An Acoustic Telemetry System Implemented for Real-Time Monitoring of the Gulf Stream with Inverted Echo Sounders

Abstract

From August 1989 until August 1990, a simple acoustic telemetry system was used for obtaining real-time data from four inverted echo sounders (IESs) deployed in the Synoptic Ocean Prediction Experiment (SYNOP) inlet array in the Gulf Stream east of Cape Hatteras. The telemetry system is based on encoding data as a time-delayed broadcast acoustic signal: the delay of the time of broadcast of the signal, with respect to a reference time, is proportional to the data value. The changes in delay time, from one broadcast signal to the next, are recorded at a remote receiving station. Moored near the sea floor, IESs are designed to emit high-frequency (10 kHz) acoustic pings toward the sea surface and receive the reflected signal. In the Gulf Stream region, the round-trip travel time of the emitted signal is proportional to the depth of the main thermocline. As the Gulf Stream meanders back and forth over the instrument, its position can be tracked with the thermocline depth changes. Every 24 h, each AES calculated a representative travel time from a set of 48 bursts of measurements (τ), and telemetered that value to a listening station on Bermuda. From the received data, a daily time series of the depth of the 12°C isotherm (our proxy for main thermocline depth) at each IES was calculated. The position of the Gulf Stream north wall through the IES array was calculated on a daily basis from the thermocline depth information at each IES site. Three of the four IESs were recovered in August 1990. Although the IES at site B2 was not recovered, its telemetered data was received at Bermuda. The rms agreement between thermocline depths, as calculated from the data on tape from the recovered IESs and as calculated from the received telemetry data, is 20 m. This compares favorably with the 19-m uncertainty in calibrating the τ's as a measure of the thermocline depth. The rms agreement between the position of the Gulf Stream path through the IESs as calculated from the tape data and the telemetry data is 5 km. This telemetry system is not IES specific. It could be used with other appropriately modified oceanographic instruments, such as current meters and pressure sensors.