Ground Based Augmentation System - How It Works
A Ground Based Augmentation System (GBAS) augments the existing Global Positioning System (GPS) used in U.S. airspace by providing corrections to aircraft in the vicinity of an airport in order to improve the accuracy of, and provide integrity for, these aircrafts' GPS navigational position. The goal of GBAS implementation is to provide an alternative to the Instrument Landing System (ILS) supporting the full range of approach and landing operations. Current non-federal (non-Fed) GBAS installations provide Category I (CAT-I) precision approach service. CAT-1 GBAS is referred to internationally as GBAS Approach Service Type-C (GAST-C) (CAT-I minima). The Federal Aviation Administration (FAA) work program is now focused on validating standards for a GBAS Approach Service Type-D (GAST-D) (CAT-III minima) service. The program currently projects a GAST-D GBAS system can be available in 2018.
A GBAS Ground Facility typically has three or more GPS antennas, a central processing system (i.e., a computer), and a VHF Data Broadcast (VDB) transmitter all locally situated on or near an airport. GBAS airborne equipment consists of a GPS antenna, a Very High Frequency (VHF) antenna, and associated processing equipment. On board the aircraft, GBAS avionics within the Multi-Mode Receiver (MMR) technology allows simultaneous implementation of GPS, GBAS and ILS using common antennas and hardware. The GBAS Ground Facility uses the VHF radio link to provide aircraft with GPS corrections, integrity, and approach path information.
The GBAS, with reference antennas in known surveyed positions, receives signals from GPS satellites. The reference receivers measure the time of transmission between the GPS satellite and the reference antenna and determines the distance the signal traveled. The GBAS Ground Facility then compares the measured distance with the actual distance based on the broadcast satellite position and the true GPS reference receiver position, and determines the error in the measurement. The average error measured by all operational reference receivers represents the correction term the GBAS avionics needs to apply to the satellite ranges measured by the GBAS avionics.
The GBAS Ground Facility also monitors general GPS satellite performance. The GBAS avionics only uses GPS satellites for which it receives valid ground corrections. When the GBAS Ground Facility determines there is a potential problem with a GPS satellite or when it cannot monitor a GPS satellite, it stops broadcasting corrections for that particular satellite, effectively preventing the GBAS avionics from using the satellite.
The GBAS Ground Facility also includes parameters which permit the GBAS avionics to determine the bound on the calculated GPS position that is only likely to be incorrect about one time per ten million calculations, thereby providing confidence the calculated position is correct.
This GBAS Ground Facility broadcasts an updated correction message two times every second through the VHF data broadcast (VDB). The VDB messages contain the corrections, integrity parameters, GBAS Ground Facility characteristics and approach path guidance for up to 48 approaches.
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 23 nautical mile radius). The GBAS service volume is designed to support the aircraft as it transitions from en route airspace, through the terminal airspace through the precision approach and landing.
The GBAS equipment in the aircraft uses the corrections to more accurately compute GPS position, velocity, and time to guide the aircraft safely to the runway. This signal provides ILS-look-alike guidance. The current system only has operational approval for use in weather minima to 200 feet above touchdown, with continued research and development to enable operational approval of the use of GBAS all the way to the runway surface.