Airplanes Can Be Networked


Ralph Yost, Sept. 2002  

The world’s air traffic control systems depend on the ability to clearly and uniquely identify aircraft. They use radar (surveillance) systems as the means to track the position of aircraft and also to uniquely identify them. Surveillance radar was instituted as part of the National Airspace System (NAS) for the purpose of providing a means of positive identification of an aircraft and its altitude in the air traffic control system. It requires extensive ground installations as well as a complementary receiver/transmitter in the aircraft.

This paper proposes an alternate method to track aircraft with a positive electronic confirmation that can be utilized by the air traffic control system. The approach is new and modern and has never before been applied to aircraft.


In July of 1999, NASA Langley Research Center held a planning conference for the Small Aircraft Transportation System (SATS). (SATS is a new transportation concept being proposed by NASA in which small aircraft (nominally 6 seats) would be used to fly to/from the 5400 small airports not currently being utilized for reliable transportation by the public.)

Government and industry participants divided into groups of personal expertise and interest. One group was the Communication, Navigation and Surveillance (CNS) Group. During this session, it was envisioned that in order for the SATS concept to be implemented, a significant broadband radio frequency (RF) connection would be required to the aircraft. This RF connection would need to be maintained over all phases of flight. It would carry all communications, navigation and surveillance data over a single, wide bandwidth channel, (The current approach is that all communications, navigation and surveillance functions are provided separately over individual radios and frequencies.) The SATS CNS vision was to integrate these functions into a single RF system. During this discussion, an analogy was made between aircraft and remote computer connections to networks, in which broadband technologies were prominently growing in number. With so much data envisioned being sent to the SATS aircraft, and a broadband solution seeming to be necessary, why couldn’t the aircraft be considered in a similar fashion to any other network device? Out of the discussion came the term “NAS Net” which was short for National Airspace System Network. When the plenary session of the SATS Planning Conference was briefed on the results of the CNS Group, the concept was described with an easier to understand term:  “airborne internet”.

A key to implementing SATS is a robust and extremely reliable automated communications system. The system must be capable of passing large amounts of data between aircraft and ground systems as well as between neighboring aircraft in a reliable manner. NASA and the FAA have been pursuing a key enabling technology area: Airborne Internet.

The NAS:

In the United States, the National Airspace System (NAS) is comprised of the air traffic control (ATC) system and all the elements that it utilizes to ensure safety of flight for nearly 6,500 aircraft simultaneously. The NAS operates continuously and reliably. Portions of it have had outages, but never the entire system. That’s because the NAS is a “system of systems” in a similar way that the Internet is a “network of networks.”

The NAS includes more than 18,300 airports, 21 air route traffic control centers (ARTCC), 197 terminal radar approach control (TRACON) facilities, over 460 airport traffic control towers (ATCT) and 75 flight service stations, and approximately 4,500 air navigation facilities. Several thousand pieces of maintainable equipment including radar, communications switches, ground-based navigation aids, computer displays, and radios are used in NAS operations. NAS components represent billions of dollars in investments by the Government. Additionally, the aviation industry has invested significantly in ground facilities and avionics systems designed to use the NAS.

The NAS relies on the FAA's 48,000 employees to provide air traffic control, flight service, security, field maintenance, certification, system acquisition, and other essential services. On the user side, there are more than 616,000 active pilots operating over 280,000 commercial, regional, general aviation, and military aircraft.

Keeping Track

The NAS consists of complex computer systems and data bases that contain aircraft flight data prior to and during flight. Aircraft flight is tracked by the ATC system after it assigns a unique 4-digit code to each aircraft that will be using ATC services (such as traffic separation).

The NAS depends on its ground radar systems, called “surveillance” radars, to track aircraft. The ground system transmits on one frequency (1030Mhz) from a rotating antenna. The antenna rotates about once every 5 seconds for short-range (terminal) systems.

Aircraft are equipped with a combination receiver/transmitter (transponder) that receives on 1030Mhz but replies on 1090Mhz. The 4-digit code (assigned by ATC) is manually entered into the transponder by the pilot, similar to the tuning of a radio station in a car. When the aircraft’s transponder receives an “interrogation” from the ground radar, it “responds”. The transponder is usually connected to the aircraft’s altimeter, and the replies include both the altitude and the 4-digit transponder code assigned to the aircraft by ATC. (Actually, the replies alternate with altitude and 4-digit transponder code). This data is “enhanced” an presented at the ATC controller’s display as aircraft position, ID, altitude and speed.

When the ground radar receives the aircraft’s reply, it processes the information and mathematically determines the distance (range) and angle (azimuth) from the radar. That information is tagged to the transponder code and is passed along to the ATC computers that drive the displays used by the air traffic controllers. 

Out of Range

Now examine what happens when an aircraft is beyond range of the surveillance radar system. This could occur by being below the horizon (elevation) of coverage of the radar or by being out of the range distance that the radar can cover. In both situations, the aircraft is referred to as “out of radar coverage.”

 Basically,  the ATC system cannot track and maintain ATC services on an aircraft that is out of radar coverage. How could it be expected to? There are significant geographic regions in the continental United States in which there is no radar coverage at all. In the less populated areas of the mid-west and northwest, there are many miles of airspace without radar coverage.

 When an aircraft is flown in an area not covered by ATC surveillance radar, the pilot is required to set the transponder to the 4-digit transponder code of 1200. The number 1200 signifies a visual flight rules (VFR) operating aircraft that is not under air traffic control. Should one of these aircraft come within radar coverage range, the ATC display would show them as “1200” and the controller would know that he does not have the same responsibility for that aircraft as he does for the others. Similarly, when an aircraft is under ATC responsibility, if he elects to terminate ATC services, he will be asked to change his transponder “squawk” to 1200. This could occur in instrument flight rules (IFR) operations in which an aircraft obtains visual sighting of his destination airport and the weather is clear enough for the pilot to land visually.   NEXT    

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Copyright©2002 Ralph Yost, All Rights Reserved.