Plane & Pilot
Monday, September 1, 2014

NextGen ADS-B Update 2014

Time to plan an upgrade: It’s cheaper than you think

Avidyne EX600
I've been covering the development of ADS-B (automatic dependent surveillance–broadcast) technology for quite a while, and my advice to readers has been the same as what I was planning: Wait until the system matures, prices come down, and there's a compelling reason to upgrade. Fellow pilots—that time is now: In April, the FAA announced that deployment of the required ground stations was complete, and that they're now being used at 100 of 230 U.S. air traffic control facilities. Price-wise, it's now possible to install certificated equipment to meet the FAA's ADS-B out broadcast mandate, and buy portable equipment for advisory-only traffic and weather, for just a few thousand dollars. And electronic traffic display is, in my opinion, more than enough reason for the upgrade, especially for those of us who already have an iPad or other portable device that will work as a display. But, to reliably receive ADS-B traffic, you also need a certificated ADS-B transmitter installed. To understand why, I'll need to review how the system works, but first, let's consider what the FAA is trying to do and why.

ADS-B is part of the FAA's NextGen initiative to upgrade the entire air traffic control system, which until now has revolved largely around ground-based radar tracking. By 2020, almost all aircraft operating in controlled airspace (aircraft originally built without an electrical system, including gliders, are exempt) must carry TSO'd equipment that can broadcast position reports (based on an approved position source—for normal category aircraft, that's a TSO'd GPS navigator) every few seconds. That has two big benefits: It provides ATC with much more accurate information on where aircraft are, which it can use to provide more efficient handling, and it offers the opportunity for pilots to know the position of other aircraft near them if they carry equipment to receive the signals being transmitted from those airplanes. Incidentally, while about half of the existing surveillance radars are scheduled for decommissioning, the rest will be kept as a cross-check and backup.

One major complication in the FAA's approach is that two separate (and incompatible) radio links are being used. Aircraft operating above 18,000 feet and/or flying internationally are required to use an upgraded "extended-squitter" Mode S transponder, operating on 1090 MHz. Those transponders are already used by most airliners and business jets—that's why those of us who buy portable ADS-B in receivers that support the Mode S link find ourselves looking at lots of high-altitude traffic. Unfortunately, for technical reasons, there's potentially a serious problem if every airplane in congested airspace is equipped with Mode S, and it has limited bandwidth that can't accommodate additional information like in-flight weather. So, for airplanes operating only in U.S. airspace below 18,000 feet, a second radio link is available, operating at 978 MHz. The equipment for using that link is called a Universal Access Transceiver (UAT). The FAA first tested UAT technology in the 1996-2006 Capstone program, which demonstrated that providing accurate aircraft position along with traffic and weather in the cockpit could significantly enhance safety in the Alaskan backcountry, where radar-based air traffic control service wasn't available.

Garmin Pilot App
To deal with the dual data links, the FAA contracted with Exelis Inc. to install over 600 ground stations covering most of the continental U.S. and Alaska. Those stations receive aircraft position reports and can also rebroadcast those reports.

For example, suppose two airplanes are approaching an airport with an ADS-B ground station. One aircraft is equipped with a UAT operating on 978 MHz, the other with a Mode S transponder on 1090 MHz. The ground station receives both positions and rebroadcasts the first aircraft position on 1090 MHz and the second aircraft position on 978 MHz. Assuming both aircraft have appropriate ADS-B in receivers and displays, they'll each see the position of the other aircraft. Now, suppose a transponder-equipped aircraft that's not ADS-B equipped joins the party. If there's a ground-based surveillance radar in the area, the position of that aircraft will be reported to nearby ADS-B ground stations, which will send the radar-based position to ADS-B in equipped aircraft.

In the last couple of years, a wide range of portable ADS-B in devices have appeared. If the transponder-equipped aircraft has one of those, it may see the signal from the ADS-B equipped aircraft, but the ground station won't automatically relay signals to it. The only way to reliably get position reports on all aircraft in the area is to be equipped with ADS-B out.


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