August 04, 2007

Future Air Navigation System

Aircraft are operated using two major methods; Positive Control and Procedural Control. Positive Control is used in areas which have radar. The controller "sees" the airplanes in the control area and uses VHF voice to provide instructions to the flight crews to ensure separation. Because the position of the aircraft is updated frequently and VHF voice contact timely, separation standards (the distance one aircraft must be separated by another) is less. This is because the air traffic controller can recognize problems and issue corrective directions to multiple airplanes in a timely fashion. Separation standards are what determines the number of airplanes which can occupy a certain volume of airspace.

Procedural Control is used in areas (such as oceanic and landmasses) which do not have radar. The FANS concept was developed to improve the safety and efficiency of airplanes operating under Procedural Control. This methods uses time-based procedures to keep aircraft separated. The separation standard is determined by the accuracy of the reported positions, frequency of position reports, and timeliness of communication with respect to intervention.

Non-FANS procedural separation uses Intertial Navigation Systems for position, flight crew voice reports of position (and time of next waypoint), and High Frequency radio for communication. The INS systems have error introduced by drifting after initial alignment. This error can approach 10 nmi. HF radio communication involves contacting an HF operator who then transcribes the message and sends it to the appropriate ATC Service Provider. Responses from the ATC Service Provider go to the HF radio operator who contacts the airplane. The voice quality of the connection is often poor leading to repeated messages. The HF radio operator can also get saturated with request for communication. This leads to procedures which keeps airplanes separated by as much as 100 nmi laterally, 10 minutes in trail, and 4000ft altitude.

These procedures reduce the number of airplanes which can operate in a given airspace. If marketing demand pushes airlines to operate at the same time on a given route, this can lead to airspace congestion which is handles the issue by delaying departures or separating the airplanes by altitude. The latter can lead to very inefficient operation.

Future Air Navigation System (FANS) is a standard developed by the air transport industry to allow more aircraft to fit into a given volume of air space. A number of generations of FANS standards are covered by the single term, progressing from simple automations of current processes up to rather futuristic approaches.

Communication Improvements

This involved a transition from voice communications to digital communications. Specifically ACARS was used as the communication medium. This allowed other application improvements. An application was hosted on the airplane known as Controller Pilot Data Link Communication (CPDLC).

This allows the flight crew to select from a menu of standard ATC communications, send the message, and receive a response. A peer application exists on the ground for the Air Traffic Controller. They can select from a set of messages and send communications to the airplane. The flight crew will respond with a WILCO, STANDBY, or REJECT. The current standard for message delivery is under 60 seconds one way.

Navigation Improvements

This involves a transition from Inertial Navigation to Satellite Navigation using the GPS satellites. This also introduced the concept of Actual Navigation Performance (ANP). Previously, flight crews would be notified of the system being used to calculate the position (radios, or intertial systems alone).

Because of the deterministic nature of the GPS satellites (constellation geometry), the navigation systems can calculate the worst case error based on the number of satellites tuned and the geometry of those satellites. (Note: it can also characterize the potential errors in other navigation modes as well). So, the improvement not only provides the airplane with a much more accurate position, it also provides an alert to the flight crew should the actual navigation performance exceed the required navigation performance.

Source: Wikipedia

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