Plane & Pilot
Tuesday, August 3, 2010

Beyond Today’s Transponder


Demystifying ADS-B



The Trig TT31 is a 1090ES-capable Mode S transponder designed to replace a KT76A. The manufacturer plans an upgrade to meet the FAA's TSO-C166b standard, required for ADS-B out.
The two different radio links operate in different frequency bands. One is an enhanced version of the 1,090 MHz Mode S link called Extended Squitter (ES). It’s the easiest to understand: Replace your existing Mode C transponder with a Mode S transponder that supports 1090ES operation, and you’re equipped to meet the FAA mandate. Unfortunately, the 1,090 MHz frequency band is congested—all existing transponders, whether Mode 3/A, C or S, respond to interrogation at 1,030 MHz with a reply on 1,090 MHz. As an alternative, the FAA and ITT also are supporting 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 still is 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. By comparing radar targets with ADS-B data, ATC will be able to prevent “spoofing” and other attempts to misuse ADS-B. Keeping radar also provides redundancy that today’s system lacks: In a GPS satellite outage, radar service will still be available; if the radar’s out, ADS-B will still work.

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, which transmits information from the ground to aircraft for display in the cockpit, offers additional capabilities. The FAA hasn’t mandated ADS-B in equipment, but supports it with two kinds of information being sent from the ground stations for use on an advisory basis by pilots of airplanes equipped to receive them.

The first is traffic information: If you’re equipped with a two-way 978 MHz UAT and display, you’ll be able to “see” other 978 MHz traffic, but not traffic that’s sending on 1,090 MHz. To deal with that, the ground stations include a feature called ADS-Rebroadcast (ADS-R). Signals from any 1090-ES equipped traffic within 17 miles of an aircraft that listen 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 within 17 miles of your position, provided you’re in line of sight of a ground station.

Airplanes with a 978 MHz UAT receiver also can receive Flight Information Services—Broadcast (FIS-B), including NEXRAD radar mosaic, current weather conditions, terminal forecasts, significant weather alerts, winds, temperatures aloft and pilot reports, along with temporary flight restrictions and other notices to airmen. Unlike today’s satellite-based weather services that require a monthly subscription, once you install the receiver and display, FIS-B is free.

There’s one more advantage of a 978 MHz UAT over a 1090ES Mode S transponder: UATs support an “anonymous” function equivalent to squawking 1,200 that generates a random 24-bit address and doesn’t reveal your N-number. That’s partly intended to combat what an FAA rep told us are “completely untrue” rumors that ADS-B will be used to enable user fees. As an avionics vendor pointed out to us: “If you think the government needed to add a 24-bit number to your airplane in order to keep track of you, you’re kidding yourself!”

FAA Mandate: What It Means
Now that I’ve got all that out of the way, here’s what the FAA has mandated: As of January 1, 2020, any aircraft operating in Class A, B or C airspace and in Class E airspace above 10,000 MSL (and above 2,500 AGL) will require both a Mode C transponder and one of the two ADS-B out links. Above 18,000 feet, the ADS-B out link must be 1090ES Mode S, which is also the accepted international standard and may be required for operations outside the U.S.

To understand what this means in practice, let me personalize things a bit: I’m a partner in a 1975 Cessna Skylane. We installed a Garmin GNS-530W IFR GPS last year, which can serve as both a position source for ADS-B out, and (hopefully) a display for ADS-B in. We don’t fly above 18,000 feet, but do take the airplane to Mexico at least a couple of times each year, so for international compatibility, our ADS-B-out link will be 1090ES. For ADS-B in, we’ll have a couple of options. At least one avionics vendor plans to offer an ADS-B-in option on Mode S transponders for under $2,000. That would give us traffic, or we could spend a similar amount for a 978 MHz receive-only device, which would give us both traffic and weather. The FAA rule doesn’t require ADS-B-in and ADS-B-out devices of the same type.



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