airspace the controller could easily become overloaded, and thereby increase the propensity to mistakes of judgement.
Therefore the issue of the workload upon the controller and its relationship to traffic densities, provides a criterion of acceptability for mixed or segregated modes of operation A principal concern is that the greater the traffic densities, and the more complex the problems are to resolve, the less resilient the system becomes to errors of judgement by the controller and pilots in mixed mode operations – the range of potential outcomes grows and so to does the range of error producing combinations. Thus the safety of the service provided is an important issue in determining the acceptability of mixed mode or segregated operations.
The efficiency of the ATS service provided will also be affected by mixed mode of operation. Separation standards between an ADS equipped aircraft and a non ADS equipped aircraft must be at least standard non-radar, potentially incurring operating penalties too ADS equipped aircraft. In high traffic density areas such as the North Atlantic and Pacific the cost to the air transport system as a whole may be too great too bear. In airspace where traffic densities are low, the efficiency of the system can be seen to be enhanced in mixed mode by offering an ADS-ATC service to aircraft so equipped, and a procedural ATC service otherwise. It should be borne in mind that ADS offers significant improvements in the safety of aircraft operations, so the option to restrict operations to either ADS or non-ADS denies this benefit to some airspace users.
Definition of traffic density
Given that traffic density is a suitable criterion to assess the applicability for mixed mode operation, as it is a surrogate for controller workload, it is necessary to find some structure with which to measure it.
The ICAO CNS/ATM concept categorises airspace into four operating domains. This is a suitable categorisation for the determination of mixed mode and segregated operations, because traffic density is implicit in the airspace structure. It is internationally accepted as part of the CNS/ATN concept, so is easily understood.
Oceanic / Continental en-route airspace with low density traffic
Continental airspace with high density traffic
Oceanic airspace with high density traffic
A mixed mode of operation between ADS and non-ADS equipped aircraft can significantly increase controller workload and thereby reduce capacity. Operating penalties can occur to ADS equipped aircraft whilst in mixed mode airspace.
In terms of the air transport industry, and the air traffic system as a whole, there are disbenefits to operating in mixed mode which can reduce overall system efficiency. However, ADS can offer benefits in terms of safety and of the service provided. The major operational factor that dictates the level of controller workload in mixed mode operations is traffic density.
Mixed mode and segregated modes are acceptable as primary operating modes as indicated in the table below:
Last Update: September 28, 2020