Filter Kinematic Constraints

A selection of kinematic constraints is available that can improve Navigation Filter performance under certain dynamic conditions. When enabling these kinematic constraints, it is important to consider the typical motion profile of the application and ensure that the assumption underlying each constraint is satisfied most of the time.

 

Zero velocity update (velocity ZUPT)

This constraint helps stabilize the navigation solution during prolonged stationary periods. This constraint is useful for vehicles which make frequent to occasional stops.

Assumption

The platform is stationary when the reported velocity falls below the user provided threshold.

Command 

Body Frame Velocity (0x13,0x2A)

Implementation

To implement the velocity ZUPT on the 3DM-CV7, the Body Frame Velocity (0x13,0x2A) is employed to afford the user more flexibility to send velocity ZUPT's on any axis. This enables the user to tune for their application specific dynamics which may not be represented in a 2D planar ground vehicle ZUPT.

The user must send this command when the scalar magnitude of the GNSS reported velocity vector is equal-to or less than the threshold value set by the user. The External GNSS NED Velocity (0x94,0x05) vector can be found in the External GNSS (0x94) descriptor set.

 

Zero angular rate update (angular rate ZUPT)

This constraint helps reduce gyro bias errors on the vertical (yaw) axis. This constraint is useful for ground vehicles which make prolonged stops, especially when no heading aid is available.

Assumption

The platform is not rotating when the angular rate estimate falls below a user provided threshold.

Command

Zero Angular Rate Update Control (0x0D,0x20)

 

Wheeled vehicle constraint

This constraint helps maintain a more accurate platform heading, especially during periods of low acceleration and poor heading solution. It is not advisable to use this constraint in applications where the platform velocity and heading routinely diverge, such as with aircraft.

Assumption

Components of velocity perpendicular to the primary axis of the platform (as indicated by the mounting transformation) are zero.

Command 

Body Frame Velocity (0x13,0x2A)

Implementation

To implement a wheeled-vehicle constraint on the 3DM-CV7, use the Body Frame Velocity (0x13,0x2A) command. Send the command at the desired rate with a Y-axis velocity of 0.0 m/s, and the X and Z-axes data marked as invalid. A minimum of 1 Hz is recommended.

Additionally, a user may implement a 2D mode by sending 0.0 m/s for the Z-axis.