IS-1500 User Guide
Thales Visionix, Inc.
MNL- 0024 (D)
Page 21 of 59
2.3
Summary
The tracking data produced by the IS-1500 reflects the InertiaCam’s position and orientation, as defined by the tracker
reference frame, relative to the world reference frame. The X, Y, and Z position of the tracker is reported in meters.
Orientation is described using Euler angles to indicate roll, pitch, and yaw.
2.2 Achieving Accurate 6-DOF Tracking
The IS-1500 uses optical-inertial tracking, meaning that it uses visual data from the camera as an external reference to acquire
accurate positional information.
The IS-1500 uses two methods of optical-inertial tracking to achieve accurate 6-DOF data - VINS to track off of natural features
and PRA to track off of InterSense fiducial constellations.
2.2.1 3-DOF Tracking from the NavChip
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The NavChip IMU is the source of high precision 3-DOF (three degrees of freedom - yaw, pitch, and roll) data.
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While IMUs provide accurate orientation data, even the high precision NavChip cannot be used alone to acquire
accurate 6-DOF positional data.
2.2.2 VINS Natural Feature Tracking
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In the context of optical-inertial tracking, natural features are those which the system determines to be still or
motionless.
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With VINS, data on each feature is only maintained for currently tracked features.
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The more the InertiaCam moves about a natural feature, the more the system will learn about it, improving the
precision of the tracking data.
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A general guideline is to keep the InertiaCam field of view pitched about 20° below the perpendicular of the horizon
line while tracking.
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For accurate tracking, if there are moving objects in the tracking area, it is generally best to keep them out of the
InertiaCam’s field of view. If this is not possible, keep the InertiaCam still so the system can discard mobile features
and replace them with immobile, natural features.
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To ensure the EKF receives more reliable data upon initialization, keep the tracker still while tracking is initializing.
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To help optimize tracking performance, it is good practice to begin tracking by exercising the InertiaCam’s NavChip.
To do this, begin by placing the InertiaCam on a level surface with the camera facing up.
Proceed to rotate the InertiaCam about 90° in each axis, leaving it still for approximately five seconds or
more between each rotation.
Be sure not to obstruct the camera during this process.
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If the optical information within the VINS filter is determined to be unreliable, data from the IMU will carry more
weight in the tracking calculations.
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If the InertiaCam’s vision is obstructed for long periods of time, the system will begin to rely almost entirely on the
IMU data from the NavChip and pose data will diverge.
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It is best to keep the tracker still after the camera has been obstructed while the system recovers from a divergent
state.
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Constellations and their fiducials serve as known, predetermined points of reference for the tracker.
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Damage to any of the three circles of a fiducial can render it unidentifiable to the system. Because the InertiaCam is
a visible light tracker reading black-and-white fiducials, uneven lighting across the constellation can also render the
fiducials unidentifiable during pattern recognition.
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In a single constellation, no two fiducials can share a fiducial ID. They must all have a unique ID and therefore a
unique pattern.
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If the constellation in use is not accurately reflected in a map file, tracking can become compromised.
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For PRA to be used, the system must be able to recognize at least four fiducials simultaneously to have enough data
points to acquire optical lock.
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In practical usage, when placed in the tracked area, a constellation serves two main purposes. The first is that the
constellation defines a fixed world reference frame. The second is that it provides another level of drift correction.
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Using a constellation is recommended for applications that require tracking over large expanses or in cases where
the world reference frame needs to be precisely defined.
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Exercising the tracker is not necessary when using a fiducial constellation.