Navigation Systems

Navigation systems are the basis for pilots to get from one place to another and know where they are and what course to follow. Since the 1930s, aircraft have navigated by means of a set of ground-based NAVAIDs. Today, pilots have access to over 2,000 such NAVAIDs within the continental U.S., but the system has its limitations:

• Constrained to fly from one NAVAID to the next, aircraft route planners need to identify a beacon-based path that closely resembles the path the aircraft needs to take to get from origin to destination. Such a path will always be greater in distance than a great circle route between the two points.
• Because the NAVAIDs are ground-based, navigation across the ocean is problematic, as is navigation in some mountainous regions.
• NAVAIDs are also expensive to maintain.

Since the 1980s, aircraft systems have evolved towards the use of SATNAV. Based on the GPS satellite constellation, SATNAV may provide better position information than a ground-based navigation system. GPS is universal so there are no areas without satellite signals. Moreover, a space-based system allows “off airway” navigation so that the efficiencies in aircraft route determination can be exacted. SATNAV is revolutionizing navigation for airlines and other aircraft owners and operators. A drawback of the satellite system, though, is the integrity and availability of the signal, especially during electromagnetic and other events that distort the Earth’s atmosphere. In addition, the signal from space needs to be augmented, especially in traffic-dense terminal areas, to guarantee the necessary levels of accuracy and availability.

The CAASD is helping the navigation system of the U.S. to evolve toward a satellite-based system. The CAASD analysts are providing the modeling necessary to understand the effects of atmospheric phenomena on the GPS signal from space, while the CAASD is providing the architecture of the future navigation system and writing the requirements (and computer algorithms) to ensure the navigation system’s integrity. Moving toward a satellite-based navigation system allows aircraft to divorce themselves from the constraints of ground-based NAVAIDs and formulate and fly those routes that aircraft route planners deem most in line with their own cost objectives.

With the advent of SATNAV, there are a number of applications that can be piggybacked to increase capacity in the NAS. Enhanced navigation systems will be capable of “random navigation,” that is, capable of treating any latitude-longitude point as a radio navigation fix, and being able to fly toward it with the accuracy we see today, or better. New routes into and out of the terminal areas are being implemented that are navigable by on-board systems. Properly equipped aircraft are being segregated from other aircraft streams with the potential to increase volume at the nation’s busy airports by keeping the arrival and departure queues full and fully operating.

The CAASD is working with the FAA to define the nation’s future navigation system architecture. By itself, the GPS satellite constellation is inadequate to serve all the system’s needs. Augmentation of the GPS signal via WAAS and LAAS is a necessary part of that new architecture. The CAASD is developing the requirements based on the results of sophisticated models to ensure the system’s integrity, security, and availability.

Data Link

The CAASD is working with the FAA and the airlines to define and test a controller-pilot data link communication (CPDLC), which provides the capability to exchange information between air traffic controllers and flight crews through digital text instead of voice messages. With CPDLC, communications between the ground and the air would take less time, and would convey more information (and more complex information) than by voice alone. Communications would become more accurate as up-linked information would be collected, its accuracy established, and then displayed for the pilot in a consistent fashion.

By using digital data messages to replace conventional voice communications (except during landing and departure phases and in emergencies) CPDLC is forecast to increase airspace capacity and reduce delays. Today the average pilot/controller voice exchange takes around 20 seconds, compared to one or two seconds with CPDLC. In FAA simulations, air traffic controllers indicated that CPDLC could increase their productivity by 40 percent without increasing workload. Airline cost/benefit studies indicate average annual savings that are significant in the terminal and en route phases, due to CPDLC-related delay reductions.

CPDLC for routine ATC messages, initially offered in Miami Center, will be implemented via satellite at all oceanic sectors. Communications between aircraft and FAA oceanic facilities will be available through satellite data link, high frequency data link (HFDL), or other subnetworks, with voice via HF and satellite communications remaining as backup. Eventually, the service will be expanded to include clearances for altitude, speed, heading, and route, with pilot initiated downlink capability added later.

Airliners

Though commercial air carrier aircraft traditionally make up less than 5 percent of the civil aviation fleet, they account for about 30 percent of the instrument operations flown in civil aviation. Commercial air carriers are the most homogenous category of airspace users, although there are some differences between U.S. trunk carriers (major airlines) and regional airlines (commuters) in terms of demand for ATC services. Generally,

U.S. carriers operate large, high performance airplanes that cruise at altitudes above 18,000 feet. Conducted exclusively under IFR, airline flights follow established schedules and operate in and out of larger and better- equipped airports. In terminal areas, however, they share airspace and facilities with all types of traffic and must compete for airport access with other users. Airline pilots are highly proficient and thoroughly familiar with the rules and procedures under which they must operate.

Some airlines are looking toward the use of larger aircraft, with the potential to reduce airway and terminal congestion by transporting more people in fewer aircraft. This is especially valuable at major hub airports, where the number of operations exceeds capacity at certain times of day. On the other hand, the proliferation of larger aircraft also requires changes to terminals (e.g., double-decker jetways and better passenger throughput), rethinking of rescue and fire-fighting strategies, taxiway fillet changes, and perhaps stronger runways and taxiways.

Commuter airlines also follow established schedules and are flown by professional pilots. Commuters characteristically operate smaller and lower performance aircraft in airspace that must often be shared by general aviation (GA) aircraft, including visual flight rules (VFR) traffic. As commuter operations have grown in volume, they have created extra demands on the airport and ATC systems. At one end, they use hub airports along with other commercial carriers, which contributes to growing congestion at major air traffic hubs. IFR-equipped and operating under IFR like other air carriers, commuter aircraft cannot be used to full advantage unless the airport at the other end of the flight, typically a small community airport, also is capable of IFR operation. Thus, the growth of commuter air service has created pressure for additional instrument approach procedures and control facilities at smaller airports. A growing trend among the major airlines is the proliferation of regional jets (RJs). RJs are replacing turboprop aircraft and they are welcomed by some observers as saviors of high-quality jet aircraft service to small communities. RJs are likely to be a regular feature of the airline industry for a long time because passengers and airlines overwhelmingly prefer RJs to turboprop service. From the passengers’ perspective, they are far more comfortable; and from the airlines’ point of view, they are more profitable. Thus, within a few years, most regional air traffic in the continental U.S. will be by jet, with turboprops filling a smaller role.

FAA and industry studies have investigated the underlying operational and economic environments of RJs on the ATC system. They have revealed two distinct trends: (1) growing airspace and airport congestion is exacerbated by the rapid growth of RJ traffic, and (2) potential airport infrastructure limitations may constrain airline business. The FAA, the Center for Advanced Aviation System Development (CAASD), major airlines, and others are working to find mitigating strategies to address airline congestion. With nearly 2,000 RJs already in use—and double that expected over the next few years—the success of these efforts is critical if growth in the regional airline industry is to be sustained. [Figure 1-8]

Managing Safety and Capacity System Design

MANAGING SAFETY AND CAPACITY SYSTEM DESIGN

The CAASD is aiding in the evolution towards free flight with its work in developing new procedures necessary for changing traffic patterns and aircraft with enhanced capabilities, and also in identifying traffic flow constraints that can be eliminated. This work supports the FAA’s Operational Evolution Plan in the near-term. Rapid changes in technology in the area of navigation performance, including the change from ground-based area navigation systems, provide the foundation for aviation’s global evolution. This progress will be marked by combining all elements of communication, navigation, and surveillance (CNS) with air traffic management (ATM) into tomorrow’s CNS/ATM based systems. The future CNS/ATM operating environment will be based on navigation defined by geographic waypoints expressed in latitude and longitude since instrument procedures and flight routes will not require aircraft to overfly ground-based navigation aids defining specific points.

Application of Area Navigation

RNAV airways provide more direct routings than the current VOR-based airway system, giving pilots easier access through terminal areas, while avoiding the circuitous routings now common in many busy Class B areas. RNAV airways are a critical component to the transition from ground-based navigation systems to GPS navigation. RNAV routes help maintain the aircraft flow through busy terminals by segregating arrival or departure traffic away from possibly interfering traffic flows.
Further, RNAV provides the potential for increasing airspace capacity both en route and in the terminal area in several important ways.
Strategic use of RNAV airways nationwide is reducing the cost of flying and providing aircraft owners more benefits from their IFR-certified GPS receivers. Several scenarios have been identified where RNAV routes provide a substantial benefit to users.

• Controllers are assigning routes that do not require overflying ground-based NAVAIDs such as VORs.
• The lateral separation between aircraft tracks is being reduced.
• RNAV routes lower altitude minimums on existing Victor airways where ground-based NAVAID performance (minimum reception altitude) required higher minimums.
• RNAV routes may allow continued use of existing airways where the ground-based NAVAID has been decommissioned or where the signal is no longer suitable for en route navigation.
• The route structure can be modified quickly and easily to meet the changing requirements of the user community.
• Shorter, simpler routes can be designed to minimize environmental impact. Dozens of new RNAV routes have been designated, and new ones are being added continuously. In order to designate RNAV airways, the FAA developed criteria, en route procedures, procedures for airway flight checks, and created new charting specifications. Some of the considerations include:
  
• Navigation infrastructure (i.e., the ground-based and space-based navigation positioning systems) provides adequate coverage for the proposed route/procedure.
• Navigation coordinate data meets International Civil Aviation Organization (ICAO) accuracy and integrity requirements. This means that all of the coordinates published in the Aeronautical Information Publication (AIP) and used in the aircraft navigation databases must be referenced to WGS 84, and the user must have the necessary assurance that this data has not been corrupted or inadvertently modified.
• Airborne systems meet airworthiness performance for use on the RNAV routes and procedures.
• Flight crews have the necessary approval to operate on the RNAV routes and procedures.

In the future, as aircraft achieve higher levels of navigation accuracy and integrity, closely spaced parallel routes may be introduced, effectively multiplying the number of available routes between terminal areas. RNAV can be used in all phases of flight and, when implemented correctly, results in:

• Improved situational awareness for the pilot.
• Reduced workloads for both controller and pilot.
• Reduced environmental impact from improved route and procedure designs.
• Reduced fuel consumption from shorter, more direct routes.

RNAV Departure Routes

For example, take the situation at Philadelphia International Airport, located in the middle of some highly popular north-south traffic lanes carrying New York and Boston traffic to or from Washington, Atlanta, and Miami. Philadelphia’s position is right underneath these flows. Chokepoints resulted from traffic departing Philadelphia, needing to wait for a “hole” in the traffic above into which they could merge. The CAASD helped US Airways and Philadelphia airport officials establish a set of RNAV departure routes that do not interfere with the prevailing established traffic. Traffic heading north or south can join the established flows at a point further ahead when higher altitudes and speeds have been attained. Aircraft properly equipped to execute RNAV procedural routes can exit the terminal area faster — a powerful inducement for aircraft operators to upgrade their navigation equipment.
Another example of an RNAV departure is the PRYME TWO DEPARTURE from Washington Dulles International. Notice in Figure 1-10 the RNAV way- points not associated with VORs help free up the flow of IFR traffic out of the airport by not funneling them to one point through a common NAVAID.