To date ATC surveillance systems have relied solely on primary and secondary radar systems, with the attendant limitations upon line of sight range etc. ADS, by utilising enabling technology in conjunction with a datalink, extends the range of surveillance systems but in a fundamentally different concept than radar systems – dependent surveillance.

This change in the derivation of aircraft position data that is supplied to the controller poses certain philosophical concerns in the provision of an ATC service.

Position derivation using independent and dependent surveillance

Primary radar systems, by their very nature, derive the position of an aircraft from there own resources – in this case reflected radar energy. The position of the aircraft is derived solely by the ground based system, without the assistance of the aircraft – other than it actually being present. Any errors in the position displayed relate solely therefore to the performance of the primary radar system. If an aircraft is not at the position that is stated, then this can be determined without the aircraft’s assistance. Thus there is an independent check of an aircraft’s position.

Secondary surveillance systems differ, in that in order for the position of an aircraft to be determined, then the aircraft must operate with a transponder. Therefore, in this case, surveillance is carried out co-operatively with the aircraft. However, the position data displayed to the controller is still determined by, and with reference too, the ground based system. Errors can occur, but if an aircraft is not at the position that is reported, there is an independence to the display of position data that enables the deviation to be identified.

Dependent surveillance, and ADS, functions quite differently. The position data displayed to the controller is constructed entirely from the data obtained form the navigation systems onboard an aircraft which is downlinked to the ground system. The position that is downlinked may come from any navigational source SATNAV, from IRS, from LORAN etc. Clearly the accuracy of some of these sources is high, others less so. However, errors in an aircraft’s position can not be determined independently.

It should be evident that ADS surveillance data is quite different from the independent and co- operative data that is supplied to the control team. In some ways it is exactly the same as the data provided in a non-radar control service – information provided to the controller by the pilot derived form aircraft systems – the principal difference being the way that the data is presented, and its efficacy.

The issue to be considered is to what extent, and what is required, for dependent surveillance to be used in a similar way to that of primary radar systems.

Control requirements for surveillance

The control process requires a surveillance function which is used to monitor the progress of traffic and to separate aircraft. These functions involve tasks which confirm an aircraft’s position, and use that knowledge to ensure the safe passage of the flight in relation to other traffic, as well as airspace constraints and route structures etc. With dependent surveillance, if errors are present in the position data that is used by the aircraft navigation systems, then they will be transmitted to the ground system and can potentially compromise these control tasks.

Required navigational performance (RNP) indexes have been established by ICAO and these greatly assist airspace and system design. The navigational data that aircraft transmit in routine or on request reports can vary in terms of its accuracy for several reasons such as satellite availability, updating rates on navigation sensors etc. and thus too its RNP index. Therefore ADS reports will include a ‘ figure of merit’ for the navigational accuracy of the navigation systems. In addition, the aircraft position displayed on an ADS workstation can vary dependent upon the frequency of updating of the position data supplied by the aircraft, it is not continuously updated as, to all intents and purposes, radar is. Therefore the volume of uncertainty around an aircraft’s position grows as a function of the reporting rate.

It is envisaged that some form of error checking will take place. (Indeed ICAO ADS panel identifies a major strength of the ADS concept as being the ability to trap ATC control loop errors and flight deck crew input errors). As the current flight plan is known to the ground system, and therefore the intended route, when position reports are downlinked they can be compared to detect deviations from the assigned route. However, It is possible for aircraft of the B757/767/747-400 types to have route discrepancies known as ‘map shift. In these circumstances the map display onboard the aircraft shows the aircraft to be on course, whilst in reality the aircraft is off course – in some cases by as much as fifty nautical miles. Another type of error that can occur is an unwanted heading change commanded by the flight management system,, but which is not shown on the navigation displays. Such heading changes are only very small, but the distance off track grows as a function of time, therefore large errors may result unless the turn is checked. Therefore there are a class of errors which can compromise the accuracy and validity of the aircraft’s position that is downlinked and the true position of the aircraft.

For the surveillance function to be carried out using dependent surveillance systems, such as ADS, the system must provide safeguards to enable these classes of errors to be identified and for corrective action to be taken by the control team in a timely manner.

Updating of position data in dependent and independent surveillance systems

Primary and secondary radar systems update the position data displayed on the radar display at a rate equal to the rotation of the antenna. In en-route operations this is a refresh rate typically of once every six to ten seconds. in the case of terminal and approach operations, the update rate is higher. Dependent surveillance systems are able to operate differently in this respect when compared to radar surveillance.

Typically, in an ADS system, the aircraft will transmit a basic ADS report at a rate that is established when the initial contract – or entry clearance into ADS airspace, is first made. Presently it is envisaged that a reporting rate of one every 300 seconds will be the norm. It will be possible to request a higher report rate, but this will have economic implications because of the institutional arrangements for providing communications links. The result of this is that for every position displayed using for example, an ADS report, the true position of the aircraft can be within a volume of uncertainty around the displayed position. Therefore there can potentially be errors and uncertainties in the data displayed for control tasks.

Dependent surveillance systems such as ADS provide position data to the controller workstation that comes solely from aircraft data sources. There is the possibility of a class of error to occur with such data which can escape detection on the flight deck.

In order for the surveillance task assigned to ATC to be carried out, dependent surveillance systems should be error tolerant and designed to capture these types of errors. Procedures should take into account the limitations in surveillance performance as a result of low reporting rates.

It is necessary for dependent surveillance systems to be designed with procedural and system defences that will counter aircraft position errors transmitted to the ground system.