  Vehicular Networks are one of the enabling technologies for cooperative
Intelligent Transportation Systems. For accurate and precise time
synchronization in vehicular networks, Global Navigation Satellite System
(GNSS) is getting increasing attention for effective use of its time
dissemination to vehicular networks. In urban road scenarios, the blockage of
satellite signals affects precise positing due to erroneous geometry. However,
for timing, it creates less deflection as long as a single satellite is
visible. With the recent development of Multi-GNSS receivers, signal reception
no longer relies on a single GNSS constellation but all the existing four GNSS
systems. This increases the number of visible satellites in the sky, thus
increasing the coverage of satellite signals. Developing a method of GNSS time
synchronization, we have conducted a systematic field-based study in this paper
to determine the attainable timing accuracy in vehicular networks using
commercial-grade GNSS receivers and off-the-shelf communication devices. Our
experimental results show that the attainable timing accuracy from GNSS
services in consumer-grade on-board devices can be as good as 2 microseconds. A
momentary outage of GNSS time solution due to signal blockage on the road can
lead a clock error of up to sub-microseconds, which still meets the desired
requirement for most vehicular applications.
