Smart Transponders

The Experimental market gets few breaks when it comes to transponder and ADS-B requirements.


As you should know by now, you’ll still need a Mode A/C transponder system in a full-up ADS-B environment. Just because your ADS-B-equipped aircraft will be transmitting GPS position, heading, and speed, the U.S. ATC system will still utilize traditional beacon codes and pressure altitude reporting.

Still, ADS-B compliance is getting easier thanks to a market shift that embraces a welcomed no-brainer solution: all-in-one smart transponders, or those equipped with mandate-approved ADS-B systems and better yet—self-contained WAAS GPS engines. This single-box breakthrough tames the installation complexity and brings cheaper and more streamlined ADS-B solutions to utilitarian panels.

In our continuing series of panel upgrade articles, we’ll scan the market of standalone and integrated ATC transponders, while focusing on the latest solutions that can satisfy the upcoming ADS-B mandate.

Standalone Models

If you haven’t shopped the transponder market in this century, you’ll be surprised to see a sizable technology leap from the ancient analog days. Gone are the high-voltage, heat-inducing cavity oscillator tubes responsible for pricey bench repairs and relatively short transponder longevity. This technology is gone for good and has been replaced by microchips, software, and digital data buses—offering greater interface potential and space-saving designs. Without a doubt, modern transponders pack a big punch. Here are the contenders.

Sandia Aerospace

Even entry-level units like the $1,850 Sandia Aerospace STX165 include advanced features like Mode C pressure altitude reporting with an integral altitude encoder, which is certifiable up to 35,000 feet. This eliminates the need for a remote altitude encoder and long runs of wiring bundles—potentially saving $500 or more in equipment and installation effort. But as the sidebar on page 25 describes, there are some drawbacks.

Sandia’s STX165 transponder has an internal altitude encoder.

Sandia also makes a remote digitizer.

The non-ADS-B STX165 (it’s a Mode A/C unit only) has a simple feature set, which consists of dual rotary function knobs, a dedicated VFR squawk button, a familiar IDENT button, plus it has a sunlight-readable and backlit LED display. The STX165’s chassis is designed for multiple installation options, including mounting in a 3-inch ATI instrument cutout. But with a shallow rectangular chassis, it can also be surface-mounted in a flat panel. Its serial databus can feed pressure altitude data to other devices that accept serial altitude, including GPS navigators, altitude alert systems, and moving map displays, to name a couple.

Trig Avionics

Scotland-based Trig Avionics was an early player with its line of 1090ES ADS-B-capable models, including the space-saving TT22 and the FDL-1090-TX model rebranded and sold by FreeFlight Systems. These transponders are compatible with Garmin WAAS navigators and FreeFlight’s remote WAAS GPS receiver for use as an external GPS position source. It’s important to remember that unless a transponder has an integral WAAS GPS receiver, you’ll still need an FAA-approved external GPS position source to complete the ADS-B Out requirement.

Trig’s TT22 two-piece Mode S ADS-B transponder is also repurposed for FreeFlight Systems (below). Both need remote GPS input.

The Trig TT22 is a two-piece system, with a control head and a remote-mount transceiver that includes an integrated altitude encoder. The panel control head houses an LCD screen and the small bezel has conventional rotary knobs and push-buttons. The control head is splash proof, making it tough enough for water ops and open cockpit applications. The TT22 draws low current—that’s a plus in applications that have minimal electrical systems like gliders, for example. For lower-end applications, Trig also offers the TT21, but it only outputs 130 watts, compared to the 250-watt TT22.

If you have a vintage BendixKing transponder, the rack-mounted $2,600 Trig TT31 transponder can work in an existing KT76A/C installation, but plan on additional wiring for piping in the GPS data from a compatible WAAS GPS source. This includes Garmin GNS and GTN navigators, plus FreeFlight’s WAAS/GPS 1201 sensor. If you have a BendixKing KT76A, you can use the existing power, ground and Mode C wiring.


Speaking of BendixKing, it makes the $2,999 KT74. This is a Mode S unit designed and built by Trig Avionics. This is a rack-mounted design with 1090ES ADS-B Out capability and is also plug-and-play with vintage KT76A/C models. But, like other slide-in replacements, additional wiring will be required when interfacing it with an approved WAAS GPS, including Garmin’s GNS530W/430W, GTN750/650 and BendixKing’s own KSN770 touchscreen navigator.

BendixKing’s KT74 is compatible with the company’s KSN770 GPS navigator for a 1090ES ADS-B solution and can use existing legacy KT76A/C wiring.

As we go to press, BendixKing still doesn’t have an ADS-B traffic or weather display interface for the KSN770 hybrid navigator, but the company says it’s working on the required TSO approval to some day make the data play on the screen. We’ll take a look at the KSN770 in a future article.

Becker Avionics

The $2,100 Becker Avionics BXP-6401 (self-contained version) and the BXP-6402 (two-piece system) are Mode S systems that Becker’s web page says can support an ADS-B extended squitter. However, Becker told me the company never earned certification for ADS-B output.

The two-piece 6402 is a popular option in small panels and has a rugged control set. For traditional rack mounting, Becker makes the $3,800 digital BXP6403. It too is designed to fit in place of a BendixKing KT76A and has the same interface potential as Becker’s 6401/6403 systems.


Although it rolled out a new line of ADS-B transponders (see the sidebar below), Garmin plans to keep the $2,000 entry-level Mode A/C GTX327 transponder in its lineup, and I think it’s a worthy choice in suites that have a remote ADS-B solution (perhaps Garmin’s GDL88, GDL84 or even third-party systems). Because the GTX327 has an RS-232 serial databus, you won’t need a dedicated ADS-B control head to keep the transponder and ADS-B codes in sync, since that logic is monitored over the databus.

Garmin says it plans to keep the proven GTX327 Mode C transponder in its lineup. It interfaces with GNS and GTN navigators for flight timer functionality.

The GTX327 is completely solid state, has flight and countdown/count-up timers, an altitude buffer with audio callouts to warn you of altitude excursions, plus it displays the current pressure altitude readout, based on the data received from the altitude encoder. When interfaced with Garmin GNS and GTN navigators, it will automatically cycle in and out of standby on the takeoff roll and start and stop the flight timer, based on GPS groundspeed.

Glass Integration

Upgraded transponder LRUs (line replaceable units) for Dynon Skyview and Garmin G3-series integrated suites can be installed to satisfy the 2020 mandate, and both companies offer upgrade paths for older systems.


For its integrated SkyView and SkyView Touch suite, Dynon offers the $1,700 SV-XPNDR-261. This is a remote 1090ES transponder that is tuned directly on the SkyView/Touch display. The LRU weighs one pound and measures 5.6 by 1.8 by 2.4 inches.

Dynon’s integrated XPNDR-261 and remote GPS-2020 antenna/receiver make for an easy ADS-B solution for new and existing SkyView suites.

But the SV-261 isn’t a self-contained ADS-B Out solution. Instead, it requires Dynon’s $590 SV-GPS-2020 GPS receiver/antenna. The newer GPS-2020 is a drop-in replacement for the company’s early-gen SV-GPS-250 without having to add or change the wiring. For existing SkyView systems not equipped for the 2020 ADS-B mandate, Dynon said the latest GPS-2020 can also be used as the suite’s primary GPS source (for map display, navigation and synthetic vision), while the older GPS-250 can be retained as a backup GPS. You’ll also need SkyView main software version 14.0 or greater for it all to work.


For Garmin’s G3X and G3X-Touch, the current $2,450 GTX23ES remote transponder has 1090ES extended squitter and 250 watts of power. It also receives TIS traffic link and displays the targets on the G3X screen. It won’t, however, receive ADS-B In data, including ADS-B traffic. You’ll need Garmin’s GDL39-series receiver for that, which is available in a remote version or battery-powered portable footprint. The GTX23ES will require a WAAS position source for completing the ADS-B Out interface. For that, Garmin sells the $845 GPS20A remote WAAS GPS receiver.

Transponder functions are accessed directly from the G3X Touch display.

The GPS20A is a 2020-rule-compliant (but non-TSO’d) position source made specifically for Experimental and LSA applications, and Garmin says it will work with a variety of 1090ES transponders over a serial databus. If equipped, you can also connect the GTX23ES with any of Garmin’s WAAS-equipped panel navigators—including the GNS530W/ 430W—and end up with a mandate-compliant solution.

The first-generation remote transponder for the G3X was the non-ADS-B GTX23, and the GTX23ES is the same form factor and can use some of the existing wiring.

Garmin’s GTX23-series remote LRU transponder works with the G3X/Touch integrated avionics.

L-3 Aviation

L-3 Aviation was perhaps the first with a full-up ADS-B transponder with its Lynx 9000. The 9000 series is more of a multifunction system than a transponder, although it has Mode A/C/S functionality. Packaged in a chassis that’s designed for radio stack mounting, the Lynx 9000 measures 2.0 inches high, has dual-band ADS-B In and extended squitter Out, internal WAAS GPS, plus traffic and weather display on a color RGB touchscreen. The entry-level $5,500 Lynx NGT-9000 can be optioned with dual antenna (diversity) and with an active TAS traffic processor, bumping the price to nearly $12,000. The Lynx outputs 1090ES, making it a player in high-flying applications.

L-3’s Lynx is really a true multi-function system, which includes a color touch display, 1090ES transponder, built-in GPS, plus weather and traffic display.

Overall, I think the Lynx feature set is nicely laid out, the menu structure is shallow (a good thing), and the user is given several screens for configuring the data. The data on the touch display is divided into left and right screens, which L-3 calls applications. You select each screen application using both drag and swipe touch actions, while basic screen position identifiers (hollow or empty circles) at the bottom of the screen show which page is active. Onscreen scroll bars navigate through lists or blocks of text, while slider bars are used for editing data and for increasing and decreasing numerical values, for example.

An optional Wi-Fi module enables wireless interface capability with iPad and Android tablets. L-3 has selected WingX Pro and SkyRadar for primary app compatibility, but says other interfaces are pending. You’ll likely make good use of the Wi-Fi module if you’re looking to view weather and traffic on a larger screen. While the 9000’s color display is bright and has a wide viewing angle, its small size may not be the best for viewing FIS-B NEXRAD images. If you’re picky, I like that the transponder supports multiple input/output protocols, including ARINC 429, RS-422 and RS-232 serial data. Check with your installer for ultimate third-party compatibility (or with L-3 directly if you are wiring it yourself).

Still, for small panels the Lynx 9000 can sufficiently work as a primary weather display, and you’ll view a wide variety of free FIS-B weather products, including AIRMETs, METARs, NOTAMs, SIGMETs, SPECI and TAF data. NEXRAD FIS-B radar can also be selected, while the age of the image is displayed at the bottom of the screen.

The Lynx has an interesting traffic information data window, which is accessed by tapping an onscreen traffic target. Once expanded, the window shows a variety of information including the flight ID of the selected aircraft, the type of aircraft, and its calculated groundspeed. Another dedicated window shows a birds-eye view of the ADS-B traffic targets the system is receiving. If you opt for the model that has built-in TAS, the system prioritizes the threat targets and only displays the ones you want to see, rather than clumping all of them at once.

Since the NGT9000 serves as a primary transponder, there is a dedicated transponder application that shares the left screen (by toggling) with traffic alerting. It contains transponder squawk code, IDENT-reply, pressure altitude readout, transponder mode control (for selecting typical Mode A/C functions including altitude encoding), plus a MSG key that flashes when a new message is received. Simply tap it to view the message and tap Done to jump back to the application page. The ON-GND indicator advises that the transponder is operating in ground mode and isn’t replying to all interrogations.

While the idea of the Lynx is to streamline the installation—and eliminate the need for an external GPS—the model with internal WAAS GPS requires a dedicated GPS antenna and can’t be combined with an existing one. Diversity models require a second directional antenna for mounting on top of the aircraft. If you happen to have an existing L-3 Skywatch system, its antenna can be used for Diversity and for TAS traffic. Pull the Sky497 processor out and save a bunch of weight when using the Lynx 9000 model with internal TAS. The system requires pressure altitude input via a serial altitude digitizer or traditional Gray code altitude encoder. It also has outputs for interfacing with compatible audio panels for traffic alerting and other prompts.

Appareo’s Stratus ESG is intended for basic panels that need a self-contained ADS-B solution.

More Models on the Way

There are two new multifunction transponders coming from Sandia Aerospace (the STX360 Sentinel) and ADS-B manufacturer Appareo (the Stratus ESG). If you aren’t familiar with the brand name, Appareo makes the Stratus series portable ADS-B systems, sold through Sporty’s.

Unlike most ADS-B transponders, the Sandia STX360 will have a built-in 978 MHz UAT transceiver, rather than 1090ES, which will limit the unit to lower-flying missions. But like the L-3 Lynx, the STX360 has a bezel display for playing weather and traffic, although the data is limited to FIS-B textual data and ADS-B traffic targets. For a more complete weather interface, Sandia said the transponder would be equipped with a wireless transmitter for outputting weather graphics to tablet computers. No specific apps have been named.

Sandia’s STX360 will have a 978 UAT receiver, plus wireless capability.

The downside is that the STX360 lacks an internal WAAS GPS receiver, although Sandia hinted it is planning on a future GPS solution, plus a remote version to interface with third-party EFIS displays. Pricing is expected to be below $3,500 when the STX360 is released later in the year.

The $2,995 Appareo ESG is a rack-mounted 1090ES transponder with built-in WAAS GPS receiver. Equipped with an OLED display, the Stratus ESG has push-button mode keys, including a dedicated VFR code button; it will display pressure altitude readout and is compatible with a variety of altitude encoders.

The Stratus portable ADS-B receiver has a wired interface with the Stratus transponder for power and GPS signal.

The Stratus doesn’t display weather or traffic. Instead, an optional wired interface is designed to connect the transponder with the popular Stratus and Stratus 2 portable ADS-B receivers. The harness basically piggybacks the portable receiver with the transponder for voltage and a GPS signal. This means the Stratus receiver can be remotely mounted, rather than placed on the glareshield and connected with remote antennas. Still, I wish for a wireless interface, and I suspect buyers will, too.

Appareo attempts to make the installation process as simple as possible, supplying everything you or your shop will need to hook it up. This includes a GPS antenna and even a prefabricated wiring harness. At 1.69 inches tall, the ESG is the same form factor as legacy transponders.

Wrap it Up

Clearly, Garmin’s latest wireless transponders (see sidebar) are positioned to simplify ADS-B equipage, especially the versions equipped with WAAS GPS. The ability to wirelessly stream weather and traffic data to a variety of portable devices—including portable GPS systems—will be appealing to many buyers.

But Garmin isn’t alone in an emerging market of smart transponders. For buyers still not sold on displaying weather and traffic on a cockpit tablet, the L-3 NGT9000 transponder—with its integral color touch display—is worth considering and has the advantage of a built-in display. It has an intuitive menu structure, fits nicely in the radio stack, and is available with GPS. Perhaps the only obstacle is its higher price—something the Experimental and LSA market just isn’t accustomed to.

On the other hand, if you’re still years away from completing your kit, it may be worth waiting. There are bound to be more options worth considering. If you have to buy now, I favor Garmin’s new wireless models for the easiest and most complete all-in-one solution.


  1. Larry
    Hi from HFD
    You installed my 430w years ago
    And your expartner Steve C since put in the Aspen 1000 PFD plus L3’s NGT 9000
    My question is:
    How can I provide an alternative pressure altitude source when the Aspen quits for any reason
    Do you recommend some switching device similar to an alternate static air device but of electrical variety?
    I have had the Aspen quit twice and lost pressure altitude for the 430 and. L3
    Clearly some alternate encoder source seems the logical solution No?
    John Hilton


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