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Acoustic Doppler Current Profiler Principles of Operation a Practical Primer

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Acoustic Doppler Current Profiler Principles of Operation a Practical Primer Acoustic Doppler Current Profiler Principles of Operation A Practical Primer P/N 951-6069-00 (January 2011) © 2011 Teledyne RD Instruments, Inc. All rights reserved. Principles of Operation Revision History: January 2011 – Corrected Equation 9, page 34. Sound Speed ratio was inverted. December 2006 – Added Phased Array Transducer section. Teledyne RD Instruments Teledyne RD Instruments Europe 14020 Stowe Drive 2A Les Nertieres Poway, California 92064 5 Avenue Hector Pintus 06610 La Gaude, France Phone +1 (858) 842-2600 Phone +33(0) 492-110-930 FAX +1 (858) 842-2822 FAX +33(0) 492-110-931 Sales – [email protected] Sales – [email protected] Field Service – [email protected] Field Service – [email protected] Client Services Administration – [email protected] Web: http://www.rdinstruments.com 24 Hour Emergency Support +1 (858) 842-2700 © 2011 by Teledyne RD Instruments. All rights reserved. No part of this document may be reproduced without permission in writing from Teledyne RD Instruments Principles of Operation Table of Contents 1. Introduction ........................................................................................................................................................ 1 History of Teledyne RD Instruments ...................................................................................................................... 1 ADCP History .......................................................................................................................................................... 1 BroadBand ADCPs .................................................................................................................................................. 2 2. The Doppler Effect and Radial Current Velocity ......................................................................................... 3 Sound ........................................................................................................................................................................ 4 The Doppler Effect ................................................................................................................................................... 5 How ADCPs use Backscattered Sound to Measure Velocity ................................................................................ 6 The Doppler Effect Measures Relative, Radial Motion ......................................................................................... 8 3. BroadBand Doppler Processing ....................................................................................................................... 9 Doppler Time Dilation ............................................................................................................................................. 9 Phase ....................................................................................................................................................................... 10 Time Dilation and Doppler Frequency Shift......................................................................................................... 10 Phase Measurement and Ambiguity ...................................................................................................................... 11 Autocorrelation ....................................................................................................................................................... 12 Modes ...................................................................................................................................................................... 12 4. Three-dimensional Current Velocity Vectors .............................................................................................. 13 Multiple Beams ...................................................................................................................................................... 13 Current Homogeneity in a Horizontal Layer ........................................................................................................ 13 Calculation of Velocity with the Four ADCP Beams .......................................................................................... 13 Error Velocity: Why it is Useful ............................................................................................................................ 14 The Janus Configuration ........................................................................................................................................ 14 5. Velocity Profile .................................................................................................................................................. 15 Depth Cells ............................................................................................................................................................. 15 Regular Spacing of Depth Cells ............................................................................................................................ 15 Averaging Over the Range of Each Depth Cell .................................................................................................... 16 Range Gating .......................................................................................................................................................... 16 The Relationship of Range Gates and Depth Cells .............................................................................................. 16 The Weight Function for a Depth Cell .................................................................................................................. 17 6. ADCP Data ........................................................................................................................................................ 18 7. Ensemble Averaging ........................................................................................................................................ 20 ADCP Errors and Uncertainty Defined ................................................................................................................. 20 Short-Term Versus Long-Term Uncertainty ........................................................................................................ 21 The Approximate Size of Random Error and Bias ............................................................................................... 21 Beam Pointing Errors ............................................................................................................................................. 21 Averaging Inside the ADCP Vs. Averaging Later ............................................................................................... 21 The Processing Cycle: Limitations on Averaging ................................................................................................ 22 8. ADCP Pitch, Roll, Heading and Velocity ..................................................................................................... 23 Conversion from ADCP- to Earth- Referenced Current ...................................................................................... 23 Measuring ADCP Rotation and Translation ......................................................................................................... 23 Self-Contained and Direct-Reading ADCPs ......................................................................................................... 24 Teledyne RD Instruments Page i Principles of Operation Data Correction Strategies for Self-Contained ADCPs ........................................................................................ 24 Vessel-mounted ADCPs ........................................................................................................................................ 25 Synchros ................................................................................................................................................................. 26 Multiple Turn Synchros for Heading .................................................................................................................... 26 Correction for Ship Velocity .................................................................................................................................. 26 Effects of Correction on Vessel-Mounted ADCP Measurements ....................................................................... 27 9. Echo Intensity and Profiling Range ............................................................................................................... 29 Sound Absorption ................................................................................................................................................... 30 Beam Spreading ..................................................................................................................................................... 31 Source Level and Power ........................................................................................................................................ 31 Scatterers ................................................................................................................................................................. 32 Bubbles ................................................................................................................................................................... 32 10. Sound Speed Corrections ...........................................................................................................................
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