Keep Your Transponder, Add ADS-B

Aircraft owners ask us a lot of questions when they’re considering how to upgrade their avionics for ADS-B. But there’s one question asked most frequently by pilots who fly below FL180:
“Do I have to replace my transponder?”

Our usual answer is, “No,” or sometimes just, “Why?”

With few exceptions, those of us who fly in the thicker air can use our current Mode A, C or S transponders alongside our ADS-B Out equipment. Sure, you can replace your older transponder with a 1090 MHz Mode S Extended Squitter (1090ES) ADS-B Out unit. But you don’t have to pull a perfectly good, functional transponder from the panel. For the large majority of piston single- and twin-engine drivers and for helicopter owners, it just doesn’t make a lot of sense.

Getting Freq-y

Pilots who fly below FL180 can use either a 1090ES transponder or a 978 MHz Universal Access Transceiver (UAT). That choice doesn’t apply to anybody who operates in Class A airspace; they must use a 1090ES transponder to support continued use of TCAS and other systems. But for everybody else a UAT – while keeping your current transponder — is the way to go:

  1. UAT technology supports expansion for weather, traffic and a host of new ADS-B services to come, as well as the long-term growth of light GA. In contrast, 1090 MHz transponder technology is from the 1940s and getting a bit long in the tooth. The frequency is already congested and is needed for the bizjet and Big Iron crowd. But for GA pilots, swapping out a working transponder for a newer one would be like replacing a big old tube TV with a fancier, big old tube TV. Go for the flat-screen, or in this case, a UAT.
  2. The adage, “if it ain’t broke…” applies. While “slide-in replacement” might sound quick and easy, the reality of installing a new transponder usually is not so pleasant. Older cabling, corroded antennas and other surprises often lie in wait. Leave it be unless you’re having trouble with your current system.
  3. You’re required to have an operational transponder as a back-up to your ADS-B equipment and to remain visible to TCAS systems. Simply keep the one you have.

Getting Out
It helps to understand how and why you can use your current transponder alongside your ADS-B Out avionics if you look at the three main pieces of required equipage:

Transponder: The transponder continues to do what it has always done and that’s report the aircraft altitude and ID.
Position Source: ADS-B Out requires precise reporting of aircraft position, velocity and time. That’s where an approved WAAS/GPS comes in. You may already have an approach-certified WAAS/GPS but it must also be approved for ADS-B and able to communicate with your transponder. The simplest solution is to add a WAAS/GPS sensor as the approved position source. It’s a small, inexpensive, remotely installed device that neither takes up panel space nor replaces your familiar IFR GPS.
Transmitter: The final required piece is the ADS-B Out transmitter. It takes data from the transponder and WAAS/GPS, adds some required info, encodes it and transmits it to the ADS-B ground station network and other aircraft. Although a few Mode S owners might upgrade their hardware and software to 1090ES capability based on their specific needs, most owners of Mode A, C or S transponders will go with a UAT.

Getting In
You’re only required to equip with an ADS-B Out transmitter. But it makes a lot of sense to equip with a transceiver – a UAT. A UAT gives you access to TIS-B traffic data no matter what type of transponder you have as well as subscription-free FIS-B weather services. You can display traffic and weather on an MFD that supports the industry-standard Capstone Protocol , and/or on an iPad via wifi. If you choose a 1090ES system you may receive TIS-B traffic data but for FIS-B weather you’ll still need to add a 978 MHz receiver. Future ADS-B In services also will use the 978 MHz frequency.

Getting It
Keep your existing transponder and put the money that you didn’t spend toward the WAAS/GPS, the UAT that also gives you ADS-B weather and traffic, and the installation costs. You’ll meet the ADS-B Out requirement and be “fully rule-compliant” in the official lingo.