Global Positioning System Reference
In-Depth Information
The navigation units used in the sensor system are composed of a DGPS, an IMU (FOG) and
a VMS (Velocity Measurement System). The DGPS is responsible for measuring the vehicle's
position using a satellite signal. The IMU, consisting of accelerometers and gyroscopes,
measures the acceleration and direction changes of the vehicle, while VMS is in charge of
measuring the vehicle's velocity with high accuracy. The combination of GPS/IMU/VMS is
complementary as the velocity from VMS, and the acceleration and direction changes from
IMU can be used to locate a vehicle's position when the GPS signal is unavailable.
Moreover, the GPS can be used to rectify the output of IMU. VMS data is more accurate
when compared with DGPS; thus, the estimation of VMS errors becomes possible when
DGPS is valid. Therefore, it is possible to acquire a more precise positioning in the pure
PDOP condition like an urban operation by means of blending VMS data. The strapdown
navigation diagram for a ground vehicle mapping system is shown in Figure 13. The
Kalman filter is processed every 10 seconds.
Fig. 12. Ground vehicle based system
IMU position
GPS position
+
‐
+
‐
RTK-GPS
IMU
200Hz
IMU
velocity
GPS
velocity
1Hz
Position error
Ve l o c i t y e r r o r
Hybrid
IMU
Kalman filter
10 Hz
200Hz
VMS
Speed
10Hz
Fig. 13. Strapdown navigation diagram with VMS
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