Garmin Appliance Data User's Guide Page 8

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2
TIS Operation and Symbology
Part One: Section 1
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 com-
prehensive. Garmin recommends the user review
the TIS Limitations section of the Aeronautical
Information Manual, Sections 4-5-6 and 4-5-8.
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 recommended 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 avoidance, it
has some system limitations that must be fully understood
to ensure proper use. Many of these limitations 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 of the client aircraft
comes between the transponder antenna (usually located
on the underside of the aircraft) and the ground-based
radar antenna, the signal may be temporarily interrupted.
Other limitations and anomalies associated with TIS are
described in the AIM, Sections 4-5-6 and 4-5-8.
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
U.S.
NOTE: TIS will be unavailable at low altitudes in
many areas of the U.S., particularly in mountain-
ous 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 “predictive 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 maneuvering will cause this algorithm to induce
errors in the 400W/500W display. These errors primarily
affect relative 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 and avoid.” Some of the more common examples of
these errors follow:
190-00356-30 Rev L
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