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Satellite Navigation - GBAS - 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-I precision approach services are enabled by a set of ICAO standards referred to internationally as GBAS Approach Service Type-C (GAST-C). The Federal Aviation Administration (FAA) contributed to the validation of ICAO SARPS for GAST-D GBAS, which will enable GBAS approaches to CAT III minima. These standards were effective in 2018, and will be the basis for any vendor wishing to pursue FAA System Design Approval for a GAST-D GBAS.

Refer to the following caption.
GBAS Architecture

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 antennas to estimate the distance the signal traveled. The GBAS Ground Facility then compares the measured/ estimated 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 use 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 broadcast also includes integrity parameters which permit the GBAS avionics to compute vertical and lateral error bounds on their calculated GPS position. This bound is commonly referred to as a protection level. The broadcast integrity values are set such that the likelihood of the actual error being larger than the computed protection level is less than 1 in 10 million. The avionics compares these computed vertical and lateral bounds to a corresponding set of alert levels. If either of the computed bounds is larger than the corresponding alert levels, the avionics determines that the aircraft positioning accuracy is not suitable for the operation. The alert limits are defined in ICAO standards and are based on the amount of allowable error for a given operation.

The GBAS Ground Facility broadcasts an updated correction message two times every second through the VDB. The VDB messages contain the corrections, integrity parameters, GBAS Ground Facility characteristics and approach path guidance for all installed approaches. The FAA-approved GAST C GBAS can provide 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 aircraft throughout the transition from en-route airspace to precision approach and landing. ICAO SARPS updates were made in 2018 to allow service providers to enable extended service volumes; this option has not yet been exercised in the U.S.

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