Near real-time transmission of reduced data from a moored multi-frequency sonar by low bandwidth telemetry

Moored, internally-recording acoustic instruments can acquire continuous profiles of echoes thro column, thus providing a low-cost method ughout the water of studying the behavior and abundance of fish and zooplankton in oceans and lakes. Calibrated sonars with several frequencies allow some information about species composition and abundance to be deduced from acoustic backscatter data. The same instrument can be configured to look up from the bottom, down from the surface, or horizontally from a CTD cage. In this presentation we describe additional capabilities of this low-power, batteryoperated, multi-frequency sonar capable of autonomously collecting data at high temporal and spatial resolution for periods of up to a year. The AZFP instrument (Acoustic Zooplankton and Fish Profiler) supports up to four frequencies in a single housing. The available operating frequencies are 38, 67.5, 125, 200, 455, 769 and 2000 kHz. The transducers are colocated, with the same nominal beam widths of 7° or 8°, except at 38 and 67.5 kHz, where the beam width is 12°. The standard AZFP can be moored at depths up to 300m, and with modified transducers as deep as 600m. The AZFP can store up to 32 GB of this data internally. The user can specify averaging in time and/or range to choose an optimum sampling scheme for the available storage. Because the AZFPs are deployed on moorings for long peri ods of time, there can be a requirement to monitor the data in near real-time. The volume of data collected is usually too great to allow monitoring of the results unless the instrument is connected by cable to shore. A capability to retrieve a subset of this data over low bandwidth satellite or other network links to provide this monitoring was developed. In this presentation we show how the stored data is further reduced to accommodate transmission over a low bandwidth network. The AZFP data is acquired in logarithmic form so compiling data averages requires conversion to linear values if a true arithmetic average is desired. Because the instrument is designed for low power consumption to allow long deployments, the onboard microprocessor has limited processing capabilities. In particular, floating point calculations are not possible and all processing is done using integer arithmetic. Conversion to and from logarithmic form is therefor done using lookup tables, but limited internal memory limits the size of the lookup tables. We will describe how the tables are constructed to provide adequate resolution. This capability is applicable to AZFPs deployed on AUVs and gliders as well as moorings where a surface buoy allows satellite or radio telemetry. If two-way serial communication is possible in a particular application, then the operation of the AZFP can be controlled by the platform or remotely by a communication link through the platform. Here we will give an example of a moored system where inductive modems are used to transmit data from the AZFP to the surface buoy for retransmission over a satellite link. In this case, because other types of data were to be transmitted over the satellite link, the fraction of the bandwidth available for the AZFP data was severely limited, so additional range averaging was required.