Avionics Academy: Why Not Portable ADS-B Out?

Two reasons: The FAA is mandating all of us to get equipped for ADS-B Out, which requires a transmitter. That's very different from portable GPS receivers. And in my humble opinion, the single most significant reason why the FAA won't approve a portable ADS-B transmitter can be given in one word: security.

As I explained last month, as of January 1, 2020, a transmitter will be required so that every airplane (and soon, every drone weighing more than 55 pounds) operating in busy controlled airspace will announce its own position once per second, providing much more accurate position information than today's air traffic control radar, and allowing those equipped with ADS-B In receivers to display the position of nearby aircraft. The result will be much safer flying, especially in high-density airspace. To make this work, however, every airplane must transmit (or, in the special cases of airplanes doing ferry flights with inoperative transmitters and antique airplanes without electrical systems, ATC will have to transmit for them).

Every civilian ADS-B transmitter has a unique, hard-coded 24-bit address. In effect, it broadcasts your N-number. From the FAA's (and other federal agencies') point of view, that's one of the main advantages of the new system---no guesswork about which airplane is being tracked. But, introduce portable ADS-B transmitters, and this advantage runs the risk of being lost. A renter who usually flies one airplane might forget to change the code when flying a different one. Someone with malicious intent might deliberately set a code to look like an airliner.

As I wrote over a year ago, in an earlier Plane & Pilot article, there's a legitimate concern with the privacy of ADS-B transmissions. They're not encrypted, allowing real-time tracking. That's a serious concern for some business aircraft owners. But it's the flip side of providing a genuinely more secure approach to aviation.

AEA President Paula Derks gave me a list of practical reasons why portable ADS-B transmitters are a bad idea: ADS-B requires at least one antenna for the transmitter and another for the GPS receiver that provides position data. For fixed installations, those go on the bottom and top of the airplane, but for a truly portable system, anywhere you put them will have problems (as I found out while flight-testing a portable system back in 2013). How do you assure that portables are properly configured with a unique 24-bit address mapped to one (and only one) N-number? How do you assure that a portable ADS-B transmitter sends the same squawk code as the mode-C transponder? How do you guarantee the integrity and availability of the GPS data used by the ADS-B receiver? There are good reasons the FAA doesn't certify portable GPS systems for IFR use, and those apply here, as well. I'll add a practical reason, after having flown with a portable setup: It was a complicated mess, with cables strung all over the cockpit---not something I'd want to live with on a long-term basis.

The real issue being raised by pilots who ask about using portables is cost. Since ADS-B equipment must be permanently installed, it has to meet FAA technical standard order (TSO) requirements, and has to be configured and installed by a licensed avionics shop. That adds up to several thousand dollars, which is a lot for pilots of piston singles to swallow. There has been a lot of discussion about potentially less expensive options, especially for sport-class and experimental airplanes. There's no technical reason why a functionally equivalent non-TSO ADS-B transmitter couldn't be built to sell for under $1,000. But it still will have to be permanently mounted in the airplane and configured to identify it as belonging to that particular airplane.