Civil Engineering Reference
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gyros. Therefore, the system is currently susceptible to the accumulation of
heading errors over time. The system also measures vertical position, but less
accurately so.
The IMU based PDR system is very accurate in measuring linear displacements
with errors being consistently less than 2% of the distance travelled (Ojeda and
Borenstein, 2007). The accuracy of the PDR system, however, degrades gradually
with extreme modes of legged locomotion, such as running, jumping, and climb-
ing. The main drawback of the PDR system is the drift error that accumulates with
the distance travelled by the mobile user. To overcome this, the authors have
developed algorithms that integrate PDR positioning systems with GPS systems
that will correct the drifting error accumulated over time.
6.5.2 Integrated tracking system (ITS)
The Integrated Tracking System (ITS) consists of components of both GPS and
PDR systems. The user wears a backpack with the GPS receiver in it. The IMU of the
PDR system is strapped to the user's foot. The computer in the PDR system is
hooked into the user's pocket (Figure 6.38). Amagnetic tracker that determines the
subject's orientation is attached to the hard hat worn by the user. The GPS, PDR
and tracker systems are connected to the serial ports of a small laptop containing
the ITS software. The accuracy of the GPS is enhanced by implementing the Real
Time Kinematic (RTK) corrections. The ITS records the mobile user's location as
dictated by the RTK-GPS and the PDR separately. However, the coordinate system
used by the PDR is different from theWorld Geodetic System84 (WGS 84) latitude,
longitude, altitude coordinate system used by the RTK-GPS.
To resolve this issue, the ITS uses Vincenty's Forward Pass Algorithm for WGS
84 to convert the user's location from a local X, Y, Z coordinate system to a location
on the WGS 84 latitude, longitude, altitude coordinate system. The accuracy of
RTK-GPS (3-5 centimeters) is much higher than the accuracy of the PDR.
Additionally, the accuracy of the PDR decreases with the distance travelled by
the mobile user. As a result, the position of the user as dictated by the RTK-GPS is
almost always inevitably more accurate than the position dictated by the PDR. The
principle behind determining the ITS coordinates is that RTK-GPS coordinates, if
available, always take precedence over the PDR coordinates.
When the mobile user enters a GPS denied environment, such as indoor
structures, urban canyons, and so on, the ITS no longer receives the mobile user's
position as dictated by the RTK-GPS system. When the ITS loses connectivity with
the RTK-GPS system, the ITS ensures that the position of the mobile user is the
position as dictated by the PDR system adjusted for drift correction. The ITS
continues to locate the mobile user's position as dictated by the PDR for the entire
duration that the user is in the GPS denied environment. As soon as the mobile
user steps out of the GPS denied environment and receives a signal from the RTK-
GPS system, the ITS switches back and the ITS mobile user's location is dictated
once again by the RTK-GPS system. The integration algorithm of the ITS
seamlessly switches between the RTK-GPS and PDR systems when required,
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