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.

GBAS Architecture

SBAS architecture

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:

• GBAS Ground Subsystem
• GBAS Aircraft Subsystem
• GNSS Satellites 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 GBAS Ground Subsystems consist 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 elemenst should be:

• 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 space segment is mainly the GNSS constellations.

User Segment

The SBAS user segment comprises all the user equipment that makes use of the SBAS Signal in Space (SIS). In fact, the SBAS User segment is not under the control of the SBAS service provider as it is driven by the SBAS application market.

In general, the SBAS service operator provides different services aiming at different market sectors, namely an Open Service, a Safety of Life service (SoL) and even a Commercial Service.

For the Safety-of-Life (SoL) service, the SBAS user equipment shall be compliant (certified) against several standards. For instance, civil aviation SBAS equipment shall demonstrate (see SBAS standards):

• Full compliance to RTCA SBAS MOPS DO-229 (airborne equipment).
• Full compliance to the RTCA SBAS MOPS 228 and 301 (antenna requirements).
• Compliance to RTCA TSO (C190, C145b, C146b) for SBAS equipment.
• Compatibility to other avionics equipment, in particular Flight Management Systems (FMS).

The SoL civil aviation certified equipment is in the highest rank with respect its cost. There exist a large number of certified receivers manufacturers worldwide both in the US (GARMIN, Honeywell, Rockwell Collins, General Avionics, etc) and in Europe (see complete list in EASA homepage).

The Open Service (OS) targets low cost, general purpose GNSS equipment that uses the SBAS SIS to provide the user with an enhanced accuracy performance in comparison with the one provided by a standalone GPS device. In comparison with the certification requirements of the user equipment above, user equipment is not necessarily compliant with the RTCA MOPS DO 229 processing rules, but might only make use of the processing algorithms that render the accuracy corrections provided by the SBAS SIS.

Finally, some SBAS service providers (see EGNOS) include the provision of the information computed by the SBAS ground element (input raw data, corrections, integrity information) by dissemination means different to the SBAS GEO link (generally, using terrestrial telecommunication networks). This constitutes the Commercial Service. This market sector comprises professional users (land and geodesy applications, maritime or terrestrial transport applications) that are not subject to the integrity and latency requirements needed in the SoL service.

SBAS Signal

Every SBAS provides ranging signals transmitted by GEO satellites, differential corrections on the wide area and additional parameters aimed to guarantee the integrity of the GNSS user:

• GEO Ranging: transmission of GPS-like L1 signals from GEO satellites to augment the number of navigation satellites available to the users.
• Wide Area Differential (WAD): differential corrections to the existing GPS/GLONASS/GEO navigation services computed in a wide area to improve navigation services performance. This includes corrections to the satellite orbits and clocks, as well as information to estimate the delay suffered from the signal when it passes through the ionosphere.
• GNSS/Ground Integrity Channel (GIC): integrity information to inform about the availability of GPS/GLONASS/GEO safe navigation service.

The SBAS delivers to the user the corrections and integrity data as well as some ancillary information (timing, degradation parameters, etc.) through messages encoded in the signal. The specification of the SBAS 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-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 SBAS satellite shall transmit a GPS-like L1 (1574.42 MHz) signal, modulated with a Coarse/Acquisition Pseudo-Random Noise (PRN) code. The SBAS L1 radiofrequency characteristics are:

Parameter	Description
Modulation	Bi-phase shift key (BPSK) modulated by a bit train comprising the PRN code and the SBAS data (modulo-2 sum).
Bandwidth	L1 ±30.69 MHz. At least 95% of the broadcast power will be contained within the L1 ±12 MHz band.
Ranging Codes	A PRN Code (Gold code) of 1 millisecond in length at a chipping rate of 1023 Kbps.
SBAS Data	500 symbols per second, module-2 modulated (250 effective bits per second)
Power	Minimum power –131 dBm at 5 degrees elevation Maximum power –119,5 dBm

A convolutional encoding of the bits is performed with the following parameters:

Code Parameter	Value
Coding Rate	1/2
Coding Scheme	Convolutional
Constraint Length	7
Generator Polynomials	G1 = 171(oct); G2 = 133(oct)
Encoding Sequence	G1 then G2
Flush	No

SBAS Performances

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.

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
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
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
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
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
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

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.
• 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. 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:
  • 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.

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).
• 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.