Low Frequency Shipping Ambient Noise Mapping For Passive Sonar Simulator.


The research note talks about an ingenious Passive Sonar Simulator (PSS) that the authors aim to develop using real time data inputs will be a critical tool for fulfilling our audacious objectives. The research note suggests that the complex tropical littoral waters of the Indian Ocean Region provide us with challenges to be able to detect adversaries with us being less vulnerable or detection. It suggests that there is a need to enhance signal to noise ratio, and assessing our vulnerability requires a real time comprehensive understanding of the prevailing noise in the region of our interest. Therefore, this real time low frequency ambient noise mapping lays the foundation for our endeavor that lies ahead, which is developing an interactive real time PSS GUI. And this research Note deals with the same.

Key highlights
  • Using a passive sonar, basically listens to the ocean mitigates our vulnerability for detection by our adversaries.
  • Given greater depths of the sound channel axis in the tropical waters of the Indian Ocean Region (IOR), an actual maritime exercise for improving the capability of submarine sonar operators in the complex littoral waters of the IOR leads to a lot of constraints and costs.
  • A sonar simulator bypasses these constraints and maximizes the capability of sonar operator and training effect by solving these problems and simulating a realistic battlefield Environment.
  • There are three stages involved for the effective deployment of the PSS. The foremost one is a comprehensive understanding of the prevailing low frequency ambient noise (noise ranging from very low frequencies to about 1kHz). This is achieved by mapping the ambient noise in the IOR.
  • The ambient noise mapping as aforementioned is the first step in the development of the Passive Sonar Simulator. Along with the signal transmitted and also the Transmission Loss associated with it due to the underwater channel model, we also get a considerable amount of noise level owing to the various factors and activities present in the underwater ocean.
  • While transient noise as the name suggests is transient which includes noise due to the local passing ships, marine animals (dealt upon in a later section), and passing rain showers (This frequency of rains dominates at 13kHz – 15kHz).
  • When our vessel is docked, the noise mapping takes place using the online data. But it has to rely upon the offline data when it is submerged in stealth mode.
Key Challenges
  • Real time online data integration: The online dynamic data from the AIS gets updated every 3 minutes. This 3-minute window can be taken advantage of by the enemy. Therefore, it is necessary that we look up alternate sources or methods so as to have a much more accurate real time noise map.
  • Optimizing the 2D Model: Due to the broadband, multi-source nature of the computational model, the model is very computationally demanding.
  • Building efficient 2D Model: The ever-growing demands and developments rely on us to make the noise models much more accurate and reliable. Therefore, it is essential for us to develop a more promising model.
Key recommendations
  • Use of Machine learning/ Artificial intelligence to predict the path of the vessels and therefore the noise to fill in the noise levels during this interval will ameliorate the simulator.
  • Use of Artificial Neural Networks to estimate the Transmission Loss component may reduce the computational time and complexity and therefore we will be able to map the noise for multiple resolutions and depths simultaneously.
  • Sea surface analysis can be used to for ship detection from optical satellite images can be away forward in this domain
  • There is a need to use the statistical classifiers for target detection. Generally, in passive sonar, the targets are detected by a sonar equation (with constant threshold) that increases the detection error in shallow water. This is a method for detecting targets in passive sonars using adaptive threshold.
  • There is a need to include marine bio acoustic noise. The presence of the marine bio acoustic noise can be a serious hindrance for the optimal working of the sonar.
  • The PSS could further be upgraded to a Combat Management System (CMS) with advanced features of undersea warfare.

“Ultimately, it is to be noted that, modeling and simulating an environment is to reduce the number of variables that affect the environment and make it as simple as possible. Very accurately predicting the environment requires more and more variables to be taken care of which in turn is the problem that we are trying to solve. Therefore, it is necessary to weigh the parameters and variables and include only them that carry more significant relative weight than including every parameter that we stumble across.”

Krishna Pranay Reddy Chennareddy, BITS Pilani and Dr. (Cdr) Arnab Das, MRC, Pune