GPS Q&A
By Larry Holtgreive

Q What is the main purpose of WAAS? What are the components of the system and how will it operate? What are the benefits? What is the time frame for implementing the complete system? What impacts, if any, will this system have on terrestrial GPS data collection?

A The stimulus for the development of Wide Area Augmentation System (WAAS) is that existing Department of Defense's (DOD) basic GPS service fails to meet the accuracy, availability, and integrity requirements critical to safety of flight.

   The Federal Aviation Administration (FAA) is planning a transition from a ground-based civil air navigation system to a satellite-based system using signals generated by the DOD Global Positioning System (GPS). GPS was designed and implemented for military purposes and does not satisfy civil air navigation requirements, including requiring that the system be available virtually all of the time. In order to meet these requirements, the FAA is developing the WAAS as a safety-critical navigation system that will provide a quality of positioning information never before available to the aviation community. WAAS will augment the satellite data that FAA receives through the DOD's GPS. It will correct the GPS signals received on the civilian portion of GPS to ensure accuracy and data integrity for civil aviation navigation. Satellite-based navigation, using GPS/WAAS, is expected to improve the safety of flight operations, allow the fuel-efficient routing of aircraft, increase airport and airspace capacity to meet future air traffic demands, and enable the FAA to phase out its costly network of ground-based navigation aids.
    The WAAS will allow GPS to be used as a primary means of navigation for en-route travel and non-precision approaches in the US, as well as for Category I approaches to selected airports throughout the nation. The system's intention is to provide the capability for the development of more standardized precision approaches, leading to fewer missed approaches, and departure guidance for approximately 4,100 ends of runways and hundreds of heliport/helipads in the National Airspace System (NAS). GPS/WAAS is expected to provide precise instrument approach navigation to all airports. Of the more than 18,000 landing facilities in the US, only approximately 635 are currently equipped with Instrument Landing System (ILS) which support aircraft precision approaches. Most general aviation airports can't afford the $1.5 million investment required to install an ILS. WAAS will allow most airports to obtain this precision approach capability at a much smaller cost. The satellite-based system will eventually replace thousands of land-based navigation aids, providing equipment savings to aircraft owners and FAA alike. WAAS will also provide the capability for increased accuracy in position reporting, allowing for more uniform and high-quality worldwide Air Traffic Management (ATM).
    WAAS comprises a network of approximately 35 ground reference stations, covering the entire US and some outlying areas such as Canada and Mexico, and geo-stationary (GEO) communication satellites. A network of precisely surveyed ground reference stations (WRS) are strategically positioned across the country including Alaska, Hawaii, and Puerto Rico to collect GPS and GEO satellite data. These reference stations are linked to form the US WAAS network. Each WRS in the network relays the data to the wide area master station (WMS) where correction information is computed. Each WMS will process data from the reference stations to determine and verify corrections for each GPS satellite. A correction message is prepared (and validated) and up-linked to a GEO via a ground up-link system (GUS). The message is then broadcast on the same frequency as GPS to receivers on board aircraft that are flying within the broadcast coverage area of the WAAS. The GEO communications satellites will transmit wide-area accuracy corrections and integrity messages to aircraft, as well as acting as additional navigation satellites for the aircraft, thus providing additional navigation signals for position determination.
    WAAS will improve basic GPS accuracy to approximately 7-meters vertically and horizontally, improve system availability through the use of geo-stationary communication, and provide important integrity information about the entire GPS constellation. Navigation benefits include providing an alternative satellite-based system to maintain required levels of safe operations in the National Airspace System. WAAS will allow the replacement of most Category 1 ILS receivers with a single WAAS receiver (en route through Category 1 precision approach.), resulting in improved safety when operating in reduced weather conditions due to the precision vertical guidance on approach. Additional benefits include a 3D position guidance for all phases of flight; an inexpensive Instrument Flight Rules (IFR) area navigation system with global coverage. This impact alone will lead to greater runway availability, reduced separation, more direct en-route paths, and reduced disruptions (delays, diversions, or cancellations). There are also significant benefits to be realized by the FAA due to the elimination of maintenance and replacement costs associated with some older, expensive ground-based navigation aids.
    There are several timelines involved in the system integration; three builds are scheduled to monitor system performance and integration. The first build was completed 9 April 1999, two weeks ahead of schedule. There are four software packages required for this operation, three of which entered formal qualification test (FQT) on or ahead of schedule. In February 1999, the FAA delayed by more than a year the deployment of WAAS development, their desire to further refine the system's fourth and final software module. The FAA originally wanted to roll out WAAS in July 1999, but it has pushed the deployment back 14 months to September 2000. The final module, the Correction and Verification System, will monitor, fix, and check functions. This module is an integrity check from the ground, validating all components of the system, requiring the interaction of more than 20 software algorithms for functions such as alarm alerts and location checks for geo-stationary satellites.
    The FAA has completed the hardware installations for Phase 1, including: 25 ground reference stations ranging from Cold Bay, Arkansas, to San Juan, Puerto Rico; two master control stations; two geo-synchronous satellite up-link stations; and two transponders on leased satellites. After the roll-out of the four modules during Phase 1, WAAS will give pilots en route navigation and vertical guidance for precision approaches to runways in a limited portion of the country. During subsequent phases, FAA will deploy WAAS nationwide. The agency's WAAS team also is working with the FAA's counterparts in Europe and Japan to build a global satellite system for improving navigational data worldwide.
    The impact of WAAS on GIS collection is undefined at this time, however, preliminary testing has achieved near 1m results (in some cases sub-meter) using both single and dual frequency receivers integrating WAAS corrections. These initial tests imply that WAAS integration may provide considerably higher accuracy than originally planned, and are equivalent to many receivers on the market today that integrate single frequency receivers with DGPS beacons. The significant advantage of WAAS to current DGPS technology is the elimination of range limitations. Many free correction signals experience spatial de-correlation, accuracy decreasing as the distance from the signal source increases. The GEO communication satellites reduce this degradation by supplying corrections from multiple sources, whereas most DGPS corrections utilize a single broadcast source. Other advantages of WAAS technology to DGPS are the absence of additional hardware, and in many cases, correction signal subscription fees. Upon completion of WAAS, benefits will be realized beyond aviation to all modes of transportation, including maritime, highways, and railroads.