1. GROUND BASED AUGMENTATION SYSTEM System Overview Christophe DEHAYNAIN Direction Générale de l’Aviation Civile FRANCE

2. GROUND BASED AUGMENTATION SYSTEM Presentation Overview
Why do we need an augmentation to GNSS ?
Local Area Differential GNSS Principle
GBAS Ground Segment
GBAS Airborne Segment
Conclusions

3. GBAS Concept System Overview
ICAO Name : GBAS for Ground Based Augmentation System
Operational Coverage :
minimum : same as ILS
recommended : Omni directional (radius 23 NM) up to FL 100 and down to 12 ft
Data-Link Frequency range : MHz (25 kHz spacing)
Operational objective :
minimum : Category 1 approach
optional : 2 D navigation (Positioning Service)
Localisation : Airport

4. GBAS Overview Do not mistake GBAS for :
DGPS, LADGPS : initial US generic names for GBAS concept
SCAT1 or Special Category 1 : US RTCA standard defining a non-interoperable system for private use
LAAS : Local Area Augmentation System : present US name for GBAS

5. WHY GNSS NEEDS AN AUGMENTATION ?
GPS Only
Civil Aviation
PERFORMANCE
CATEGORY I
Requirements
H. 13 m
V. 23 m
ACCURACY (95%)
H 16.0 m
V 4.0 m
99% (RAIM)
AVAILABILITY
99.75 %
3,5.10-7/ approach
Time to alarm 6 s ?
INTEGRITY ?
CONTINUITY OF SERVICE
10-4 / approach
(10-5 / 15 s)

6. LOCAL AREA DIFFERENTIAL PRINCIPLE I - Basic Principle
Measurements made by two receivers are affected by the same errors as long as these two receivers are not too far from each other
RX2
RX1

7. LOCAL AREA DIFFERENTIAL PRINCIPLE II - Differential Correction Calculation
Broadcast SV Position
Actual SV Position
Measured Pseudoranges
Calculated Range
REF
USER
Corrections
Calculation
Differential Message Broadcast
Known Reference Location
The first receiver in a reference station can calculate these errors knowing its exact location (corrections calculated by the GBAS ground station)
The second receiver (the user) will use these corrections to correct its own measurements and increase the accuracy of these measurements
Calcul simplifié pour la démonstration
Modèles atmosphériques non appliqués

8. GBAS ground segment I - Basic architecture of a reference station
GPS/GLONASS Antennas
Differential Message Broadcast Antenna
Receiving Unit
(up to 4 Reference Receivers)
Data Broadcast Unit
Data processing for broadcast
Raw Data
PR, ephem.,time
Unités rassemblées dans le même ensemble ou éclatées sur le terrain
Data Processing Unit
Differential Corrections Calculation
Integrity Monitoring Functions
GBAS Messages Elaboration

11. GBAS ground segment II - VHF Data Broadcast (VDB)
GBAS VDB characteristics :
VHF NAV band ( MHz), channel spacing 25 kHz
D8PSK (Differential 8 States Phase Shift Keying) modulation
2 Hz update rate (Pseudorange Corrections)
8 slots Time Division Multiple Access technique (16 per second)
Horizontal Polarisation or Elliptical Polarisation (Recommendation)
50W ground transmitter power (Typically power for HPOL)
Omni-directional antenna
Coverage: 23 NM radius

12. GBAS ground segment III - VDB message content
The current SARPS require the transmission of three message types :
Differential Code Corrections and integrity Data
Reference Point and GBAS Data
Final Approach Path description

13. GBAS ground segment IV - GBAS Ground System Classification
The global performances of Ground System are linked to :
the Number of installed GNSS receivers (2 to 4)
The quality of the GNSS signal reception
A Ground Accuracy Designator (GAD) will qualified the ground segment

15. GBAS User segment I-Basic operations
GBAS will be basically used as an ILS (ILS Look-alike concept)
RNAV 2D operations may be developed if the Ground System support the Positioning Service

16. GBAS User segment II-Airborne Equipment
Multi-mode Receivers (MMR) and their associated GPS and VHF antennas will be used for GBAS approaches
No hardware update is foreseen (Software only)

17. Conclusions
The GBAS system as standardised in ICAO Annex 10 is able to serve Cat-I operations. Advanced operations such as Cat-II/III or A-SMGCS are under consideration by ICAO GNSS Panel
GBAS ground stations
are being built,
are considered in several implementation programmes in the World
GBAS airborne equipment
are being developed within Multi-Mode Receiver
are considered in several aircraft manufacturer programmes
Standards Status
ICAO Standards (SARPS) are available
Other Standards (Doc 4444, Doc 8071, PANS OPS) are being finalised
Industry Standards (MOPS) are being developed and will be soon available