Time Spreading at High Frequency in a Shallow Water Channel

• Main Authors: Hines, Paul C, Collier, Arthur J, Hutton, J. S
• Format: Publication
• Language: English
• Published: 05.03.1997
• Subjects: ACOUSTIC DATA; ACOUSTIC DETECTION AND DETECTORS; ACOUSTIC MEASUREMENT; ACOUSTIC TRACKING; ACTIVE SONAR; CANADA; COASTAL REGIONS; FOREIGN REPORTS; HIGH FREQUENCY; MULTIPATH TRANSMISSION; NOVA SCOTIA; OCEAN BOTTOM TOPOGRAPHY; SHALLOW WATER; SONAR ARRAYS; SONAR ECHOES; SONAR PULSES; SONAR RECEIVERS; SONAR SIGNALS; TIME DEPENDENCE; TIME SPREADING; UNDERWATER ACOUSTICS; WAVE PROPAGATION

Time spreading measurements provide an indirect measure of the acoustic bandwidth that can be supported by the water channel, which is critical to the design of sonar systems. Time spreading measurements were collected in a water channel 100 m deep, off the coast of Nova Scotia. Data were collected at frequencies of 20-22 kHz, 27-29 kHz, and 35-37 kHz using linear FM pulses 25 in duration. The experiments were part of a collaborative TTCP trial known as Trial Scotian (HF) organized by the Environmental Acoustic specialists group of GTP-11, Underwater Weapons and Countermeasures. Canada, the US, and the UK participated in the trial. Canada's SEAHORSE array, an anchored, high frequency active sonar was employed for the source-receiver, and a UK free drifting echo repeater was employed for the target. Source-receiver and target position were recorded using a portable tracking range operated by the US. In the paper, time spreading measurements are compared with modelled estimates obtained using the Generic Sonar Model (GSM). The GSM estimates of time spreading due to multipath propagation compare favourably with the experimental data. However, time spreading of individual paths beyond the model predictions is also evident. Additionally, the data were used to compute the standard deviation of the received echo intensity at each frequency. The standard deviation was computed two different ways. First it was computed using the peak echo level from each of the pulses at a given frequency. Then, it was computed from the total energy received from each of the pings. At all three frequencies, the standard deviation was lower by 1 to 2 dB when computed from the total received energy.