Overview of ICAO Doc 9849 Global Navigation Satellite System

1. Doc 9849 Global Navigation Satellite System

2. 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.

3. 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).

4. 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.

5. 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.

6. 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.

7. 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.

9. 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.

10. 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.

11. 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 a
particular phase of flight.

13. 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.

14. 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.