Unmanned Systems Technology 019 | Navya Autonom Cab | Batteries | UGVs Insight | UAV Factory UAV28-EFI | Swiss Aerobotics Hummel | UMEX 2018 report | Antennas | Oceanology International 2018 report

84 Focus | Antenna systems environment, given that metals are often unsuitable as hull materials for the thinnest ‘peel and stick’ patch antennas. Blade antennas, most commonly shaped as a trapezium plane, can be designed with an aerodynamic trim to minimise their effect on drag, potentially acting similarly to winglets. At the same time, they reduce the number of antenna apertures required on the vehicle body as typically they are dipole, omnidirectional systems that combine multiple elements to cover distinctly different bands within a single set-up. On the other hand, blade antennas have been known to hamper aerodynamics and increase weight. They also suffer from interference from surrounding materials and subsystems if full consideration has not been paid to their design and placement. Integrating an antenna inside the hull is possible thanks to the greater availability of RF-permeable materials for radomes. This can offer better aerodynamics for unmanned aerial systems, or protect UGV antennas from obstacles and debris. Mounting it externally though will still generally give better performance by ensuring that its radiating elements are kept well away from any interference with the onboard materials and subsystems. It also helps to ensure that all line-of- sight data links from the GCS to the UAV remain clear when the aircraft banks or rolls. Keeping antennas as far away as is feasible from the other electronics on the vehicle – including camera systems – is important for maintaining a desirable SNR free from electromagnetic interference (EMI). Other subsystems may even be impacted in turn by the antenna, as improperly shielded cabling can pick up spurious RF signals. Antenna systems installed sub-optimally and too close to payloads have been known to send false movement instructions to cameras, causing them to pan and zoom erratically. GNSS antennas Such considerations warrant even closer scrutiny in the case of antennas designed to resonate at GNSS frequencies. The inherent weakness of GNSS signals relative to data links for command, control and payload telemetry makes satellite navigation and localisation even more dependent on optimal antenna integration. All GNSS antennas must be installed so that they have a clear view of the sky, and ideally the element should be placed vertically higher on a UAV than any other part in order to receive a signal well. That tends to be problematic with multi-rotor systems, as the rotors will occasionally be designed at a high elevation relative to the hub of the aircraft, where the antenna would logically be placed. The noise generated by the rotors, and the interference that can result, mean the rotors should not be placed above or near the GNSS antenna. It follows that other subsystems can cause even more harm to GNSS links than to control and the data links. GNSS antennas generally use a low-noise amplifier (LNA) that boosts the weak satellite signal from the element to a level that can be used by the receiver. However, the likelihood of EMI from inadequately shielded onboard electronics such as sensors, motors or high-speed processors still needs to be considered, to avoid losses of navigation data at critical junctures. Lastly, when integrating a GNSS antenna, care must be taken not to use too large a ground plane, as that may actually create destructive waves and make the antenna completely ‘deaf’ to GNSS signals. For example, an autonomous road vehicle using its own car roof as a ground plane for a GNSS antenna can prove problematic or helpful, depending on the geometry and size of the roof. The environment surrounding an unmanned vehicle can also cause significant interference with the satellite link, beyond the simple multi-path concerns that particularly affect UGVs and USVs. In particular, the advent of LTE and 5G comms in urban areas is leading to increased rates of intermodulation distortion (IMD), as airspace becomes congested with signals that happen to fall within the GNSS bands. These can saturate the first stage of the LNA, April/May 2018 | Unmanned Systems Technology Satellite link antennas are designed to point upwards with a clear view of the sky, to enable a clear signal to Iridium, GPS, GLONASS and other constellations (Courtesy of Tallysman)