Sea Technology

JAN 2014

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Page 43 of 71

phase changes) limits accuracy. The fundamental Doppler calculation is: Fd = 2 Fs*(V/C)* cos(A) shift of 0.5 hertz versus a carrier frequency of 307.2 kilohertz. Early methods used simple continuous wave pulses (known as narrowband Doppler). Each measurement was one sample of a statistical data set. To signifcantly increase the accuracy, a method was devised and patented by Dr. Blair Brumley, Kent Deines and Ramon Cabrera of RoweDeines Instruments Inc. (now Teledyne RD Instruments), and Eugene Terray of Woods Hole Oceanographic Institution. Patent Number 5,483,499, titled "Broadband Acoustic Doppler Current Profler," was granted January 9, 1996. This Broadband Doppler method relied on combining multiple sets of coherent coded pulses riding on the carrier wave, allowing one broadband ping to improve accuracy by orders of magnitude. This single enhancement propelled the utility of the technology forward. This also enabled the use of sensors as Doppler velocity logs (DVLs) on moving platforms, such as AUVs and ROVs. Accuracy and update rates allowed DVLs to be used as a navigation feedback system to perform tasks, such as station-keeping, which were not possible with narrowband methods. With some of the core challenges addressed, limitations based on the underlying physics were the next hurdles. Many of these factors were at opposing ends of a desired solution. First, the size of the sensors needed to be practical for use in smaller platforms. Four-beam piston sensors increased signifcantly in size as range increased, thereby limiting the size of vehicles on which sensors could be used. To keep size practical, frequencies were kept higher, (Top) Sample system performance. (Middle) 75-kilohertz sacrifcing range. The next adADCP. (Bottom) Phased array transducer compared to a Janus vent—the phased array senpiston array transducer. sor—helped overcome some of these limitations. 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 diffcult at best. To achieve a 0.25 percent precision, we would need to measure a frequency 44 st / January 2014 The Phased Array Lowering frequency to gain range would be practical, except that size and weight increase with this reduced frequency. A 38-kilohertz sensor that achieves 1,000 meters of current profling range would be roughly 3 meters across, weigh nearly 1,000 pounds and require a top-side electronic chassis to supply signifcant power. While this could potentially ft on an oil drilling platform, it does not lend itself well to AUVs or ROVs, or to long-term current profling applications on moorings or buoys.

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