Ionospheric specification algorithms for precise GPS-based aircraft navigation

The Federal Aviation Administration (FAA) is implementing an aircraft navig ation scheme for the United States using the Global Positioning System (GPS ) L-1 signal (1575.42 MHz). To achieve position accuracies of a few meters, sufficient to allow precision airfield approaches, it will be necessary to broadcast corrections to the direct GPS signal. A significant component of these corrections is the delay in the GPS signal introduced by its propaga tion through the ionosphere. Ionospheric delay corrections will be derived using a ground network of at least 24 dual-frequency GPS receivers distribu ted across the continental United States. This network is part of the FAA's wide area augmentation system (WAAS) and will provide real-time total elec tron content (TEC) measurements. We present a technique for converting thes e TECs into gridded vertical delay corrections at the GPS L-1 frequency, wh ich will be broadcast to users every 5 min via geosynchronous satellite. Us ers will convert these delays to slant corrections for their own particular lines of sight to GPS satellites. To preserve user safety, estimates of th e error in the user delay corrections will also be broadcast. However, the error algorithm must not resort to excessive safety margins as this reduces the expected accuracy, and thus utility, of the navigation system. Here we describe an error algorithm and its dependence on various factors, such as user location with respect to the WAAS ground network and ionospheric cond itions.