Matched-filter loss due to reflection from the sea surface: measurements and simulation

Acoustic reflection from the sea surface can cause signal distortion owing to the time-varying nature of the sea surface, leading to matched-filter loss and degraded sonar per-formance. Surface reflections were measured and simulated to quantify matched-filter loss as a function of signal duration (up to 8 s) for a linear frequency modulated (LFM) waveform spanning the 2000–4000 Hz band. The ‘exact’ boundary integral equation (IE) method was used for the simulation, following the 2D treatment in the literature. The IE method was extended to 3D and a comparison was made against the 2D method for a scaled down example; further testing will require more computational resources. A new IE-p method was also implemented in 2D and 3D where the surface is partitioned, each partition is solved using the IE method, and the solutions for each of the partitions is combined to form the total solution. The widely used Kirchoff approximation was also implemented in 2D and 3D. A 2D simulation using the IE, KA, and IE-p methods was performed to recreate an experiment done during the Littoral Continuous Active Sonar sea trial in 2018 where a source and receiver were separated by 110 m. The three methods gave similar results at this close range, whereas the KA method is not expected to perform as well at longer ranges with lower grazing angles and increased shadowing. The simulated matched-filter loss generally agreed with the measured loss as a function of duration.