Global Navigation Satellite System (GNSS), based on GPS and additional satellite augmentations, could eventually replace most of these ground-based systems.

Civilian pilots have been utilizing GPS in uncontrolled airspace for applications such as crop dusting, aerial photography and surveying, search and rescue, and basic point-to-point navigation for some time.¹³ On June 9, 1993, the Federal Aviation Administration (FAA) approved GPS for supplemental use in the domestic, oceanic, terminal, and non-precision approach phases of flight in controlled airspace as well. This supplemental use required that another navigation source, such as a ground-based radio aid, must still be monitored while using GPS as the primary system. Once initial operating capability was declared for GPS by the DOD and DOT (Department of Transportation) on December 8, 1993, the monitoring of another navigation system for integrity purposes became unnecessary, provided that the GPS receiver utilized meets the FAA's TSO C-129 criteria for Receiver Autonomous Integrity Monitoring (RAIM).¹⁴ In addition, traditional navigation sources such as VORs and TACANs must still be operational, and their associated receiver equipment must be on board the aircraft as a backup. Several GPS receivers have already been certified under the FAA's TSO C-129 criteria.

The use of GPS as the primary means of navigation for the domestic en route through non-precision approach phases of flight will require better availability and continuity of service (reliability) than is currently available from the stand-alone system. Phase I of the FAA's Wide Area Augmentation System (WAAS), which is scheduled to be in place by 1997, will make this possible. Table 2-4 contains the quantitative performance requirements that the WAAS is being designed to meet.¹⁵

In the near future, the FAA hopes that GPS also will be used for Category I precision approaches. Precision approaches are required when the weather conditions at a given airport reduce the ceiling, or height of the base of a cloud layer, and the visibility, or the distance a pilot can see visually, to levels that are below non-precision approach criteria.¹⁶ Phase II of the FAA's WAAS implementation, scheduled for completion in 2001, will improve GPS-derived accuracy enough to allow the system to be used for these types of approaches. This increased accuracy requirement, which was also derived from the WAAS request for proposal (RFP), is included in Table 2-4.

Testing by the FAA and several contractors is currently underway to determine the feasibility of also using GPS to conduct Category II and III approaches and landings, and the results to date have been very promising. These approaches are flown when the weather conditions at an airport are even worse than those described previously for Category I.¹⁷ As can be expected, the accuracy, integrity, and continuity of service requirements are stricter than those for Category I landing systems, and therefore, the concepts currently under development utilize local-area differential GPS augmentations, rather than the WAAS. The requirements for Category II and III, which were derived from the Federal Radionavigation Plan and existing International Civil Aviation Organization (ICAO) requirements for instrument landing systems (ILS), are listed in Table 2-4.¹⁸

GPS also shows promise for use in Traffic Alert/Collision Avoidance Systems (TCAS) and Automatic Dependent Surveillance (ADS) systems. TCAS is already used by U.S. airlines and by many airlines in Europe.¹⁹ Testing of an updated TCAS, which broadcasts an aircraft's position and velocity derived from GPS on the existing Mode-S datalink, has proven to be more accurate than the existing system.²⁰ The requirements for this application are listed in Table 2-4.

ADS systems, which are still under study and development, would automatically broadcast an aircraft's GPS-derived position to the air traffic management (ATM) system via geostationary communications satellites in oceanic airspaces, and via terrestrial-based communications links in domestic airspace.²¹ This would allow for more efficient ocean crossings than are currently possible using the existing ATM reporting system. ADS would also be useful in the domestic en route and terminal phases of flight, where current aircraft separation is primarily the responsibility of air traffic controllers who utilize secondary surveillance radars. ADS systems are also being considered for monitoring the land-based operations of an airport, such as aircraft taxiing, and service-vehicle collision avoidance. The requirements listed for ADS in Table 2-4, which are based on current radar-based surveillance requirements, should be considered preliminary because the FAA is in the early phases of studying how to use GPS in performing the surveillance function.