DVL
28
© 2017 Nortek AS
8.3
Bottom Track
In order to detect the bottom the transmitted ping must have return characteristics that provide a
sufficiently high enough score to qualify it as a valid bottom detection. Once the bottom detection is
made then the velocity along the beam is calculated. This means that any any number of beams
may have valid estimates of velocity, however in order to report a complete XYZ estimate of velocity,
the DVL needs to have a minimum of three valid bottom detections.
The lack of consistent bottom detection may be attributed to a variety of reasons. The following is a
list of reasons:
Acoustic interference from other acoustic instrumentation operating in the same frequency band
(25% band centered on DVL transmit frequency). Other acoustic instrumentation may also
produce harmonics which interferes with bottom detection. In order to identify acoustic (or
electronic sources) of noise, one may consider using the Spectrum Analyzer function of the DVL.
This works together with the MIDAS software. Further documentation of this functionality is found
in the MIDAS software.
Highly reverberating environments such as buckets, tanks, and other small enclosures.
Poor acoustic return from bottom tracking surface such as (a) soft sediment bottoms, (b) seaweed
rich bottoms, (c) or extreme bottom slopes.
Highly scattered transmit pulse for environments that are rich in bubbles: (a) propeller wash, (b)
breaking waves may produce bubbles in the first several meters of the water column, (c) schools of
fish.
Extremely close to the bottom.
Bottom is out of range or configured range of DVL.
false detections may occur from time to time, particularly at short range when traveling near the
surface where there normally is high particle (or bubble) concentrations near the surface (first 10 m
or so), and especially when the true bottom is out of range
One way of avoiding false detects is to configure the blanking distance and/or the maximum range
so that it looks for the bottom within a known window. For example, if the vehicle is not going to
operate closer than 5 m from the bottom, the blanking distance can be set to e.g. 3 m. This will
eliminate all detections within the first 3 meters. The larger the blanking distance, the lower the
probability of false detections. Just remember that anything inside the blanking distance is invisible
to the DVL.
Highly reverberant environments may also benefit by reducing the transmit power. This is of course
done at the expense of the bottom detection at greater ranges.
9
Appendices
9.1
Glossary
Accuracy
A value giving the degree of closeness of a velocity measurement to the actual
velocity. Refer to the data sheet for specific minimum accuracy.
AD2CP
Nortek's broadband signal processing platform.
.ad2cp
Nortek raw binary data file
Amplitude (*)
See signal strength
ASCII output
Data is displayed in ASCII format (text).
Bandwidth
Wider signal bandwidth is used to get more information and improve the velocity
precision.
Beam
Coordinates
Bottom track: The reported velocities are positive when the motion is towards the
transducer
.
Current profile: The reported velocities are positive when the motion is
away from the transducer.
Blanking
Specifies the distance from instrument head to the start of the measurement cell
