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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. Other limitations and anomalies are
associated with the TIS predictive algorithm.
(a) Intruder Display Limitations. 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 described in paragraph
4-5-2.
(b) TIS Client Altitude Reporting Requirement.
Altitude reporting is required by the TIS client
aircraft in order to receive TIS. If the altitude encoder
is inoperative or disabled, TIS will be unavailable, as
TIS requests will not be honored by the ground
system. As such, TIS requires altitude reporting to
determine the Proximity Coverage Volume as
2/14/08 AIM
Surveillance Systems 4-5-13
indicated in FIG 4-5-4. TIS users must be alert to
altitude encoder malfunctions, as TIS has no
mechanism to determine if client altitude reporting is
correct. A failure of this nature will cause erroneous
and possibly unpredictable TIS operation. If this
malfunction is suspected, confirmation of altitude
reporting with ATC is suggested.
(c) Intruder Altitude Reporting. Intruders
without altitude reporting capability will be displayed
without the accompanying altitude tag.
Additionally, nonaltitude reporting intruders are
assumed to be at the same altitude as the TIS client for
alert computations. This helps to ensure that the pilot
will be alerted to all traffic under radar coverage, but
the actual altitude difference may be substantial.
Therefore, visual acquisition may be difficult in this
instance.
(d) Coverage Limitations. Since TIS is
provided by ground-based, secondary surveillance
radar, it is subject to all limitations of that radar. If an
aircraft is not detected by the radar, it cannot be
displayed on TIS. Examples of these limitations are
as follows:
(1) TIS will typically be provided within
55NM of the radars depicted in FIG 4-5-5, Terminal
Mode S Radar Sites. This maximum range can vary
by radar site and is always subject to “line of sight”
limitations; the radar and data link signals will be
blocked by obstructions, terrain, and curvature of the
earth.
(2) TIS will be unavailable at low altitudes
in many areas of the country, 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.
(3) TIS will be temporarily disrupted when
flying directly over the radar site providing coverage
if no adjacent site assumes the service. A
ground-based radar, like a VOR or NDB, has a zenith
cone, sometimes referred to as the cone of confusion
or cone of silence. This is the area of ambiguity
directly above the station where bearing information
is unreliable. The zenith cone setting for TIS is
34degrees: Any aircraft above that angle with
respect to the radar horizon will lose TIS coverage
from that radar until it is below this 34 degree angle.
The aircraft may not actually lose service in areas of
multiple radar coverage since an adjacent radar will
provide TIS. If no other TIS-capable radar is
available, the “Good-bye” message will be received
and TIS terminated until coverage is resumed.
(e) Intermittent Operations. TIS operation
may be intermittent during turns or other maneuvering,
particularly if the transponder system does not
include antenna diversity (antenna mounted on the
top and bottom of the aircraft). As in (d) above, TIS
is dependent on two-way, “line of sight” communications
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.
(f) TIS Predictive Algorithm. TIS information
is collected one radar scan prior to the scan
during which the uplink occurs. Therefore, the
surveillance information is approximately 5 seconds
old. In order to present the intruders in a “real time”
position, TIS uses a “predictive algorithm” in its
tracking software. This algorithm uses track history
data to extrapolate intruders to their expected
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