Plane & Pilot
Tuesday, November 5, 2013

ADS-B From Portable Equipment


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ADS-B Update

Three years ago, the FAA mandated that as of January 1, 2020, aircraft operating in U.S. class A, B or C airspace and in class E airspace above 10,000 MSL (and above 2,500 AGL) must be equipped with one of two ADS-B out data links: an enhanced form of the Mode-S transponder already used by most airliners, especially on international flights; or an alternative technology called Universal Asynchronous Transceiver (UAT) that was developed for the FAA's Capstone program.

The two radio links operate in different frequency bands. One is an enhanced version of the 1090 MHz Mode-S link called Extended Squitter (ES). It's the easiest to understand: replace your existing transponder with a Mode-S transponder that supports 1090ES operation, and you're equipped to meet the FAA mandate.

Unfortunately, the 1090 MHz frequency band is congested—all existing transponders, whether Mode 3/A, C or S, respond to interrogation at 1030 MHz with a reply on 1090 MHz. As an alternative, the FAA supports ADS-B on the less congested 978 MHz frequency band, which requires a completely new piece of equipment called a Universal Asynchronous Transceiver (UAT). A UAT doesn't replace your transponder it's a separate piece of equipment installed
in addition to your transponder, which is still required because the FAA plans to keep about half of today's surveillance radar sites as a backup and to verify integrity of the ADS-B system.

The one-way link from aircraft to ATC described so far is ADS-B out, which is being mandated by the FAA. ADS-B in is an optional system that transmits information from the ground to aircraft for display in the cockpit. This includes traffic information: With a UAT and display, you can "see" other 978 MHz traffic, but not traffic that's sending on 1090 MHz. To deal with that, ADS-B ground
stations include a feature called ADS-Rebroadcast (ADS-R). Signals from any 1090-ES-equipped traffic near an aircraft that listens on a UAT are copied and transmitted on 978 MHz and vice-versa. Another feature called Traffic Information Service-Broadcast (TIS-B) does the same thing for transponder-equipped aircraft not equipped with ADS-B, if you're flying in a radar service volume.
The effect is that with either a 1090ES or 978 MHz ADS-B receiver and display, you'll have a complete picture of all traffic near your position, provided you're in line of sight of a ground station.

Airplanes with a UAT can also receive Flight Information Services–Broadcast (FIS-B), including NEXRAD radar mosaic, current weather conditions, terminal forecasts, significant weather alerts, winds and temperatures aloft and pilot reports, along with temporary flight restrictions and other notices to airmen. Unlike satellite-based weather services that require a monthly subscription, once you install the receiver and display, FIS-B is free.

An issue to consider: coverage. In the continental U.S., ADS-B coverage is good along the Atlantic and Pacific coasts, throughout the South and in the Great Lakes region—but much more limited in the Central and Mountain regions—Wyoming, Montana and Colorado have little or no coverage, because very few ADS-B ground stations haven't been activated yet in those states. None have been installed in the Hawaiian islands or Puerto Rico at this writing.





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