Global Positioning System Reference
In-Depth Information
14
Hybrid Positioning and Sensor Integration
Masahiko Nagai
Asian Institute of Technology
Thailand
1. Introduction
Utilization of a mobile platform is important for effectively acquiring spatial data over a
wide area (Zhao & Shibasaki, 2000). Although mobile mapping technology was developed
in the late 1980s, the more recent availability of Global Positioning Systems (GPSs) and
inertial measurement units (IMUs), the latter being a combination of accelerometers and
gyroscopes, has made mobile mapping systems possible, particularly for aerial surveys and
ground vehicle surveys (Manandhar & Shibasaki, 2002). Remote sensors—such as image
sensors or laser scanners—are instruments that gather information about an object or area
from a distance. Using these sensors for surveying and collecting information from mobile
platforms has become a valuable means of disaster mapping, environmental monitoring,
and urban mapping, amongst others.
Trajectory tracking of a mobile platform is considered part of directing the movement of a
platform from one place on Earth to another. Although GPS gives excellent trajectory
tracking performance, it is not adequate to use for mobile mapping in terms of its lack of
attitude information and low data acquisition frequency. On the other hand, an IMU is a
closed system that is used to detect attitude and position with high frequency.
An IMU exhibits position errors, called drift errors that tend to increase with time in an
unrestrained manner. This degradation is due to errors in the initialization of an IMU and
inertial sensor imperfections such as accelerometers bias and gyroscope drift. By mitigating
this growth and bounding, the errors update the inertial system periodically by fixing
external reference sources. The combination of GPS and IMU has become increasingly
common as the characteristics of these two mobile positioning technologies complement
each other. Firstly, an IMU provides continuous positioning drifts, whereas GPS
measurements do not drift, but are not continuously available. Also, GPS, as external data, is
used not only for position updates but also for error correction of inertial components such
as attitude, heading, velocity, gyro bias, and accelerometer bias. However, the integration of
IMU and GPS is restricted due to the cost of high quality inertial components.
To obtain both the wide area coverage of remote sensors and the high levels of detail and
accuracy of ground surveying at low costs, a mobile mapping system has been developed in
this research. All the measurement tools are mounted on a mobile platform to acquire
detailed information. This mobile platform integrates and combines equipment such as
digital cameras, a small and cheap laser scanner, an inexpensive IMU, GPS, and VMS
(Velocity Measurement System). These sensors are integrated by a high-precision
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