GLOBAL POSITIONING SYSTEM (GPS)

The use of the Global Positioning System (GPS) has changed aircraft navigation both in the en-route phase and approach (landing) phases of flight.

Aircraft have traditionally flown from one radio navigation aid ("navaids") to the next (e.g., from VOR to VOR). The paths between navaids are called airways. While this is rarely the shortest route between any two airports, the use of airways was necessary because it was the only way for aircraft to navigate with precision in instrument conditions. The use of GPS has changed this, by allowing "direct" routing, allowing aircraft to navigate from point to point without the need for ground-based navigation. This has the potential to save significant amounts of both time and fuel while en-route.

However, "direct-to" routing causes non-trivial difficulties for the air traffic control (ATC) system. ATC's basic purpose is to maintaining appropriate vertical and horizontal separation between aircraft. The use of direct routing makes maintaining separation harder. A good analogy would be vehicular traffic: Roads are comparable to airways. If there were no roads and drivers simply went directly to their destination, significant chaos would ensue (e.g., large parking lots without barriers or lines). ATC does give clearance for direct routing on occasion, but its use is limited. Projects like free flight propose to computerize ATC and allow greater use of direct routing by identifing potential conflicts and suggesting maneuvers to maintain separation. This is much like the existing Traffic Collision Avoidance System, but on a larger scale and would look further forward in time.

GPS has also significantly changed the approach phase of flight. When horizontal visibility and vertical cloud ceilings are below visual flight rules (VFR) minimums, aircraft must operate under instrument flight rules (IFR). Under IFR, aircraft must use navigational equipment for horizontal and vertical guidance. This is particularly important in the approach and landing phases of flight. The path and procedure used to land on a particular runway is called an instrument approach.

IFR approaches traditionally required the use of ground-based navaids such as VOR, NDB and ILS. GPS offers some significant advantages over traditional systems in that no ground-based equipment is required, reducing cost. This has allowed many smaller airports that cannot justify ILS equipment to now have instrument approaches. GPS receivers for aircraft are also less expensive, use a single small antenna, and require virtually no calibration.

The downside to GPS approaches is that they have higher minimum visibility and ceiling requirements. ILS typically require a cloud ceiling no lower than 200 feet above ground level and horizontal visibility greater than 1/4 mile, while GPS minimums are typically never less than 400 feet and 1 mile. This difference in minimums is because GPS approaches offer horizontal guidance only. Vertical guidance is possible, but GPS accuracy in the vertical is not as high as in the horizontal. To solve this problem, the FAA has implemented the Wide Area Augmentation System (WAAS). GPS receivers with WAAS capability have typical vertical accuracy of 2-3 meters. This is sufficient for ILS-type approaches, i.e., those with vertical navigation. GPS/WAAS receivers certified for vertical navigation GPS approaches are slowly coming to the market.

Although the FAA was initially slow to allow the use of GPS in IFR approaches, the number of published GPS approaches is climbing significantly. However, because ILS has lower minimum visibility and ceiling requirements, ILS remains the "best" type of approach, and the FAA has committed to maintaining ILS installations.