Methods, systems, and apparatus, including computer programs encoded on computer-storage media, for obtaining initial parameters for ultrasonic transducers around a calibration target. The calibration target can include a fish-shaped structure, sensors placed at different locations of the fish-shaped structure, a processor that receives sensor values from the sensors, and a transmitter that outputs sensor data from the calibration target based on the sensor values. The calibration target can be fixed at a particular position relative to the ultrasonic transducers by a filament coupled to both the calibration target and a support structure. Sensor data can be obtained from the calibration target at the particular position relative to the ultrasonic transducers, and relative locations of the sensors can be determined. Parameters for the ultrasonic transducers around the calibration target can be adjusted based on the sensor data and the respective locations of the sensors.