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A Slow Adoption

Lincoln Labs finished a comprehensive proposal and design for the Mode S system and delivered it to the FAA in 1975. In addition, Lincoln Laboratory assisted the FAA in developing and testing three commercial prototype Mode S sensors. These prototypes clearly demonstrated that the finished design could be used to manufacture commercial sensors. However, despite Lincoln Labs' attention to interoperability and efforts to minimize transition costs, Mode S was not promptly deployed.

Deployment of the Mode S transponders ultimately depended on the manufacturing companies. After all, as Drouilhet said, ``The lab exists to be a capable technical resource to help figure out how to solve the problems...then industry builds it.'' Companies contracted to produce the Mode S sensors altered Lincoln Labs' design so that they could be more easily produced using existing production facilities. The resulting modifications caused problems, creating delays in producing the sensors. For instance, a contract with Texas Instruments (TI) to produce Mode S sensors fell apart when TI decided to implement the system software using smaller, less powerful TI machines instead of a mainframe. This change required a software rewrite, and subsequent problems were so severe that the contract was terminated. Similar problems occurred with other contractors such as Westinghouse and UNISYS. Because of these setbacks, few Mode S ground sensors and no commercial Mode S transponders were made available before 1980.

An Accident Spawns Change

A tragic mid-air collision over Cerritos, CA on August 31, 1986 prompted a dramatic change. During the initial approach to Los Angeles International Airport, an Aeromexico DC-9 passenger plane collided with a small Piper aircraft carrying a family of three, killing all 67 passengers aboard the planes. An additional fifteen people on the ground were also killed. The accident was blamed on inadequate automatic conflict alert systems and surveillance equipment, and Congress responded by passing the Airport and Airway Safety and Capacity Expansion Act in December, 1987. This law required that all carrier aircraft operating within U.S. airspace with more than 30 passenger seats would have to be equipped with TCAS (Traffic Collision Avoidance System) II by 1993. Aircraft with 10 to 30 seats were required to employ TCAS I.


The Traffic Collision Avoidance System is the product of a joint effort between Lincoln Laboratory and MITRE. Like Mode S, TCAS was supported by Lincoln Labs from proof of concept through design and testing to publication of final international standards. Formerly called BCAS (for Beacon Collision Avoidance System), TCAS is an instrument integrated into other systems in an aircraft cockpit. It is designed to ``provide a set of electronic eyes so the pilot can 'see' 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. This system is 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.

The first implementation of TCAS, TCAS I, allows the pilot to see the relative position and velocity of all aircraft within a 10-20 mile range. More importantly, TCAS has a traffic advisory capacity which 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 the pilot with important data for him to make the maneuvers.

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 suggests an evasive maneuver necessary to avoid the other aircraft. If both planes in conflict are equipped with TCAS II, then the evasive maneuvers will be well-coordinated via air-to-air transmissions over the Mode S datalink; the proposed maneuvers will not cancel each other out.

The Mode S Legacy

``TCAS was what made Mode S real, and what entrenched Mode S as a lasting technology.'' -- Jonathan Bernays, Lincoln Labs researcher

Due to the Congressional mandate, TCAS became a pervasive system for air traffic control centers around the world. Because TCAS uses Mode S as the standard air-ground communication datalink, the widespread international use of TCAS has helped Mode S become an integral part of air traffic control systems all over the world.

An additional advantage of Mode S is its flexible design, which has made it a possible platform for a variety of other applications. In the July, 1980 OTA Seminar on DABS, Quentin Taylor of the FAA notes, ``[Mode S] adds improved surveillance quality, discrete aircraft addressing function, and the technical base for a digital communication exchange system. The latter is obviously a very important feature.'' 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 several services available to the general aviation community which were previously accessible only to commercial aircraft. Two of these services, the Graphical Weather Service and the Traffic Information Service, were developed by Lincoln Labs.

Traffic Information Service

The Traffic Information Service (TIS) provides many of the functions available in TCAS; unlike TCAS, TIS is a ground-based service available to all aircraft equipped with Mode S transponders. TIS takes advantage of the Mode S data link to communicate collision avoidance information to aircraft and presents this information to a pilot in a cockpit display. This cockpit display shows traffic within five nautical miles and a 1200 feet altitude of Mode S-equipped aircraft. The TIS system uses track reports provided by ground-based Mode S surveillance systems to retrieve traffic information. Because it is available to all Mode S transponders, TIS is an inexpensive alternative to TCAS; its availability makes collision avoidance technology more accessible to the price-sensitive general aviation community.

Graphical Weather Service

The Graphical Weather Service provides a graphical representation of weather information that is transmitted to aircraft and displayed on the cockpit display unit. The service is derived from ground-based Mode S sensors and offers information to all types of aircraft, regardless of the presence of on-board weather avoidance equipment. The general aviation community has been very pro-active in evaluating this technology, as they have already participated in field evaluations in Mode S stations across the U.S.

Mode S Extended Squitter

The most recent and most compelling use of Mode S has been in the area of Automatic Dependent Surveillance-Broadcast (ADS-B). Under ADS-B, each aircraft periodically broadcasts its identification, position, and altitude. These broadcasts can be received by ground sensors and other aircraft for surveillance. This system of broadcast addresses the major deficiency of TCAS -- accuracy. In the TCAS system, aircraft positions are only accurate to a few degrees; thus, the accuracy of TCAS decreases with distance. Moreover, the reliance on transmission timing for range data in TCAS is error-prone. The method used by ADS-B avoids this problem.

The Mode S extended squitter (also known as the GPS squitter) is a component of ADS-B which was proposed, developed, and demonstrated by Lincoln Labs. A participating aircraft broadcasts (``squits'') positional information using a modified Mode S transponder. The positional information comes from a source of global navigation, such as a Global Positioning Satellite (GPS) receiver. The Mode S extended squitter is a smooth upgrade from traditional Mode S. According to Steve Bussolari, project leader at Lincoln Labs, ``The notion behind the Mode S extended squitter is that it works seamlessly with the existing system.'' Paul Drouilhet adds, ``The GPS squitter is an expansion of the signal...What has changed is that GPS squitter adds to the broadcast, GPS info...the previous Mode S squitter [just] broadcasts altitude. The GPS Squitter has taken [Mode S], added more bits and in those bits, transmits information as derived from GPS.'' The Mode S extended squitter was demonstrated by Lincoln Labs and the FAA in Boston and the Gulf of Mexico in 1994, and is one of three candidates being considered by the FAA for ADS-B.

The Story of Mode S: An Air Traffic Control Data Link Technology last modified: 12.06.2000