medium. Properties of seawater that affect sound speed
are salinity, temperature, and pressure. Output from this
program is classified and should be labeled as required.
PASSIVE ACOUSTIC PROPAGATION
LOSS (PPL)
LEARNING OBJECTIVES
Identify
applications, limitations, and assumptions of
the PPL program. Interpret PPL outputs.
The PPL program calculates transmission loss as a
function of range, frequency, source depth, and receiver
depth. The calculations from this program will be used
in the prediction of ASW sensor systems performance.
The purpose of this program is to define the acoustic
propagation conditions within the ocean area of interest.
It is intended as an interface between environmental
data read from the ocean environmental file (OEF) and
operational data.
The program will translate
information about existing oceanographic conditions
into an assessment of PPL versus range that is necessary
for sensor system performance predictions.
APPLICATION
The PPL program computes the loss of sound
intensity in traveling from the selected source (for
example, submarine) to the receiver (for example,
passive sonobuoy) for ranges (kyd) out to the maximum
range specified. The operator-selectable input is the
desired frequencies, source and receiver depths, and
maximum range. The propagation-loss curve aids the
operator in computing detection ranges and possible
detection paths.
The range should be chosen to include the first CZ.
A typical maximum range value varies from 60 to 100
kyd.
When the propagation-loss curve is displayed on the
monitor, the operator can see the transmission loss
associated with each range. In general, the propagation
loss increases with range but may decrease rapidly
(spike) when environmental conditions allow formation
of CZs. If the operator knows the figure of merit
(FOM), in decibels (dB), detection ranges can be
determined instantly by looking at the display. At any
range point where the FOM is greater than the
propagation loss, the probability of detection (POD) is
at least 50 percent.
LIMITATIONS AND ASSUMPTIONS
The restrictions as well as the principles taken for
granted in using the PPL program are as follows:
l The PPL program incorporates low-frequency
bottom-loss (LFBL) data base processes, assumptions,
and correction factors. Viability of output depends upon
the degree of difference between the model and actual
seabed conditions.
. System correction factors are preset to define an
omnidirectional/vertical-line array DIFAR (VLAD)
sonobuoy.
. Maximum range should include the first CZ.
l Horizontal homogeneity y is assumed. Therefore,
the output should be used with caution in areas of high
variability (for example, fronts and eddies).
l The SSP program must be used to create the
environmental data set used by RAYMODE. The SSP
program stores a sound speed profile, bottom depth,
high- and low-frequency bottom-type information,
wind speed, and so on, in the OEF.
. Propagation-loss curves may be generated for
target frequencies in the range of 1 to 35,000 Hz. Due
to the limitations of the LFBL data base, reliable output
is constrained to frequencies >30 Hz.
FUNCTIONAL DESCRIPTION
The RAYMODE propagation-loss model was
developed at the Naval Underwater Systems Center,
New London, Corm. The original version of this model
has been updated to incorporate factors for determining
bottom-loss and system correction factors. This model
considers the ocean bottom as a varying sound receptor
and not simply a reflector. Computation of losses in the
bottom sediment are a feature of PPL that treats the
bottom of the ocean as a continuation of the water
column, and computes the contribution of the bottom
sediment to propagation loss, considering refracted
paths through the sediment and reflections at the
basement.
Locations beyond the coverage of the LFBL data
base use the COLOSSUS data base model to estimate
propagation loss. Output from this program is classified
and should be labeled as required.
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