Doc 9849 Global Navigation Satellite System

The Global Air Navigation Plan for CNS/ATM Systems (Doc 9750) recognizes the Global Navigation Satellite System (GNSS) as a key element of the Communications, Navigation, and Surveillance/Air Traffic Management (CNS/ATM) systems as well as a foundation upon which States can deliver improved aeronautical navigation services.

Global Positioning System (GPS) and Global Navigation Satellite System (GLONASS) have the capability to provide accurate position and time information worldwide. The accuracy provided by both systems meets aviation requirements for en-route through non-precision approach, but not the requirements for precision approach. Augmentation systems can be used to meet the four basic GNSS navigation operational performance requirements that are stated in the GNSS SARPs in Annex 10 — Aeronautical Telecommunications, Volume I (Radio Navigation Aids).

Integrity, availability and continuity can be provided by using on-board, ground- or satellite-based augmentation techniques. Accuracy can be enhanced by using differential techniques. The total system, including core satellite constellations (i.e. GPS and GLONASS), and all augmentation systems, is referred to as GNSS. Efforts to bring the full benefits of satellite navigation to users are focused on developing these augmentations and certifying them for operational use.

Accuracy is the degree of conformance between the measured or estimated position and the true position of an aircraft at a given time.

Integrity is the ability of a system to provide timely warnings to users when the system should not be used for navigation.

Continuity is the probability that a system will perform its function within defined performance limits for a specified period of time given the system is operating within the defined performance limits at the beginning of the flight operation.

Availability is the fraction of time that the services of the system are usable by the flight crew.

Accuracy

GNSS position accuracy is the difference between the estimated and actual aircraft position.

GNSS errors however can change over a period of hours due to satellite geometry changes, the effects of the ionosphere and augmentation system design.

Integrity and Time-to-Alert

Integrity is a measure of the trust which can be placed in the correctness of the information supplied by the total system. Integrity includes the ability of the system to alert the user when the system should not be used for the intended operation (or phase of flight). The necessary level of integrity for each operation is established with respect to specific horizontal/lateral (and for some approaches, vertical) alert limits.

When the integrity estimates exceed these limits, the pilot is to be alerted within the prescribed time period.

Continuity

Continuity is the capability of the system to perform its function without unscheduled interruptions during the intended operation. This is expressed as a probability. For example, there should be a high probability that the service remains available throughout a full instrument approach procedure.

Availability

The availability of a service is the portion of time during which the system is simultaneously delivering the required accuracy, integrity and continuity. The availability of GNSS is complicated by the movement of satellites relative to a coverage area and by the potentially long time it takes to restore a satellite in the event of a failure. The level of availability for a certain airspace at a certain time should be determined through design, analysis and modeling, rather than through measurement.

The core satellite constellations alone cannot meet the stringent aviation requirements of accuracy, integrity, continuity and availability discussed in this chapter. Table 4-2 shows the potential of aircraft-based augmentation system (ABAS), SBAS or GBAS to meet the navigation requirements for aparticular phase of flight.

The integrity service of ICAO compliant GNSS systems may currently be provided by the three normalized augmentations known under the terms ABAS (Airborne Based Augmentation System), GBAS (Ground Based Augmentation System) and SBAS (Satellite Based Augmentation System).

In addition to integrity service, GBAS and SBAS also provide to the user differential corrections to improve the precision in a restricted area around a single reference station for GBAS and over a wide area defined by a network of reference stations for SBAS.

Finally, the SBAS geo satellites also transmit a ranging navigation signal similar to a GPS satellite.

Therefore, the SBAS integrity service which is addressed here should protect the user from both:

•failures of GPS/GLONASS/GEO satellites (drifting or biased pseudo ranges) by detecting and excluding faulty satellites through the measurement of GPS signals with the network of reference ground stations •transmission of erroneous or inaccurate differential corrections.

These erroneous corrections may in turn be induced from either:

- undetected failures in the ground segment,- processing of reference data corrupted by the noise induced by the measurement and algorithmic process.