Effectiveness of the ground-based transceiver (GBT) parrot system for monitoring GPS integrity for Alaska ATC "radar-like services" using ADS-B

In January of 2001, the Federal Aviation Administration (FAA) commissioned the first use of automatic dependent surveillance broadcast (ADS-B) based on the global positioning system (GPS) to support air traffic control (ATC) "radar-like services (RLS)." These services are available via ATC to nearly 200 equipped aircraft operating in the vicinity of Bethel, Alaska as part of the FAA\´s Capstone program to improve aviation safety in Alaska. RLS allow ATC to support instrument flight rules (IFR) separation of 5 nautical miles (nmi) based only on ADS-B surveillance. Since ADS-B information is being used by ATC for aircraft separation, some method to verify the integrity of the GPS/ADS-B data must be continually provided. The ADS-B concept, as defined in RTCA standards, relies on aircraft to "self-report" the integrity of the navigation data reported in the ADS-B message. System evaluations leading to commissioning of RLS showed the ADS-B data to be easily equivalent to radar surveillance in terms of accuracy, latency, and update rate. However, it highlighted some difficulties with the self-reported integrity concept for this "first generation" of ADS-B avionics, due mainly to excessive receiver autonomous integrity monitoring (RAIM) holes - from the ATC perspective. The resulting excessive number of false integrity alerts led to an FAA decision to disregard ADS-B self-reported integrity in the Bethel area and, instead, to monitor GPS integrity through a network of new ground position "parrot" monitors implemented along with the existing ADS-B receiving stations. The objective of this paper is to determine how effective this monitor system is at detecting integrity failures of the GPS satellites that could cause hazardously misleading information (HMI) over the Bethel, Alaska area. Although our results show that the existing parrot system is generally effective for conducting RLS in the Bethel area, we offer recommendations to the FAA to improve the performance of integrity monitoring in the area.