GBAS Fundamentals

Fundamentals
Title	GBAS Fundamentals
Author(s)	GMV
Level	Basic
Year of Publication	2011

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).

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 Ground Based Augmentation System (GBAS) will augment 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.

Signals from GPS satellites are received by the GBAS GPS Reference Receivers (4 receivers for each GBAS) 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. Included in this message is suitable 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 20-30 mile radius). The signal coverage is designed 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 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.

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.