12 Essential Technical Considerations For Selecting Your Next UAV Transponder

Every day, UAV manufacturers are sifting through new and emerging technology requirements for transponder surveillance solutions to enable missions in civil and military airspace. What are the Minimum Operational Performance Standards (MOPS)? What FAA, FCC, military AIMS, and other compliances are required? And, in addition to this evolving landscape, there are the technical parameters themselves – what should you evaluate when selecting surveillance transponders for your next UAV?

Technical considerations are of serious consequence. We’ve all heard the horror stories – UAVs lost for days, requiring airspace shutdown. Or, UAVs who lost transponder power and therefore went invisible to other aircraft while still in flight. Transponders are critical to the safety of manned and unmanned aircraft alike.

We sat down with our CTO and microelectronics industry expert, Matthew Hamilton, to create the definitive list of technical considerations for selecting a mission-critical transponder. The good news is that with careful attention to transponder specifications, you can avoid letting your program become a history lesson to future programs.

According to Matt’s hands-on experience, and, based on in-depth research with our military and civil customers, we’ve compiled the top 12 parameters of UAV transponders that should be evaluated as part of your technical due diligence. We’ve captured all of this information in a .pdf whitepaper, and here is a quick summary of the issues.

1.       Reliability. Integrating a transponder and syncing it with your autopilot and other systems requires significant engineering. Ensure you only need to do it once by selecting a highly reliable, proven product. Is your application for the military? Best to select equipment that is purpose-built to survive the most severe military environments.  If it’s mission-critical civil applications, such as firefighting, consider selecting a commercial-grade model that’s built on proven military heritage technology.

Matt also suggests, “In addition to performance, ask your supplier about quality. For example, over the last decade, what was the unit return rate for quality issues?”

2.       Output power over the full temperature range. “Time and again, we counsel customers to watch for output power and power droop issues,” explains Matt. This is essential for a transponder to be seen by ATC- or TCAS-equipped aircraft. If your transmission droops out of spec (2dB), then ATC or TCAS ADS-B receivers will reject your transmissions.  If you ensure your transponder maintains full power over the specified operating temperature range and never falls below 2dB droop, your aircraft will remain visible.

Rejected ADS-B Out Message from non-Sagetech transponder, due to power droop.

Rejected ADS-B Out Message from non-Sagetech transponder, due to power droop.

Sagetech MXS/XPS transponder ADS-B out message success; power within spec.

Sagetech MXS/XPS transponder ADS-B out message success; power within spec.

3.       Multiple communication ports. Essentially, plan for every current or future need. “Having more than one communication port gives you more flexibility for integration, such as one for GPS and one for command and control,” details Matt.


Autopilot Support

4.       Autopilot integration. Integrating with various autopilots can cost serious time and money. To get your program off the ground faster, ensure your transponder brings integration with every major autopilot so you can integrate with any option.

5. Multi-mode capabilities. Many UAV platforms perform civil and military missions; look for one transponder that can support both. To satisfy both sets of requirements, the transponder will need military modes 1,2,3 and 5, and civilian modes A, C, S and ADS-B out. Mode 5 is the newest NATO requirement for military programs, where the transponder communicates with a crypto computer to provide secure Identify Friend or Foe (IFF) functionality.  See our article explaining Mode 5.

6. Integrated ADS-B In. Often customers try to piece together a transponder with an external  ADS-B In module, requiring time-consuming integration. Look for a transponder that includes this capability within the unit to simplify your system in terms of size, weight, and connections. .

“We’re seeing an increased demand for ADS-B In,” continues Matt. “ADS-B In provides airspace situational awareness to enable proper decision-making, helping to avoid dangerous mid-air collisions.  Having a certified ADS-B In receiver is the first step in making a safety case to fly BVLOS. Certified ADS-B In capability is an FAA requirement for Detect and Avoid Systems.”

7. Ethernet connectivity. “Ethernet provides a more robust and reliable solution than RS-232 or even some traditional military bus architectures. With ethernet, it is easy to implement communication redundancy and have more advanced diagnostics for failure isolation. We have seen a recent shift to ethernet by most major OEMs for a more flexible system design and a lower overall SWaP solution,” says Matt. When reliability is a priority, make sure your transponder provides ethernet capability.

8. Visibility by ATC. Not all ADS-B messages can be seen by ATC. If the transponder and the GPS do not hold TSOs, then there are certain integrity fields in an ADS-B message that must be set to represent that. These fields are the Source Integrity Level (SIL) and System Data Assurance (SDA). If your system is uncertified, these fields must be set below a 3 and a 2 respectively. However, when these fields are below a 3 and a 2, ATC discards the ADS-B message as not trustworthy. If you don’t have certified equipment, then the probability of you transmitting a false position significantly increases, which could lead to a mid-air collision. It is better just to rely on the traditional secondary surveillance radar using Mode A/C functionality than it is to potentially transmit a false and misleading position and have ATC use that info.

9. Visibility by TCAS. Unlike ATC, TCAS systems will still receive ADS-B messages if the SIL and SDA values are below 3 and 2. “The important parameter here is to look at power droop during a transmission. TCAS (and ATC) systems will reject any transmission that droops more than 2dB during a transmission,” explains Matt. This requirement is easy to meet at high temperatures but as temperature goes down, the effective capacitance is decreased. Transponders use large capacitor banks to store energy for the very high power and short transmissions. A lot of micro transponders remove too much capacitance to shrink their size and weight, which can result in too much power droop at colder temps.  Study the datasheet to ensure power droops do not occur at colder temps in your transponder.

10. Antenna diversity. Top and bottom antennas provide more visibility above and below your aircraft.  This enables the aircraft to be visible by space-based ADS-B systems as well as air traffic control on the ground. According to Matt,“It gives everyone visibility of your aircraft’s location while satisfying air traffic control with full situational awareness.” Some transponders use a dipole antenna; however, the radiation is significantly reduced which provides less distance in each direction. “Dedicated dual antennas provides greater reach and visibility for any mission-critical UAV, including reception by satellite-based ADS-B systems.”


Single rear dipole antenna creates visibility issues.

Single rear dipole antenna creates visibility issues.

Top and bottom monopole antenna provides greater visibility.

Top and bottom monopole antenna provides greater visibility.

Table -- 12 Essential Technology Considerations for Selecting a UAV Transponder.PNG

Market forces are coming together to make transponders an increasingly important decision in your next program. With the mix of transponders available, emerging standards and requirements, and the rapidly growing types of vehicles, this list should help you navigate the complexities and select the best transponder for your program needs.

Learn more about our XP Series transponders for basic requirements, and our MX Series transponders for next-gen, including Mode 5, solutions.