Abstract: A Doppler device including a transducer and processing circuitry is provided. The transducer transmits underwater ultrasonic wave and receives a reflected wave of the ultrasonic wave. The Doppler device generates a first echo signal from the reflected wave in a first direction making a depression angle ? with a receiving surface of the transducer, generates a second echo signal from the reflected wave in a second direction making the depression angle ? with the receiving surface, the second direction being different from the first direction, and generates a third echo signal in a third direction perpendicular to the receiving surface. The processing circuitry further calculates a first Doppler frequency of the first echo signal, a second Doppler frequency of the second echo signal, and a third Doppler frequency of the third echo signal. The processing circuitry further calculates the depression angle ? from the first, second and third Doppler frequencies.

Abstract: A fishfinder is provided for identifying with high precision a single fish under the ship and for measuring the length of the fish precisely. An operator of the fishfinder is assisted to grasp the depth and the length of the fish. A fishfinder transmits an ultrasound pulse having a high carrier frequency and an ultrasound pulse having a low carrier frequency by means of a pulse transmission and reception device at predetermined time intervals. A single fish echo is determined and tracked for a plurality of transmission intervals of ultrasound pulses emitted into the water. A target strength of single fish is measured based on the signal strength of a low carrier frequency echo signal from the single fish. The length of the fish is measured based on the target strength. A fish mark is displayed on the screen of an indicator, with the size of the fish mark depending on the fish length.

Abstract: A fishfinder is provided for identifying with high precision a single fish under the ship and for measuring the length of the fish precisely. An operator of the fishfinder is assisted to grasp the depth and the length of the fish. A fishfinder transmits an ultrasound pulse having a high carrier frequency and an ultrasound pulse having a low carrier frequency by means of a pulse transmission and reception device at predetermined time intervals. A single fish echo is determined and tracked for a plurality of transmission intervals of ultrasound pulses emitted into the water. A target strength of single fish is measured based on the signal strength of a low carrier frequency echo signal from the single fish. The length of the fish is measured based on the target strength. A fish mark is displayed on the screen of an indicator, with the size of the fish mark depending on the fish length.