Geography Reference
In-Depth Information
measures changes in orientation and acceleration, the INS determines changes in position
and attitude, but the initial values for these parameters must be provided from an external
source. Once these parameters are known, an INS is capable provide an autonomous
solution with no external inputs. However, because of errors in the IMU measurements that
accumulate over time, an inertial-only solution degrades in time unless external updates
such as position, velocity or attitude are supplied.
The GPS receiver provides auxiliary information for the INS, and it is reciprocally aided by
feedback from the INS to improve signal tracking. The feedback from the INS to the GPS
engine is the deeply coupled aspect of the system (Fig. 14).
Output
RTK-GPS
INS
Figure 14.
Bidirectional INS/GPS Coupling
The combined GPS/INS solution of the SPAN (Synchronized Position Attitude Navigation)
integrates the raw inertial measurements with all available GPS information to provide the
optimum solution possible in any situation. By using the high accuracy GPS solution, the
IMU errors can be modelled and mitigated. Conversely, the continuity and the relative
accuracy of the INS solution enable faster GPS signal reacquisition and RTK solution
convergence.
GPS signal reacquisition is dramatically improved when running SPAN. This is a key
performance feature in restricted coverage environments, such as urban canyons, where the
user may have only a few seconds of satellite visibility before another blockage occurs. With
SPAN technology, the user will be able to get GPS measurements in that small window of
visibility. That means the INS will have shorter periods of free navigation and smaller
errors, since the GPS is available more often for aiding (Kennedy and Rossi, 2005).
4.3.1. The IMU unit montage conditions
It is necessary to mount the IMU unit in a fixed location where the distance from the IMU to
the GPS antenna phase center is constant. Also, the use must ensure that the orientation,
with respect to the vehicle and antenna, is also constant. For the attitude output to be
meaningful, the IMU should be mounted such that the positive Z-axis marked on the IMU
enclosure points up and the Y-axis points forward through the front of the vehicle, in the
direction of track and X pointing to right. (IMAR-iTraceRT-F200, 2008).
The body coordinate system is defined as given in figure 15:
4.3.2. GPS antenna montage conditions
Mount the GPS antenna close to the IMU housing. It is recommended to mount the
antenna in top of the IMU if the system is mounted on a car, truck, ship or aircraft. In the
