The FAA’s William J. Hughes Technical Center is conducting research that could lead to improved GPS integrity allowing further development of low-cost vertically guided instrument approaches.
The current integrity system for lateral guidance only is receiver autonomous integrity monitoring (RAIM) and it doesn’t depend on ground stations or other infrastructure. “RAIM allows aviation receivers to detect a GPS satellite fault and in many cases isolate the offending satellite and remove it from usage by the receiver,” according to the FAA. For vertical guidance integrity, the FAA’s wide-area augmentation system (WAAS) uses ground stations to ensure accuracy of the GPS signals.
Advanced RAIM (ARAIM) simplifies the vertical integrity aspect by eliminating the need for WAAS ground stations, instead using another civilian global navigation satellite system (GNSS) such as Europe’s Galileo to help improve accuracy. ARAIM does this by using the second GNSS to allow the GPS receiver to directly measure ionospheric error, according to the FAA. “The ionosphere is in most cases the largest source of error in a GNSS signal, and the ionosphere can also reduce the integrity of GNSS signals.”
The goal of ARAIM is to allow LPV-200 approaches (200-foot minimums) anywhere in the world, the FAA explained. ARAIM thus would become an aircraft-based augmentation system, vastly increasing the utility of GNSS-based approaches.