The GNSS can be defined as a world-wide position and time determination system that includes one or more constellations of navigation satellites augmented as necessary, by means of ground and space systems, to support the required navigation performances for the selected flight operations.

Aside from all the technical and political issues involved in the implementation of the GNSS it is important to examine the operational aspects related to the provision of the Air Traffic Services by means of such system.

The ICAO Annex 11 states that:

The application of the provision of information to ATS units for existing navigation aids is described in the ICAO Doc 9426 “ATS Services Planning Manual”.

The navigation sensors that are selected by the aircraft will be determined by the RNP for the flight operations to be conducted. It is important for both flight crews and ATCOs to know the status of the system at present and, to be able at any time to predict what the status of the system will be in the future, in order to evaluate if each of the intended flight operations are practicable.

GNSS scenario

On the GNSS implementation a lot of options have been proposed. Currently there are two main satellite constellations which could be used in the field of civil aviation: GPS and GLONASS. According to the technical specifications, these systems, if fully operational, can offer a great positioning accuracy, especially when operated in differential mode. Since it has been proved that limited performances are obtainable in terms of integrity, which is a fundamental requirement for safety critical applications, at present both GPS and GLONASS are only capable to support en-route and Non-Precision Approaches RNP as stand alone systems.

It has been demonstrated that it is possible to meet all the RNP parameters through the implementation of a system architecture supported by ground and space based augmentation systems as currently envisaged for the augmentation system projects: WAAS, LAAS, EGNOS, MT-SAT etc.

In the near future, GNSS equipped aircraft will therefore navigate with high accuracy This offers the opportunity to design complex instrumental procedures (e.g.: holdings SIDs and STARs) and a more efficient airspace organisation in terms of routings and off-set implementation.

GNSS derived navigation data can be used to improve the quality of ADS position reports. Assuming that the rate of ADS position reports would be comparable to the rate of radar derived positions it should be investigated the possibility to adopt ATC operational procedures similar to those applied in radar environment.

Where the use of GNSS is approved it becomes necessary to develop specific operational procedures and support tools in order to enable all the required strategic and tactical actions, including those required in case of system failures or degradations.

Consequences of a change in the operational status of a navigation aid

When a navigation aid becomes unavailable several consequences can occur. First of all a “re-routing” via a less convenient route could become necessary, or delays can arise until the system unavailability has been overcome. Secondly a capacity reduction could be easily foreseen especially if ( minimum ) separation has to be increased. At times of peak traffic any capacity reduction impacts on the efficiency of the overall ATM operations.

In such circumstances the workload for the operational personnel may increase considerably. ATCOs may need to issue further instructions, to change the strategic planning and to execute additional co-ordinations. The aircraft crews may need to re-program their navigation computer in order to select alternative navigation sensors or to deviate from their required flight profile.

After that the tactical actions have been undertaken, it becomes necessary an intervention in terms of traffic flow management in order to match the expected traffic demand, in the near and medium term, with the new system capacity. Technical information on system status must be first converted in operational information and then delivered to the users, as they require, by direct links and Aeronautical Information Service messages.

GNSS failure effects

The evaluation of the effects on ATM operations in case of a GNSS system failure must take in account a number of factors: the system architecture ( satellite constellation features and ground augmentation systems ), aircraft avionics, availability of other navigation aids, airspace RNP, timeframe.

In case of a satellite outage there may be no significant effects due to the availability of other satellites or the presence of augmentation systems (ground pseudo-satellites or different navigation sensors) so that the aircraft can still fly on the routes previously assigned. On the other hand, since the satellite availability for position computation is a function of position and time, where no ground based navigation aids are available, all the aircraft concerned will experience short periods of reduced navigation performances and this circumstance can require the ATC to re-route the aircraft and/or to increase relevant separation criteria.

It is interesting to notice that the areas affected by the reduced navigation performances can be visualised like moving holes around the earth. Accurate predictions on the behaviour of these moving holes can be carried out well in advance.

In case of a ground augmentation system outage (pseudo satellite, transmitter for differential corrections, local integrity monitor), the affected area is well defined but, the reduction in terms of navigation performance for those aircraft flying in that airspace may be considerable because the main supports required for All Weather operations could be not available. In such circumstances ATC may decide, if practicable, to make use of instrument procedures having RNP parameters which are less restrictive or, in the worst case, to co-ordinate a diversion to alternate airports.

Further important circumstances that can seriously limit the GNSS performances are represented by the electromagnetic interference (intentional or accidental) and all the environmental factors such as irregular propagation delays and solar maximum conditions. In such conditions a system degradation may occur in some areas producing deleterious effects on the aircraft navigation capability, especially where no ground augmentation systems, radio aids or other navigation methods are available.

Once a GNSS system degradation occurs, it must be considered that the ATC workload immediately increase when new instructions or co-ordinations are needed in order to cope with the situation. At the same time the ATC capacity can be negatively affected during the application of the alternate procedures introducing unavoidable delays on the air traffic.

The ICAO plan for the global use of the GNSS will allow States to remove ground based aids where GNSS can provide a replacement service. In the context of this replacement process it should be considered that some conventional aids, where practicable, can be retained and used as a back-up during failures.

System status information

GNSS users achieve their required navigation performances by a combination of inputs from different system elements and therefore, failures in the space or in the ground segment are likely to cause a degradation in the level of service, rather than a total loss of navigation capability. Another GNSS feature is that, apart from the planned service interruptions, it is possible to predict, in the space and time domain, the satellite availability to perform navigation computations and integrity checks.

The amount and the complexity of technical information involved in the definition of a GNSS operational status is such that all data, especially those coming from monitoring subsystems, need to be processed and appropriately displayed in understandable way before being used by ATC and Flight crews. There is no reason to inform the operational personnel about the technical aspects of a system failure, as in the form provided by the monitoring sub-systems, (i.e. a specific satellite outage) unless the information has a known impact on the air traffic management system concerned.

An effective solution may be the interpretation of technical information in terms of operational parameters which are related to any possible failure or degradation situation. In this way, according to the current or predicted parameter values, the corresponding ATC operational procedures can be therefore used.

Users requirements

The development of a GNSS interpretation service about any system status information, as well as any GNSS monitoring and augmentation sub-system, will be carried out on regional and local basis according to the services to be provided. The main ATM Services hereby concerned are: the flow management and the air traffic control.

Flow management is a planning function which task is to match the traffic demand with the airspace capacity. In case of a system degradation, ATFM requires to be informed on the portion of airspace affected, the time duration, the expected consequences on aircraft navigation performances and about new ATC Unit capacities.

ATC requires information on the current system status and any changes that are going to occur in the near future. The information needed may be different according to the operational scenario.

The approach phase requires up to date information on the current status on the airfield and in its vicinity especially where local ground augmentation systems (with pseudo-satellites) are implemented to support All Weather operations. In the more general case of landing procedures relying on the space segment signals only, it must be considered that the satellite availability, producing the RNP for such procedures, may evolve in the time domain. ATCOs must therefore be provided with a status prediction covering a period at least equal to the duration of the approach procedure to be performed.

During terminal area procedures, such as arrival and departure, a reduction in the RNP parameters may influence the separation between aircraft flying on near trajectories, as well as the approach and departure sequences. Apart from real time warnings, it is also necessary a longer term GNSS performance prediction and this may be at least as long as necessary to complete the procedure considered.

In particular over desert or oceanic areas, during the en-route phase, issue of ATC clearances could not necessarily be influenced by the status of the available navigation aids. The reasons for this are mainly the relaxed RNP placed on this phase of flight which can be satisfied by every navigation technique. Anyway if GNSS is used by some aircraft for a particular route or if it is used in ADS reports, ATCOs must be made aware of any reduction of service provided that can influence the aircraft spacing or the applicable separations. This information, as in the other phases of flight, shall be provided in real time and must include a prediction of the future status (where appropriate time frame are to be decided) that might limit the operations.

Presenting information to the air traffic controller

It has been stated as IFATCA policy that ATCOs need to be properly informed about the status of all GNSS components and about the operational impact of this kind of information. It has been identified that the interpretation of technical information on GNSS, current and predicted status, shall produce information in terms of navigation performances. It is also expected that the application of ATC operational procedures based on GNSS will be strongly dependent on these parameters and therefore, in this paper, it has been found that the basic information on which such operational procedures could be referred are the following :

• RNP in the airspace and in the control sector under the controller responsibility;
• Aircraft RNP compliance (or aircraft navigation performances);
• GNSS status and predictions in terms of navigation performances.

Given an airspace volume, a specific type or level of RNP can be assigned to the entire portion considered or to each of the single components (i.e. : instrumental procedures) in accordance to the complexity of the operational scenario. This procedure will carry out a reference model where every information on the navigation systems status, provided in the same form, can then be easily evaluated in real time operations. These data shall be reported in AIS documentation and shall be easily made available to operational personnel, possibly in a graphical form or in a listed form.

The RNP assignment process will take in account a lot of factors as: the expected traffic density, the route spacing, the presence of obstacles, the type of instrument procedures to be implemented and other relevant factors.

Furthermore it is necessary to consider aircraft avionics, GNSS status, the presence of other navigational aids and their operational status. It is therefore convenient to summarise all those data in a single parameter which accounts for the RNP value that the aircraft can satisfy at present or in the near future. This value can be indicated as the aircraft navigation performance.

At present there are no clear ideas on how to manage this kind of information (by means of the flight plan, via voice communication or via data-link) but it is clear that aircraft navigation performances indication shall be made available on the working position, for instance it might be sufficient to display (in a labelled form on radar screens) if the aircraft is compliant or not with airspace RNP.

Data on GNSS present status and system warnings shall be converted in terms of RNP values that can be granted by the system over the airspace volume concerned or for the areas concerned with specific instrumental procedures and then shall be displayed to the ATC in a very compact form.

A possible solution may be to display, by means of a status monitor, the list of instrumental procedures which are usable, showing clearly their level of availability. Another way to depict the areas of reduced RNP might be a visualisation with shaded colours. Any solution shall take in account Human Machine Interface.

For those applications where predictions on GNSS status are needed, they must include:

• for short term applications:
  • the period of validity of the RNP declared on the specific area concerned;
  • the time frame to perform a specific instrument procedure.
• for longer term applications:
  • the expected variations of the RNP.

Once the technical information have been interpreted and delivered to the AIS, an user oriented NOTAM shall be issued. NOTAM must include the airspace portion concerned, the corresponding RNP, the time of validity, information on variation of RNP or expected RNP for a specified period of time. Where no further data processing is required, for the purpose to produce an object where just a simple reading is required by the operational personnel to get the situation awareness, also for NOTAMs relevant information could be represented in a graphical form, such as in the weather charts, otherwise in a list form where it is specified what kind of procedure is available or what level of RNP can be granted or is expected within the airspace considered.

Operational ATC procedures in GNSS environment

Once the ATC units are provided with tools which are able to present all the information that have been previously described, there is the need to develop all the operational procedures regarding the use of GNSS in ATC which will be included in ICAO documentation.

In this paper, in order to give some guidelines for the definition of such operational procedures, it is suggested that a relationship between the existing or declared RNP and the corresponding procedures available or the actions to be carried out shall be clearly established.

It must also be considered that, even if the GNSS will be certified as sole means of navigation, aircraft navigational capabilities will always be related to their on board avionics features. This concept establishes a key point in the development of such procedures because, apart from any GNSS information, every ATC (but also pilots) decision must also take in account the aircraft specific navigation performances.

It can be observed that a comparison between a set of RNP values related to a generic airspace and the GNSS navigation performances which can be granted inside the airspace considered, will give indications on the degree of usability of GNSS based procedures in that airspace. Such indications are therefore of great importance when strategic actions must be carried out.

In the same comparison, if current aircraft navigation performances are considered in place of GNSS navigation performances, more detailed indications can be given about the impact of any GNSS service degradation on the ATM system. According to the aircraft behaviour in such circumstances, the corresponding tactical actions to be performed will therefore appear more evident.

It is also well clear that, in case of a system degradation occurs, an aircraft which is still compliant with airspace RNP can be accepted at all in that specific airspace if the application of uniform ATC procedures, for all the remaining aircraft, is not much influenced. Other aircraft reliant on GNSS, according to the circumstances or the pertinent GNSS procedures, may be delayed, re-routed, spaced and instructed to fly alternate instrumental procedures. It is anyway to be noted that, in this particular event, the overall ATM capacity will strongly and definitely depend upon an external factor like airborne equipment.

A further consideration in developing ATC operational procedures based on GNSS is the opportunity to carry out an analysis on the characteristics of each control sector or airspace, including the RNP for each single portion, the phases of flight involved, the route arrangement and the typology of ATC instruction that are usually imparted, in order to research what specific ATC actions are required in case that a modification in the level of GNSS service occurs.

The result of such analysis is permanently stored and it has to show, from one side, the requirements in terms of RNP, for each GNSS based procedure which has been established within the airspace considered (it means: instrument procedures, GNSS routes, aircraft spacing, aircraft sequencing, operational techniques based on GNSS etc.) and, from the other side, the corresponding ATC actions which can be executed according to the current/expected level of the navigation system RNP or the aircraft navigation performance.

In real time operations, ATC will match these stored information with current and predicted GNSS status information displayed at the working position in order to decide the strategic actions to be carried out within the airspace concerned. Information on aircraft current navigation performances will be therefore used for tactical actions.

The implementation of the satellite technology in the field of air navigation will imply improvements in terms of safety, efficiency and regularity of flights. Since the complexity of these new systems and the amount of technical information involved in their use, in particular those concerning the GNSS operational status, it is necessary first to interpret and then to present the essential information needed by pilots and ATCOs in a form adherent to the operational context in which they will be used.

It has been identified that information on the operational scenario, on the degree of RNP available from the GNSS and on current aircraft navigation performances are essential for the application of future operational ATC procedures. Further analysis is then needed in order to find convenient forms and tools for presenting such information and to develop relevant operational procedures.

ICAO documents:

• ANNEX 11;
• Manual on RNP (Doc 9613);
• ATS Planning Manual (Doc 9426);
• Manual of RNAV Operations (Doc 9573);
• Guide lines for the introduction and operational use of GNSS (Circ. 267).

EUROCONTROL documents:

• GNSS/ATM INTERFACE Drafting Group;
  • Draft;
  • First Meeting;
  • Second Meeting;
  • Third Meeting.

IFATCA documents:

• RGCSP Report – IFATCA 98;
• Review of Technical Policy: ADS General Issues – IFATCA 98;
• Automatic Dependent Surveillance – chapter 1 – IFATCA Technical Manual;
• Display of GNSS status to ATC – chapter 2 – IFATCA Technical Manual;
• RNP for approach and landing – chapter 2 – IFATCA Technical Manual;
• WAAS – chapter 4 – IFATCA Technical Manual.

Working Papers:

• The Interface Between GNSS and ATC – ION GPS 96;
• Advising ATCOs of the status of GNSS in their control sector – GNSSP 96;
• GNSS performance predictions for pilots and controllers – GNSSP 97.