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

86 preventing the signal and navigation information from reaching the receiver. A prime example of that is in Japan, where the LTE signal runs precariously close to the GNSS signal. Antennas may therefore receive no signal at all if they are close to a transmission tower. Using a pre-filter on the GNSS antenna is therefore becoming more popular across unmanned systems and other applications. The pre-filter feeds into the first section of the LNA and provides mitigation against near-frequency signals, multi-path interference or IMD by blocking them out before they can pass to the receiver. Users seeking further accuracy may also opt for one of the many correction services now available. Post-processed kinematic services are finding favour for applications such as civil and commercial photogrammetry, that require precise geolocation only after the fact. However, for more time-critical applications such as pipeline inspections, security surveillance or search-and- rescue operations, real-time kinematic (RTK) processing may be necessary to provide real-time accuracy. In general, there are two classes of RTK systems. Single-frequency RTK GNSS uses only the upper GNSS bands, which run from 1542 to 1575 MHz. Dual- frequency RTK GNSS meanwhile, which is necessary for centimetre-level precision, uses both upper and lower band frequencies, such as GPS L1 and L2, the latter of which is centred on 1227.60 MHz. Recent advances The utility of software algorithms to identify and ignore multi-path signals and other disruptions is limited by the extent to which a bad GNSS signal can be corrected. As the number of GNSS constellations and therefore sources of interference expands, the need arises to find alternatives to the dominant single- pin ceramic patch antennas for GNSS. Single-pin antennas were ideal when GPS L1 was the sole global navigation system. However, modern systems aim to receive from upper and lower bands such as GPS L1 and L2, and from other countries’ constellations such as Russia’s GLONASS, China’s BeiDou and the EU’s Galileo. Similar systems for regional navigation satellites, such as Japan’s QZSS and India’s NAVIC, are also on the horizon. As a result, unmanned vehicle developers looking to develop systems for use across multiple nations and continents increasingly face the task of finding and integrating antenna systems that can access multiple constellations and broader bands. GPS L1, as a key example, is effectively no longer just a signal at 1575.42 MHz, but instead is centred on that frequency and can be accessed at tens of MHz above or below it (depending on the craft’s bandwidth). If an antenna designer using a single- feed antenna wanted to access both GPS and GLONASS, the latter of which is centred on 1602 MHz, they might tune it to 1590 MHz. Here, the antenna would have a perfectly circular response owing to both signals being right-hand circularly polarised (RHCP). The problem however is that there is no usable signal at 1590 MHz. Were the designer to tune to where the signals were usable – at 1606 and 1575 MHz – the antenna would exhibit a linear response. A linear antenna cannot distinguish or differentiate between an RHCP signal and a left-hand circularly polarised (LHCP) signal, the latter being the multi-path ‘bounced’ signal. Single-pin antennas therefore end up April/May 2018 | Unmanned Systems Technology Post-processed kinematic services are finding favour for applications that need precise geolocation only after the fact Using air as the dielectric core of an antenna reduces weight and size for UAVs and other vehicles with major SWaP constraints (Courtesy of Maxtena)