Doppler Velocity Log DVL deadreckoning

A Doppler velocity log (DVL), integrated with a precise heading reference, is another standard instrument for underwater robotics. As a standalone solution, DVL navigation provides a dead-reckoning estimate of position based on discrete measurements of velocity over the seafloor. To produce this dead-reckoning estimate in local coordinates sequential DVL measurements are related to a common coordinate system. Because the raw measurements are made relative to the sensor, the attitude (heading, pitch and roll) of the sensor relative to the common coordinate system must be measured. Once compensated for attitude, the velocity measurements are accumulated to estimate position. The position uncertainty for standalone DVL dead-reckoning grows with both time and distance. Fig. 6 illustrates a simple example of this error growth based on a vehicle moving at a constant speed along the x-axis. Velocity uncertainty causes uniform error growth in both directions while heading uncertainty dominates the error growth in the across track direction. To further quantify the dynamics of uncertainty in such a situation we propose an observation model compatible with the LBL uncertainty model presented above.

Dvl Doppler Velocity Log
Fig. 6. Illustration of odometry uncertainty dynamics. The ellipses illustrate the 1-ff uncertainty in the along track (x) and across track (y) directions. Five discrete vehicle positions are shown, indexed by i. The distance between consecutive positions is indicated by d.

The DVL instrument provides independent measurements of velocity (zVk ) in each of three dimensions (indexed by fc).

We characterize the uncertainty as mutually independent additive, zero-mean, Gaussian white noise.

Transforming these sensor frame measurements into a local coordinate frame requires knowledge about sensor and vehicle attitude. Heading is the most important and difficult to accurately observe measurement for this coordinate rotation. Again we use a simple additive Gaussian noise model to represent the heading measurement.

It is possible to carry forward the complete three dimensional (k = {1,2,3}) formulation (Eustice, Whitcomb, Singh, & Grund, 2007), but it is non-limiting to simplify this representation to a two dimensional representation. In particular we assume the pitch and roll are transformations that do not affect the uncertainty growth. We also consider the uncertainty along-track to be independent of the uncertainty across track. These considerations capture the dominant dynamics of error growth (velocity and heading uncertainty) and allow us to simplify our two-dimensional model, preserving intuition. The resulting odometry measurement model considers discrete observations of incremental distance (z0.), where j is the temporal index for sequential velocity measurements.

The additive noise is characterized by a two-dimensional covariance matrix (E0) in the along track and across track directions.

ttf O

The diagonal matrix in equation (9) is a consequence of the independent along track and across track uncertainty growth. The along track term, in the upper left, captures growth of position uncertainty as a function of velocity uncertainty, based on random walk uncertainty growth. The across track term, in the lower right, is dominated by heading uncertainty; therefore, the across track uncertainty grows linearly with distance travelled. Returning to Fig. 6 we can predict how the odometry error will grow for a straight line vehicle trajectory. The figure shows the along track uncertainty the x direction and across track uncertainty in the y direction. The aspect ratio of error ellipses increases with time, illustrating combination of linear growth of the along track uncertainty (growing with distance travelled) and growth proportional to the square root of time of the along track position.

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