Acoustic Doppler Current Profiler (ADCP): An instrument that obtains profiles of water velocity by transmitting sound of known frequency into the water and measuring the Doppler shift of reflections from scatterers, which are assumed to be passively moving with the water.
Acoustic Window: A covering for the hull-side opening of a sea chest that is transparent to sound. A vessel mounted ADCP is typically mounted in a sea chest and the acoustic window helps to isolate it from biofouling organisms and also the flow noise generated by the vessel.
ADCP: Acoustic Doppler Current Profiler
ADCP Coordinates: Profile data is reported in an orthogonal coordinate system as referenced to the instrument. Beam 3 is forward. Sideways is to the right of forward (beam 2 for a down-looking ADCP, beam 1 for an up-looking ADCP).
Ambiguity: ADCPs determine the radial motion between a source and scatterer by measuring the phase change of the reflected signal. Because phase is periodic, this solution is multi-valued. For example, all three of the displacements shown below will return the same phase measurement, which results in ambiguity:
Ambiguity Resolution: A method to count the number of wavelengths included between two points where phase is measured, thereby removing the ambiguity associated with measuring only phase.
Ambiguity Velocity: The maximum allowable radial motion for phase measurements to be unambiguous, corresponding to a maximum observable velocity, beyond which ambiguity resolution is required.
Autonomous Underwater Vehicle (AUV): An unmanned submersible with propulsion, generally capable of navigation and accomplishing specific tasks (such as data gathering).
AUV: Autonomous Underwater Vehicle
Backscatter: 1) The portion of a sound wave that is reflected by scatterers directly back toward the source. 2) A qualitative measurement (in decibels) of scatterer concentration which is calculated in the WinRiver software. This calculation corrects the echo intensity data for sound absorption, beam spreading, transducer temperature, etc. and provides an excellent means of tracking relative concentration (e.g. ìIs most of the sediment in the water here or is it there?î). Obtaining a quantitative measurement (e.g. ìHow many mg/L of bottom sediment are in this parcel of water?î) requires a field calibration at the measurement site. There is a third-party software package, Sediview, designed for this task.
BBBatch: Utility program to allow automated conversion of a series of raw binary ADCP files to ASCII files.
BBCheck: Utility program that checks the integrity and quality of raw binary ADCP files.
BBConv: Utility program to convert some portion of a raw binary ADCP file into an ASCII file using a decoder file. Several decoder files are included in RDITools, for example: extract only distance to first bin, extract only navigation data, etc. Complete documentation on these decoder files as well as information on how to write your own is available.
BBList: Utility program for viewing and converting raw binary ADCP data files. It is a menu-driven program offering a step-by-step process to convert raw binary files to ASCII files.
BBMerge: Utility program to convert comma-delimited ASCII files back into raw binary ADCP format.
BBSlice: A raw data sub-sectioning utility program. BBSlice converts a raw data file into a series of ASCII files, opening a new ASCII file whenever there is a jump in the sequence of ensemble numbers.
BBss: Utility program to calculate the speed of sound when given temperature, salinity and depth.
BBSub: A raw data sub-sectioning utility program. BBSub allows you to extract subsets of raw data from a large raw data file by choosing the starting and ending ensemble number.
BBTalk: Terminal emulator program for direct communication with the CPU of an ADCP or DVL.
Beam Angle: The angle between a transducer beamís main axis and the vertical axis of the ADCP or DVL (typically 20 or 30†).
Beam Coordinates: Profile data is reported as referenced along each beam (i.e. no coordinate transformation is performed upon the raw data).
Beam Spreading: The extent to which the main lobe of energy generated by a transducer fans out, or spreads as an acoustic wave front, with distance from the transducer. This is proportional to ?/d where ? is the wavelength of sound generated and d is the diameter of the transducer. Note: this is why ADCP transducer diameter increases with decreasing operating frequency (and increasing wavelength).
Bin (Depth Cell): A measurement within a profile, generally equivalent to a single-point current meter on a mooring.
Bin Mapping (Depth Cell Mapping): When the ADCP is tilted; the measurements taken at equal distances along each beam are no longer in the same horizontal layer of water. For example, in the image below the tilted ADCP bins do not line up horizontally, they are offset by one. In this case the tilted ADCP will offset the bins on the ìrightî beam by one bin in order to line them up horizontally with the bins on the ìleftî beam before combining the measurements to compute the velocity.
Blank Zone: The area near the head of an ADCP in which no measurements are taken. This is usually the minimum distance required to avoid collecting data that is potentially contaminated by ringing, but is sometimes extended for other reasons (e.g. to begin measurement well beyond the flow influence of a mounting structure).
Bottom Discharge: When using an ADCP to measure river discharge, it is not possible to measure all the way to the bottom (due to sidelobe contamination and the finite resolution of the depth cells). In order to get an accurate approximation of the total discharge, the flow in this area must be estimated and included, usually by extrapolating the measured velocities to the bottom using a power curve fit.
Bottom Track: In moving platform applications where the bottom is within range of the ADCP or DVL, a special ping can be transmitted to measure the Doppler shift of the signal return from the bottom. If the bottom is not moving, this measurement is a very accurate measurement of the platform velocity. For ADCPs this velocity is typically used to extract the true water velocity profile from the measured velocity profile (by removing the vehicle motion from the measurements). For DVLs, this IS the desired velocity.
Bottom Track Modes: There are currently four bottom tracking modes of operation available from RDI:
- Bottom Mode 4: Uses ambiguity resolution, and adjusts lags for reduced variance at higher elevations (or deeper depths).
- Bottom Mode 5: Specifically designed for reduced variance in shallow water/low elevations. It transmits several pings with computations to best determine depth and speed. This mode is the default mode, but it will automatically switch to Bottom Mode 4 if conditions warrant.
- Bottom Mode 6: Offers special narrow bandwidth operation to reduce the potential for interference or for stealth. No ambiguity resolution, instead bottom mode 6 requires user input of an approximate operating elevation (or depth).
Bottom Mode 7: Similar to bottom mode 5, except that it is optimized for slow-moving platforms in very shallow, high backscatter environments.
Break: A wake up command to an ADCP or DVL which places the instrument in command mode.
Broadband ADCP: An ADCP that uses broadband processing.
Broadband Processing: Use of coded pulses to make multiple measurements of phase with a single ping, and thereby greatly increase the precision of the measurement.
ChannelMaster: Model name for an ADCP designed to use horizontal profiling for flow monitoring in inland waterways.
Command Mode: The state into which an ADCP or DVL goes upon receiving a break. In this mode the ADCP or DVL is waiting to receive a command. It draws relatively high power, so the ADCP or DVL will go to sleep if no command is received for five minutes.
Correlation: A key quality control parameter, this is essentially a measurement of how much the particle distribution has changed between phase measurements. The less the distribution has changed, the higher the correlation, and the more precise the velocity measurement.
Dead Reckoning: A navigation method where position is estimated by measuring velocity, heading and time from the last known position.
Degaussing: Technique used to remove the magnetic field from Teledyne RDI battery packs before installation, done to minimize any effects the batteries will have on the magnetic compass.
Depth Cell (Bin): A measurement within a profile, generally equivalent to a single-point current meter on a mooring.
Depth Cell Mapping (Bin Mapping): When the ADCP is tilted; the measurements taken at equal distances along each beam are no longer in the same horizontal layer of water. For example, in the image below the tilted ADCP bins do not line up horizontally, they are offset by one. In this case the tilted ADCP will offset the bins on the ìrightî beam by one bin in order to line them up horizontally with the bins on the ìleftî beam before combining the measurements to compute the velocity.
DGPS: Differential Global Positioning System
Differential Global Positioning System (DGPS): Satellite-based navigation aid for precise measurement of location. When the bottom is out of range or moving, calculating the distance between DGPS position fixes and dividing by the time between those fixes can be used to measure the platform velocity.
Direct-Reading ADCP: An ADCP intended for real-time operation. Direct-Reading ADCPs do not have internal batteries or an internal recorder.
Discharge: The total flow through a section of a river. Rio Grande ADCPs obtain discharge measurements by transecting the river to measure water velocities, boat velocity, and the cross sectional area of the river and combining these measurements with estimates for the flow in the areas that can not be measured (edge estimates, bottom discharge and topdischarge).
Distance Made Good: When measuring transects of data with a moving platform, this is a measure of the actual distance between the platform and the start point (i.e. variations in course track are removed).
Doppler Shift: Named for Johann Doppler (1803-1853), the German physicist who first predicted it: it is the shift in frequency caused by radial motion between a source and an observer. Specifically, fD= fS (v/c): Where D is the Doppler-shifted frequency, fS is the source frequency, v is the relative velocity between source and observer, and c is the speed of sound.
Doppler Velocity Log (DVL): An instrument designed to measure the velocity and elevation of a moving platform with bottom tracking. Most DVLs will switch to measuring velocity relative to the water when the bottom is out of range.
DVL: Doppler Velocity Log.
Earth Coordinates: Profile data is reported in an orthogonal coordinate frame as referenced to the Earth (East, North and Up). ìNorthî can mean magnetic or true, depending on the heading input.
Echo Intensity: A key quality control parameter, echo intensity is a measure of the signal strength intensity returned to the transducer. High echo intensity can show solid targets (e.g. a boundary, obstruction or fish), while low echo intensity can show insufficient scatterers or the limits of profiling range for the environment.
Edge Estimate: When measuring river discharge with an ADCP it is not possible to measure to zero depth at the banks of the river. The flow through this unmeasured area must be approximated in order to obtain an accurate estimate of the total discharge.
Ensemble: A group of measurements (pings) considered together. An ensemble is usually the average of the individual measurements, and has a higher precision than any individual measurement.
Error Velocity: A key quality control parameter that derives from the four beam geometry of an ADCP. Each pair of opposing beams provides one measurement of the vertical velocity and one component of the horizontal velocity, so there are actually two independent measurements of vertical velocity that can be compared. If the flow field is homogeneous, the difference between these vertical velocities will average to zero. To put the error velocity on a more intuitive footing, it is scaled to be comparable to the variance in the horizontal velocity. In a nutshell, the error velocity can be treated as an indication of the standard deviation of the horizontal velocity measurements.
Fish Detection Threshold: Used to identify and mark as bad any velocity measurement that was potentially contaminated by a fish (because fish are generally not passively following the flow). It is a flag on the maximum allowable value for the measured echo intensity return.
Frequency: The number of wave crests passing a given point per unit time.
Gimbals: Frame that will support the weight of an object but allow its free rotation. Gimbals can be constructed to allow free rotation in one, two, or three axes.
GO-DVL: DOS batch file used to configure a Navigator and start data collection with Shiptrack.
Gyro: A rapidly spinning device mounted on gimbals to maintain a constant orientation. These devices are commonly used to measure heading on ships because, unlike magnetic compasses, they are unaffected by ferrous metals or by varying electromagnetic fields. They can also be used to measure pitch and roll because, unlike liquid level sensors, they are unaffected by accelerations.
Homogeneity: The extent to which the current measured by all four beams is the same. A key assumption of all ADCP processing is that the currents are horizontally homogeneous across the four beams. This assumption is checked for each measurement by using the error velocity measurement.
Horizontal ADCP (H-ADCP): Instrument designed to measure velocity profiles in a horizontal plane.
Inertial Navigation: Method for estimating the attitude and position of a moving platform (of primary interest here are AUVs) by integrating measurements from gyros and accelerometers. This integration is subject to large errors over time, so DVLs and pressure sensors are commonly incorporated as external inputs to measure and correct these errors.
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Lag: A time delay between pulses or pings.
Long Ranger: Model name for Workhorse ADCPs of frequency 75 kHz.
Lowered ADCP (L-ADCP): Technique whereby one or two ADCPs are lowered through the water column (typically on a rosette) to obtain velocity profiles over the full ocean depth. Note that a whole body of research exists on how to properly remove the motion of the rosette from the velocity measurements obtained in this manner.
Main Lobe: The main focus of energy emitted from a transducer. If the transducer were a flashlight, the main lobe would be the visible beam of light.
Mariner: Model name for a Workhorse Monitor ADCP configured for underway current measurement in shallow water (as opposed to the deep water Ocean Surveyor systems).
Modes: Teledyne RDI offers several modes of operation that are optimized for certain conditions. There are currently five modes of operation for water profiling available from RDI:
- Mode 1: General purpose water profiling. This is the most robust profiling mode, designed for the widest variety of measurement applications. Mode 1 can resolve high velocities and can measure over long ranges.
- Mode 5: For high resolution profiling in shallow water with low flows. Mode 5 should be used with bottom tracking. It offers the lowest standard deviation per measurement, but is also the most limited in its allowed performance envelope (e.g. low velocity flow, slow platform movement, no high shears or turbulence).
- Mode 8: Also for high resolution profiling in shallow water. Mode 8 should also be used with bottom tracking. It has higher standard deviation per measurement than mode 5, but allows a somewhat wider performance envelope.
- Mode 11: The latest in our high resolution profiling modes. Intended for the shallowest water, it is also well suited for boundary layer studies. Mode 11 should be used with bottom tracking. It allows smaller depth cells (1 cm), more depth cells, and has improved signal processing which allows faster ping rates and a wider performance envelope than modes 5 and 8. Mode 11 is intended to supercede mode 5.
- Mode 12: Offers increased resolution (1 cm depth cells) and uses multiple sub-pings to improve the standard deviation of each measurement. Mode 12 allows measurement of fast moving, shallow water; and can also be used to improve the standard deviations of any measurement (any range, any velocity) when the ADCP heading is fixed or reasonably steady.
Monitor: Model name for a direct reading Workhorse ADCP.
Moving Bottom: Some rivers carry such a heavy sediment load that they do not have a clearly identifiable bottom. In essence, the mud just keeps getting thicker and slower with depth. In such environments it is not uncommon for bottom tracking measurements to lock onto a sediment layer that is still moving, resulting in a bias to the bottom tracking velocity. This is especially important for river discharge measurements, where the vesselís navigation must be substituted for the bottom track velocity to obtain accurate results.
Navigator: Model name for the Teledyne RDI Doppler Velocity Log.
Narrowband ADCP: An ADCP that uses narrowband processing.
Narrowband Processing: Uses a single pulse per ping to measure velocity. The lack of coding in the pulse makes a narrowband measurement much less precise, but it allows profiling over a longer range. Narrowband processing generally requires much larger ensembles to get a precise measurement.
Ocean Observer: Low frequency Phased Array ADCP for cabled deployment, usually from an oil rig.
Ocean Surveyor: Low frequency Phased Array ADCP for vessel mounted operations.
Percent Good: A key quality control parameter, percent good indicates what fraction of the pings passed the various error thresholds. Each depth cell reports four values for percent good, and the meaning depends on the coordinate frame. If data is collected in beam coordinates, then the four percent good values represent the percentage of the pings collected by each beam for that depth cell whose correlation exceeded a low correlation threshold. In the other coordinate frames (ADCP, Ship and Earth Coordinates), the four Percent Good values represent (in order): 1) The percentage of good three beam solutions (one beam rejected); 2) The percentage of good transformations (error velocity threshold not exceeded); 3) The percentage of measurements where more than one beam was bad; and 4) The percentage of measurements with four beam solutions.
Phase: An engineering measure of the propagation delay caused by radial motion between scatterer and source. Phase is ambiguous in that it is cyclical (e.g. 10 † is the same phase as 370 †).
Phased Array Transducer: A single, flat, multi-element transducer that uses an Teledyne RDI proprietary technique to simultaneously form all four beams. Available phased array transducers are generally low frequency (38 kHz, 75 kHz and 150 kHz) long range devices.
Ping: The entirety of the sound generated by an ADCP transducer for a single measurement cycle. A broadband ping contains a coded series of pulses and lags, while a narrowband ping contains a single pulse.
Ping Mode: Power conserving mode for a deployed ADCP or DVL, where only the power needed for the immediate deployment task is drawn (as opposed to command mode, where the ADCP consumes considerable power while simply waiting for input). This mode saves the deployment configuration so that, in the event of a power interruption, the ADCP or DVL will be able to automatically resume the configured deployment upon return of power.
PlanADCP: Windows-based software package allowing the user to configure, and evaluate the consequences of, a deployment command-set for an ADCP.
Profile: A series of regularly spaced depth cells in which the ADCP measures velocity along with several quality control parameters.
Pulse: A sound wave generated by a transducer.
Propagation Delay: The change in the travel time of sound between a source and scatterer, generally due to radial motion. As an example: if it takes longer for sound to reflect back from a scatterer than it did a short while ago (and the speed of sound has not changed), then the scatterer must be getting farther away.
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Radial Motion: Movement which alters the distance between source and scatterer.
Range: The maximum profile length of an ADCP, it depends on several factors (note that these factors are inter-related in a complex way, and the generalizations below are intended only as rules of thumb ñ use PlanADCP to check specific combinations):
- Frequency: the lower the frequency, the longer the range.
- Depth cell size: the larger the cell, the longer the range.
- Mode of operation: mode 1 has the longest range.
- Bandwidth: the narrower the bandwidth, the longer the range.
- Concentration of scatterers: generally, the more scatterers, the longer the range.
- Temperature: generally, the colder the water, the longer the range.
- Salinity: generally, the fresher the water, the longer the range.
Range Gating: After sending a ping into the water, the ADCP transducers listen for returned signal. The time series of the returned signal is then broken into a sequence of time segments, or range gates. Each segment is equivalent to a depth cell, with the last segment coming from the farthest range from the ADCP.
RDITools: Software package containing all of the BB* programs (BBTalk, BBList, etc.) as well as several commonly used decoder files.
Reference Layer: In moving platform applications where the bottom is out of range, the vessel motion can be approximated by assuming the water is motionless at some point in the measured profile. This point is the reference layer.
Remotely Operated Vehicle (ROV): unmanned submersible controlled by an operator via a tethering cable.
Ringing: After transmission the ADCP electronics, transducer and immediate surrounding equipment (particularly in vessel mounted ADCPs) all require some finite time to dampen the transmit energy, during which time any signal return from scatterers will be contaminated.
Rio Grande ADCP: The model name for an ADCP optimally configured for measurement of river discharge. A distinguishing feature of Rio Grande ADCPs is that they are designed to operate from a 12 VDC power supply (all other ADCPs operate from 20 ñ 60 VDC).
Rosette: Oceanographic instrumentation package found on most research vessels. Rosettes are designed to be lowered to depths of interest while collecting data of various types. Most Lowered-ADCPs are mounted on rosettes.
ROV: Remotely Operated Vehicle
Scatterers: Small particles or plankton in the water which reflect sound waves.
Sea Chest: Cavity in the hull of a vessel to allow stream-lined, recessed mounting of equipment such as a Vessel Mount ADCP.
Sediview: Third party software designed to use ADCP echo intensity measurements and in situ sampling to obtain quantitative estimates of sediment concentration.
Self-Contained ADCP: An ADCP equipped with internal batteries and an internal recorder for autonomous operation.
Sentinel: The model name for a self-contained Workhorse ADCP.
Ship Coordinates: Profile data is reported in an orthogonal coordinate frame as referenced to the ship (if beam 3 is forward then ship coordinates are the same as instrument coordinates).
Shiptrack: DOS program to integrate and display Navigator data.
Sidelobes: Peaks in sound intensity generated by a transducer found to the side of the main lobe.
Sidelobe Contamination: This only need be considered when operating an ADCP near a boundary (e.g. in shallow water). The beam angle of the main lobe of an ADCP transducer is 20 or 30† off the vertical, which means that the distance to the boundary along the ADCP centerline is shorter than the distance to the boundary along a beam. Because most boundaries will reflect very strongly (much more strongly than the scatterers), sidelobe energy can travel the shorter path directly to the surface and thereby include the ìvelocityî of the boundary with the velocity measurements taken along the beams at any longer distance. This potential for interference depends strictly on the beam angle. An ADCP with a 20† beam angle has the potential for sidelobe contamination at (distance to the boundary)*cos(20†), or equivalently, the last 6% of the profile. Note: Sidelobe contamination is not relevant for DVLs, which specifically look for the bottom.
Software Break: When using radio or acoustic telemetry, it is usually not possible to send a break signal. Under these circumstances an ADCP can be configured to recognize a series of keystrokes (i.e. = = =) as a break.
Source: Originator of sound of known frequency, here typically the transducer of an ADCP.
StreamPro: Model name for a miniaturized ADCP packaged into a streamlined float designed to be towed by hand across very shallow streams for quick and efficient discharge measurements.
Top Discharge: When using an ADCP to measure river discharge, it is not possible to measure all the way to the surface (due to the blank zone and to the need to mount the transducers at sufficient depth to remain submerged with no air entraining past the transducers). In order to get an accurate approximation of the total discharge, the flow in this area must be estimated and included, usually by extrapolating the measured velocities to the surface.
Transducer: A device to convert electrical energy into sound waves, and vice versa.
Unmanned Underwater Vehicle (UUV): Generic term referring to both AUVs and ROVs.
UUV: Unmanned Underwater Vehicle
Vessel Mount: An ADCP mounted to the hull of a vessel, typically in a sea chest, and having inputs from the vesselís navigation equipment.
VM-DAS: Windows-based data acquisition package for vessel mount ADCPs. This package includes the ability to incorporate the shipís navigation equipment.
Wavelength: The distance between successive wave crests in a sound wave.
WinADCP: Windows-based post-processing package for ADCP data.
WinH-ADCP: Windows-based data acquisition and playback package for Horizontal ADCPs.
WinRiver: Windows-based software package for real time ADCP data gathering. It is designed primarily for measurement of river discharge and allows integration of the platformís navigation equipment. It also converts echo intensity measurements to qualitative estimates of backscatter.
WinSC: Windows-based software package for self-contained ADCPs includes configuring, testing, data recovery and viewing options.
Workhorse ADCP: The generic model name for all of the non-phased array broadband ADCPs currently produced by RDI.
Xdcr: A common abbreviation for transducer.
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ZedHed: Teledyne RDI trade name for a transducer designed to minimize ringing.