What Is WAAS?
In 2007, the FAA completed and certified a significant upgrade to the GPS system. This new system, dubbed Wide Area Augmentation System (WAAS) uses a network of over 25 precision ground stations to provide corrections to the GPS navigation signal. The network of precisely surveyed ground reference stations is strategically positioned across the country including Alaska, Hawaii, Puerto Rico, Canada and Mexico to collect GPS satellite data. Using this collected error information, a message is developed to correct any signal errors. These correction messages are then broadcast through communication satellites to the airborne GPS receiver using the same frequency as GPS.
WAAS is designed to provide the accuracy, availability and integrity necessary to allow flight crews to rely on GPS for all phases of flight, from en route through GPS precision approach for all qualified airports within the WAAS coverage area. This provides a capability for the development of more standardized precision approaches, missed approaches and departure guidance for approximately 4,100 ends of runways and hundreds of heliport/helipads in the U.S. airspace.
WAAS will also provide the capability for increased accuracy in position reporting, allowing for more uniform and high-quality worldwide air traffic management. WAAS is a critical part of the FAA’s NextGen program.
Perhaps this summary will make it easier:
- Every IFR-certified and installed GPS unit allows the pilot to descend to LNAV (or Straight-in) and circling approaches.
- Baro-VNAV-equipped GPS systems can also descend to LNAV/VNAV minima.
- WAAS GPS receivers can descend to LNAV, LNAV/VNAV and LPV minima.
- Now, the real push for GPS LPV Precision Approaches has begun.
WAAS is here now and available for your aircraft.
A Condensed History
First, a few acronyms explained (see also Key Terms):
- WAAS – GPS Wide Area Augmentation System.
- LPV Approach – Localizer Performance with Vertical Guidance approach offering the lowest minimums of GPS systems (2008).
- LP Approach – Localizer Performance, no vertical guidance from GPS.
- LNAV / VNAV Approach – Designed for GPS/FMS equipment using altimeter data for vertical (2003).
GPS has been widely used for aircraft navigation since the 1980s. While GPS is dramatically more accurate than the VLF/Omega/Loran systems in use at that time, GPS was originally designed as a military system and was not FAA certified for sole means use. As the cold war was nearing the end, President Reagan released the system for civil use and most recently President Clinton ended the use of Selective Availability (SA).
With the much improved accuracy of GPS without SA and the demise of VLF/Omega, the FAA published Advisory Circulars that allowed GPS to be used for primary means navigation, domestic and oceanic. But a greater need for GPS was evident, that of GPS precision approaches. On its own, GPS accuracy and reliability was not adequate to supplement or replace ILS/Localizer approaches.
The FAA proposed two upgrades to the GPS system. The first was for WAAS, a ground station, satellite linked correction system with improved accuracies over wide geographic areas (SBAS). The second is LAAS, a local ground station broadcasting (GBAS) radio signals for a very limited area with better accuracy than WAAS. The FAA underestimated and Congress under funded both WAAS and LAAS programs so both programs struggled in development.
In time, the FAA refined their plan and Congress funded WAAS. After years of hard work, the FAA announced WAAS as operational in 2003. WAAS leased space on a couple of existing satellites to prove the concept was valid and to work out the bugs. Those existing satellites were not ideally located or equipped to make WAAS useable for precision approaches. (However, GPS was being used for overlay approaches.)
The most important news on WAAS came in September of 2007. FAA and prime contractor, Raytheon, announced the commissioning of two new geostationary satellites for WAAS. These satellites are ideally positioned so that operators in North America have redundancy. In addition, each of these satellites broadcast an additional signal that GPS receivers will use to improve their overall accuracy. As the United States Air Force replaces the earlier GPS satellites with new Block II/III versions that have higher power and added reliability, better positional accuracy will be implemented.
( Return to top )