Garmin GDL 88 Series Pilots Guide - Page 18

The following table describes the aircraft that are observed by a GDL 84/88 equipped aircraft according to the level of equipment installed in the other aircraft. Other Aircraft Equipage Viewable by GDL 84/88 Equipped Aircraft 1090ES Out Equipped Yes UAT Receive Only Capable No UAT Transmit Equipped Yes No Transponder, No ADS-B No Non ADS-B Equipped, but with Mode C or S Transponder Yes (see Note) Table 3-1 Aircraft Available for Viewing by an ADS-B Equipped Ownship NOTE: Only when in ADS-B ground station coverage and when the other aircraft is in ATC radar coverage. 3.2 Self-Interrogation When there is a digital interface between the GDL 84/88 and the transponder, the transponder acts as a single control to enter the transponder squawk code, IDENT, and emergency status. When a digital interface is not available, the GDL 84/88 utilizes self-interrogation by using a small, ultra low power 1030 MHz transmitter integrated into the unit to interrogate the transponder. This interrogation imitates interrogations received from ground based surface search radars causing the transponder to reply including the squawk code, IDENT status, and emergency status. The GDL 84/88 receives this reply and sets the corresponding ADS-B Out message with the same squawk code, IDENT status, and emergency status. 3.3 Pressure Altitude Broadcast Inhibit (PABI) A method is required to be available to the flight crew to inhibit the transmission of pressure altitude in the ADS-B Out message. This control is either integrated into the controlling display or as a stand-alone switch in the cockpit. While conducting operations within airspace requiring ADS-B Out compliant transmissions, per 14 CFR 91.225, PABI should not be activated unless directed by ATC. 3-2 Garmin GDL 84/88 ADS-B Transceiver Pilot's Guide 190-01122-03 Rev. E