Garmin GNC 420W Pilots Guide - Page 193

SECTION 11 ADDITIONAL FEATURES 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. Other limitations and anomalies associated with TIS are described in the AIM, Section 1-3-5. TIS is unavailable at low altitudes in many areas of the U.S., 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 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 GNC 420 display. These errors primarily affect relative bearing information and traffic target track vector (it lags); intruder distance and altitude remain relatively accurate and may be used to assist "see and avoid". Some of the more common examples of these errors follow: • When client or intruder aircraft maneuver excessively or abruptly, the tracking algorithm may 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 0.25 nm, TIS may display the intruder on the wrong side of the client. The preceding errors are relatively rare occurrences and are corrected within 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: • Time of observation • Location, type, and identity of aircraft • Condition observed • Type of transponder, processor, and software in use 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 may be obtained at FAA FSS's, General Aviation District Offices, Flight Standards District Offices, and General Aviation Fixed Based Operations. 190-00140-20 Rev. K GNC 420(A) Pilot's Guide and Reference 11-3