UAS Lost Link

Summary of the category

Unmanned aircraft systems (UAS) are operated remotely through commands sent by light of sight radio, satellite relay, or by following pre-programmed plans saved to the autopilot program. There are two components to the remote signal: the uplink that transmits command and control (C2) instructions from the ground control station to the flight control system, and the downlink that relays the status and location of the aircraft to the ground control station. If either link is disabled or malfunctions, the resulting condition is called a “lost link” (FAA, 2016).

Legislation and technical requirements

In the recently released Part 107 FAA regulations for commercial operation of small UAS, there are no specific requirements for lost link procedures or equipment. Prior to Part 107, the only permitted commercial operations outside of restricted category airspace were required to follow the conditions and limitations of a Certificate of Waiver or Authorization (COA). A COA is specific to a particular type of aircraft and area of operations and includes specific pre-programmed actions that would occur in the event of a lost link event. Part 107 specifies that the remote pilot in command is responsible to ensure that the “small unmanned aircraft will pose no undue hazard to other people, other aircraft, or other property in the event of loss of control of the aircraft for any reason” (FAA, 2016). In the FAA’s response to comments following the release of Part 107 in the Federal Register, their stated position is to avoid mandating specific responses to emergencies, and also acknowledge that lost link events may not constitute an emergency if there are no people or manned aircraft nearby.

For public or civilian commercial UAS weighing over 55 pounds, a special airworthiness certificate and/or COA are still required to operate. On October 11, 2016, the FAA issued an update to FAA Order 7110.65W that directs the national Air Traffic Control system, adding a procedure for ATC radar facilities to monitor UAS with a loss of link condition. Under notice JO 7110.724, UAS may squawk transponder code 7400 to alert ATC of a lost link. With this procedure in place, ATC facilities will be alerted to UAS lost link conditions and be able to direct manned traffic away from the path of the UAS (U.S. DOT, 2016).

Related technology

Most sUAS flight control systems follow pre-programmed courses or commands when a link loss is detected. One example is the Ardupilot Mega (APM) flight control software, an open-source flight control system that is based on the Arduino controller system. APM flight controllers default to a “return to launch” (RTL) plan when the C2 link is lost. By default, the aircraft will climb or descend to a pre-determined altitude and fly towards the GPS location where the aircraft was armed for takeoff. If installed in a rotor-wing aircraft, the autopilot will land the aircraft. If installed in a fixed-wing aircraft, the airplane will loiter at a set radius from the marked GPS point. Alternately, the autopilot can be programmed to complete a mission (such as a mapping survey flight) before conducting the RTL plan. “Rally” points offset from the launch point may also be located at specific locations along the flight plan, and the aircraft will perform the RTL plan to that point (Ardupilot, 2016). This is important if flying near a towered airport, restricted airspace, or in the vicinity of obstacles such as towers or power lines.

There are a small number of manufacturers that produce Mode S transponders for UAS. These transponders emit an extended “squitter” at frequencies that are used in manned aircraft for traffic collision avoidance systems (TCAS) and automatic dependent surveillance systems-broadcast (ADS-B). Sagetech produces an FAA-approved (via technical service order, TSO) Mode S transponder that weighs less than 140 grams (5 oz) (Sagetech, 2010).

Expected future developments

The FAA’s approach to relatively limited regulation of sUAS provides many opportunities to commercial operations within visual line of sight (VLOS). However, in order to utilize the full potential of UAS, beyond VLOS or extended VLOS operations are essential. Three companies are coordinating with the FAA to test EVLOS and BLOS scenarios, including lost link procedures (FAA, 2016). These types of operations are more akin to manned instrument flight rules, and will almost certainly include limitations and equipment required to prevent and detect lost link events.

References:

Ardupilot Development Team. (2016). RTL mode (return to launch). Retrieved from http://ardupilot.org/plane/docs/rtl-mode.html#rtl-mode

Federal Aviation Administration. (2016, June). Focus area Pathfinder program. [Press release]. Retrieved from https://www.faa.gov/uas/programs_partnerships/focus_area_pathfinder/

Operation and Certification of Small Unmanned Aircraft Systems; a rule by the Federal Aviation Administration, 81 Fed. Reg. 42063 (August 29, 2016) (to be codified at 14 C.F.R. pts. 21, 43, 61, 91, 101, 107, 109, 133, and 183).

Sagetech Corp. (2010, December). Mode S transponder XPS-TR. [Brochure]. Retrieved from https://sagetechcorp.com/xp-transponders.html

U.S. Department of Transportation. (2016, October). 5-2-9 unmanned aircraft systems (UAS) lost link (Air Traffic Organization Policy 7110.724). Retrieved from http://www.faa.gov/documentLibrary/media/Notice/N_JO_7110.724_5-2-9_UAS_Lost_Link.pdf