Underwater Countermeasures : A Test Bed for an Acoustic Echo Repeater

Sammanfattning

Modern naval operations depend on detecting and tracking potential underwater threats. Active sonar systems transmit acoustic signals and analyse returning echoes to estimate a target’s position, but this also exposes the transmitting platform to detection. To exploit this vulnerability, acoustic countermeasure systems can manipulate reflected echoes to deceive tracking systems regarding a target’s location, velocity, or size. This work focuses on the evaluation of an acoustic echo repeater developed by the Swedish Defence Research Agency (FOI), which receives incoming sonar pulses, processes them in real time, and retransmits modified echoes for research on sonar countermeasure techniques. This thesis aims to develop a test framework designed to quantify the performance of the bearing estimation algorithm, provide a framework for future development, and enable systematic evaluation under different scenarios. To achieve this, four different modules were implemented. Two of the modules use synthetic data generated in controlled simulation environments, enabling systematic variation of parameters such as pulse types, source-receiver geometries, and signal-to-noise ratio. A third module processes recorded sea-trial data, allowing evaluation under real-world operating conditions. Furthermore, a hardware-in-the-loop functionality for generating heading-adaptive synthetic signals was implemented, allowing synthetic echoes to respond dynamically to the platform orientation The results of evaluations using synthetic data show that pulse tapering has the largest impact on estimation accuracy, while interference patterns and time resolution also affect performance. Certain bearing angles consistently produced larger estimation errors, indicating geometry-dependent limitations in the estimator. For moving-source scenarios, Doppler effects reduced the accuracy of the raw bearing estimates, while the incorporation of a Kalman filter to predict the trajectory of the source together with the time difference of arrival algorithm used for the bearing estimation improved the overall prediction accuracy.