Avionics Academy: Which Data Link Should You Get For ADS-B Out?

The system of Mode-C altitude reporting transponders we have today goes back to the 1950s. From an air traffic control perspective, it's severely limited by the number of squawk codes; that's why you have to check in and announce your N-number each time you change from one sector to another. An obvious improvement would be to code the N-number (or airline flight number) into the transponder, a feature provided by Mode-S, which appeared in the late 1960s.

Mode-S has since become the worldwide standard for airliners and business jets, which made it inevitable that any major upgrade to the air traffic control system would have to support it. An upgrade to Mode-S called Extended Squitter (ES) is the international (and U.S. high-altitude) standard ADS-B data link. It expands the standard Mode-S signal to include GPS-based position reports, a hard-coded unique digital address and other data, along with the altitude and flight number. So if you ever expect to fly your airplane in the flight levels or outside U.S. airspace, Mode-S is the way you'll have to go.

A second data link option developed out of the FAA's Capstone Program of 2000 to 2006. Mountainous Alaskan terrain limits radar coverage and, combined with unpredictable weather, leads to a high accident rate. Capstone tested whether advanced avionics could help. Airplanes participating in the program were provided (at government expense) with a Universal Access Transceiver (UAT), which downlinks aircraft position to ATC, and uplinks weather and traffic for in-cockpit display. It was a great success, leading to a drop by some 59% in the fatal accident rate for air taxi and commuter operators.

The success of Capstone led the FAA to offer the UAT as an alternative to Mode-S for U.S.-controlled airspace. In addition to offering weather in the cockpit, since UATs operate in a different frequency band from conventional 1090 MHz transponders, they don't contribute to transponder signal congestion, which is already a serious problem in busy Class B airspace. So, if you're not required to use Mode-S, the FAA hopes you'll get a UAT.

Regardless of which data link you choose, any ADS-B Out solution requires a position source, typically a GPS. When the mandate rolled out, most people (including me) assumed this would be a panel-mount. The catch is that the GPS has to be compatible with whatever ADS-B equipment you buy. And what about airplanes that don't have a GPS in the panel?

Many ADS-B vendors now offer GPS position sources with their equipment. That's an option worth considering, not only for pilots who don't have a panel-mount GPS, but also for those who might be thinking about upgrading their GPS at some point, because there's no guarantee that the ADS-B unit you get today will be compatible with an upgraded GPS you add several years down the road.

If you choose a UAT, you're also required to have a functioning Mode-C transponder. The FAA will use ADS-B and radar to back up each other, and while an ADS-B compatible Mode-S transponder is visible to both systems, a UAT doesn't show up on radar. The UAT and transponder are required to share the same altitude source and squawk code (otherwise, ATC might see signals that look like two airplanes flying a dangerously close formation).

My friend with the Cirrus turns out to have a complicated decision to make. His airplane is old enough to have a first-generation glass panel, with displays from one well-known avionics vendor and dual GPS navigators (and a Mode-S transponder) from another vendor. My first suggestion was to upgrade the transponder to an ADS-B compatible version. That would work, but he's thinking about upgrading the GPS units, and if he doesn't stay with the same vendor, there's no guarantee they'd work as position sources for the transponder. He'd also like to get ADS-B traffic and weather on his cockpit displays, which raises another question I'll discuss in another column.

Here's some free advice: Take a long, hard look at the transponder (and panel-mount GPS, if you have one) in your airplane and think about how long you can expect them to last. You might also want to look at the antennas, then find an avionics shop to help select and install the best ADS-B option for your situation.