  In this paper, we develop an integrated markerless gait tracking system with
three Kinect v2 sensors. A geometric principle-based trilateration method is
proposed for optimizing the accuracy of the measured gait data. To tackle the
data synchronization problem among the Kinect clients and the server, a
synchronization mechanism based on NTP (Network Time Protocol) is designed for
synchronizing the server and Kinect clients' clocks. Furthermore, a time
schedule is designed for timing each Kinect client's data transmission. In the
experiment, participants are asked to perform a 60 s walk while the proposed
tracking system obtains the participant's gait data. Six joints (including left
hip, right hip, left knee, right knee, left ankle and right ankle) of the
participants are tracked where the obtained gait data are described as 6000
{movements} of joint positions (1000 {movements} for each joint). The results
show that the trilateration tracking result by the three Kinect sensors has a
much higher accuracy compared with the accuracy measured by a single Kinect
sensor. Within a randomly sampled time period (67.726 s in the experiment),
98.37% of the frames generated by the gait tracking system have timing errors
less than 1 ms, which is much better than the default NTP service embedded in
the Windows 8.1 operating system. The accuracy of the proposed system is
quantitatively evaluated and verified by a comparison with a commercial medical
system (Delsys Trigno Smart Sensor System).
