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Doppler Log

The Doppler shift between the transmitted sound wave and the received reflected wave from the water bottom is utilized to measure the ship’s speed and cumulative range relative to the water bottom. This type of meter has good accuracy and high sensitivity and can measure both longitudinal and transverse speeds, but it is expensive. They are mainly used to provide accurate data on the longitudinal and transverse movements of huge ships while navigating in narrow channels, entering and leaving ports, and docking and leaving terminals. The Doppler rangefinder is limited by its depth of action, and when it exceeds several hundred meters, it can only use the water mass in the water layer as a reflective layer, turning it into a water-to-water rangefinder

Introduction

Doppler rangefinder is a kind of precision speed measurement and range accumulation instrument made according to the principle of Doppler effect of sound waves in water [1]. It is a computational positioning and navigation equipment, computational positioning and navigation is an ancient technology, it must rely on the important parameter of the ship’s speed, the accuracy of the ship’s speed directly affects the positioning accuracy.

The Doppler shift between the emitted sound wave and the received reflected wave from the water bottom is utilized to measure the ship’s speed relative to the water bottom and the accumulated voyage. This type of meter has good accuracy and high sensitivity and can measure both longitudinal and transverse speeds, but it is expensive. They are mainly used to provide accurate data on the longitudinal and transverse movements of huge ships while navigating in narrow channels, entering and leaving ports, and docking and leaving terminals. The Doppler rangefinder is limited by its depth of action, and when it exceeds several hundred meters, it can only use the water mass in the water layer as a reflective layer, turning it into a water-to-water rangefinder.

History

With the development of technology, the towed tachymeter appeared in the 18th century. In the middle of the nineteenth century, people made use of the ship’s movement around the fluid to push the impeller rotation of the rationale for the rotating wheel type tachymeter. In the twenties of the twentieth century, in the Pitto made on the fluid dynamic pressure and fluid velocity square proportional to the experimental conclusions, inspired by the development of water pressure type tachymeter. To the mid-twentieth century, the use of Faraday electromagnetic induction principle made of electromagnetic range meter, make up for the hydraulic range meter can not measure the backward speed and measurement of the defects of low sensitivity. In this later with the development of electronic technology, the rotating wheel type and dynamic pressure type tachymeter made corresponding improvements. By the end of the sixties and early seventies electromagnetic type tachymeter has further development. With the increase of ship tonnage and other navigation and positioning system requirements, not only require the provision of the ship forward and reverse speed, but also in the ship in and out of the dock, berthing wharf, but also require the provision of the ship’s bow or stern of the speed of the traverse, and the need to determine the speed is not relative to the water current, but must be the ship’s speed in relation to the ground, “the measurement of the forward, reverse or traverse speed threshold is also a higher requirement. As a result, the Doppler Velocimeter (DVL), designed to utilize the Doppler effect of sound waves, which is a characteristic of the relative motion of the carrier and the receiver, and the Correlation Velocimeter (CVL), which utilizes correlation techniques to process the echo information, appeared in the 1970’s. The two types of Velocimeters, as well as the Doppler effect, were designed for the purpose of determining the speed of a ship. The emergence of these two types of rangefinders and echo sounder constitute the hydroacoustic navigation system, which makes the ship’s underwater navigation system used in civil transportation ships, with its independent mode of operation, is other surface navigation and positioning systems can not be replaced, by the maritime community’s appreciation and a wide range of applications.

Frequency Estimation Methods

Velocity measurement is the main function of a Doppler range finder. The Doppler Meter utilizes the Doppler shift of the echo to solve the velocity of the carrier, therefore, the accuracy of the frequency measurement has a direct influence on the accuracy of the velocity measurement, and the frequency measurement becomes the main task of the Doppler Meter. It is especially important to select a frequency algorithm with high accuracy and small operation volume for the Doppler range finder.

The most commonly used frequency estimation method in Doppler range finders is the zero crossing detection method. In this method, the frequency is estimated by counting a very high number of clock pulses at a zero crossing point to determine the time required for N cycles. This method is simple to implement with a small amount of arithmetic, but it is too sensitive to noise and has a large time quantization error when the observation length is short.

Applications

Doppler Log are particularly suitable for determining the speed of ships in shallow water and at low speeds . Because of the high accuracy of the speed measured by the Doppler range finder, the combined navigation system consisting of the Doppler range finder, the compass, and the satellite navigation or the precision radio positioning system has a high accuracy and can be used for surface ships, various engineering operations, and oceanographic research vessels. In addition, the velocity measured by the Doppler log can also be used as the velocity damping signal of the inertial guidance system. The combination of Doppler range meter and inertial guidance is one of the more ideal combination modes for marine inertial navigation system.

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