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
55 NM 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
34 degrees: 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

Surveillance Systems

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
positions consistent with the time of display in the
cockpit. Occasionally, aircraft maneuvering will
cause this algorithm to induce errors in the TIS
display. These errors primarily affect relative bearing
information; 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 are as follows:
(1) When client or intruder aircraft maneuver excessively or abruptly, the tracking algorithm
will report incorrect horizontal position until the
maneuvering aircraft stabilizes.
(2) When a rapidly closing intruder is on a
course that crosses the client at a shallow angle (either
overtaking or head on) and either aircraft abruptly
changes course within  NM, TIS will display the
intruder on the opposite side of the client than it
actually is.
These are relatively rare occurrences and will be
corrected in a few radar scans once the course has
stabilized.
(g) Heading/Course Reference. Not all TIS
aircraft installations will have onboard heading
reference information. In these installations, aircraft
course reference to the TIS display is provided by the

4-5-13