March 1, 2001


Digital data link is the key to the successful future of the Air Traffic Control System

By Jim Sparks

March 2001

While driving home from the airport after a particularly long day I observed that at 10:00 at night, the travel time to my home was about half of what it took me to get to work that morning. I followed exactly the same route; however, at this late hour, there were very few other cars on the road. It was also evident that the computer-controlled traffic lights were in my favor.
Like many in the aircraft maintenance field, I have to travel frequently by commercial airlines. Just like highways on the ground, skyways tend to reach a point of saturation. When this occurs, air traffic control has to meter the flow in a fashion similar to computerized or synchronized traffic lights on the way to work.

Stretching the radio frequency spectrum
With aircraft manufacturers producing new aircraft at a rapid pace and new maintenance techniques keeping older aircraft flying longer, certain actions are needed to avoid gridlock. Additional aircraft promote development of new airports and expansion of those that already exist. Of course, airport growth puts additional demands on the radio frequency spectrum. It has been anticipated by the Federal Aviation Administration that in the US alone, the number of airport operations will increase almost 15 percent and airlines are already gearing up for a 100 percent increase in the number of passengers within the next 15 years.
Congestion is also present when it comes to Very High Frequency (VHF) radio communications. Flight crews may now converse with several air traffic controllers, Approach Control, Ground Control, and in numerous situations, Flight Operations departments. Each of these communications will generally require use of a different radio frequency. In certain geographic areas where many major airports are located within a short distance of each other, situations arise that can overload the radio waves.

FM Immunity
FM Immunity is another current issue. It has been determined that commercial FM broadcast stations may have the ability to interfere with the FM Navigation Receivers in the aircraft. This could possibly desensitize the deflection of course indicators and may cause actual aircraft displacement from a desired course. As of January 2001, most ICAO countries have restricted operation in their airspace by non-FM Immune aircraft. The modification to ensure compliance is accomplished by the radio manufacturer or an authorized shop facility. Unfortunately, receivers not certified as FM Immune do not provide flight crews with a warning of erroneous signals.
Compliance with this rule is recognized by a note on a flight plan that will identify the aircraft eligibility. It is this note that will enable the regional Air Traffic Control to determine placement of the aircraft in a suitable place within the network.

New methods for improvement
Current Air Traffic Control is not, as some believe, an organization subsidized by the railroads for the purpose of discouraging travel by air. New methods are being reviewed and imposed to enhance Air Traffic Management (ATM). These include improvements in Communications, Navigation and Surveillance (CNS). Some prime examples of improvements in navigation include Reduced Vertical Separation Minimums (RVSM), which will enable aircraft to fly at 1,000 feet versus 2,000-feet vertical distance to another aircraft. To qualify for this capability, operators must show compliance of tight tolerances with altitude indicating equipment as well as demonstrate the aircraft’s ability to fly at a specific altitude without significant deviation. In addition, there is the Reduced Navigation Performance (RNP) specification, which enables aircraft to operate with reduced horizontal distances and is predicated on the aircraft being equipped with redundant position sensing equipment. Compliance with RNP and RVSM are part of the foundation for the future Free Flight Concept, which involves the automation of many tasks now carried out by air traffic controllers. Also, some of the authority that controllers now possess would be shifted to pilots.

"Free" thinking
The Aeronautical Telecommunications Network (ATN) is a major player in the resolution of the current perceived dilemma and is working with ICAO to help in bringing about the "Free Flight" concept. Being a driver of a four-wheel drive vehicle, I am often tempted to follow unoccupied and sometimes undeveloped roads to avoid the delays caused by saturation of the public highways. What if an aircraft could takeoff and would be free to fly the most direct route to the destination — all without the delays associated with following the published and heavily traveled routes?
Also, suppose that all the while, the flight crew were free from the burden of having to tune communication radios and stay in constant contact with an Air Traffic Controller. Instead, any message of significance would be transmitted to the aircraft using a digital format and the crew would have readable displays strategically located in the flight deck. Areas of high density traffic would no longer require extensive assigned frequencies.

Tuning in
In many commercial/business aircraft, radio tuning can be accomplished by the Flight Management Systems (FMS) and they, like the Radio Transmitters and Radio Tuning Units (See sidebar "Dividing VHF"), will require in most cases, a software change to be 8.33 kHz- certified. Any aircraft that is non-compliant will have to either be re-routed or will have altitude restrictions imposed when flying in 8.33 kHz-approved airspace. It is anticipated that this will meet the need for the next five years, however most in the business realize ongoing increases in the number of channels is not the ultimate solution. Currently, there is no mandate in the United States to require the 8.33 kHz change. The answer will most probably be the introduction of Data Links otherwise known as Digital VHF Transmissions.

Going digital
Definition of compliance has already been realized for the flight deck of an ATN-equipped aircraft. The most noticeable difference will be the installation of Data Communication Display Units (DCDU). In addition, an "ATC Message" annunciator will occupy a prominent place. All radio communication systems including VHF, HF and even Satellite will be connected into a Data Router. It is this device that will determine the most effective downlink with the ground and will alleviate the crew’s having to make the selection of communication mode or frequency selection.
The obvious advantages here are not without cost. It is anticipated that outfitting one aircraft to this level could cost as much as US $500,000. To accommodate this new technology, several International agreements will have to be adopted. These will include the specifications for a universal digital data link, consistency in worldwide Air Traffic Control systems, and reduced political barriers.

Implementing Free Flight
Some modernization of the Air Traffic System is taking place in both North America and Europe, which will hopefully lead to realization of the Free Flight concept. In the US, the FAA has implemented a two-phase program. The first initiative that began in 1998, provides Air Traffic Controllers with the ability to manage user requests for specific airspace. This system employs a method to identify inappropriate convergence between aircraft and provide up to a 20-minute advanced warning, enabling controllers to create specific arrival sequence tactics. Incorporation of a Final Approach Spacing Tool (FAST) will help make the most of runway use. In fact, the FAA currently is evaluating an Automatic Dependent Surveillance-Broadcast (ADS-B). This is a proposed method that enables pilots to unmistakably locate other air traffic in their airspace and negotiate themselves in an orderly and safe fashion to the ultimate destination. A Surface Movement Advisor (SMA) is another concept that can supply airlines and other flight operations with aircraft present position information to allow better synchronization with ground support. This initiative is planned to be completed by the end of 2002.
Once the first phase of the plan is complete, "Free Flight - Phase 2" is scheduled to start. This phase will concentrate on expanding good methods and tactics and eliminating practices that are not effective.

Key to the future
It certainly appears that the digital data link is the key to the successful future of the Air Traffic Control system. Voice communications between air and ground will have to be drastically modified to avoid restricting future growth. The need for a worldwide seamless Air Traffic Management system is well realized. Despite the fact that there are few who oppose this concept, implementation has been a slow process.