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|Category=Fundamentals
|Title={{PAGENAME}}
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|Authors=GMV
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|Level=Basic
|YearOfPublication=2011
|YearOfPublication=2011
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A [[Ground-Based Augmentation System (GBAS)]] is a civil-aviation safety-critical system that supports local augmentation –at airport level– of the primary GNSS constellation(s) by providing enhanced levels of service that support all phases of approach, landing, departure and surface operations. While the main goal of GBAS is to provide [[Integrity|integrity]] assurance, it also increases the [[Accuracy|accuracy]] with position errors below 1 m (1 sigma).<ref name="GNSS Aug">[[Wikipedia:GNSS augmentation|GNSS augmentation in Wikipedia]]</ref><ref name="Kaplan">E.D. Kaplan, C.J. Hegarty, ''Understanding GPS Principles and Applications”, 2nd Ed., Artch House, ISBN 1-58053-894-0, 2006.</ref>
The Ground Based Augmentation System (GBAS) is intended primarily to support precision approach operations. It consists of a GBAS Ground Subsystem and a GBAS Aircraft Subsystem.
==Introduction: How a GBAS works==
(The information in this section has been taken from [http://www.faa.gov/ FAA]  homepage.)
A Ground Based Augmentation System (GBAS) augments the Global Positioning System (GPS) to improve aircraft safety during airport approaches and landings. It is expected that the end-state configuration will pinpoint the aircraft's position to within one meter or less with a significant improvement in service flexibility and user operating costs. GBAS is comprised of ground equipment and avionics. The ground equipment includes 4 reference receivers, a GBAS ground facility, and a VHF data broadcast transmitter. This ground equipment is complemented by GBAS avionics installed on the aircraft.<ref>[http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/laas/howitworks/ LAAS-How it works] by FAA.</ref>
Signals from GPS satellites are received by the GBAS GPS Reference Receivers at the GBAS-equipped airport. The reference receivers calculate their position using GPS. The GPS Reference Receivers and GBAS Ground Facility work together to measure errors in GPS-provided position. The GBAS Ground Facility produces a GBAS correction message based on the difference between actual and GPS-calculated position which includes as well integrity parameters and approach path information. This GBAS correction message is then sent to a VHF data broadcast (VDB) transmitter.
The VDB broadcasts the GBAS signal throughout the GBAS coverage area to avionics in GBAS-equipped aircraft. GBAS provides its service to a local area (approximately a 30 kilometer radius). The signal coverage is designed to support the aircraft's transition from en route airspace into and throughout the terminal area airspace.


A Ground-Based Augmentation System (GBAS) is a civil-aviation safety-critical system that supports local augmentation – at airport level – of the primary GNSS constellation(s) by providing enhanced levels of service that support all phases of approach, landing, departure and surface operations. While the main goal of GBAS is to provide [[Integrity|integrity]] assurance, it also increases the [[Accuracy|accuracy]] with position errors below 1 m (1 sigma).<ref name="GNSS Aug">[[Wikipedia:GNSS augmentation]]</ref><ref name="Kaplan">E.D. Kaplan, C.J. Hegarty, ''Understanding GPS Principles and Applications”, 2nd Ed., Artch House, ISBN 1-58053-894-0, 2006.</ref>
The GBAS equipment in the aircraft uses the corrections provided on position, velocity, and time to guide the aircraft safely to the runway. This signal provides ILS-look-alike guidance as low as 200 feet (60 m) above touchdown. GBAS will eventually support landings all the way to the runway surface.


The Ground Based Augmentation System (GBAS) is intended primarily to support
<gallery>
precision approach operations. It consists of a GBAS Ground Subsystem and a
Image:Laas_step1.jpg‎
GBAS Aircraft Subsystem.  
Image:Laas_step2.jpg‎
Image:Laas_step3.jpg‎
Image:Laas_step4.jpg‎
</gallery>


==GBAS Architecture==
==GBAS Architecture==
[[File:SBAS_architecture.png|SBAS architecture|300px|thumb|right]]
[[File:LAAS_Architecture.png|GBAS architecture|350px|thumb|right]]


A SBAS is a safety critical system designed to augment one or several satellite navigation systems. The main components of a general GBAS architecture are:
A GBAS is a safety critical system designed to augment one or several satellite navigation systems. The main components of a general GBAS architecture are:
* GBAS Ground Subsystem
* GBAS Ground Subsystem
* GBAS Aircraft Subsystem
* GBAS Aircraft Subsystem
Line 24: Line 39:
===GBAS Ground Subsystem===
===GBAS Ground Subsystem===


The main purposes of the GBAS Ground Subsystem are: signals-in-space receive and decode, carrier smoothed code and differential corrections and integrity computation, integrity monitoring, generation and broadcasting of GBAS messages.
The main purposes of the GBAS Ground Subsystem are:
*reception and decoding of signals-in-space;
*computation of the differential corrections to the carrier-smoothed codes;
*integrity monitoring;
*generation and broadcasting of GBAS messages.


The GBAS Ground Subsystems consist of  
The GBAS Ground Subsystem consists of:
*2 to 4 GNSS Reference Receivers and their respective geographically separated antennas
*2 to 4 GNSS Reference Receivers and their respective geographically separated antennas;
* A VHF data broadcast (VDB) transmitter
* A VHF data broadcast (VDB) transmitter;
*A monitor system
*A monitor system;
*Approach Database (FAS data)
*Approach Database (FAS data);
*Ground processing functions
*Ground processing functions.


===GBAS Aircraft Subsystem===
===GBAS Aircraft Subsystem===
Line 44: Line 63:
* provide guidance signals and integrity information.
* provide guidance signals and integrity information.


The GBAS aircraft subsystem essential elemenst are:
The GBAS aircraft subsystem essential elements are:
* An Aircraft GNSS Receiver Function that receives, tracks, and decodes the GNSS satellite signals.
* An Aircraft GNSS Receiver Function that receives, tracks, and decodes the GNSS satellite signals.
* A VHF Data Broadcast Receiver Function that receives and decodes the messages broadcast by the GBAS Ground Subsystem.
* A VHF Data Broadcast Receiver Function that receives and decodes the messages broadcast by the GBAS Ground Subsystem.
Line 51: Line 70:
===GNSS Satellites Subsystem===
===GNSS Satellites Subsystem===


The GBAS Satellites Subsystem is mainly the [[GNSS systems description
The GBAS Satellites Subsystem is mainly the GNSS constellations declared operational for civil use. The minimum requirements of a GBAS system are limited for the use of GNSS satellites ranging sources, with additional ranging sources being supplied by SBAS as an option.
|GNSS constellations] declared operational for civil use. The minimum requirements of a GBAS system are limited for the use of GNSS satellites ranging sources, with additional ranging sources being supplied by SBAS as an option.


==GBAS Signal==
==GBAS Signal==
Line 58: Line 76:
The GBAS Signal in Space is defined to be only the data broadcast from the ground to the aircraft subsystem. The Satellite Signals in Space are part of the basic GNSS satellite constellations.
The GBAS Signal in Space is defined to be only the data broadcast from the ground to the aircraft subsystem. The Satellite Signals in Space are part of the basic GNSS satellite constellations.


The GBAS ground subsystem differential computed corrections, (contained in Type 1 message), GBAS ground subsystem  related data (contained in Type 2 message) and Final Approach Segments (FAS) (contained in Type 4 message) are transmitted to the aircraft users via VHF Data Broadcast (VDB) Signal. The GBAS Messages are encoded in this signal. The specification of GBAS messaged data format is contained in the ..........
The GBAS ground subsystem differential computed corrections (contained in Type 1 message), GBAS ground subsystem  related data (contained in Type 2 message) and Final Approach Segments (FAS) (contained in Type 4 message) are transmitted to the aircraft users via VHF Data Broadcast (VDB) Signal. The GBAS Messages are encoded in this signal. The specification of GBAS message data format is contained in the [http://www.icao.org ICAO] SARPS Appendix B for the aspects related with the signal in space, as well as in the RTCA MOPS DO-253C for the minimum operational performance requirements applicable to the airborne GBAS receiver equipment.  
''SBAS-> "The specification of the SBAS message data format is contained in the [http://www.icao.org ICAO] SARPS Appendix B for the aspects related with the signal in space, as well as in the RTCA MOPS DO-229D for the minimum performance requirements applicable to the airborne SBAS receiver equipment. The format of the messages is thoroughly explained in the article [[The EGNOS SBAS Message Format Explained]]."''
 
The VDB radio frequencies used shall be selected from the radio frequencies in the band 108-117.975 MHz. The lowest assignable frequency shall be 108.025 MHz and the highest frequency assignable shall be 117.950 MHz. The separation between assignable frequencies (channel spacing) shall be 25 kHz.
 
The GBAS message types are summarized in the following table:
 
{| class="wikitable" align="center"
!Message type identifier
!Message Name
|-align="center"
|0
| Spare
|-align="center"
|1
| GBAS Differential Corrections
|-align="center"
|2
| GBAS Related Data
|-align="center"
|3
| Reserved for ground–based ranging source
|-align="center"
|4
| Final Approach Segment (FAS) data
|-align="center"
|5
| Predicted ranging source availability
|-align="center"
|6
| Reserved
|-align="center"
|7
| Reserved for national applications
|-align="center"
|8
| Reserved for test applications
|-align="center"
|9-255
| Spare
|}


==GBAS Performances==
==GBAS Performances==




The GBAS performances are defined with respect to the level of service that the system is designed to provide. The main source for GBAS performances comes from civil aviation navigation safety requirements and they are different for each civil aviation operation. <ref name="ICAO_SARPS">[http://www.icao.org ICAO] Standards and Recommended Practices, Annex 10, Volume 1 Radio Navigation Aids, July 2006</ref>


The SBAS performances are defined with respect to the level of service that the system is designed to provide. The main source for SBAS performances comes from civil aviation navigation safety requirements and they are different for each civil aviation operation.<ref name="ICAO_SARPS"/>
{| class="wikitable"
{| class="wikitable"
|-
|-
!|Typical Operation||Horizontal Accuracy (95%)||Vertical Accuracy (95%)||Integrity||Time-To-Alert (TTA)||Continuity||Availability  
!Typical Operation
!Horizontal Accuracy (95%)
!Vertical Accuracy (95%)
!Integrity
!Time-To-Alert (TTA)
!Continuity
!Availability  
|-
|-
|En-route || 3.7 km (2.0 NM) || N/A || 1 – 1 × 10-7/h || 5 min || 1 – 1 × 10-4/h to 1 – 1 × 10-8/h || 0.99 to 0.99999
|Initial approach, Intermediate approach, Non-precision approach (NPA), Departure
|align="center"|220m (720ft)
|align="center"|N/A
|align="center"|1–1×10<sup>-7</sup>/h
|align="center"|10s
|align="center"|1–1×10<sup>-4</sup>/h to 1–1×10<sup>-8</sup>/h
|align="center"|0.99 to 0.99999
|-
|-
|En-route Terminal || 0.74 km (0.4 NM) || N/A || 1 – 1 × 10-7/h || 15 s || 1 – 1 × 10-4/h to 1 – 1 × 10-8/h || 0.99 to 0.99999
|Non Precision Approach with vertical guidance (NPV-I)
|align="center"| 220m (720ft)
|align="center"| 20m (66ft)
|align="center"| 1–2×10<sup>-7</sup> per approach
|align="center"| 10s
|align="center"| 1–8×10<sup>-6</sup> in any 15s
|align="center"| 0.99 to 0.99999
|-
|-
|Initial approach, Intermediate approach, Non-precision approach (NPA), Departure ||220 m (720 ft)||N/A||1 –1x10-7/h||10 s ||1 – 1x10-4/h to 1 – 1x10-8/h||0.99 to 0.99999
|Non Precision Approach with vertical guidance (NPV-II)
|align="center"| 16m (52ft)
|align="center"| 8m (26ft)
|align="center"| 1–2×10<sup>-7</sup> per approach
|align="center"| 6s
|align="center"| 1–8×10<sup>-6</sup> in any 15s
|align="center"| 0.99 to 0.99999
|-
|-
|Approach operations with vertical guidance (APV-I) || 16 m (52 ft) || 20 m (66 ft) || 1 – 2 × 10-7 per approach || 10 s || 1 – 8 × 10-6 in any 15 s || 0.99 to 0.99999
|Category I (CAT-I) Precision Approach
|-
|align="center"| 16m (52ft)
|Approach operations with vertical guidance (APV-II) || 16 m (52 ft) || 8 m (26 ft) || 1 – 2 × 10-7 per approach || 6 s || 1 – 8 × 10-6 in any 15 s || 0.99 to 0.99999
|align="center"| 6.0m to 4.0m (20ft to 13ft)
|-
|align="center"| 1–2×10<sup>-7</sup> per approach
|Category I precision Approach  || 16 m (52 ft) || 6.0 m to 4.0 m (20 ft to 13 ft) || 1 – 2 × 10-7 per approach || 6 s || 1 – 8 × 10-6 in any 15 s || 0.99 to 0.99999
|align="center"| 6s
|align="center"| 1–8×10<sup>-6</sup> in any 15s
|align="center"| 0.99 to 0.99999
|}
|}
Usually a GBAS system is designed to fulfil CAT-I Precision Approach.


As indicated in the table above, the performance requirements are expressed in terms of four quantitative concepts, many of them to be interpreted as probabilistic figures:
As indicated in the table above, the performance requirements are expressed in terms of four quantitative concepts, many of them to be interpreted as probabilistic figures:
* '''Accuracy''': is expressed in terms of Navigation System Error (NSE) as the difference between the real position of the aircraft and the position provided by the airborne equipment. A SBAS assures the compliance with respect the accuracy requirements by providing to the user corrections to the satellite orbit and clock errors as well as to the ionospheric residual propagation error.
* '''Accuracy''': is expressed in terms of Navigation System Error (NSE) as the difference between the real position of the aircraft and the position provided by the airborne equipment.
* '''Integrity''': is defined by [http://www.icao.org ICAO] as a measure of the trust that can be placed in the correctness of the information supplied by the system. This general statement is expressed at the SBAS system level as the maximum allowable probability that the navigation position error exceeds alarm limit and the navigation system does not alert the pilot in a time less than the time to alert. The SBAS assures the integrity requirements by:
* '''Integrity''': is defined by [http://www.icao.org ICAO] as a measure of the trust that can be placed in the correctness of the information supplied by the system.
** Providing to the user satellite and/or ionospheric alarms in order to inform the user to reject the corresponding satellite/ionospheric corrections in its positioning computation.
** Providing to the user Horizontal and Vertical Protection Level information (HPL, VPL) in order to assess the availability of the system, by comparing these PLs with the corresponding Alarm Limits (AL) for a given phase of flight (see next tabel). The SBAS computes and broadcasts integrity bounds to the satellite orbit and clock (UDRE) corrections as well as to the ionospheric corrections errors (GIVE) so that the user is able to compute a PL that over bounds the navigation system error experienced by the user with the integrity risk requirement.
 
{| class="wikitable"
|-
!Operation || Horizontal AL || Vertical AL
|-
|En-route (oceanic/continental) || 7.4 Km (4 NM) || N/A
|-
|En-route (continental) || 3.7 Km (2 NM) || N/A
|-
|En-route, Terminal || 1.85 Km (1 NM) || N/A
|-
|NPA || 556 m (0.3 NM) || N/A
|-
|APV-I || 40 m (130 ft) || 50 m (164 ft)
|-
|LPV200 || 40 m (130 ft) || 35 m (200 ft)
|-
|APV-II || 40 m (130 ft) || 20 m (66 ft)
|-
|Category I || 40 m (130 ft) || 15 to 10 m (50 ft to 33 ft)
|}


* '''Continuity''': is the probability that the specified system performance will be maintained for the duration of a phase of operation, presuming that the system was available at the beginning of that phase of operation and was predicted to operate throughout the operation. Lack of continuity means that the operation must be aborted (with the associated risk).  
* '''Continuity''': is the probability that the specified system performance will be maintained for the duration of a phase of operation, presuming that the system was available at the beginning of that phase of operation and was predicted to operate throughout the operation. Lack of continuity means that the operation must be aborted (with the associated risk).  
* '''Availability''': is the probability that the navigation service is available at the beginning of the planned operation. A SBAS is considered available when the accuracy, integrity and continuity requirements are met and it is measured in terms of probability of the system being available for any given user at any given time. In practice, the availability is computed by measuring the probability of a protrection level being below its corresponding alarm limit. It should be noted that a lack of availability is not a safety concern but prevents the nominal operation of the system, and implies an associated impact on the service operation status.
* '''Availability''': is the probability that the navigation service is available at the beginning of the planned operation. A GBAS system is considered available when the accuracy, integrity and continuity requirements are met throughout the coverage region.


==Notes==
==Notes==
Line 121: Line 181:


[[Category:Fundamentals]]
[[Category:Fundamentals]]
[[Category:GBAS]]

Latest revision as of 14:43, 24 July 2018


FundamentalsFundamentals
Title GBAS Fundamentals
Edited by GMV
Level Basic
Year of Publication 2011
Logo GMV.png

A Ground-Based Augmentation System (GBAS) is a civil-aviation safety-critical system that supports local augmentation –at airport level– of the primary GNSS constellation(s) by providing enhanced levels of service that support all phases of approach, landing, departure and surface operations. While the main goal of GBAS is to provide integrity assurance, it also increases the accuracy with position errors below 1 m (1 sigma).[1][2]

The Ground Based Augmentation System (GBAS) is intended primarily to support precision approach operations. It consists of a GBAS Ground Subsystem and a GBAS Aircraft Subsystem.

Introduction: How a GBAS works

(The information in this section has been taken from FAA homepage.)

A Ground Based Augmentation System (GBAS) augments the Global Positioning System (GPS) to improve aircraft safety during airport approaches and landings. It is expected that the end-state configuration will pinpoint the aircraft's position to within one meter or less with a significant improvement in service flexibility and user operating costs. GBAS is comprised of ground equipment and avionics. The ground equipment includes 4 reference receivers, a GBAS ground facility, and a VHF data broadcast transmitter. This ground equipment is complemented by GBAS avionics installed on the aircraft.[3]

Signals from GPS satellites are received by the GBAS GPS Reference Receivers at the GBAS-equipped airport. The reference receivers calculate their position using GPS. The GPS Reference Receivers and GBAS Ground Facility work together to measure errors in GPS-provided position. The GBAS Ground Facility produces a GBAS correction message based on the difference between actual and GPS-calculated position which includes as well integrity parameters and approach path information. This GBAS correction message is then sent to a VHF data broadcast (VDB) transmitter.

The VDB broadcasts the GBAS signal throughout the GBAS coverage area to avionics in GBAS-equipped aircraft. GBAS provides its service to a local area (approximately a 30 kilometer radius). The signal coverage is designed to support the aircraft's transition from en route airspace into and throughout the terminal area airspace.

The GBAS equipment in the aircraft uses the corrections provided on position, velocity, and time to guide the aircraft safely to the runway. This signal provides ILS-look-alike guidance as low as 200 feet (60 m) above touchdown. GBAS will eventually support landings all the way to the runway surface.

GBAS Architecture

GBAS architecture

A GBAS is a safety critical system designed to augment one or several satellite navigation systems. The main components of a general GBAS architecture are:

  • GBAS Ground Subsystem
  • GBAS Aircraft Subsystem
  • GNSS Satellites Subsystem

GBAS Ground Subsystem

The main purposes of the GBAS Ground Subsystem are:

  • reception and decoding of signals-in-space;
  • computation of the differential corrections to the carrier-smoothed codes;
  • integrity monitoring;
  • generation and broadcasting of GBAS messages.

The GBAS Ground Subsystem consists of:

  • 2 to 4 GNSS Reference Receivers and their respective geographically separated antennas;
  • A VHF data broadcast (VDB) transmitter;
  • A monitor system;
  • Approach Database (FAS data);
  • Ground processing functions.

GBAS Aircraft Subsystem

The main element of GBAS Aircraft Subsystem is the aircraft GBAS Receivers.

The primary functions of the GBAS aircraft subsystem are:

  • receive and decode the GNSS satellite and GBAS signals;
  • determine the aircraft position;
  • provide availability of the service;
  • compute deviations from the desired flight path calculated from the Final Approach Segment (FAS) data;
  • provide guidance signals and integrity information.

The GBAS aircraft subsystem essential elements are:

  • An Aircraft GNSS Receiver Function that receives, tracks, and decodes the GNSS satellite signals.
  • A VHF Data Broadcast Receiver Function that receives and decodes the messages broadcast by the GBAS Ground Subsystem.
  • An Aircraft Navigation Processing Function that receives the measurement of the pseudo-ranges from the GNSS Receiver Function, applies the differential corrections received from the VHF Data Broadcast Receiver Function and calculates the differentially corrected aircraft position. The Aircraft Navigation Processing Function extracts from the different FAS path construction data received, the one having the Reference Path Selector selected by the crew through the GLS Channel Number Selector. The Aircraft Navigation Processing Function also calculates deviations from the selected FAS path based on its differentially corrected position.

GNSS Satellites Subsystem

The GBAS Satellites Subsystem is mainly the GNSS constellations declared operational for civil use. The minimum requirements of a GBAS system are limited for the use of GNSS satellites ranging sources, with additional ranging sources being supplied by SBAS as an option.

GBAS Signal

The GBAS Signal in Space is defined to be only the data broadcast from the ground to the aircraft subsystem. The Satellite Signals in Space are part of the basic GNSS satellite constellations.

The GBAS ground subsystem differential computed corrections (contained in Type 1 message), GBAS ground subsystem related data (contained in Type 2 message) and Final Approach Segments (FAS) (contained in Type 4 message) are transmitted to the aircraft users via VHF Data Broadcast (VDB) Signal. The GBAS Messages are encoded in this signal. The specification of GBAS message data format is contained in the ICAO SARPS Appendix B for the aspects related with the signal in space, as well as in the RTCA MOPS DO-253C for the minimum operational performance requirements applicable to the airborne GBAS receiver equipment.

The VDB radio frequencies used shall be selected from the radio frequencies in the band 108-117.975 MHz. The lowest assignable frequency shall be 108.025 MHz and the highest frequency assignable shall be 117.950 MHz. The separation between assignable frequencies (channel spacing) shall be 25 kHz.

The GBAS message types are summarized in the following table:

Message type identifier Message Name
0 Spare
1 GBAS Differential Corrections
2 GBAS Related Data
3 Reserved for ground–based ranging source
4 Final Approach Segment (FAS) data
5 Predicted ranging source availability
6 Reserved
7 Reserved for national applications
8 Reserved for test applications
9-255 Spare

GBAS Performances

The GBAS performances are defined with respect to the level of service that the system is designed to provide. The main source for GBAS performances comes from civil aviation navigation safety requirements and they are different for each civil aviation operation. [4]

Typical Operation Horizontal Accuracy (95%) Vertical Accuracy (95%) Integrity Time-To-Alert (TTA) Continuity Availability
Initial approach, Intermediate approach, Non-precision approach (NPA), Departure 220m (720ft) N/A 1–1×10-7/h 10s 1–1×10-4/h to 1–1×10-8/h 0.99 to 0.99999
Non Precision Approach with vertical guidance (NPV-I) 220m (720ft) 20m (66ft) 1–2×10-7 per approach 10s 1–8×10-6 in any 15s 0.99 to 0.99999
Non Precision Approach with vertical guidance (NPV-II) 16m (52ft) 8m (26ft) 1–2×10-7 per approach 6s 1–8×10-6 in any 15s 0.99 to 0.99999
Category I (CAT-I) Precision Approach 16m (52ft) 6.0m to 4.0m (20ft to 13ft) 1–2×10-7 per approach 6s 1–8×10-6 in any 15s 0.99 to 0.99999

Usually a GBAS system is designed to fulfil CAT-I Precision Approach.

As indicated in the table above, the performance requirements are expressed in terms of four quantitative concepts, many of them to be interpreted as probabilistic figures:

  • Accuracy: is expressed in terms of Navigation System Error (NSE) as the difference between the real position of the aircraft and the position provided by the airborne equipment.
  • Integrity: is defined by ICAO as a measure of the trust that can be placed in the correctness of the information supplied by the system.
  • Continuity: is the probability that the specified system performance will be maintained for the duration of a phase of operation, presuming that the system was available at the beginning of that phase of operation and was predicted to operate throughout the operation. Lack of continuity means that the operation must be aborted (with the associated risk).
  • Availability: is the probability that the navigation service is available at the beginning of the planned operation. A GBAS system is considered available when the accuracy, integrity and continuity requirements are met throughout the coverage region.

Notes


References

  1. ^ GNSS augmentation in Wikipedia
  2. ^ E.D. Kaplan, C.J. Hegarty, Understanding GPS Principles and Applications”, 2nd Ed., Artch House, ISBN 1-58053-894-0, 2006.
  3. ^ LAAS-How it works by FAA.
  4. ^ ICAO Standards and Recommended Practices, Annex 10, Volume 1 Radio Navigation Aids, July 2006