Most air traffic control communications between pilots and controllers today are conducted via voice. Each air traffic controller uses a radio frequency different from the ones used by surrounding controllers to communicate with the aircraft under his or her jurisdiction. With the increased traffic, more and more controllers have been added to maintain safe separation between aircraft. While this has not diminished safety, there is a limit to the number of control sectors created in any given region to handle the traffic. The availability of radio frequencies for controller-pilot communications is one limiting factor. Some busy portions of the U.S., such as the Boston- Chicago-Washington triangle are reaching toward the limit. Frequencies are congested and new frequencies are not available, which limits traffic growth to those aircraft that can be safely handled.


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.


The first comprehensive proposal and design for the Mode S system was delivered to the FAA in 1975. However, due to design and manufacturing setbacks, few Mode S ground sensors and no commercial Mode S transponders were made available before 1980. Then, a tragic mid-air collision over California in 1986 prompted a dramatic change. The accident that claimed the lives of 67 passengers aboard the two planes and fifteen people on the ground was blamed on inadequate automatic conflict alert systems and surveillance equipment. A law enacted by Congress in 1987 required all air carrier airplanes operating within U.S. airspace with more than 30 passenger seats to be equipped with Traffic Alert and Collision Avoidance System (TCAS II) by December 1993. Airplanes with 10 to 30 seats were required to employ TCAS I by December 1995.

Due to the congressional mandate, TCAS II became a pervasive system for air traffic control centers around the world. Because TCAS II uses Mode S as the standard air-ground communication datalink, the widespread international use of TCAS II has helped Mode S become an integral part of air traffic control systems all over the world. The datalink capacity of Mode S has spawned the development of a number of different services that take advantage of the two-way link between air and ground. By relying on the Mode S datalink, these services can be inexpensively deployed to serve both the commercial transport aircraft and general aviation communities. Using Mode S makes not only TCAS II, but also other services available to the general aviation community that were previously accessible only to commercial aircraft. These Mode S-based technologies are described below.


The traffic alert and collision avoidance system (TCAS) is designed to provide a set of electronic eyes so the pilot can maintain awareness of the traffic situation in the vicinity of the aircraft. The TCAS system uses three separate systems to plot the positions of nearby aircraft. First, directional antennae that receive Mode S transponder signals are used to provide a bearing to neighboring aircraft ? accurate to a few degrees of bearing. Next, Mode C altitude broadcasts are used to plot the altitude of nearby aircraft. Finally, the timing of the Mode S interrogation/response protocol is measured to ascertain the distance of an aircraft from the TCAS aircraft.

TCAS I allows the pilot to see the relative position and velocity of other transponder-equipped aircraft within a 10 to 20-mile range. [Figure 1-14] More importantly, TCAS I provides a warning when an aircraft in the vicinity gets too close. TCAS I does not provide instructions on how to maneuver in order to avoid the aircraft, but does supply important data with which the pilot uses to evade intruding aircraft.

TCAS II provides pilots with airspace surveillance, intruder tracking, threat detection, and avoidance maneuver generations. TCAS II is able to determine whether each aircraft is climbing, descending, or flying straight and level, and commands an evasive maneuver to either climb or descend to avoid conflicting traffic. If both planes in conflict are equipped with TCAS II, then the evasive maneuvers are well coordinated via air-to-air transmissions over the Mode S datalink, and the commanded maneuvers do not cancel each other out.

TCAS and similar traffic avoidance systems provide safety independent of ATC and supplement and enhance ATC’s ability to prevent air-to-air collisions. Pilots currently use TCAS displays for collision avoidance and oceanic station keeping (maintaining miles-in-trail separation). Recent TCAS technology improvements enable aircraft to accommodate reduced vertical separation above FL 290 and the ability to track multiple targets at longer ranges. The Airborne Collision Avoidance System (ACAS) is an international ICAO standard that is the same as the latest TCAS II, which is sometimes called “Change 7” or “Version 7” in the United States. ACAS has been mandated, based on varying criteria, throughout much of the world.