What to Look For When Buying ADS-B Systems
A transponder might be advertised as ADS-B capable, but that doesn't necessarily mean it's ready to make your aircraft ADS-B compliant.
When the FAA's ADS-B Out mandate took effect in January 2020, thousands of aircraft owners upgraded their transponders and installed new equipment to meet the requirements.
Today, the used avionics market offers plenty of options for aircraft owners looking to replace failed equipment or complete an ADS-B installation without purchasing everything new.
But there's one important detail that occasionally gets overlooked.
ADS-B compliance involves more than just the transponder itself.
Before purchasing used equipment, it's worth understanding what makes a complete ADS-B Out system and why two seemingly identical transponders may not offer the same installation options.
Not Every Mode S Transponder Has ADS-B Out
Let's start with one of the most common misconceptions.
Just because a transponder operates in Mode S doesn't mean it provides ADS-B Out.
Mode S transponders have been around for decades, long before the ADS-B mandate. While some newer Mode S transponders incorporate ADS-B Out functionality, others do not.
For ADS-B Out, there are two primary transmission technologies used in general aviation:
1090ES (Extended Squitter): Uses 1090 MHz and is commonly integrated into a compatible Mode S transponder. This is the required option for aircraft operating at or above 18,000 feet MSL.
978 MHz UAT (Universal Access Transceiver): Provides an alternative ADS-B Out solution for qualifying aircraft operating below 18,000 feet within U.S. airspace.
The FAA allows either technology for applicable operations below 18,000 feet, but 1090ES is required for operations in Class A airspace.
You can review the requirements on the FAA's ADS-B installation page. Federal Aviation Administration
The takeaway?
Don't assume a transponder is ADS-B compliant simply because it's digital, relatively modern or equipped with Mode S.
The GPS Position Source Is Just as Important
Here's where used ADS-B equipment can become a little more complicated.
An ADS-B Out transmitter needs accurate position information to broadcast.
That information generally comes from a compatible GPS position source.
Some ADS-B transponders have an internal GPS receiver designed to provide the required position data.
Others rely on an external GPS source.
That external source might be an appropriately configured GPS navigator or a dedicated position-source receiver.
And here's the important part.
Not every GPS is an acceptable ADS-B position source, even if it displays your aircraft's position perfectly.
The FAA specifies performance requirements for ADS-B Out position information, and the transmitter and position source must work together in an appropriate installation.
For type-certificated aircraft, the transmitter and GPS pairing also needs the necessary FAA installation approval. Federal Aviation Administration
This means a used ADS-B transponder advertised without an internal GPS might require additional equipment before it can be installed as part of a compliant system.
That additional equipment could significantly change the total cost of your project.
The Cheapest Transponder Isn't Always the Cheapest Installation
Imagine finding a used ADS-B transponder advertised for $1,000.
Another unit is listed for $1,600. At first glance, the $1,000 option looks like the obvious choice. But what if the less expensive unit requires a separate approved GPS position source that your aircraft doesn't already have?
What if it also needs a different antenna, installation rack or connector kit?
Suddenly, the cheaper transponder might not be the more economical option.
This is why we encourage buyers to consider the entire installation rather than focusing exclusively on the price of the equipment.
A few hundred dollars saved on the purchase can disappear quickly when additional components and installation labor are factored in.
Before purchasing, have your avionics installer review the proposed equipment and confirm what your aircraft actually needs.
ADS-B In and ADS-B Out Are Two Different Things
Another area that creates confusion is the difference between ADS-B In and ADS-B Out.
They sound similar, but they serve different purposes.
ADS-B Out broadcasts information about your aircraft, including its position, altitude and identification.
ADS-B In receives available traffic information and, depending on the equipment and services available, weather information.
ADS-B Out is required for most aircraft operating in designated ADS-B rule airspace. ADS-B In is not required by the FAA's ADS-B Out mandate.
Some transponders provide both functions, while others provide only ADS-B Out.
For example, the Garmin GTX 335 provides ADS-B Out, while the Garmin GTX 345 offers both ADS-B In and Out. However, that additional functionality comes at a price. Depending on the condition, configuration and market availability, the difference between these two units on the used market can be upwards of $2,000.
Does that mean everyone should spend the extra money on a GTX 345?
Not necessarily.
An aircraft owner looking to meet the ADS-B Out requirement could choose a GTX 335 and pair it with a portable ADS-B In receiver, such as a Sentry, to display traffic and weather information on an iPad using ForeFlight.
This combination can provide much of the same practical traffic and weather awareness at a significantly lower equipment cost, particularly for pilots who already fly with an iPad.
Of course, there are advantages to having ADS-B In integrated directly into the aircraft's avionics. A GTX 345 can provide traffic and weather information to compatible panel-mounted displays, eliminating the need to rely solely on a portable receiver and tablet. Actual display capabilities depend on the installed equipment and configuration.
Neither option is necessarily better for everyone. It comes down to your budget, how you use your aircraft and where you prefer to view traffic and weather information.
Understanding these differences can help aircraft owners avoid spending thousands of dollars on features they may not actually need.
Don't Forget About Software and Configuration
Even after you've selected compatible equipment, there's still work to be done.
ADS-B systems require proper configuration to transmit accurate information about the aircraft.
Among other things, that includes the aircraft's ICAO address, flight identification and appropriate equipment settings.
An incorrect configuration can result in a system that appears to operate normally while transmitting inaccurate information.
The FAA identifies several common ADS-B installation problems, including incorrect ICAO addresses, missing altitude information, flight identification errors and position-reporting problems. Federal Aviation Administration
Software also matters. Some older ADS-B equipment requires software updates to meet current performance requirements. The FAA specifically notes that certain early ADS-B Out installations used an older link version that does not satisfy the applicable rules without updating. Federal Aviation Administration
When buying used equipment, verify the software and configuration requirements with your installer. Don't assume that because a transponder worked in one airplane, it will automatically work correctly in another.
How Do You Know Your ADS-B System Is Working Properly?
Here's something that surprises some aircraft owners.
Your transponder can appear to operate normally in the cockpit while your ADS-B Out system is transmitting incorrect information.
Fortunately, the FAA provides a useful tool for checking ADS-B performance.
It's called the Public ADS-B Performance Report, or PAPR.
After an appropriate flight within FAA ADS-B coverage, aircraft owners can request a report showing how their installed system performed. The report evaluates transmitted information and can identify potential compliance or configuration problems.
It's free and available through the FAA.
The report is particularly useful after installing or replacing ADS-B equipment, but it can also be helpful for checking an existing installation. It supplements rather than replaces required installation inspections and testing. ADSB Performance
What Should You Check Before Buying?
If you're shopping for a used ADS-B transponder, there are several questions worth answering before placing an order.
Does it actually support ADS-B Out? Not every Mode S transponder does.
Does it have an internal GPS position source? If not, determine what external equipment is required.
Is it compatible with your aircraft? Verify voltage, installation approval, interfaces and other requirements.
What's included? Check for the rack, connectors, backplate, GPS antenna and other necessary components.
Does it have the appropriate software? Confirm whether updates are required or available.
What's its operating condition? Understand whether the equipment is bench tested, working when removed or being sold in unknown condition.
Has your installer approved the proposed equipment combination? This is especially important when mixing components from different manufacturers.
Answering these questions before purchasing can help prevent unnecessary expenses and installation delays.
Used ADS-B Equipment Can Still Be a Great Value
The ADS-B mandate created a substantial market for new equipment, but it also helped establish a large installed base of systems that will eventually need maintenance or replacement. As aircraft continue to receive panel upgrades, serviceable ADS-B equipment enters the used market.
For aircraft owners who already have a compatible installation, replacing a failed transponder with an appropriate used unit can be considerably less expensive than starting over with an entirely new system.
And for aircraft owners planning their first ADS-B installation, used equipment may still offer savings when the complete installation cost makes sense.
The key is knowing exactly what you're purchasing.
At Skyman Avionics, we work with aircraft owners and avionics shops to help identify and source used equipment, including transponders, GPS navigators and other components commonly used in ADS-B installations.
We also help shops and aircraft owners sell equipment removed during upgrades, giving perfectly useful avionics an opportunity to serve another aircraft.
Want to know which ADS-B offerings we currently have available? Just click here.
Email: info@skymanavionics.com
Call: 541-604-9573
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