Autonomous Receivers
The autonomous receivers (Model N201, Sonic Concepts, Inc) are self-contained and consist of a PVC housing with a threaded coupling and O-ring to join the upper and lower portions (Figure 1). The lower housing holds lithium battery packs capable of powering the receiver for 30 days. The upper housing has an externally mounted omnidirectional hydrophone on the top (Figure 2) and internal analog and digital circuit boards. Each receiver is fitted with an external beacon tag that transmits a unique code every 15 seconds, as well as a high-impact plastic fin to reduce drag and increase stability under high flow conditions. Receivers record pressure and water temperature data every 15 seconds. All data are stored on Compact Flash media (1 GB, SanDisk Extreme III). Autonomous receivers are positively buoyant and typically deployed 3 to 5 m above the riverbed or ocean floor (Figure 3). System components designed specifically for use in the nearshore ocean are under development and have been tested successfully in the Pacific Ocean off the coast of Washington State.
Figure 1. Autonomous receiver and associated mooring components as used in the Columbia River basin. Receiver beacon (A), hydrophone (B), acoustic release (C), and bungie mooring and anchor (D) are shown.
