35TH ANNUAL CONFERENCE, Tunis, Tunisia, 15-19 April 1996
WP No. 86
Display of GNSS Status to ATC
During the Committee B deliberations at the Jerusalem conference, there was considerable discussion on the requirement for the monitoring of GNSS by ATC. This paper provides background on this subject and proposes policy for the development of a monitoring service and the display of GNSS status.
The GNSS will consist of a number of elements. These will include the navigation satellites, and the ground based monitoring stations which, together with communication satellites, will form a Wide Area Augmentation System (WAAS). Ground based differential stations with a datalink may be used at specific locations to form a Local Area Augmentation System (LAAS). The aircraft will select which elements of the system are to be used depending on the operations to be undertaken.
For en-route navigation and for non-precision approaches it is possible that just the satellite signals may be used, but it is expected that some type of augmentation will be required. The WAAS is being introduced to provide additional integrity information for en-route operations and to permit Cat I precision approaches. A LAAS will probably be required for Cat II and Cat III approaches.
The systems that are selected by the aircraft will be determined by the Required Navigational Performance (RNP) for the operation to be conducted. Not only is it important for the aircraft to know the status of the system at present, but also be able to predict what the status of the system will be in the future. For example, the pilot needs to know that the GNSS will be capable of providing the required precision or non-precision approach guidance at the time he arrives at the destination airport.
It must be appreciated that the controller may not know what systems the aircraft is using (e.g. WAAS or LAAS) and the controller therefore will not know what the effect of any particular failure will be on the aircraft. It is also important for the controller to be given advance warning of a degradation in the service so that he is aware when, for example, precision approaches can no longer be made.
The first consideration is the effect of a failure of one of the navigation satellites . A monitoring system could inform the controller that a particular satellite had failed. It might possibly inform him if that satellite could be received in the vicinity of the airfield, or if it was over the other side of the world. Unfortunately, this information is of no practical value when we consider how this could be used by the controller or the pilot.
The GNSS receiver will automatically select the satellites that give the most accurate position calculation. There are many factors that are taken into account in this selection. The signal strength of the satellite signal is important, but so is what is known as the geometry of the satellites. Choosing to use the signals from satellites that are in advantageous positions as far as the calculations are concerned can enhance the position accuracy considerably.
It can be seen from this, that even if the controller knew which satellites were in the sky above his airspace, he would not know which aircraft was using which satellites. In fact, the pilot of the aircraft is not even aware of which satellites are being used. The choice is made by the receiver and as one satellite disappears below the horizon, the receiver will automatically start using the signals from another without the pilot knowing anything has happened.
This situation highlights the importance of the GNSS receiver in the aircraft being able to detect faulty satellites and discard them from the position calculation. The FAA claim that GPS receivers have always detected the failure of a GPS satellite and that an undetected failure has never occurred. The use of WAAS considerable enhances this situation, as it provides a means to independently detect faulty satellite signals and pass this integrity information back to aircraft receiver.
The next consideration is a failure of the WAAS. The most likely failure to affect an airport where Cat I approaches were taking place would be the failure of the local ground monitoring station. It is possible that this would result in a loss of Cat I operations, but this would depend on a number of factors such as the number of satellites that can be received in the area and any adjacent ground stations.
Even with all the systems operating correctly , it is envisaged that factors such as poor geometry will limit GNSS availability at certain locations for short periods of time, and planned or unplanned satellite outages will lead to further limitations. The determination of the effect of any failure will require detailed information on all parts of the system, including the geometry of the satellites. In practice, computer facilities operated by suitably trained staff will be required to interpret this information and present it in a form that pilots and controllers can understand and make use of.
It is the responsibility of the state Aviation Authority to ensure the accuracy and availability of the navigational aids used over their territory and report any deficiencies. They must obtain information on the status of GNSS and determine the availability of navigational performance over the territory. For example, should a satellite fail, it will be possible to predict that at some time in the future it will not be possible to maintain the required RNP in a certain airspace. This information must be disseminated in order to give advance warning of any changes in the RNP for each airspace.
The important consideration for the controller is not the detailed information about any failures that may have occurred, but rather the interpretation of the status of the GNSS in a form that is relevant to the operation being conducted. For example, if the reduced number of satellites changes the RNP of the airspace, then the controller must implement certain procedures such as increased separations as a result.
Information about specific failures of the items that make up the GNSS are of little or no use to ATC and to pilots. What is important is the effect of these failures on the operations that are being conducted. Some operations such as precision and non-precision approaches may not be possible. Sometimes the operation may be restricted due to a change in the RNP value achievable.
This will require a monitoring and interpretation service to gather information on the GNSS from a number of sources such as the satellite operators and the WAAS providers. This service must then interpret the information and present it to pilots and controllers in the form that they require. This must be stated in operational terms such as the RNP value in the airspace, or the category of precision or non- precision approaches that are possible. As many of the components of the GNSS will be developed on a regional basis, such as the EGNOS augmentation system to be operated by Eurocontrol. It can also be expected that this monitoring role may be carried out on a regional basis.
In cases where ATC require immediate information on changes to the status of the GNSS, for example where aircraft are conducting precision approaches, the time taken to disseminate information through the monitoring service may be too slow. In these cases, an electronic display of GNSS status may be required at the controllers operational position. This should also display the information in operational terms, such as the category of approach available.
ATC procedures must be established that cover the effects of failure or degradation of the GNSS. When information regarding the status of the GNSS is received from the monitoring service or via a controllers display, the appropriate procedures must be implemented.
The equipment on board each aircraft varies, and many factors affect the navigational performance. When any failure occurs, it is to be expected that some aircraft will fail to meet the RNP and require special action to be taken, while others will be able to continue to meet the RNP. Information provided by the monitoring service and issued to pilots is advisory. The final decision on using the GNSS or undertaking any specific operation must rest with the pilot.
It is recommended that:
A monitoring and interpretation service should be established to monitor the status of all elements of the GNSS and interpret this information in a manner that provides relevant information to pilots and ATC. The information disseminated from the monitoring service or displayed at controller positions must be expressed in operational terms.
ATC procedures must be established for the use of GNSS and must cover the failure or degradation of the system. When ATC is informed of a change in the status of the GNSS by the monitoring service or by display equipment, specific procedures associated with that change must be implemented. Should it not be possible to achieve the RNP in an airspace, an alternative RNP should be declared.
Last Update: September 28, 2020