  In regions where global navigation satellite systems (GNSS) signals are
unavailable, such as underground areas and tunnels, GNSS simulators can be
deployed for transmitting simulated GNSS signals. Then, a GNSS receiver in the
simulator coverage outputs the position based on the received GNSS signals
(e.g., Global Positioning System (GPS) L1 signals in this study) transmitted by
the corresponding simulator. This approach provides periodic position updates
to GNSS users while deploying a small number of simulators without modifying
the hardware and software of user receivers. However, the simulator clock
should be synchronized to the GNSS satellite clock to generate almost identical
signals to the live-sky GNSS signals, which is necessary for seamless indoor
and outdoor positioning handover. The conventional clock synchronization method
based on the wired connection between each simulator and an outdoor GNSS
antenna causes practical difficulty and increases the cost of deploying the
simulators. This study proposes a wireless clock synchronization method based
on a private time server and time delay calibration. Additionally, we derived
the constraints for determining the optimal simulator coverage and separation
between adjacent simulators. The positioning performance of the proposed GPS
simulator-based indoor positioning system was demonstrated in the underground
testbed for a driving vehicle with a GPS receiver and a pedestrian with a
smartphone. The average position errors were 3.7 m for the vehicle and 9.6 m
for the pedestrian during the field tests with successful indoor and outdoor
positioning handovers. Since those errors are within the coverage of each
deployed simulator, it is confirmed that the proposed system with wireless
clock synchronization can effectively provide periodic position updates to
users where live-sky GNSS signals are unavailable.
