Federal Aviation Administration GPS Augmentation Systems

Loni Czekalski

Office of the Associate Administrator for Research and Acquisition

Federal Aviation Administration


The FAA has the statutory mission to establish, operate and maintain navigation systems for the safe operation of aircraft in the United States. Navigation capabilities with appropriate accuracy, integrity, and reliability allow airspace users to safely travel from one location to another and operate in the different phases of flight: approach/departure, terminal, enroute and landing. At the present time the FAA has a ground based air navigation system. The agency is beginning to transition to a space based system because it will likely provide equal or better service and be more cost effective. This is important for the users, because the FAA is currently in the process of being reinvented into an agency supported by user fees and off the federal budget. The cost of operating and maintaining the ground based system is believed to be approximately $200M per year, and a space based system will work well and cost less. The purpose of FAA GPS augmentation systems is to provide the navigation capabilities required in a cost effective manner. When they become operational they will replace existing radio navigation aids to support instrument navigation in the enroute environment and many instrument approach procedures. The WAAS signal is not secured or controlled and is available to anyone with a suitable receiver. To date, a number of antennas and aircraft receivers are under development by various companies.


The WAAS system augments the DoD provided GPS Standard Positioning System (SPS) signal in space. The WAAS will provide sufficient accuracy for precision approaches, availability and continuity of service for sole means navigation requirements as well as improvements in GPS integrity. Through a ground station network linked by FAA communications systems the WAAS will provide navigation corrections to airborne users by means of geostationary satellites.

A contract was awarded in August 1995 to Wilcox Corp. with teammates Hughes and TRW to develop and furnish the WAAS. The system is currently under development and is intended to provide a signal by early 1998.

An initial WAAS system will begin operations in late 1998 and the end state full system is planned to become operational in 2001.

The WAAS acquisition strategy was to build an expandable system with a ground component that uses already developed software with new software for integrity, availability and accuracy in independent modules. The phased introduction of the system would start with a functional verification system for testing, and use the FAA's terrestrial communications network to link ground facilities. The space component of the system will use leased communications satellite service to allow for expansion of the coverage area, new technology and phased implementation of improvements to meet performance requirements. The standard broadcast format for the WAAS augmentation message will allow for global utilization.


As shown in figure 1 the WAAS is made up of eight functional parts. These functions are performed at reference stations and master stations to generate the WAAS message for broadcast to users. Each WAAS reference station (WRS) collects independent sets of data including geosynchronous satellite observables, GPS satellite observables and local troposphere observables and transmits the data to each master station. Independence of data sets is ensured by gathering observable parameters through independent sets of hardware. Data is collected at a rate consistent with the expected level of variation. For example, slowly changing troposphere data is collected less frequently than GPS satellite data. Prior to transmitting data to the master station each reference station verifies the reasonableness of the data. All data is time tagged. WAAS master stations (WMSs) provide correction processing, satellite orbit determination, verification/validation and generate the WAAS messages. WMSs collect all data received from WRSs and process it once per second. After the 250 bit WAAS message is generated it is transmitted via an FAA private data network using T-1 lines at 56KB/sec to ground earth

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