Unmanned Systems Technology 017 | AAC HAMR UAV | Autopilots | Airborne surveillance | Primoco 500 two-stroke | Faro ScanBot UGV | Transponders | Intergeo, CUAV Expo and CUAV Show reports

78 Focus | Transponders In many cases, a UAV will be tracked by only a single receiver on the ground. Where to place the antenna on a UAV should therefore take the transponder’s architecture into account. If the ADS-B signal is vertically polarised, for example, then the antenna is best placed vertically, either above or below the aircraft, depending on whether the transponder signal is intended for ground-based receivers or aircraft flying below or above. As a possible solution, a dipole or doublet antenna can be mounted on the side or rear of a UAV. It can take the form of a top-mounted mast and a bottom-mounted component to enable equal strength of transmission above and below the craft, although signals might still be blocked by airframe shadow if that issue is not investigated and addressed during flight tests. Such antennas could actually be a requirement for UAVs above a certain maximum airspeed or weight, depending on the airspace or future regulations. GNSS Equal care must also be taken when selecting and integrating GNSS with a UAV’s ADS-B transceiver. While it is commonly assumed that a UAV’s autopilot GPS can be relied on to drive the ADS-B output, it would not actually result in a legal configuration with respect to the FAA’s ADS-B mandate, which requires various metrics showing the reliability of the ADS-B data to be embedded into the GNSS and broadcast by the transponder. That could then inform ATC operators and nearby aircraft as to the accuracy of the positioning information they are receiving. As laid out in the FAA Part 91 regulations, this information includes figures for Navigation Accuracy Category for Position (NACP), signifying positioning accuracy; Navigation Accuracy Category for Velocity (NACV), signifying accuracy of velocity data; and Navigation Integrity Category (NIC), specifying an integrity containment radius around the reported position of the aircraft. Also included are System Design Assurance (SDA), which is the probability of false or misleading transmissions owing to malfunctions; and Source Integrity Level (SIL), which signifies the probability of the reported horizontal position exceeding the containment radius defined by the NIC on a per- sample or per-hour basis. All of this information would be transmitted along with conventional details such as a craft’s latitude and longitude, length, width and barometric altitude. The figures reported must also fit within key constraints in order to meet performance standards. For example, NACP must be less than 0.5 nautical miles, NACV must be below 10 m/s, NIC below 2 nautical miles. Most existing aviation GPS broadcasts none of this additional information. An autopilot’s GPS can be connected to a transponder, and will broadcast the UAV’s position as part of the ADS-B Out protocol, but without the above accuracy information, authorities might lower those targets, defeating the original aims of UAV conspicuousness and safety. Spectrum congestion Another disadvantage with ADS-B, and one that has posed a high risk for more than a decade, is the congestion in its frequencies. Transponders and ADS-B both communicate over the 1090 MHz band, and there can be thousands of messages being transmitted through a small corridor in airspace at any given instant. UAVs will soon be added to this space, exacerbating the problem. In addition, excessive noise reduces the ability to actually receive and decipher the transponders’ replies. As the detection range falls, so does the amount of time available to respond to an approaching aircraft, so the risk of a collision rises. That may not be a concern when conducting surveillance missions in areas with relatively low airspace activity, such as deserts or tundra, but as UAVs operate increasingly near busy airports and in other highly populated areas where there are a lot of other ADS-B transponder-equipped aircraft, it becomes a dangerous weakness. One proposed solution involves lowering the power output of ADS-B transponders in small UAVs, enabling them to maintain a suitable detect- December/January 2018 | Unmanned Systems Technology As UAVs proliferate near congested airspaces, the use of ADS-B In receivers will be critical for GCS operators to avoid dangerous collisions (Courtesy of uAvionix)