How Tis Differs From Tcas; Tis Limitations - Garmin GNC 420W Pilot's Manual Addendum

Part One: Section 1
TIS Operation and Symbology

How TIS differs from TCAS

The main difference between TIS and TCAS is the
source of surveillance data. TCAS uses an airborne
interrogator with a one-second update rate, while TIS
uses the terminal Mode S ground interrogator and its
Data Link to provide about a five-second update rate.
The range accuracy of TIS and TCAS is similar.

TIS Limitations

NOTE: This section on TIS Limitations is not compre-
hensive. Garmin recommends the user review the TIS
Limitations section of the Aeronautical Information
Manual, Section 1-3-5.
TIS is NOT intended to be used as a collision
avoidance system and does not relieve the pilot of
responsibility to "see and avoid" other aircraft. TIS
should not be used for avoidance maneuvers during
IMC or other times when there is no visual contact
with the intruder aircraft. TIS is intended only to assist
in visual acquisition of other aircraft in VMC. No rec-
ommended avoidance maneuvers are provided for, nor
authorized, as a direct result of a TIS intruder display
or TIS advisory.
While TIS is a useful aid to visual traffic avoid-
ance, it has some system limitations that must be fully
understood to ensure proper use. Many of these limi-
tations are inherent in secondary radar surveillance.
In other words, the information provided by TIS will
be no better than that provided to ATC. TIS will only
display aircraft with operating transponders installed.
TIS relies on surveillance of the Mode S radar,
which is a "secondary surveillance" radar similar to the
ATCRBS. TIS operation may be intermittent during
turns or other maneuvering. TIS is dependent on
two-way, "line-of-sight" communication between the
aircraft and the Mode S radar. Whenever the structure
2
of the client aircraft comes between the transponder
antenna (usually located on the underside of the air-
craft) and the ground-based radar antenna, the signal
may be temporarily interrupted. Other limitations and
anomalies associated with TIS are described in the
AIM, Section 1-3-5.
Garmin is not responsible for Mode S geo-
graphical coverage. Operation of the ground
stations is the responsibility of the FAA. Refer
to the Aeronautical Information Manual for a
Terminal Mode S Radar Site Map covering the
NOTE: TIS will be unavailable at low altitudes in many
areas of the U.S., particularly in mountainous regions.
Also, when flying near the "floor" of radar coverage
in a particular area, intruders below the client aircraft
may not be detected by TIS.
TIS information is collected one radar scan prior to
the scan during which the uplink occurs. Therefore,
the surveillance information is approximately five
seconds old. In order to present the intruders in a "real
time" position, the TIS ground station uses a "predic-
tive algorithm" in its tracking software. This algorithm
uses track history data to extrapolate intruders to
their expected positions consistent with the time of
display in the cockpit. Occasionally, aircraft maneuver-
ing will cause this algorithm to induce errors in the
400W/500W display. These errors primarily affect rela-
tive bearing information and traffic target track vector
(it will lag); intruder distance and altitude will remain
relatively accurate and may be used to assist in "see
U.S.