Garmin GDL 88 manual Self-Interrogation, Pressure Altitude Broadcast Inhibit Pabi

The following table describes the aircraft that will be observed by a GDL 88

equipped aircraft according to the level of equipment installed in the other aircraft.

Other Aircraft Equipage

Viewable by GDL 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.

3.2Self-Interrogation

For installations with a digital interface between the GDL 88 and the installed

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 88 is configured to utilize self-interrogation. The GDL 88 has a small, ultra low power 1030 MHz transmitter integrated into the unit that interrogates the transponder installed in the aircraft. This interrogation looks exactly like the interrogations received from ground based surface search radars and the transponder replies like it is designed to do, to include the squawk code, IDENT status, and emergency status. The GDL 88 receives this reply and sets the corresponding ADS-B Out message values to match the received data.

3.3Pressure Altitude Broadcast Inhibit (PABI)

Much like with Mode C transponders, 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 could be used for certain surface operations, in the event of a failure of an altitude source in the ownship, etc. This control may either be integrated into the controlling display or as a stand-alone switch installed in the cockpit.

While conducting operations within airspace requiring ADS-B Out compliant Out transmissions, per 14 CFR 91225, PABI should not be activated unless directed by ATC.

3-2 GDL 88 ADS-B Transceiver Pilot’s Guide 190-01122-03 Rev. A