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AIM 

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 
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, 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 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 
interrupted. 

(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 
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 
Mode S radar. The radar only determines ground 

Surveillance Systems 

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