April
2018
Pathfinder DVL Guide
Page 26
EAR-Controlled Technology Subject to Restrictions Contained on the Cover Page.
System Integration Introduction
This chapter is intended to provide both the basic operating theory and the necessary and often critical
implementation and integration considerations for the use of DVL systems on various platform types. In
addition, the procedures described provide a step-by-step guide that will enable the client to configure
the Pathfinder DVL sonar for optimum utilization.
DVL Theory
BroadBand Doppler
Doppler sonars by definition, measure the Doppler shift, i.e. the frequency shift due to the relative motion
of the DVL to either the seabed or suspended scatter particles in the water column. TRDI uses a more pre-
cise method of measurement that is a refinement to standard Doppler phase shift measurements called
BroadBand. The methodology uses time dilation, by measuring the change in arrival times from succes-
sive acoustic pulses. TRDI uses phase to measure time dilation instead of measuring frequency changes
because the phase measurement gives a more precise Doppler shift measurement.
TRDI also incorporates a technique called autocorrelation for error checking and thus a further refine-
ment of velocity measurements. Autocorrelation works by transmitting a series of coded pulses, all in se-
quence and inside a single long pulse. The resulting received signal is composed of many echoes from
many scatterers, all combined into a single echo. The propagation delay is extracted by computing the au-
tocorrelation at the time lag separating the coded pulses. The success of this computation requires that
the different echoes from the coded pulses (all buried inside the same echo) be correlated with one an-
other.
Bottom Tracking
Bottom tracking is implemented using separate pings from water profiling. The transmit pulse is a longer
duration, and the received acoustic signal incorporates a different processing scheme.
While water-profiling uses short transmit pulses to obtain vertical resolution, Bottom Tracking requires
long duration pulses. Long pulses are utilized because this provides ensonification over a lager bottom
area for each individual pulse (Figure 14).
If the pulse is too short or long, the echo returns first from the leading edge of the beam, followed later by
the trailing edge. Because the beam has a finite beam width, the angle of the beam relative to the horizon-
tal is different on these two edges, thus resulting in a Doppler shift that is different from one side of the
beam to the other. By illuminating the bottom across the beam all at once, a long pulse produces an accu-
rate and stable estimate of velocity, more accurate than is typically obtained from water profiles.
The disadvantage of long transmit pulses is that a considerable part of the echo can come from water-
mass echoes. Where water-mass echoes are weak relative to the bottom echo, there is no adverse impact.
For environments with high concentrations of suspended sediment (i.e. in some rivers) the water-mass
echoes can introduce significant water bias. The added water bias causes an undesirable shift of the bot-
tom-track velocity toward the ambient water velocity.