Pathfinder DVL Guide
April
2018
EAR-Controlled Technology Subject to Restrictions Contained on the Cover Page.
Page 49
System Synchronization
Typically, data from the Pathfinder is fed into a larger user supplied acquisition and control system, which
collects data from other sources as well. In such situations, it is important to have all the data accurately
referenced to a common time base.
The Pathfinder data ensembles in
PD0
format contain a time stamp having a resolution of 10 ms that is
from a reading of the real-time clock before the ping. This time stamp is of limited utility because it is
difficult to keep the Pathfinder clock synchronized to the navigation system clock and also because
the 10 ms resolution is arguably too coarse when the Pathfinder is combined with an INS. It is recom-
mended instead that synchronization be achieved using a hardware trigger (CX1 or CX2 command)
with the data time-tagged with the navigation system time recorded at the time of the hardware trig-
ger. Care must be taken so that the time tag represents the correct edge of the trigger and that the time
tag be attached to the correct data ensemble, because the data ensemble arrives after the next ping has
been triggered.
Note that the minimum time interval between pings (which is the maximum velocity update rate) is alti-
tude dependent, thus the triggering scheme should be tested over the full working range of altitudes (in-
cluding an altitude beyond bottom track range). These tests should be made with water mass layer meas-
urement both on and off.
Operational and Setup Considerations
Unfavorable Environments
There are certain environmental conditions where the DVL’s errors may be large or where the instrument
does not function at all.
•
In the surf zone where waves are actively breaking, the acoustic beams may not be able to pene-
trate the bubble clouds, and even if they do, the sound speed may be significantly affected by the
high concentration of bubbles. This changes the Doppler scale factor.
•
The dense growth of weeds or kelp on the bottom may prevent the DVL from detecting the true
bottom. If the DVL locks onto the top of the weeds by mistake, they may have motion relative to
the bottom due to wave action, which would give inaccurate velocity measurements.
•
In areas of high salinity (>35ppt), the absorption of the sound by the water column will reduce
the altitude capability of the DVL. Absorption (maximum range) can also be affected by water
temperature, as outlined in
•
In areas where the salinity varies as a function of location and/or time, the Doppler scale factor
will be varying and it may be necessary to integrate a speed-of-sound sensor into the navigation
system to keep the velocity measurement errors to an acceptable value. Temperature gradients
can cause similar issues, as outlined in
. This is only for the Z-axis ve-
locity, as SoS does not impact 2D velocities on a Phased Array Transducer.