WAAS User Segment

WAAS
Title	WAAS User Segment
Author(s)	GMV.
Level	Basic
Year of Publication	2011

The Wide Area Augmentation System (WAAS) is the United States Satellite Based Augmentation System. The programme, started in 1992, is being carried out by the Federal Aviation Agency (FAA) and is specially developed for the civil aviation community. The system, which was declared operational in late 2003, currently supports thousands of aircraft instrument approaches in more than one thousand airports in USA and Canada. WAAS service area includes CONUS, Alaska, Canada and Mexico. The WAAS programme is continuously in evolution; two development phases have been already covered, a third is in progress, and there are plans to improve the capability of the system in parallel with the evolution of the SBAS standards towards a dual-frequency augmentation service.

WAAS User Segment

The user segment is any aircraft with approved WAAS avionics, i.e. with a WAAS-enabled GPS receiver that used the WAAS corrections from the Space segment. The WAAS user equipment shall be compliant (certified) against several standards, i.e. RTCA MOPS DO 229 (see article SBAS Standards).

The WAAS User segment is not under the control of the WAAS service provider, FAA, as it is driven by the WAAS application market. In general, the WAAS 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, WAAS users include all aircraft with approved WAAS avionics using the WAAS for any approved phase of flight. The WAAS user equipment shall be compliant (certified) against several standards, i.e. RTCA MOPS DO 229 (see article SBAS Standards). 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, in the US: GARMIN, Honeywell, Rockwell Collins, General Avionics, etc. The Open Service (OS) targets low cost, general purpose GPS equipment that uses the WAAS Signal-In-Space (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 SoL user equipment, 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.

In the WAAS receiver, the two types of WAAS correction messages received (fast and slow) are used in different ways. The GPS receiver can immediately apply the fast type of correction data, which includes the corrected satellite position and clock data, and determines its current location using normal GPS calculations. Once an approximate position fix is obtained the receiver begins to use the slow corrections to improve its accuracy. Among the slow correction data is the ionospheric delay. As the GPS signal travels from the satellite to the receiver, it passes through the ionosphere. The receiver calculates the location where the signal pierced the ionosphere and, if it has received an ionospheric delay value for that location, corrects for the error the ionosphere created. While the slow data can be updated every minute if necessary, ephemeris errors and ionosphere errors do not change this frequently, so they are only updated every two minutes and are considered valid for up to six minutes.