IS-1500 User Guide
Thales Visionix, Inc.
MNL- 0024 (D)
Page 14 of 59
2.2
Achieving Accurate 6-DOF Tracking
The IS-1500 uses optical-inertial tracking, meaning that it uses visual data from the camera along with inertial data from the NavChip
IMU to acquire more accurate positional information. As the IS-1500 Natural Feature Tracker name indicates, it uses natural features
it identifies in the environment to track off of. In this context, natural features are those which the system determines to be still or
motionless. However, the IS-1500 can also use fiducial tracking, which was originally developed for earlier optical-inertial trackers like
the IS-1200 and HObIT.
2.2.1
3-DOF Tracking from the NavChip
As previously mentioned, the InertiaCam tracker included with the IS-1500 kit is a 6-DOF (rotation and position) tracker with a
monocular camera and a built-in NavChip IMU. The NavChip IMU is the source of high precision 3-DOF (three degrees of freedom-
yaw, pitch, and roll) tracking data. The accuracy of the data produced by the NavChip contributes greatly to the precision performance
of the IS-1500 as a whole.
The NavChip measures changes in acceleration and rotation by using an array of individually characterized, semi-redundant
accelerometers and gyroscopes. The angular rate and linear acceleration data from the sensors is integrated to produce the standard
ΔΘ (angular displacement) and ΔV (change in velocity) IMU outputs. These outputs are integrated at 200Hz to track orientation and
position.
This data allows the NavChip to also function as an attitude and heading reference system (AHRS) to measure yaw, pitch, and roll.
The accelerometers provide measurements of pitch and roll by calculating the proportion of gravity along those vectors. However,
yaw lies in the XY plane and gravity is along the Z axis, so accelerometers cannot be used to measure yaw. Because of this, gyroscopes
are also used, which are capable of measuring rotation about each axis. While gyroscopes are less accurate than accelerometers on
the whole, they serve the dual purpose of measuring yaw and providing supplemental pitch and roll data to increase overall accuracy.
This is why IMU specifications for pitch and roll accuracy and drift are typically listed separately from yaw, which is prone to drift if not
corrected by external data.
While IMUs provide accurate orientation data, even the high precision NavChip cannot be used alone to acquire accurate 6-DOF
positional data. Every IMU is prone to a certain degree of drift, especially in yaw, and while the NavChip has been specifically designed
to minimize drift, it does not eliminate it. The drift errors are not as problematic when performing a single integration to find
orientation. However, acquiring position information requires a double integration of the sensor data, allowing drift to accumulate
quadratically with time. An example of this is illustrated towards the end of the VINS Natural Feature Tracking subsection.
In order to obtain accurate 6-DOF tracking, an external source of positional reference is also required.