AGRICULTURE
Modernizing a Safety Critical System
In civil aviation there’s no room for navigation error. Precise positioning information is vital for aircraft, whether during en route navigation, airport departures, or approaches and landings—and NovAtel has been helping the Federal Aviation Administration (FAA) provide this information through the GPS Wide Area Augmentation System (WAAS) since the 1990s.
The WAAS system began in 1992 after approval of a mission need for improved satellite navigation capability, said Gregory Thompson, FAA program management analyst. WAAS became an official program two years later, with the actual development process beginning after the first contract was awarded in the mid-1990s. NovAtel was involved from the beginning, internally designing and funding the first reference receiver for this safety critical system. The FAA purchased and fielded the NovAtel receivers in the late 1990s.
“NovAtel approached the FAA and its prime contractor with a receiver that integrated WAAS signal tracking, provided robust semicodeless processing, and multipath mitigation technology that met the FAA’s needs at the time,” Thompson said. “The FAA has developed a very good relationship with NovAtel over the years. We benefit from the overall support that NovAtel has been willing to provide to a technically demanding and admittedly niche market.”
The receiver is now in its third generation. The FAA determined the G-II receiver needed a technology update in 2009, Thompson said; so, NovAtel went to work developing the latest iteration. The G-III, which the FAA began fielding in 2015, provides the upgrades the agency envisioned, including additional processing for modernized GPS signals (see sidebar, “New Civil Signals,”) as well as the ability to accommodate future enhancements. These potential enhancements include handling positioning signals from Europe’s Galileo or another Global Navigation Satellite System (GNSS).
WAAS is designed to provide augmentation information to GPS receivers, enhancing the accuracy and reliability of position estimates. It removes the errors that typically come through GPS signals, and unlike traditional groundbased navigation aids, WAAS covers most of the National Airspace.
“When GPS was in its earlier days we thought we could use it to navigate and land aircraft, but then we realized that was only true to a certain extent,” said Jonathan Auld, NovAtel director, safety critical systems. “How do you know you can trust the satellites to work to their required level? That’s kind of where WAAS came from. We needed a second system to monitor the first system so we could verify the data received is healthy and that pilots are able to make good decisions in the cockpit.”
So, how does the system improve positioning accuracy? By providing correction data to the receiver that’s inside the aircraft, Auld said. This includes correction data for GPS satellite orbits, GPS satellite clock corrections and ionosphere delays. The WAAS network calculates this data then provides it to the aircraft receiver and pilot over a geostationary satellite. This takes the achievable accuracy down to a level of a few metres rather than 10 metres.
The data WAAS provides also helps the aircraft receiver and pilot determine the protection level, Auld said, and if they can trust the data coming from a specific GPS constellation. If the receiver in the aircraft thinks the constellation is accurate to a metre but it is actually in error by 10 metres, for example, that will lead to problems.
Through the WAAS system, pilots know if a constellation isn’t healthy, minimizing and even eliminating the risk of receiving Hazardous Misleading Information (HMI). In fact, users are notified within six seconds of any issuance of HMI that would cause an error in the GPS position estimate, according to the FAA.
“WAAS makes it possible for the flight crew to make the best decisions based on the best data available,” Auld said. “It’s better the system says don’t trust me, I can’t be depended on right now, than for it to mislead you. Then you can fall back to other systems and take the necessary actions to do something differently or follow a different procedure. You don’t want to depend on data you can’t trust.”
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.
Thus far, the G-III has met or exceeded the GII’s performance, with NovAtel’s latest version of the receiver certified to RTCA/DO-178B Software Considerations in Airborne Systems and Equipment Certification Level D. The increased level of rigor imposed by DO-178B, particularly in the requirements for development and testing phases, contributed to the improved end product.
The transition to the new receiver began last summer and has proven relatively seamless. As of June of this year, all 38 wide area reference stations have been converted. In that time, the FAA has experienced very few issues with the G-III, Thompson said, and when issues do arise, they work closely with NovAtel to make any necessary changes or adjustments.
“The FAA and NovAtel jointly work to correct any issues that might arise over time,” Thompson said. “Subsequent generations of the receiver have captured these improvements to increase reliability and performance.”
The Benefits of GNSS Modernization
GNSS modernization is important to WAAS for two primary reasons, Thompson said. First, the Department of Defense announced in a federal register notice that they would discontinue support for the GPS L2 P(Y) signal in the future. WAAS requires two frequencies (currently L1 and L2 P(Y)) to calculate signal delays caused by the ionosphere.
“The G-III incorporates the new GPS L5 signal into WAAS, allowing for the replacement of the L2 P(Y) signal,” Thompson said. “Quite simply, if FAA did not replace the L2 P(Y) signal, the existing single frequency service provided by WAAS would cease.”
Not only that, adding L5 to WAAS also provides more precise correction for ionospheric delay, Thompson added, which is currently the largest source of error within a GPS signal. WAAS uses models to calculate the estimated error at any given location within the WAAS service area. Because both L1 and L5 are within the protected spectrum allocated for aeronautical radionavigation service, future-dual frequency users will be able to use the L1 and L5 GPS signals to calculate ionospheric delay for their exact location, in real time, within the WAAS avionics.
“In addition, this will enable use outside of the existing service area,” Thompson explained. “For example, users in Hawaii do not currently have LPV service. Dual-frequency avionics will enable this.”
The FAA has developed a strong relationship with NovAtel over the years and knew they could trust them to deliver an exceptional receiver with the necessary performance and capabilities. NovAtel developed a quality product from the beginning and over the years has demonstrated that the company can work well with the FAA to make further enhancements.
The FAA wanted a receiver that was easy to integrate, required very few changes to the operational system, and ensured backwards capability aspects within WAAS, Thompson said. That’s what NovAtel gave them in the G-III.
“For over a decade, NovAtel’s G-II receiver has been the reference receiver used by the FAA and has performed very well,” Thompson said. “One of the key reasons to proceed with NovAtel was to ensure the G-III was a functional replacement for the G-II unit.”
NovAtel is a respected name in precision navigation and has worked alongside the FAA to ensure the WAAS receiver delivers. NovAtel has a reputation for high-quality products and outstanding customer support, and that means the FAA isn’t the only agency turning to them for satellite-based aviation positioning needs—something Thompson says is another benefit of working with NovAtel to develop the G-III.
“Another key benefit is the potential use of this product across the globe,” Thompson said. “The FAA has a vested interest to cooperate with other SBAS providers to ensure that our respective systems are interoperable. A common reference receiver makes this job easier.”