Avionics Report

February 2009 Issue

Garmin’s GTX330ES Transponder For ADS-B

Propaganda hints at an affordable ADS-B option. While it will multi-task TIS traffic input and basic ADS-B output, itís too early to bank on.

In the avionics technical world, the transponder squitter is the transmission or reply format that sends data to ATC facilities over the 1090 MHz frequency. Itís the backbone of airborne transponder pulse theory. In the ADS-B world, the technical thread revolves around 1090-ESó for 1090 MHz Extended Squitter. Thatís not to be confused with the UAT, or Universal Access Transceiver. With ADS-B comes a handful of abbreviations that substantiate the layers of ADS-BóVDL 4, TIS-B, FIS-Bóthe list goes on. Itís no surprise that many pilots are confused and misinformed on ADS-B. The 1090-ES and UAT handle the communication with ground stations and/or other aircraft in different ways. The 1090-ES interface is common in commercial aircraft that routinely fly in the flight levels, while the UAT link was designed for lower-altitude, smaller-craft applications. Extended squitter streams data including aircraft address information, GPS-based position, velocity, and projected heading data to the ADS-B ground stations and other ADS-B equipped aircraft via the 1090 MHz pulse. The key here is automatic broadcast, resulting in better surveillance, rather than old-school interrogation theory (a.k.a. request and reply, which suffers from a slow refresh rate). The other piece of the ADS-B puzzle is TIS-B (not to be confused with TIS traffic as we know it now). TIS-B means Traffic Information Service Broadcast, and is integral to a so called "free flight" ATC system of NexGen. Garminís GTX330ES model will do TIS-B, but understand that it doesnít receive any ADS-B (including TIS-B) data. It strictly broadcasts aircraft data. It must be interconnected with approved Garmin panel-mounted GPS systems, such as the GNS400W- and 500W-series, GNS480/CNX80, and G1000s with GDU software 9.11 and higher. The GTX330 is a Level 2 transponder, which provides downlink of the aircraft information in which itís installed. ATC ground stations interrogate the unit (and other Mode S models) using a unique, 24-bit Mode S address. This capability includes the ability to recognize the programmed aircraft tail or flight ID number, as well as its maximum speed capability for interface with airborne TCAS traffic systems.

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