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MODE S
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.
TRAFFIC ALERT AND
COLLISION AVOIDANCE SYSTEM
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
1-21
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.
TRAFFIC INFORMATION SERVICE
Traffic Information Service (TIS) provides many of the
functions available in TCAS; but 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. Information is pre-
sented to a pilot in a cockpit display that shows traffic
within 5 nautical miles and a 1,200-foot altitude of
　
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