Unmanned Systems Technology 022 | XOcean XO-450 l Radar systems l Space vehicles insight l Small Robot l BMPower FCPS l Prismatic HALE UAV l InterDrone 2018 show report l UpVision l Navigation systems

92 Correction and augmentation While PPK (post-processed kinematic) correction does not provide real- time guidance, as RTK processing does, it can offer major advantages in photogrammetric work. For example, while both techniques require a GNSS base station, RTK requires that the station be located at a known control point, whereas UAV operators flying with PPK GNSS can simply set up their own base station at the vehicle’s ground control station. Furthermore, in the event of satellite signal interruption (or ‘loss of lock’), post-processing can resolve any georeferencing ambiguities by looking at mapping or point cloud data before and after the receiver lost its lock. RTK, on the other hand, can only access data before the event and therefore cannot compensate so well. Both corrections services are well- established. In recent years, however, a few others have become more common for unmanned systems, or are experiencing ongoing development. Satellite-based augmentation systems are now a common integration on GNSS receivers. These use a combination of ground stations and satellites to send differential signal messages to end-users to provide corrections for improving the reliability and accuracy of navigation data. For example, EGNOS in the EU uses 40 ground stations to receive and monitor GPS signals, four mission control centres for processing data and differential corrections, and a further six stations to transmit the corrections to three satellite transponders in geostationary orbit. Vehicle users can then access them on an EGNOS- enabled receiver. The FAA’s WAAS and the privately operated RTX networks provide similar services in the US, with the Japanese equivalent QZSS due to come online in November 2018. WAAS generally provides position and altitude accuracy within 1 and 1.5 m respectively (far higher than its target of 7.6 m 95% of the time). RTX is rated to provide 2.5 cm accuracy 95% of the time, correcting for errors such as ionospheric distortion and minor errors in satellite position and time reporting. These systems can overcome a key limitation of RTK: the finite availability of base stations to provide real-time corrections, in the US, Europe and elsewhere around the world. This limitation might become markedly less pronounced in the future, however, with a privately developed and managed cloud correction network of RTK stations October/November 2018 | Unmanned Systems Technology GNSS receiver architecture consists of a series of key components. First, the satellite’s signal is detected by a GNSS antenna, which converts it into an electrical signal. That then typically goes through a preamplifier to make the signal strong enough for processing. The receiver takes that signal and passes it through a series of bandpass filters to prevent interference from other signals (particularly those from nearby frequencies, such as BeiDou and Galileo, which are less than 100 MHz apart). The carrier frequency is then combined with a local frequency (produced from a local oscillator), converting and reducing it to a more useable intermediate frequency (a process known as ‘heterodyning’). Tracking loops then lock on to the ranging code and carrier wave from the satellites as they move through the sky. With heterodyning, the signals are then differentiated and separated among the receiver’s channels by reducing them to each channel’s base frequency, of which there can be dozens or even hundreds in a GNSS module. Increasing the number of channels ensures each signal has a channel to handle it, speeding up and securing satellite signal acquisition. It also improves position accuracy and sensitivity in remote areas, where GNSS signals can be difficult to receive. The channels then send the signals to a microprocessor, which is responsible for collecting and organising the positioning data for storage, or (more commonly in the age of software-defined radio) conducting further processing of signal data (such as satellite locations, timing, health and ionospheric information). This additional processing can take place within the receiver or on the UAV’s autopilot, with or without systems such as an integrated IMU, or a nearby reference base station. GNSS receiver anatomy GNSS receivers commonly incorporate multiple constellations including current corrections services such as SBAS, and future ones such as Japan’s QZSS (Courtesy of Trimble) Focus | Navigation systems