GNSS stands for Global Navigation Satellite System and currently consists of 6 different satellite systems (GPS, GLONASS, Galileo, BeiDou, IRNSS, QZSS), some of them still under development. The principle of localization is the same for all of them, though. By receiving a satellites signal, which is an electromagnetic wave propagating at the speed of light, a GNSS receiver measures the time required for the signal to propagate from the satellite to the receiver. Multiplying this time with the speed of light yields an approximation of the distance between the receiver and the satellite. To determine the location of the receiver using a technique known as trilateration at least four distances to different satellites are necessary, three for the coordinates of the receiver on earth and the fourth to determine its clock error.
GNSS and ADAS
In the beginning of satellite navigation, receivers used GPS signals only, which under average conditions resulted in a positioning accuracy not worse than 5m. Due to clock errors, atmospheric effects or reflections / blocking of a signal by the surroundings the calculation of the actual distance to a satellite is nearly impossible in general. Now, that doesn’t sound that good but one of the main reasons why manufacturers kept GPS (and now several other satellite systems) for ADAS around is its availability and unaffectedness by most environmental conditions, i.e., if a signal is not completely blocked, absolute positioning is still possible. Furthermore, using several satellite systems, signal frequencies and correction services centimeter-accurate positioning is achievable today. Eventually, combining GNSS and inertial measurement unit (IMU) data can lead to a highly accurate and reliable position in harsh weather conditions when optical sensors fail even if GNSS signals are blocked completely for a short amount of time.