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Surveillance Systems

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 dis-

played 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


(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


(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

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 maneuver-

ing, 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” communica-

tions 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


(f) TIS Predictive Algorithm. TIS informa-

tion 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 maneu-

ver 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 ¼ 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


(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