Budget Transponders: Sandia Versus Garmin

With ADS-B, Mode A/C transponder performance is critical. Sandia’s STX 165 and Garmin’s GTX 325 both pack a modern punch.

Now that you’ve equipped for the ADS-B mandate, you’ll need to make sure the transponder remains in good working order. Ones that aren’t are on the FAA’s enforcement list.

There are plenty of aircraft for sale that need transponder (and ADS-B) upgrades, and as we’ve advised for years, the age and health of the existing unit should be a factor when selecting the right ADS-B Out solution. It’s simple—an ADS-B transponder is the best solution for aircraft with vintage models—and that’s still the advice moving forward.

But maybe the airplane already has a compliant 978 MHz UAT ADS-B solution (the uAvionix skyBeacon/tailBeacon products come to mind), but you’re now faced with replacing a failed transponder. Luckily there are a few modern and budget-friendly options for doing so and in this article we’ll look at them. We won’t cover 1090ES ADS-B models—we’ll hit those in an upcoming ADS-B buyer’s guide. Here we’ll focus on basic Mode A/C transponder upgrades for those who already have ADS-B, and even for those who don’t need to comply with the mandate. 


To know for sure, use FAR 91.413 test standards as a guide. These are the rigors a unit is put through during the two-year transponder inspection. Yes, complying with biennial certification is still required with ADS-B, whether you fly VFR or IFR. Transponder testing and certification itself (not counting pitot static and altimeter certification required by FAR 91.411) might run $150 to $200 without additional bench work. It’s worth the labor costs (when done by an experienced tech) because it’s an opportunity to gauge the health of the unit, even if you think the unit is healthy. Sure, it’s a hassle to drag the aircraft to the shop for this test, but you’d rather the shop spot a problem than the FAA and its resulting enforcement letter.

The Sandia STX 165, has a built-in altitude encoder and a small footprint

Now that ADS-B ops are widespread we’re hearing reports of ADS-B systems failing (and the FAA sending letters advising of such) because the transponder isn’t up to snuff. This includes both Mode A and Mode C altitude reporting functionality. Again, just because you have a functioning ADS-B Out system doesn’t mean you’re in the clear if the transponder’s Mode A and Mode C (altitude reporting) circuitry has problems. 

The testing required per FAR 91.413 ensures the transponder system has sufficient power output (most transponders pulse 250-plus watts of power), the correct pulse framing, SLS (sidelobe suppression), proper Ident function and of course outputs the correct squawk codes. The shop will first test the transponder as installed in the aircraft, which includes the antenna system—critical to overall performance. If there’s a problem, or if the unit is on the ragged edge of passing, the shop will remove the unit and put it on the test bench if it has the capability to work with it. Some do, some don’t. Ask before showing up especially if you sense there is a problem brewing. 

Garmin’s GTX 325, replaces the reliable GTX 327

When it comes down to it, a proper bench test (with the correct calibrated test equipment) is perhaps the only real way to determine just how healthy it is. Got an old analog transponder left over from the 1980s or earlier? It’s worth the hour or so of bench labor to determine if the unit is a long-term keeper. If the shop is installing a non-transponder ADS-B solution (maybe a uAvionix skyBeacon/tailBeacon or Garmin GDL 82), we wouldn’t proceed unless the transponder is in top working order. If it isn’t, and long-term functionality is questionable, put the brakes on and look at ADS-B transponders as the ultimate solution.


Garmin’s all-digital GTX 327 Mode A/C transponder was a huge seller and was often paired with GNS 430/530 navigators. As we describe in the sidebar on page 7, we think the right preowned GTX 327 could be the best solution going. But the GTX 327 has been discontinued and replaced by the GTX 325.

Priced at $2295, Garmin pitches the GTX 325 to buyers who already have an ADS-B Out solution and to those who don’t fly in ADS-B mandate airspace. 

The GTX 325 is sized 1.68 by 6.30 by 10.07 inches and that makes it easy to replace an existing analog transponder without having to reconfigure the radio stack, although we wish it had a more shallow chassis for saving space behind the panel. For panels where the transponder is displaced away from the fingertips, the unit has a digital databus for onscreen code and Ident selection on Garmin’s current GTN-series navigators.

Garmin designed the GTX transponders, including the GTX 325, so that the miniature Garmin altitude encoder, mounts to the transponder’s connector plate.

One feature we like about the GTX 325 that trickles down from Garmin’s flagship models is how the unit accommodates Garmin’s $249 GAE 12 pressure altitude sensor. Think of it as a smart encoder. The GAE 12 is literally the size of a quarter and its static fitting is actually larger than the chassis it sits on.

The GAE 12 screws to the back of the transponder tray and the static line connects directly to it. The advantage is that if the transponder has to be removed for repair or replacement, the GAE 12 encoder stays put and the static system remains undisturbed. That avoids the hassle and expense of recertifying the static system. It saves time and money.

From a feature set standpoint, the GTX 325 brings many of the smart functions that are familiar on other units in Garmin’s lineup, including a sunlight readable LCD display. It also has useful features including a pressure altitude readout so you always know what the altitude encoder is outputting. When interfaced with an optional temperature sensor, the unit will display density altitude and of course static air temperature.

Carried over from the GTX 327 is a full-featured timer including count up, count down, trip time and flight time. There’s also an altitude monitor with aural alerting (for tying into the audio panel) that displays the current deviation from a selected altitude. Wander off the altitude (which has presettable limits) and you’ll get an attention-getting “leaving altitude” warning.   

Aside from basic push-button code entry there’s an automatic VFR button, while the unit holds the previous squawk code in memory. Contact www.garmin.com.


New Mexico-based Sandia has a variety of products, from cooling fans, altitude encoders and EFIS, and for a couple of years it has been selling the $1950 STX 165 transponder. 

If there’s any doubt about how healthy any unit is, we think it’s worth a trip across a test bench. Ask your shop if it has the capabilities, like the oscilloscope shown.

It’s a space-saver (and weighs 1.3 pounds) with a ½ 3 ATI bezel, and has an OLED display that does we’ll in sun-splashed cabins. The display is small, and it displays a lot of data including pressure altitude and a three-function timer, and has an option for OAT probe input for additional displayed data. 

As transponders go, the STX 165 is somewhat untraditional in form factor, but with a simple user control set that’s worth a run-through for those only accustomed to using the more common design.

Controls are limited to a power/mode control knob, a VFR key, an Ident push-button and a data knob with a push-button for use as an Enter key. This is a dual concentric knob with push-button action. There’s also a photocell for automatic display dimming, plus you can set the minimum and maximum levels for which it will adjust. 

Sandia STX 165 function display

The Power button has two additional functions. One is as an Escape mode. If the operator is in a programing mode and wishes to escape to the main page, he can press the Power button momentarily. This will cancel all programming and show the normal display page. This is smart given the need for no-nonsense on-the-fly transponder functions, like changing the code immediately.

The other use for the Power button is controlling the transponder’s three timer functions. These include elapsed flight time, count up and count down. To select a timer rotate the Data knob either clockwise or counterclockwise. 

The other knob on the lower right of the bezel (Data knob) is used for data entry. The larger Select knob is a momentary switch that is spring loaded. It can be turned clockwise and counterclockwise and will return to center when released. It is used to move the cursor during code selection and to toggle through the display options in the lower left display (PALT for pressure altitude, DALT for density altitude and OAT for static air temperature). The transponder will display temperature in either degrees Fahrenheit or degrees Celsius.

And like the Garmin GTX 325’s optional probe, it’s worth mentioning that these functions are only available if the optional Sandia STP 78 OAT probe ($350) is installed. For many we think the accessory may be worth the effort to install. Best of all, the STP 78 OAT probe will provide an attention-getting icing alert on the transponder. The icing alert is a snowflake that appears in the upper right corner of the display whenever a user preset temperature is reached. The snowflake symbol will blink for 10 seconds once the icing alert temperature is reached, then remain solid until the temperature goes above the preset temperature. Smart and useful.

The smaller inner knob is the Data knob and is used to select the digit value when programing the squawk code and to select the counter function in the lower right display (when not in the code selection mode).

As for the display, the upper left portion of the screen will show the current transmitter mode. The upper right portion of the display will show the last squawk code that was selected prior to shutdown; the pressure altitude is displayed in the lower left and elapsed flight time in the lower right of the display.

Perhaps what we like the most is the integral 35,000-foot TSO-certified altitude encoder. It has a feature where the user can set the transponder to come on in the altitude reporting mode at startup or at a predetermined altitude differential after takeoff. The system also outputs pressure altitude data on an RS232 databus. 

Last, there’s a remote version of the transponder, the STX 165R. It’s priced at $1800, geared toward experimental applications and doesn’t have a built-in altitude encoder. It displays on Dynon, Advanced Flight and GRT experimental displays. Contact www.sandia.aero.


As you can see, the market for traditional non-ADS-B transponders is stark perhaps because manufacturers assume the majority of buyers would opt for a Mode S ADS-B model. That’s what Becker Avionics told us when we questioned why it dropped its line of compact, basic Mode A/C units.

That leaves the interesting question of whether you should you buy an ADS-B Out equipped transponder even if you don’t use it for ADS-B, which means not connecting it to a WAAS GPS position source. If you already have ADS-B Out via a 978 UAT system, we don’t see the need to spend the extra money for one unless you’re still concerned about the Canadian ADS-B requirement for transponder-based systems, which has been put on hold. 

As we reported in the October 2019 Aviation Consumer, not only did the original spec call for a 1090ES transponder, but it also required Diversity, which means a sizable jump in price. We’ll keep an eye on that.

As for choosing between Garmin’s GTX 325 and Sandia’s STX 165—which we think are two excellent products—how you choose might depend on panel space and what you have for other equipment in the panel.

The Sandia has more flexible mounting options in panels that are space challenged. This includes modern and legacy LSA models where mounting a 6-inch-wide transponder in the radio stack isn’t an option.

On the other hand, we like that the Sandia has a built-in 35,000-foot altitude encoder because face it, if you are going through the effort and expense of replacing the transponder, we think you should replace the encoder, too. 

Garmin’s GTX 325 interfaces with the company’s GTN-series touchscreen navigators, which could come in handy if the existing transponder is out of easy reach on the panel and you’re looking for more integration.

As an alternative and perhaps the ultimate money saver, we think it’s worth shopping for a well-cared-for used GTX 327. They’re out there.      

Larry Anglisano
Editor in Chief Larry Anglisano has been a staple at Aviation Consumer since 1995. An active land, sea and glider pilot, Larry has over 30 years’ experience as an avionics repairman and flight test pilot. He’s the editorial director overseeing sister publications Aviation Safety magazine, IFR magazine and is a regular contributor to KITPLANES magazine with his Avionics Bootcamp column.