ADP Principles of Operation

Contents: Introduction Beam Geometry and 3D Velocity Measurements Profiling Range and Range Cell Location 2D Horizontal Current Profiling

SonTek ADPs

1. Introduction

SonTek/YSI ADPs belong to a group of instruments known as acoustic Doppler current profilers. Over the past decade, current profilers have greatly expanded the ability to make detailed current measurements in challenging field applications. Since its introduction in 1994 as the first profiler designed specifically for shallow water environments, the ADP has revolutionized the current profiler market. This document provides a basic introduction to the operating principles of the SonTek ADP. For detailed information, contact SonTek/YSI.

The ADP measures the velocity of water using a physical principle called the Doppler shift. This states that if a source of sound is moving relative to the receiver, the frequency of the sound at the receiver is shifted from the transmit frequency.

F_doppler = -2F_source ( V / C )

In this equation, V is the relative velocity between source and receiver (i.e.; motion that changes the distance between the two), C is the speed of sound, F_doppler is the change in the received frequency at the receiver (i.e., the Doppler shift), and F_source is the frequency of the transmitted sound.

Figure 1. Monostatic Doppler System

Figure 1 illustrates the operation of a monostatic Doppler current meter such as the ADP (monostatic indicates fact the same transducer is used as transmitter and receiver). SonTek transducers are constructed to generate a narrow beam of sound where the majority of energy is concentrated in a cone only a few degrees wide. Each transducer generates a pulse of sound at a known frequency. As the sound travels through the water, it is reflected in all directions by particulate matter (e.g., sediment, biological matter, bubbles). Some portion of the reflected energy travels back along the transducer axis toward the transducer where the processing electronics measure the change in frequency. The Doppler shift measured by a single transducer reflects the velocity of the water along the axis of the acoustic beam.

Again referring to Figure 1, if the distance between the transducer and the reflecting object is decreasing, frequency increases. If the distance is increasing, frequency decreases. Motion perpendicular to the line connecting source and receiver has no effect on the frequency of received sound.

The measurement location is a function of the time at which the return signal is sampled. By measuring the return signal at different times following the transmit pulse, the ADP measures the profile of water velocity at different distances from the transducer. The profile of water velocity is divided into range cells, where each cell represents the average of the return signal for a given period of time. For example, a 1-m range cell corresponds to an averaging time during which the acoustic pulse moves 1 m.

Figure 2 shows a simplified animation of how the ADP transmits its signal and collects data from the return signals. (Click on the figure for an expanded view.)

Figure 2. Simplified Animation of ADP Operation
(Click for expanded view.)

2. Beam Geometry and 3D Velocity Measurements

The velocity measured by each ADP transducer is the projection of the 3D velocity onto the axis of its acoustic beam. The ADP operates using three transducers generating beams with different orientations relative to the flow of water. These beam velocities are used directly only in specialized applications. More commonly, beam velocities are converted to XYZ (Cartesian) velocities using the relative orientation of the acoustic beams, giving the 3D velocity field relative to the orientation of the ADP. Since it is not always possible to control instrument orientation, the ADP often includes an internal compass and tilt sensor to report 3D velocity data in Earth (East-North-Up or ENU) coordinates, independent of instrument orientation.

3. Profiling Range and Range Cell Location

The profiling range of an ADP is determined by the acoustic frequency and the conditions in the water (specifically the amount of scattering material present). The ADP is available in three frequencies for operation in water depths from 1 m to over 100 m. The maximum profiling range of an ADP will depend upon the conditions in which it is operated; for each frequency a typical spread of profiling ranges is shown in the table below.

All current profilers have a region immediately in front of the transducers (called the blanking region) where no measurements can be made. This region is required for the transducers and electronics to recover from the high energy transmit pulse. The blanking distance is a function of the acoustic frequency, as shown in the table below.

After the blanking region, the ADP averages the return signal in time intervals corresponding to the user specified range cell size. The minimum range cell size is a function of the acoustic frequency as shown in the table below. The user can also select the total number of range cells to be recorded, up to a maximum of 100 cells.

The ADP records nine values with each depth cell: three signal strength values (one for each acoustic beam), three velocity values (one for each velocity component), and three standard deviation values (one for each velocity component). The velocity values are of foremost interest; signal strength and standard deviation are used to determine the effective profiling range of the instrument and to estimate the accuracy of the velocity measurements. Signal strength and standard deviation data allow the user to determine what portion of the velocity profile is returning accurate data. ADP velocity data within the effective profiling range can be used directly without any additional processing.

Figure 3. ADP Profiling Range

Figure 3 shows the ADP profiling range split into several regions. The first portion of the profile is lost while the system recovers from acoustic transmission (the blanking region, discussed earlier). After the blanking distance, the ADP makes velocity measurements in range cell sizes selected by the user. As the profile approaches the boundary, there are two potential sources of interference: direct reflection of the pulse from the boundary, and reflection from sidelobe energy taking a shorter path to the boundary.

Although the ADP transducers concentrate most of the acoustic energy in a narrow beam, some energy is transmitted in all directions. As pictured in Figure 3, a portion of this energy will take a direct path to the boundary; this is called sidelobe energy and the reflections are called sidelobe interference. Although sidelobe energy levels are much lower than the main beam, the boundary reflection is much stronger than the reflection from particles in the water and can potentially bias velocity measurements. Sidelobe interference may affect the last 10% of the velocity profile. The extent to which the sidelobe reflection may contaminate the velocity measurements is a function of the boundary conditions, the scattering return strength from the water, and the acoustic properties of the transducers.

SonTek has invested considerable effort to design ADP transducers with reduced sidelobe energy levels, and hence decrease the possibility of sidelobe interference. Our experience has shown that in many conditions, the ADP does not see any evidence of sidelobe interference and hence does not lose the last 10% of the velocity profile. However, there is always a potential for sidelobe interference, and any near-boundary data should be analyzed carefully.

4. 2D Horizontal Current Profiling

The standard ADP configuration uses three beams for vertical current profiling, oriented looking either up or down. There are a number of situations where a current profiler can be used looking horizontally. For these applications, a two beam ADP measures the 2D velocity profile in a horizontal layer. Figure 4 illustrates a typical horizontal profiling application.

Figure 4. 2D Horizontal Current Profiler

A 2D-ADP is typically mounted from some type of underwater structure (bridge piling, channel wall, etc.) in water that is relatively deep compared to its width. The two beams are mounted to stay in a plane parallel to the surface and bottom. The ADP operates in exactly the same manner as the standard ADP except that the two beams measure a horizontal profile of water velocity, and return only the two horizontal velocity components.

The primary limitation of horizontal profilers is the maximum range relative to the total water depth. This is expressed as the aspect ratio between profiling range and the distance to the nearest boundary (R / H as pictured in Figure 4). Recent experience has shown that with SonTek's oversized and shaded transducers we are obtaining accurate measurements at aspect ratios of 40 or more.

More details about the ADP can be found at:

• ADP Overview
• ADP Description
• ADP Applications
• ADP Specifications

• Ask questions or request ADP/ADCP literature
• Listing of all SonTek/YSI products