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.

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.

The restrictions as well as the principles taken for granted in using the PPL program are as follows:

. 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.

. Horizontal homogeneity y is assumed. Therefore, the output should be used with caution in areas of high variability (for example, fronts and eddies).

. 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.

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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