Pathfinder DVL Guide
April
2018
EAR-Controlled Technology Subject to Restrictions Contained on the Cover Page.
Page 47
FFT can be useful in catching continuous interference. However, its sampling is dependent on the number
of FFT samples selected by the user. It may therefore update too slowly to detect an intermittent acoustic
interference source. On the other hand, the long water profile may detect interference from an intermit-
tent source as it listens for a longer period, although this method may not be effective for detecting
self-interference or continuous low level source interference. When using the water profiling tech-
nique to identify interference, the user must monitor both RSSI and Correlation in order to distinguish
external interference from the DVL’s own echo signal.
If interference is present from another device on the vessel, the most efficient mitigation technique is to
use a triggering scheme to coordinate their operation. For example, an external trigger commands the ex-
ternal device and the DVL to alternate their transmissions, and this external synchronization ensures that
the two devices will never interfere with each other.
Reference, Data & Timing Considerations
Coordinate Frames
, a DVL can output the measured velocity vector in one of the
following coordinate frames: beam, instrument, ship, leveled-ship, and geographic (“earth”) frames. The
distinction between instrument and ship frames is that the latter takes into account intentional align-
ment angles such as the azimuth angle of Beam 3 relative to the fore-aft axes of the vehicle. The coor-
dinate frame may be selected.
There are two main advantages to using instrument or ship coordinates. First, if the instrument has
been calibrated so that the beam-to-instrument transformation matrix contains instrument-specific
corrections for beam pointing errors, then these corrections will have already been applied by the DVL
firmware. Second, it is easier to measure and account for separate horizontal and vertical scale factor
errors if the velocity is already in instrument or ship coordinates. For the majority of applications instru-
ment or ship coordinates will be the optimum solution.
For more advanced applications, the instrument can output velocity in beam coordinates. The ad-
vantage of using beam coordinates is that beam velocities are the fundamental quantities that the Path-
finder measures. Even if only one beam is able to give a valid measurement, it can still provide useful in-
formation whereas a minimum of three valid beams are necessary for measurements in other coordinate
frames. Using this technique an external transformation would have to be performed to obtain velocity in
instrument or earth coordinates. The beam-to-instrument transformation matrix can be extracted from
the Pathfinder using the &V command.
Data Screening
Pathfinder measurements suffer from occasional data outliers which, although rare, can still be far more
frequent that would be expected from a normal (Gaussian) probability function. To prevent the navigation
system from giving undue weight to data outliers, it is important to screen the Pathfinder data to detect
and reject them.
Data outliers are often the result of ambient data environmental factors. The Pathfinder DVLs have sev-
eral internal routines to detect and account for the false data described. Unless the bottom is detectable
and the correlation value is above a threshold criterion, bottom track velocity will be marked bad (0x8000
which equates to the full-scale negative value of -32768 decimal). The presence of fish in one or more
beams will also cause data outliers. The Pathfinder also can screen for fish using the information in the
intensity data for all four beams.
For operations in coordinate frames other than beam coordinates, if all four beams pass these screening
tests, the Error Velocity (representing the redundant information among the four beam velocities) is cal-
culated and its magnitude compared to a threshold as an additional screen. Measurements exceeding this