Garmin GPSMAP 495 Owner's Manual page 142

TIS > TIS Limitations
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, the TIS ground station 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 causes this algorithm to induce errors in the
GPSMAP 495 screen. These errors primarily affect relative bearing
information and traffic target track vector (it will lag); intruder
distance and altitude remains relatively accurate and may be used to
assist in "see and avoid." Some of the more common examples of
these errors include:
• When client or intruder aircraft maneuvers excessively or
abruptly, the tracking algorithm can report incorrect horizontal
position until the maneuvering aircraft stabilizes.
• When a rapidly closing intruder is on a course that crosses the
client aircraft course at a shallow angle (either overtaking or head
on) and either aircraft abruptly changes course within ¼ nm, TIS
may show the intruder on the opposite side of the client than it
actually is.
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These are relatively rare occurrences and are corrected in a few
radar scans once the course has stabilized.
Users of TIS can render valuable assistance in the correction
of malfunctions by reporting their observations of undesirable
performance. Reporters should identify the time of observation,
location, type and identity of aircraft, and describe the condition
observed; the type of transponder processor and software in use can
also be useful information. Since TIS performance is monitored
by maintenance personnel rather than ATC, it is suggested that
malfunctions be reported in the following ways:
• By telephone to the nearest Flight Service Station (FSS) facility.
• By FAA Form 8000-7, Safety Improvement Report, a postage-
paid card designed for this purpose. These cards can be obtained
at FAA FSSs, General Aviation District Offices, Flight Standards
District Offices, and General Aviation Fixed Based Operations.
GPSMAP 495 Owner's Manual