With the continuous growth of the unmanned aircraft industry, and the anticipated trajectory to come with urban air mobility solutions, commercial civil applications, and more, suppliers are often asked, “Does my UAS need a transponder?” Vehicle designers need to understand the current and anticipated requirements to make an informed decision on incorporating transponders into an aircraft.

The Standards Today

In a word, the standards today are “emerging.” There is no single chart from any global agency that sums up the requirements for transponders on unmanned aircraft of all types, for all classes of airspace, in all geographies. Therefore, this article brings together the current best practices and knowledge from around the industry to serve as a reference for unmanned aerial systems and vehicles that are above 55 pounds and operating under 14 CFR 91 (“Part 91”) aviation requirements.

Does An Unmanned Aircraft Need A Transponder?

An aeronautical transponder is an electronic device that produces a response when it receives a radio-frequency interrogation. Transponders provide vital information to Air Traffic Control (ATC) and other aircraft’s Airborne Collision Avoidance Systems (ACAS or TCAS). Collision avoidance systems use transponder transmissions as a means of detecting aircraft at risk of colliding with each other. The transponder receives interrogations from ground-based secondary surveillance radars and ACAS systems on 1030 MHz and replies on 1090 MHz.

Additionally, ADS-B Out was made a requirement on all aircraft flying in most controlled airspace. According to the FAA, “Real-time precision, shared situational awareness, advanced applications for pilots and controllers alike – these are the hallmarks of ADS-B NextGen surveillance. Real-time ADS-B is now the preferred method of surveillance for air traffic control in the National Airspace System (NAS).” Advanced transponders today offer these technologies, ideally integrated into a single electronic device, to satisfy aircraft airworthiness requirements.

In answer to the question, “Does a UAS need a transponder,” the short answer is “It depends.” A mix of mission parameters and airspace requirements plays into a transponder decision:

  1. How big is the aircraft? Typically, aircraft that need to fly in controlled airspace are of significant size and weight, usually more than 55 pounds. Smaller aircraft, perhaps down to 20 pounds, may also operate under Part 91 rules if they are flying beyond visual line of sight (BVLOS) in National Airspace, above 400 feet AGL. The U.S. Department of Defense classifies UAS into five groups, shown in the chart, with each category increasing in capability. Civil UAS that align with groups 2-5 can have missions requiring integration into controlled airspace and if so, will require a transponder for visibility by ATC and TCAS. For military UAS, transponder need is determined by military mission controllers.

  2. What kind of aircraft is it? Beyond size and weight, UAS can come in many formats, ranging from the familiar military surveillance drone to weather balloons, high altitude pseudo-satellites, eVTOLs, and a whole host of other aircraft classes that are currently flying or in design. With airspace increasingly crowded, if an aircraft of any kind will fly or operate in or through controlled airspace, its operators and others will need to keep track of it for safety reasons and will need a transponder, unless otherwise directed by the FAA.

  3. How far will the aircraft fly? UAV operation falls into two scenarios – those flown within line of sight and those flown BVLOS. If a program requires flight BVLOS a transponder will most likely be required for keeping track of it as well as increasing the safety of nearby aircraft.

  4. How high will the aircraft fly? U.S. airspace is organized by altitude and proximity to airports. According to the FAA, “Operations in Class G airspace are allowed without air traffic control permission. Operations in Class B, C, D and E airspace need ATC authorization.” In addition to a transponder, the FAA mandates: “You must be equipped with ADS-B Out to fly in most controlled airspace.” If an aircraft flight plan requires ATC services, it will need a transponder to provide visibility to ATC.

  5. What airspace rules apply? The military defines its own Military Operations Area (MOA) airspace requirements for unmanned drone operation. In the United States, civilian and military pilots have equal rights to MOA airspace, and both have equal responsibility to see and avoid other air traffic. MOAs serve as a warning, since military aircraft often fly at high speeds and are intentionally difficult to see. If an aircraft will fly through a MOA, it should include a transponder.

Airspace Requirements Chart

For civil programs, the FAA offers rules under 14 CFR 107 and 14 CFR 89 (the new Remote ID rule) that define the rules for aircraft flying at low (< 400 ft AGL) altitudes. Operations under Part 107 or Part 89 do not require a transponder. Some unmanned aircraft fly under FAA Part 91 guidelines (or Part 135 for cargo aircraft) in Class A-E airspace, or BVLOS above 400 ft AGL in Class G airspace. If this is the case, in all situations a type certification or FAA waiver will probably be required as well as a transponder with ADS-B.

These airspace requirements are the base set of requirements in the United States. Outside of the US, UAV operators should check with their own country’s air authority to ensure compliance with their requirements.

What Transponder Functionality Will My UAV Need?

Aviation transponders provide several RF communications protocols, or “Modes,” for communication. Select the transponder type according to FAA and DoD AIMS requirements for your mission. Additionally, ADS-B In and Out functions can ideally be packaged together in an integrated transponder. Whereas ADS-B Out is a requirement on UAVs in manned airspace, Sagetech also recommends equipping the aircraft with integrated ADS-B In to provide the additional situational awareness of other aircraft operating in the vicinity of the UAV.

A transponder implementation in a UAV, showcasing the dual antenna option. (Source: Sagetech Avionics)

Additional features to consider in transponders include integrated pressure-based altitude encoder and, if a UAV will be communicating with both ground- and space-based ADS-B, dual antennas. Finally, ensure a transponder carries the necessary communication protocols for a UAV, including the newer Ethernet as well as the heritage RS-232 and RS-422 for applicability across an OEM’s entire UAV portfolio.

Looking Ahead To Future Requirements

New Remote ID Requirements. The FAA has recently published the final Remote ID rule via the Federal Register, which will take effect in mid-Q1 2021. This rule defines an “electronic license plate” technology for aircraft broadcast identification, and also specifies that ADS-B will NOT be used for Remote ID.

UAS operating under Part 91 or Part 135 rules, with a flight plan and in communication with ATC, will not utilize Remote ID and will need transponders, ADS-B, and probably an appropriate ACAS-based detect and avoid system in order to achieve aircraft type certification. Steve Dickson, the FAA Administrator, gave an update at the Commercial UAV Expo on September 15, 2020: “Now what’s becoming clear is that anyone can do Part 107, but that’s not enough. We’re seeing its limits now. When we talk about integration, the future of expanded business opportunities will be in repeatable, scalable, and economically viable Beyond Visual Line of Sight operations; what we call BVLOS. Routine BVLOS and operations over people will need to meet the higher standards of Part 91 and Part 135, where type certification is required.”

Sagetech Mode S ADS-BTransponders

Urban Air Mobility (UAM). As the industry considers the integration of UAM aircraft into manned airspace, it is believed that transponders will be a core technology enabler for future detect and avoid (DAA) systems required on autonomous aircraft. At the Commercial UAV Expo in September of 2020, Steve Dickson, the FAA Administrator, explained, “Expanded drone operations will also require detect and avoid capability. Manned aircraft have always operated under the principle of see and avoid to remain clear of other manned aircraft, and now in some cases to remain clear of unmanned aircraft as well. Thus far, unmanned aircraft can’t do this. Reliable detect and avoid systems will enable us to integrate more complex unmanned operations along with traditional manned operations. This will be a game changer.”

Conclusion

Unmanned aviation is exciting, with emerging standards, new technologies, and a new world of possibilities. For every program, research and consideration are essential to determine if a program will require a transponder for enhanced visibility and, ultimately, safety in the skies.

This article was written by Tom Furey, CEO, Sagetech Corporation, (White Salmon, WA). For more information, visit here .


Aerospace & Defense Technology Magazine

This article first appeared in the May, 2021 issue of Aerospace & Defense Technology Magazine.

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