New-Tech Europe Magazine | March 2018

The path to high GNSS accuracy

Thomas Nigg, u-blox

For fully autonomous driving to become reality, several technologies will have to reach maturity and be rolled out in concert. One of them is affordable, scalable, and reliable high precision positioning. The past decades have seen substantial improvements in the performance of Global Navigation Satellite System (GNSS) technology. In the early 2000s, the time it took to get a first accurate position went from minutes to under thirty seconds. In the latter half of the decade, receiver sensitivity improved dramatically – from -130dBm to -167dBm. By 2015, the number of functional positioning satellite constellations had gone from one global constellation in 2000 (the USA’s GPS) to four (GPS, Russia’s GLONASS, China’s BeiDou, and the European Union’s Galileo), which

are complemented by two regional systems (India’s NAVIC and Japan’s QZSS). This opened the doors to multi-constellation GNSS receivers. The satellite signals, too, have been modernized, and as of 2018, multi- band GNSS will become affordable. These advances set the stage for the next big theme in GNSS: achieving decimeter- or centimeter-level accuracy. GNSS receivers triangulate their position using their distance from at least four GNSS satellites. Because they measure this distance based on the time it takes a satellite signal to reach them, even the slightest errors – down to a few billionths of a second – can negatively impact accuracy. Errors in satellite orbit position can lead to around 2.5 meters’ loss of accuracy. Satellites clock errors

can add another 1.5 meters. And perturbations in the troposphere and the ionosphere can add another one and five meters respectively – even more if the satellite is close to the horizon or during periods of intense solar activity. By far the largest error is caused by multipath effects, in which satellite signals reach the receiver on multiple or indirect trajectories, for example by bouncing off building walls in urban canyons. In open sky conditions, standard accuracy GNSS receivers are accurate to within about two meters. High precision GNSS systems dramatically improve precision using GNSS correction data to cancel out GNSS errors. One way to obtain this data involves monitoring GNSS signals from a base station at a known location. Deviations from the

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