Unmanned Systems Technology 006 | ECA Inspector Mk2 USV | Antenna systems | Northwest UAV NW-44 | Unmanned ground vehicles | Navigation systems | Lunar X challenge

65 terrestrial radio networks, geostationary satellites and even UAV-based pseudo satellites (‘pseudolites’), which do jobs normally associated with spacecraft. Japan’s Quasi-Zenith Satellite System (QZSS) is a satellite-based augmentation system designed to fill coverage shadows created by the urban canyons of Japan’s cities. By 2018, the full capability will be provided by three satellites in highly elliptical orbits plus a geostationary spacecraft covering Japan, Australia, the Indonesian archipelago and large areas of the Asia-Pacific region. Other systems are also under development, for example the seven- satellite Indian Regional Navigation Satellite System, which is expected to be in orbit by March 2016. Generally, the more GNSS satellites that can be ‘seen’ by Earth-based vehicles and platforms, the faster and better the navigation solution derived from the satellite signals will be. Discussions of GNSS accuracy get complicated very quickly. Following US Department of Defense (DoD) practice, all constellations provide two accuracy standards through different ranging signals transmitted by the satellites – known as the Standard Positioning System (SPS) and Precise Positioning System (PPS) in the case of GPS. The Navstar satellites in the GPS constellation provide their SPS with a Coarse/Acquisition (CA) code ranging signal on the L1 frequency that is available to anyone, and the PPS through a Precision (P) code that is reserved for authorised use on the same frequency. Furthermore, the P code is normally encrypted into what is called the Y code, which can only be accessed by users with the right decryption software in their receivers. Navstar satellites also transmit a second P or Y code signal on the L2 frequency for comparison to compensate for atmospheric distortion. In the fourth edition of the official GPS performance standard, the DoD quotes a best global average user range error for 95% global coverage with new data – or zero age of data – of less than or equal to 2.6 m for the signals in space for a healthy satellite, but this is without any augmentation, and of course, the 3D position is a function of the receiver’s range from several satellites. The equivalent figure for the SPS is 7.8 m. Augmentation systems can improve significantly on these figures, sometimes driving position errors down to a matter of centimetres. Accuracy is only one measure of Required Navigation Performance established by civil aviation, which has the most critical interest in it. The others are availability (defined as a percentage of the time that the system’s services are available), continuity (defined as the ability of the complete system to maintain that accuracy without interruption) and integrity, which is a measure of the trust that users can place in the information, taking into account the timeliness of any warnings from the system itself not to rely on it. The delay between switching a receiver on and obtaining an accurate position, known as Time To First Fix (TTFF), provides an additional performance measure. Navigation systems | Focus Unmanned Systems Technology | February/March 2016 Building up to a constellation of 30, the European Galileo GNSS has ten spacecraft in operation and two more in orbit and preparing to go online (Courtesy of the ESA) Generally, the more GNSS satellites that can be ‘seen’ by Earth-based vehicles, the faster and better the navigation solution derived from their signals will be