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Surveillance Systems

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



(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 indicated in FIG 4


5. 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:


TIS will typically be provided within 55 NM of the radars depicted in FIG 4


6, 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.


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.


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, similar to 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 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: