Garmin GPSMAP 396 User Manual download pdf (Page 156)

148 GPSMAP 396 Owner’s Manual

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 396 screen. These errors primarily affect relative bearing

information and trafc 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 Ofces,

Flight Standards District Ofces, and General Aviation Fixed

Based Operations.

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Garmin GPSMAP 396 User Manual Page 156