International Civil Aviation Organization

World-wide CNS/ATM Systems

Implementation Conference

Rio de Janeiro, 11 – 15 May 1998

WW/IMP-WP/37

11/5/98

Item 1

Agenda Item 1:	Overview of CNS/ATM systems implementation

RESULTS OF GNSS ASSESSMENT FOR APPLICATION IN APPROACH,

LANDING AND DEPARTURE

(Presented by the Secretariat)

Information Paper

SUMMARY

The appendix to this paper presents an abbreviated version of the report that was developed by the All Weather Operations Panel (AWOP) as a result of feasibility studies to evaluate GNSS applications in approach, landing and departure.

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WW/IMP-WP/37

APPENDIX

APPENDIX

GNSS ASSESSMENT FOR APPLICATION IN APPROACH, LANDING AND DEPARTURE

1.	Introduction

1.1	This document addresses the technologies assessed by the All Weather Operations Panel (AWOP) in early 1997. Only those architectures and systems which showed promise for meeting users requirements for a global system were considered.

1.2	Global navigation satellite systems

1.2.1	Global navigation satellite system (GNSS) satellite constellations. In the context of this document, the term global navigation satellite system (GNSS) refers to the global positioning system (GPS) and the global navigation satellite system (GLONASS). Key characteristics of the GPS and GLONASS provided to the civil community are given in Table 1.1 and more detailed information is provided in ICAO Guidelines for the Introduction and Operational Use of the Global Navigation Satellite System (Circular 267).

Table 1.1. GPS/GLONASS technical characteristics

Characteristic	GPS	GLONASS
Full constellation	24 satellites	24 satellites
Geometry	6 orbital planes

55° inclination	3 orbital planes

64.8° inclination
Altitude	20 200 km	19 100 km
Orbit period	11 hr, 56.9 min	11 h, 15 min
Carrier frequency (MHz)	1 575.42 MHz	(1 602 + 0.5625 x n) MHz (where n=1 to 24)
Chipping rate	1.023 MHz	0.511 MHz
co-ordinate system	WGS-84	PZ 90
System time reference	UTC-USNO	UTC-SU

1.2.2	GNSS augmentations. For most approach and landing operations, the performance of both GPS and GLONASS is inadequate and augmentations to the basic constellation are needed to meet accuracy, integrity, continuity and availability requirements. GNSS augmentations can be in the aircraft or external (non-aircraft) including ground equipment or space vehicles or a combination of all of the above.

1.2.3	Ground-based augmentation system (GBAS). A ground-based augmentation provides additional information, locally or within a small region, in order to enhance the integrity and/or availability and/or accuracy and/or continuity of the GNSS service without augmenting the basic GNSS constellation with additional space vehicles. A local area data broadcast is required to carry the augmentation services to a user. Details of this broadcast are not required for the technology assessment and are not included within this document, although the appendix provides a brief description of the data broadcasts used in the assessed architectures. The frequency band used by any fielded system must be commensurate with the intended operation.

1.2.4	Local area differential GNSS. Local area differential GNSS is differential GNSS in which the differential corrections are usable for the supported phases of operation within a limited geographical area. It involves transmission of corrections to the pseudo-range code measurements associated with each satellite. The corrections are derived from measurements collected at a site within approximately 150 km (80 NM) of the user aircraft and are typically transmitted via a line-of-sight data link. In order to meet the integrity requirements, local area differential systems generally include a second ground station which monitors both the satellite and differential corrections and provides an integrity notification when warning thresholds are violated.

1.2.5	Kinematic local area differential GNSS. Kinematic local area differential GNSS is a modified form of local area in which the corrections are in the form of satellite signal carrier phases, as opposed to just the code, thereby allowing the aircraft to employ interferometric principles for navigation. This technique requires that certain integer wavelength values be determined, as part of an initialization process, before full interferometric operation is achieved.

1.2.6	Pseudolites. Pseudolites (PL) are ground-based transmitters which generate and distribute a signal which is compatible with those transmitted directly from the satellite. Pseudolites therefore enhance the availability of ranging signals. PL may provide code ranging as well as carrier phase measurements. A specific realization of PLs are integrity beacons.

1.2.7	Satellite-based augmentation system (SBAS). Satellite-based augmentation provides additional information over a wide area in order to enhance the integrity/availability/accuracy/continuity of the GNSS service. The simplest form of this augmentation is the broadcast of satellite integrity status via a geostationary satellite, using data collected at ground monitor stations. The wide area differential GNSS augmentation supplies differential corrections valid over an extensive geographical area. It also involves a network of data collection ground stations. Typically, these ground stations might be separated by 1 000 km (540 NM). Information collected at several ground stations is transmitted to and processed at a central facility to derive corrections to satellite pseudo-range error component sources (e.g. satellite clock, satellite ephemeris, and ionospheric delay). These corrections are then broadcast to users via a data link which provides coverage in the intended service area (e.g. geostationary satellite or network of ground transmitters).

1.2.8	Wide area augmentation system (WAAS). The WAAS is intended to provide:

a)	an integrity broadcast to users;

b)	an additional ranging source; and

c)	differential corrections.

1.2.9	The integrity broadcast provides status information on each satellite. An additional ranging source will be implemented in the same geostationary satellites that provide the down link of WAAS information to the user. Differential corrections will be employed to enhance accuracy.

1.2.10	A network of reference stations are sited uniformly over a coverage area. Satellite and weather data collected at each reference station is transmitted to a central processing facility called the wide area monitor station. This facility generates the GNSS integrity message, determines GPS ephemeris and clock corrections, and calculates ionospheric and tropospheric correction parameters. This information is transmitted to the earth station for uplink to the geostationary satellite constellation, then downlinked to the user within the GPS frequency band.

1.2.11	European geostationary navigation overlay system (EGNOS) and MTSAT satellite augmentation system (MSAS). Beside the WAAS, two other satellite-based augmentation systems are currently under development, EGNOS and MSAS.

1.3	Aircraft based augmentation system (ABAS)

1.3.1	Airborne integrity monitoring. Airborne integrity monitoring is based on the use of redundant navigation information available on the aircraft to perform an integrity check on the GNSS position information. RAIM (receiver autonomous integrity monitoring) is a technique whereby an airborne GNSS receiver/processor autonomously monitors the integrity of the navigation signals from GNSS satellites. RAIM uses only GNSS pseudo-ranges (or differentially corrected pseudo-ranges) to perform the integrity check. Pseudo-ranges are required to perform the function. The basic RAIM function will identify the presence of an unacceptably large position error for a given flight operation. The more advanced versions of RAIM provide fault detection and exclusion, whereby the source of the error (i.e. specific satellite) is identified and excluded from the position solution. There is also a function used in some applications called predictive RAIM, which is the ability to look ahead and determine if constellation geometry will be sufficient to support the use of RAIM for the intended operation.

1.3.2	A variation on RAIM is aircraft autonomous integrity monitoring (AAIM) which utilizes GNSS pseudo-ranges (possibly differentially corrected) in conjunction with other airborne data as inputs to the integrity check. AAIM techniques make use of integrated navigation systems which generally provide data from sources other than satellites that can be used to enhance the integrity checking capability. Typical AAIM implementations include integration with inertial and altimetry data.

1.3.3	Inertial systems. There are two basic inertial systems: inertial reference unit (IRU) and inertial navigation system (INS). IRUs sense the host vehicle's body angular rates and body accelerations, and generate continuous outputs of attitude, azimuth and navigation data relative to the Earth (including latitude, longitude, altitude, velocity, track and wind data), based on pre-flight initialized position data. INSs have a similar internal capability to the IRU and in addition, store and process way-points and generate lateral steering data. Typically, the IRU and INS are interfaced with, and support, other avionics equipment.

1.3.4	Altimetry aiding. Altimetry aiding uses altitude data as if it were a GNSS satellite directly over the receiver antenna thereby reducing the number of satellites required for a given function. Barometric altimetry (calibrated pressure) was used in an architecture evaluated because it is required in civil aircraft.

2.

2.1	Architecture considerations

2.1.1	Requirements. Candidate GNSS architectures were assessed against the requirements for a specific operation or phase of flight. For any given operation and set of associated requirements, one or more GNSS augmentations have been proposed as an architecture capable of fulfilling that operation. It is recognized that there are a large number of augmentation combinations that may be possible. For the purposes of this document, preference was given to the simplest combination of augmentations (an architecture) that has the potential to meet the phase of flight requirements.

2.1.2	Selection factors. The selection process first examined the three basic architecture categories that represent systems either certified or under development in several states (i.e. GNSS, GNSS + SBAS, and GNSS + GBAS). Augmentation categories were defined based on the generally accepted definitions of SBAS, ABAS and GBAS. When necessary, distinctions were made in the method of augmentation if it resulted in significant performance and/or installation differences in the over-all architecture (e.g. kinematic vs. code-based DGPS). Finally, given a form of external augmentation, variations in aircraft-specific augmentations were considered. The assessment only took into account architectures for which data existed.

2.2	Architecture synthesis

2.2.1	Table 2.1 contains a matrix of the candidate GNSS architectures assessed Column 1 indicates the operation for which the assessment was made. Column 2 presents generic augmentations used in the evaluation. Column 3 assigns a letter designator to the combination of specific systems which constitutes an architecture. If an architecture contains variations in the involved systems, this is indicated by subscripts. Column 4 indicates the corresponding specific systems nominated on which the evaluation was based. Results of the evaluation of data from the assessment of systems in column 4 was used to formulate conclusions regarding the feasibility of the generic augmentations of column 2.

Table 2.1. Candidate GNSS architectures

Operation	Augmentations

architecture	Designator	Systems evaluated
NPA	ABAS

SBAS + ABAS	A

B	GPS + RAIM

GPS + WAAS + RAIM
Instrument approach with vertical guidance (IPV)	ABAS

SBAS + ABAS	A1

B	GPS + RAIM/AAIM + Barometric VNAV + (IRU)

GPS + WAAS + RAIM
CAT I	SBAS + ABAS

GBAS

GBAS + ABAS	B

C

D	GPS + WAAS + RAIM

GPS + Local code-based DGPS

GPS + Local code-based DGPS + IRU
CAT II	GBAS +ABAS

GBAS + ABAS	C1

E	GPS + Local code-based DGPS + IRU

GPS + Local kinematic DGPS + IRU
CAT III	GBAS + ABAS

C 2

F

GPS + Local code-based DGPS + IRU

GPS + Local DGPS with integrity beacon + RAIM
DEPARTURE without take-off guidance	ABAS

SBAS + ABAS	A

A1

B	GPS + RAIM

GPS + RAIM/AAIM + Barometric VNAV + (IRU)

GPS + WAAS + RAIM
DEPARTURE with take-off guidance	GBAS + ABAS

C2

F

GPS + Local code-based DGPS + IRU

GPS + Local DGPS with integrity beacon + RAIM

2.3	Architecture descriptions

2.3.1	Non-precision approach (NPA). Two basic architectures were proposed for the non-precision approach phases of flight.

2.3.1.1	Architecture A, GNSS + ABAS. The general case of the GNSS is considered where integrity is provided through RAIM or other aircraft autonomous integrity methods. Continuity and availability are addressed for the basic GNSS constellation (e.g. GPS and/or GLONASS).

2.3.1.2	Achitecture B, GNSS + ABAS + SBAS. The basic GNSS constellation is augmented through the addition of SBAS ranging satellites which incorporate an integrity message. Integrity performance is enhanced through the additional ranging signals as well as the presence of the integrity message (dependent on GEO redundancy rather than constellation geometry as in RAIM).

2.3.2	Instrument approach with vertical guidance (IPV). Two architectures were assessed for this operation.

2.3.2.1	Architecture A1, GNSS + ABAS. This architecture is similar to architecture A with the addition of barometric vertical navigation (VNAV) and, in some cases, inertial reference unit (IRU) augmentation.

2.3.2.2	Architecture B, GNSS + ABAS + SBAS. This architecture is similar to the architecture described in paragraph 2.3.1.2 except that a vertical satellite navigation capability was included.

2.3.3	Category I approach and landing. Three basic architectures were selected for Category I: satellite- and aircraft-based, ground-based, and a combination of ground-based and aircraft-based augmentation. The first architecture is again denoted B as in the NPA and IPV evaluations.

2.3.3.1	Architecture C, GNSS + GBAS. This architecture incorporates the simplest of local differential GNSS architectures. Code or carrier-smoothed code differential is baselined. Integrity performance is provided by the ground station through proper detection and correction of SV errors prior to transmission of differential data. In general, availability and continuity are provided through redundancy of the equipment.

2.3.3.2	Architecture D, GNSS + GBAS + ABAS. This architecture builds upon architecture C and includes aircraft-based augmentation such as that provided by an IRU. Various IRU/GNSS integration techniques have been realized with this architecture.

2.3.4	Category II approach and landing. For Category II, two variations of ground augmentation are considered.

2.3.4.1	Architecture C1, GNSS + GBAS + ABAS. This architecture is identical to Architecture C for Category I, except for the following considerations: the stricter accuracy, integrity, and continuity requirements for CAT II may impact the differential method and/or implementation and require the use of inertial data.

2.3.4.2	Architecture E, GNSS + GBAS (Kinematic) + ABAS. Unlike the code-based architecture C variants, this architecture employs a kinematic carrier phase ambiguity resolution technique. The techniques rely only on a single frequency. Additional aircraft augmentation is also utilized. Inertial velocity data compensates for latency in the data and attitude information is used for the non-collocated reference point correction.

2.3.5	Category III approach, landing, and autolanding. Three variations of ground augmentations were considered. These variations fundamentally differ in the position technique employed. For Category III, it is assumed that a type of inertial reference is used along with radar altimeter. Although these are "aircraft augmentations", they do not directly aid the GNSS solution for an ILS look-alike integration and no credit in performance for these aircraft systems will be taken (no differentiation is currently made between CAT III A, B and C; since the requirements do differ significantly these distinctions may be made in the future).

2.3.5.1	Architecture C2, GNSS + GBAS + ABAS. Like architecture C1, this architecture continues to build on the code or carrier-smoothed code solution of architecture C. The stricter accuracy, integrity, and continuity requirements for CAT III may impact the differential method and/or implementation as well as the type of implementation of inertial data. For example, receiver architectures and stricter siting requirements which significantly mitigate multipath may be required. The primary integrity function is provided by the ground system.

2.3.5.2	Architecture F, GNSS + GBAS (integrity beacon) + ABAS. This architecture employs both a code-based and a carrier-phase differential scheme employing integrity beacons. The integrity beacon is a special type of L1 transmitter or pseudolite which is sited under the approach path similar to a marker beacon. These "integrity beacons" provide the basis for cycle ambiguity resolution and improve system geometry.

2.3.6	Departure. Three variants of the aircraft-based augmentations, one with satellite-based augmentation as well, were considered for departure operations without take-off guidance. The architectures are those considered for NPA and IPV operations (A, A1, and B) and are described above. The most appropriate architectures considered for Category III operations were also considered for departure operations with take-off guidance. These are architectures C2 and F.

3.

3.1	Introduction

3.1.1	Evaluation methodology was developed to enable the assessment of the applicability of new technologies to approach, landing, and departure operations to be carried out in a uniform and consistent manner. Guidance was also developed on the information to be provided to enable the assessment to be made.

3.1.2	This evaluation methodology was used to explicitly evaluate particular emerging new technologies to perform approach, landing and departure operations. The assessment was primarily against accuracy, integrity, continuity and availability parameters for each type of operation but also considered airworthiness certification and general operational requirements. Feasibility implies that there is a high level of confidence that the particular architecture can be certified to perform its intended operation. In this regard, the needs of both the manufacturers and operators must be considered during the assessment. One factor in the assessment methodology is, therefore, the extent to which the proposed system can be implemented as an ILS look-a-like configuration since this has been identified by the manufacturers as a desirable feature to meet retrofit considerations. In general, the technology being assessed is the navigation system and may include some equipment on board the aircraft. Where specific aircraft augmentations are necessary to achieve the required performance, e.g. inertial navigation systems this is identified in the assessment.

3.2	Assessment objectives

3.2.1	The methodology must satisfy the objectives of the evaluation. These objectives include:

a)	determination of the feasibility of GNSS for approach, landing and departure operations as the first step leading to the development of Standards and Recommended Practices (SARPs);

b)	providing information to assist in the development of global and regional strategies for the introduction and application of non-visual aids to approach and landing; and

c)	providing information to the manufacturers and operators on the assessment of new technologies.

3.3	Definition of feasibility

3.3.1	In the context of this assessment, proven feasibility means that a high confidence exists that there are no technical issues which would preclude certification for the intended operation.

3.4	Evaluation philosophy

3.4.1	The evaluation methodology was developed* from the beginning of the assessment. Ideally, the future assessment should be made in accordance with this agreed methodology, provided that sufficient information could be made available in the required form. In practice, it was recognized that in many cases, the information available was not sufficient to carry out a full assessment using the agreed evaluation methodology. The assessment was, therefore, made using the information available and provided to AWOP, following the agreed methodology as far as possible. In some cases, there are therefore outstanding issues which have been identified in the assessment conclusions.

3.4.2	The main assessment criteria used was compliance with the four RNP parameters: accuracy, integrity, continuity and availability. The intention was that the assessment should consider a common architecture and a common set of assumptions. Each architecture was therefore assessed to ensure that it can simultaneously comply with all four RNP parameters. For the purpose of this assessment, it was assumed that the system is providing guidance to a single runway only. The availability requirement is therefore identical for all systems and does not take account of the potential problem of wide area, multiple runway outages with GNSS-based systems. This problem has not been addressed at this time and availability assessment criteria have not yet been developed.

4.

4.1	The architecture assessment is summarized in the table below. Within a row/column combination the following entries are made:

a)	a "Yes" in a row/column indicates that the architecture is feasible to satisfy that operation;

b)	a "No" in a row/column indicates that the architecture is not feasible to satisfy that operation; and

c)	a "UE" in a row/column indicates that analysis and testing of the architecture is under way. Insufficient results currently exist to enter either a "Yes" or a "No". Testing and analysis continue.

Table 4-1. Generic architecture augmentations

Operation	ABAS	SBAS + ABAS	GBAS	GBAS + ABAS
Non-precision approach	Yes

Note 1

Note 5	Yes	Yes

Note 4	Yes

Note 4
Instrument approach with vertical guidance	Yes

Note 1

Note 5	Yes

Yes

Note 4	Yes

Note 4
Category I approach & landing	No	Yes

Note 2	Yes	Yes

Category II approach & landing	No	No	Not being pursued

UE

Note 3
Category III approach & landing	No	No	No	UE

Note 3
Departure without take-off

guidance	Yes

Note 1	Yes

Yes

Note 4	Yes

Note 4
Departure with take-off guidance	No	No	No	UE

Note 3

Note 1.— Aircraft-based augmentation systems provide limited availability of the service. This may lead to operational restrictions (e.g. prediction of the satellite constellation status before departure).

Note 2.— Satellite-based augmentation systems may provide limited vertical accuracy availability due to integrity requirements.

Note 3.— Accuracy has been demonstrated and requirements met for these types of operations. No data or insufficient data has been presented for a complete integrity and continuity assessment.

Note 4.— This architecture is more than the minimum for this operation.

Note 5.— Without proper airborne integrity implementations, potential for integrity failures may exist.

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