Perception sensors and GNSS have their strengths and weaknesses in different areas and can complement each other to deliver optimal safety for automated piloting. The table below was inspired by a presentation given by Curtis Hay, Technical Fellow General Motors during the Munich Satellite Navigation Convention 2019.

	Technology	Strength	Weakness
Perception	Radar	Good ranging accuracy	Cannot detect road markings
	Lidar	Highly accurate ranging	Higher cost  Less effective in featureless areas (country roads)
	Camera	Good object detection  Good in tunnels/urban canyons	Less effective on featureless roads  Less effective in low visibility (snow, rain, darkness, fog)  Roads without lane markings, construction areas
Absolute Localization	HD map + Perception	Good accuracy	High acquisition and maintenance costs  Requires continuous updates  Difficulty in changing surroundings (highways)  May not be available everywhere
	GNSS+IMU	Good accuracy  Common global reference between vehicles  Good in low visibility, featureless roads and changing surroundings (highways)	Less reliable in high urban canyons  Not usable in long tunnels (a few km) due to high drift rate of IMU

 

Featureless roads, rain, darkness and snow are environments where sensor-based perception struggles, while GNSS positioning remains accurate.

 

SSR centimeter accuracy

      GPS in the average infotainment system today has approximately 5 m (16 feet) accuracy which is enough for simple navigation. High-precision GNSS receivers, on the other hand, can place you on the map with centimeter accuracy.

      This exceptional precision can be achieved by combining GNSS signals with corrections from local reference stations. Such reference station networks, such as SSR networks, are being setup today in the US and in many other parts of the world. To create such a reference network, a stationary GNSS receiver is needed about every 300 km (186 miles) along the road. It can be attached to existing infrastructure such as telecommunication towers or to roadside units. “When the customer starts the engine in the vehicle, we need to deliver continuous GPS corrections to the car, and that’s to ensure we know which lane the vehicle is operating in”, said Curtis Hay, Technical Fellow at GM in an interview at the International Motor Show, 2018.

Integration with IMU and odometry

      High degree of reliability is possible in specialized GNSS receivers due to their multi-frequency, multi-constellation GNSS technology. This means that the receiver is using all the signals being sent out by all GNSS satellites available world-wide: GPS, GLONASS, Galileo, BeiDou and QZSS. GPS receivers need a line-of-sight to at least 4 satellites to know where they are, and they need even more satellites for centimeter-level positioning. When the view of the sky is partially blocked, in urban areas or under trees, having access to more GNSS signals can make a big difference for availability and accuracy. When the sky is temporarily blocked, other sensors such as IMU or wheel odometry continue localization by providing relative position to the last known GNSS location.

      What if the sky is completely blocked, like under thick trees or bridges? An IMU provides relative positioning to a receiver that has lost satellite visibility, so that a position can be continuously maintained as the vehicle moves under bridges or through underpasses.

 

GNSS reliability and integrity indicators

      But how can a driverless vehicle be sure that it can rely on GNSS positioning? The answer lies in monitoring the receiver’s integrity indicators. By outputting the maximum uncertainty of the current position, the receiver indicates to the ECU (Electronic Control Unit) whether it is safe or not to execute the planned maneuvers.

Tried, tested and true

      GNSS brings global positioning along with its integrity information to vehicle localization solutions. Other safety-of-life applications such as aviation have already been using GNSS for decades. In the ADAS system, precise GNSS positioning can serve as localization back-up in instances where sensor-based perception has difficulty. It is no wonder that today automotive leaders such as GM and Bosch are investing in high-performance GNSS as part of their advanced auto piloting solution.

Septentrio and high-precision GNSS

      Septentrio offers a wide range of solutions geared to the automotive market. Their advanced GNSS chipset, high-quality measurement engine software and positioning engine software are available for seamless integration into the ECU. Together with an IMU and wheel odometry they provide localization and integrity information needed for safe assisted driving. AsteRx multi-frequency, multi-constellation receivers have been serving for years as reference stations in various RTK, PPP and SSR networks around the world. Septentrio’s technology and experience in the automotive market makes it your perfect partner for safe ADAS localization solutions.   

Download this insight in print ready pdf here.

摘錄自：https://www.septentrio.com/en/learn-more/insights/role-gnss-localization-safe-assisted-driving