The hydrophone used on the Otter is a TBR 700 RT from Thelma Biotel. This provides detections of acoustic tag, as well as measurements taken in the hydrophone. Depending on the software version ordered from the manufacturer, the hydrophone can detect tags transmitting on 63-77 kHz. Typically, lower frequencies are only suitable for larger tags, while higher frequencies are mostly used in smaller tags. It detects tags that transmit using using Differential Pulse Position Modulation (DPPM), and can use different protocols to give meaning to the time delays between pulses.
The communication with the controlling computer is through a RS-485 interface, delivering messages in a NMEA 0183 inspired format. Bluetooth communication is also present, but not used for real-time operations like those the Otter target. Instead, it can be used to download lists of detections after a mission has been performed.
Power is supplied through the same cable as the RS-485 uses, but in a separate lead. It accepts a DC voltage of 5-12V, so 5V is used in the Otter because it was available and unused by the other components. Battery power is also possible, but not used in the Otter project.