  Time synchronization is critical for the operation of distributed systems in
networked environments. It is also demanded in vehicular ad-hoc networks
(VANETs), which, as a special type of wireless networks, are becoming
increasingly important for emerging cooperative intelligent transport systems.
Global navigation satellite system (GNSS) is a proven technology to provide
precise timing information in many distributed systems. It is well recognized
to be the primary means for vehicle positioning and velocity determination in
VANETs. However, GNSS-based time synchronization is not well understood for its
role in the coordination of various tasks in VANETs. To address this issue,
this paper examines the requirements, potential benefits, and feasibility of
GNSS time synchronization in VANETs. The availability of GNSS time
synchronization is characterized by almost 100% in our experiments in high-rise
urban streets, where the availability of GNSS positioning solutions is only
80%. Experiments are also conducted to test the accuracy of time
synchronization with 1-PPS signals output from consumer grade GNSS receivers.
They have shown 30-ns synchronization accuracy between two receivers of
different models. All these experimental results demonstrate the feasibility of
GNSS time synchronization for stringent VANET applications.
