DSA Volume 1 Issue 4 December 2010 - Defence Science and ...

Improved sonar pictures
of a noisy changeable ocean
Ocean forecast modelling is being applied by
DSTO to assist sonar target detection.
In anti-submarine warfare, the sonar
operator plays an integral part in monitoring
in-water active sonar returns to locate
potentially hostile targets and warn of
approaching attack from any given direction.
Conventional sonar operator consoles
typically comprise a screen with a plan
position indicator (PPI) display that depicts
the range and bearing of active sonar returns.
The work of interpreting what an active
return signal indicates is made difficult
and time-consuming due to the fact that
transmitted sonar energy propagates
in highly variable ways, being affected
by oceanographic properties such as
density, temperature and salinity that
vary from place to place and over time.
For the detection and identification
of faint return signals, this problem is
further exacerbated by the presence of
background noise produced by various
natural and human-made causes.
Correspondingly, sonar operators are
commonly faced with complex and changing
displays that require intense scrutiny and
skilled interpretation to distinguish potential
targets from environmental clutter.
To alleviate these difficulties, sonar
performance modelling is applied using
oceanic parameter inputs to produce output
predictions of how sound will travel through
water plus estimates of probability of signal
detection against submarines. This modelling
aids the process of both operating the sonar
system and interpreting active sonar returns.
Environmental data sources
Conventional sonar performance modelling
tools have been reliant on monthly-averaged
oceanographic data amassed in climatology
databases, along with in situ data gathered
via expendable bathythermographs
(XBTs) that provide a measure of ocean
temperature for a single place and time
from the sea-surface to a certain depth.
For some operational scenarios, however,
the sonar performance modelling outcomes
obtained with these data inputs do not
adequately capture the variability of
the changing ocean environment.
A recently instituted oceanic forecasting
service, called BLUElink, provides a
means of significantly improving on
this situation. BLUElink is an Australian
Government initiative involving the Bureau
of Meteorology, Navy and CSIRO.
The service produces three-dimensional
ocean models of the Australian maritime
region based on data from climatology
databases, satellite remote sensing
and measured at-sea data, featuring
parameters that include ocean currents,
wind stress, temperature and salinity.
The modelling results are available either
as forecasts for use in preparing for future
sonar operations, or hindcasts that can be
used for post-trial analysis, by estimating
what the state of the ocean was most likely
to have been at a particular time in the past.
Contained within BLUElink is a capability
called the Relocatable Ocean Atmosphere
Model (ROAM) for limited area high-resolution
modelling. This has sufficient resolution to
accurately depict smaller-scale shortlived
ocean circulation features, including frontal
boundaries and eddies, that dramatically
impact on sonar system performance through
the effects of refraction and energy scattering.
Seeing great potential in the BLUElink
resource, DSTO researchers Jarrad Exelby
and Han Vu have sought to harness
the higher fidelity modelled data on
offer for better quality predictions
of signal detection probability.
The work has been developed as an
experimental ‘concept demonstrator’,
aimed at proving the validity of the
approach to establish grounds for
applying further time and effort to
deliver a version for operational use.
A plethora of information
at fingertip reach
DSTO’s concept demonstrator currently
operates on a stand-alone laptop that
can be used during at-sea exercises or in
the laboratory for post-trial analysis.
The eventual intended use is to incorporate
both measured and modelled undersea
environmental information into the
real-time displays that sonar operators
view, presented as an overlay on the
conventional sonar display picture.
The demonstrator system has been designed
with functionality that enables a range
of different environmental data inputs
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Left: BLUElink predictions for ocean temperature, height and surface currents for eastern Australia.
Above: DSTO researchers Jarrad Exelby and Han Vu.