International Civil Aviation Organization

World-wide CNS/ATM Systems

Implementation Conference

Rio de Janeiro, 11 – 15 May 1998

International Civil Aviation Organization

World-wide CNS/ATM Systems

Implementation Conference

Rio de Janeiro, 11 – 15 May 1998

WW/IMP–WP/36

11/5/98

Item 1

Agenda Item 1:	Overview of CNS/ATM systems implementation

GNSS SYSTEM STATUS AND STANDARDIZATION IN PROGRESS

(Presented by the Secretariat)

Information Paper

SUMMARY

This paper presents an overview of global navigation satellite system (GNSS) and the work carried out by the GNSS Panel on development of Standards and Recommended Practices (SARPs) for the system.

1.	Introduction

1.1	Standards and Recommended Practices (SARPs) for global navigation satellite system (GNSS) are being developed to satisfy a variety of performance levels defined by RNP types for all phases of flight operations.

1.2	The RNP is defined by ICAO as a statement of navigation performance necessary for operations within a defined airspace. RNP types for en-route operations are identified by a lateral accuracy value.

1.3	The RNP types for approach, landing and departure operations (yet to be adopted by ICAO) are defined in terms of required accuracy, integrity, continuity and availability of navigation. These RNP types include some types with accuracy specification of lateral performance only (i.e. similar to en-route) and other types which include lateral and vertical performance specifications. The types similar to en-route specification are intended for operations such as non-precision approach or departure, while most RNP types for precision approach and landing operations require vertical component.

2.

2.1	The GNSS, as defined by ICAO, is a world-wide system of position and time determination that includes one or more satellite constellations, aircraft receivers, and system integrity monitoring, augmented as necessary to support the required navigation performance for the actual phase of operation.

2.2	The global positioning system (GPS), owned by the United States, and the global orbiting navigation satellite system (GLONASS), owned by the Russian Federation, were offered to ICAO as a means to support the evolutionary development of GNSS. In 1994, the ICAO Council accepted the United States offer of the GPS and in 1996, it also accepted the Russian Federation offer of GLONASS.

2.3	To overcome inherent system limitations and to meet the RNP requirements (accuracy, integrity, availability and continuity) for all phases of flight, GPS and GLONASS need varying degrees of augmentation. Augmentations are classified by ICAO in three broad categories: aircraft-based, ground-based and satellite-based.

2.4	Aircraft-based augmentations. One type of aircraft-based augmentation system(ABAS) is called receiver autonomous integrity monitoring (RAIM), which can be used if there are five or more satellites with suitable geometry in view. Other aircraft-based augmentations can also be implemented and are usually termed aircraft autonomous integrity monitoring (AAIM). Augmentation techniques, particularly useful for improving availability of the navigation function, also include inertial and altimetry-aiding, more accurate time sources or some combination of sensor inputs.

2.5	Ground-based augmentation. For ground-based (also referred to as local area) augmentation system (GBAS), a monitor is located at or near the airport where precision GNSS-based operations are intended. Signals are sent through a data link to aircraft in the vicinity to increase the position accuracy locally along with satellite integrity information.

2.6	Satellite-based augmentation. The satellite-based augmentation (also referred to as wide area or regional augmentation) system (SBAS) makes use of satellites to transmit augmentation information as necessary to enhance the position accuracy and integrity.

3.	WGS-84 coordinate system

3.1	The successful global implementation of satellite navigation is predicated on the existence of a coordinate and procedures database of a very high quality. In support of evolving satellite-based technology, ICAO adopted WGS-84 as the common geodetic reference datum for civil aviation. Implementation of WGS-84 involves, among other things, WGS-84 surveying or transformation of existing coordinates and reference datums to WGS-84. The latter technique requires less effort as compared to the surveying. However, it can be applied preferably when original accuracy of a local geodetic datum complies with accuracy requirements for intended operations.

3.2	Failure to implement WGS-84 will prevent application of GNSS-based operations in terminal area, particularly for instrument approach and landing including non-precision approach.

4.	Gnss integrity monitoring and notams

4.1	With the terrestrial radio navigation aids, monitoring equipment is designed to cease the navigation signal should the signal deviate outside specified tolerances. The fact that the navigation aid should no longer be used for navigation is indicated promptly to the pilot and ATC as appropriate. Interruptions, unreliability or irregularity of service is to be promulgated through a NOTAM.

4.2	In GNSS, a receiver with RAIM capability needs to process signals from a minimum of five satellites in order to detect and to alert to inaccurate signals. There are some GNSS navigation receivers that have a fault exclusion capability in addition to the fault detection. With a minimum of six satellites in view, a receiver having the fault detection and exclusion (FDE) capability is able to determine which of the satellites is transmitting an anomalous signal and reject the information from its navigational computations.

4.3	Due to the satellite constellation geometry, there are occasions at different places on the Earth when an aircraft's receiver will have less than five or six satellites in sight and thus there may not be sufficient integrity in the navigational guidance provided. These occasions are commonly called RAIM outages. For any given location, it is possible to forecast if and when a RAIM outage is going to occur and the duration of the outage.

4.4	Some States provide, or are in the process of providing, a briefing service that includes RAIM outages using automated systems that can forecast RAIM outages for any particular location. The software in their computer programs also allows them to exclude specific satellite vehicles, should they be notified to be out of service. Additionally, some GNSS receivers can determine when RAIM outages are likely to occur at the aircraft's destination. Relevant amendments to ICAO documents are under development by the GNSS Panel.

5.

5.1	The ICAO strategy for introduction and application of non-visual aids to approach and landing considers GNSS as alternative system which would be introduced in addition to and in place of present ICAO standard systems, i.e. ILS and MLS. Pending GNSS implementation in support of precision approach and landing, the strategy suggests to continue ILS operations as long as operationally acceptable and economically beneficial and to implement MLS where operationally required and economically beneficial. It also promotes multi-wide receiver (MMR) or equivalent airborne capability to maintain aircraft interoperability.

5.2	The following summary is intended to show the interrelationship between typical approach and landing operations, associated RNP types and GNSS augmentations expected to support these operations and RNP types:

•	ABAS is intended to support non-precision approach without and with vertical guidance (RNP 0.3 and RNP 0.3/125 respectively). Some restrictions may apply as a result of RAIM outages (see para 4 above);

•	SBAS (in combination with ABAS) is intended to support "near" Category I and Category I precision approach and landing operations (RNP 0.03/50 and RNP 0.02/40 respectively);

•	GBAS alone or in combination with other augmentations is intended to support all precision approach and landing operations down to Categories II (RNP 0.01/15) and III (RNP 0.003), and surface operations (RNP 0.003) as well.

6.	Evolutionary introduction

6.1	GNSS implementation will be carried out in an evolutionary manner, allowing gradual system improvements to be introduced. Near-term applications of GNSS are intended to enable the early introduction of satellite-based en-route navigation, non-precision approaches and departures using the existing satellite systems (GPS and GLONASS) and primarily aircraft-based augmentations.

6.2	Medium-term applications will make use of existing satellite navigation systems with an augmentation or combination of augmentations required for precision approach and landing down to Category I Longer-term applications relate to low visibility operations, i.e. precision approach and landing down to Category III, and surface operations.

6.3	Three approval levels are generally accepted for introduction of GNSS-based operations:

a)	supplemental-means: GNSS must be used in conjunction with a sole-means navigation system. The supplemental means GNSS must meet accuracy and integrity requirements for a given operation or phase of flight; availability and continuity requirement may not be met. Sole means navigation system supporting a given operation or phase of flight must be on board and may be monitored by cross-checking;

b)	primary-means: GNSS must meet accuracy and integrity requirements, but need not meet full availability and continuity of service requirements for a given operation or phase of flight. Safety is achieved by limiting operations to specific time periods and through appropriate procedural restrictions. There is no requirement to have a sole-means navigation system on board to support the primary means GNSS; and

c)	sole-means: GNSS must allow the aircraft to meet, for a given operation or phase of flight, all four, accuracy, integrity, availability and continuity of service requirements.

7.	GNSS standardization

7.1	A number of air navigation panels are involved in GNSS-related work. They are progressing their respective tasks in the following areas:

a)	development of GNSS SARPs for Annex 10 – Aeronautical Telecommunications;

b)	assessment of GNSS for approach, landing , departure and aerodrome surface operations; and

c)	procedures and criteria for GNSS-based approach and landing operations.

7.2	The Global Navigation Satellite System Panel (GNSSP) was established by the ICAO Air Navigation Commission in 1993 with the basic objective to develop ICAO SARPs and Guidance Material as required to support GNSS introduction world-wide. At the first stage of its work, the panel produced guidelines for the introduction and operational use of the GNSS (published as ICAO Circular 267), and then focussed on developing the first package of GNSS SARPs for Annex 10. In general, this package will include ICAO specifications for the following elements of GNSS:

-	global positioning system (GPS);

-	global orbiting navigation satellite system (GLONASS);

-	aircraft-based augmentation systems (ABAS);

-	satellite-based augmentation systems (SBAS); and

-	ground-based augmentation systems (GBAS).

7.3	GNSS/SBAS as envisaged at this stage of system development will be capable of supporting at least "near" Category I precision approach and landing operations. Category I capability performance down to the 60m (200 ft) decision height will be defined by GNSS/GBAS SARPs as part of the first package of GNSS SARPs. The third meeting of the GNSSP (GNSSP/3) is now envisaged in Spring of 1999 to complete this initial package of GNSS SARPs and recommended them for ICAO's adoption.

– END –