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Advancements and Applications For Doppler Sonar

By Alan Kenny

Acoustic Doppler current profilers (ADCPs) and Doppler velocity logs (DVLs) are underwater acoustic sensors that apply basic principles of Doppler shift to measure three-dimensional velocity of particles in water, or the velocity of vehicles moving through the water with respect to the ocean floor. The advent of the Doppler sonar proved to be one of the most significant advances in navigation and current measurement technology in history. These sensors enabled us to fully understand in real time the motion and speed of ROVs; to understand current flows throughout the water column in oceans, lakes, rivers and streams; and were critical in enabling the autonomy of AUVs. With 30 years of evolution and development by companies like Teledyne RD Instruments (Poway, California), these are now the primary sensors used to measure water in motion and motion in water. They are, in essence, the speedometers of the sea.

In the beginning, these sensors were large, power hungry and suitable only for surface vessels. But today's systems, especially new phased array technologies, have become more efficient and have enabled applications that were not previously possible.

Increased depth rating also advanced the areas of application for these sensors, and less power hungry, remote operated systems are making the task of velocity measurements more cost effective and more manageable.

History of Doppler Sonar
The journey of the Doppler was kicked off through two patents. First, for measuring speed of a surface ship versus the bottom, Jacob A. Kritz and Seymour D. Lerner of the Sperry Rand Corp. filed a patent in 1969. Their patent was titled 'Digital Sonar Doppler Navigator' and utilized four focused acoustic beams spaced 90 degrees around a circle, pointing at a constant angle offset from the vertical (generally 20 to 30 degrees, a Janus array configuration). Combining the piezoelectric ceramic transmit and receive elements with a series of pulse counters and phased delay circuits allowed for calculating a Doppler shift when the acoustic signal was reflected off of the bottom of the ocean. Pairing the fore, aft, starboard and port beams provided directional information.

Expanding on this concept, David G. Shave of the Western Geophysical Co. of America (now WesternGeco, based in London, England), filed a patent application in October, 1977, claiming extended application of the Doppler devices to include measurement of currents relative to a moving ship. This system used reflectors, such as plankton and other materials, in the water and gated the return signals, discretized in time, to measure current profiles. By taking bottom-referenced Doppler measurements, the vessel's velocity could be removed, and a complete absolute velocity current profile could be obtained.

Overcoming Challenges
The key challenges to the Doppler sensors have long been balancing accuracy, precision and range against the fundamental physics that define size, power and weight. These factors are at odds with each other. Higher frequencies result in shorter range as they are absorbed quickly by the water, but yield smaller sensors and better resolution. And even with today's modern computing power, the ability to discern extremely small Doppler frequency shifts (i.e., phase changes) limits accuracy. The fundamental Doppler calculation is:

Fd = 2 Fs*(V/C)* cos(A)

Fd is the measured Doppler frequency shift. Fs is the sonar's source frequency. V is the horizontal velocity component of the moving vessel (or the water) in meters per second. C is the speed of sound in meters per second, and A is the angle of the beam relative to the horizontal.

Doppler sonars typically operate between 38 kilohertz and 2 megahertz, with range being inversely proportional to frequency. At 300 kilohertz, a vessel traveling at 8 knots (i.e., 4 meters per second) sees a maximum Doppler shift of 280 hertz, or less than 0.3 percent of the center frequency.

Resolving speed to another three or four decimal places with environmental and system noise and other nonlinearities, we realize quickly that an accurate measurement of a small velocity change is difficult at best. To achieve a 0.25 percent precision, we would need to measure a frequency shift of 0.5 hertz versus a carrier frequency of 307.2 kilohertz. To continue this article please click here.

Alan Kenny is the sales manager and acting director of the navigation product line at Teledyne RD Instruments. With a background from the U.S. Naval Academy and University of Minnesota, Kenny has been involved in vibration and acoustics in various industries, in and out of the water, since 1984.

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