A Closer Look at WAAS
WAAS is an extremely accurate navigation system made up of the equipment and software that augments the Department of Defense (DoD) GPS Standard Positioning Service, Thompson said. The system, which is comprised of many ground stations as well as geostationary satellites, provides WAAS users with a Signal-In-Space (SIS) that delivers corrections to make GPS more accurate, as well as integrity data and alerts.
Before WAAS was developed, the U.S. National Airspace (NAS) couldn’t provide horizontal and vertical navigation for approach operators in all locations, according to the FAA. Now it can, and that includes vertically guided landing approaches under instrument f light rules at all qualified locations throughout the NAS during adverse meteorological conditions.
WAAS reached its initial operating capability in July 2003 and it now includes 38 reference stations, three master stations, and six uplink stations that support three L1/L5 signals on Geostationary Earth Orbit (GEO) satellites, Thompson said. WAAS reference stations are located throughout North America, including sites at northern latitudes of Alaska and Canada and southern latitudes of Mexico.
The broadcast SIS provides three functions that are critical to precise aviation navigation—service availability, position accuracy, and integrity monitoring, Thompson said. The ranging function improves service ability and continuity, its ability to provide differential GPS corrections improve positioning accuracy and its signal integrity monitoring capability enhances safety. The safety case for WAAS is incredibly complex, Thompson said, with a fault tree that culminates in the integrity processing that’s performed within the WAAS safety computer.
The G-III reference receiver is a vital part of this system, Thompson said, and NovAtel has been a key player in making WAAS what it is today.
“The reference receiver is a critical component installed at the Wide Area Reference Stations (WRS), providing range measurements and navigation information to the master stations for corrections and integrity processing,” Thompson said, noting that “Since its inception, the WAAS program has relied on reference receivers from NovAtel.”
Other satellite based augmentation systems (SBAS) providers around the world have chosen to use NovAtel’s receiver, too, Thompson said, further illustrating the reliability and robustness the solution offers. The European Geostationary Navigation Overlay Service (EGNOS), Japan’s MT-SAT Satellite Based Augmentation System (MSAS), and India’s GPS Aided Geo Augmented Navigation (GAGAN) systems all invested in NovAtel receivers for their reference stations. MSAS and GAGAN were developed in partnership with Raytheon, the prime WAAS developer.
Evolution of a WAAS Receiver
The requirements for the WAAS reference receiver have evolved over time, Thompson said, and the G-II (second generation WAAS receiver, that was also developed directly with FAA at NovAtel) needed an update to continue to meet those requirements. For example, the receiver must provide support to three new civil signals, L1C, L5, and L2C, as part of GPS modernization—something the G-II receiver simply wasn’t equipped to handle.
Of course, as the FAA and NovAtel have worked with the receivers over the years, they’ve learned a lot about what they can do and what improvements need to be made. Both continually monitor the receivers in the field and in the lab, and the experience gleaned from working with the existing units helped the NovAtel team develop an even more robust, reliable receiver with a host of enhanced capabilities.
“The G-III marks a major upgrade for the unit,” said Thompson, who managed the $11.6-million, third-generation WAAS reference receiver development contract. “In addition to overall performance improvements, the G-III added the ability to process the GPS L1C, L2C, and L5 signals. The G-III can process all of the current as well as planned GPS civil signals plus L2 semi-codeless. Processing power has increased to handle the additional signals as well as track additional satellites.”
But the improvements don’t end there. Other upgrades include temperature compensation, adaptive phase lock loop tracking, digital pulse blanking, an Ethernet interface and an improved front display panel, Thompson said. And, of course, the receiver still features the many benefits the second-generation receiver offered. Robustness measures developed with the G-II receiver, such as improved RF interference rejection and signal deformation measurements, are also part of the enhanced design.