YSI ADV6600 - Manual (Page 112)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 102
(sediment, small organisms, bubbles, etc.). Some portion of the reflected energy travels back along
the receiver beam axes. The reflected signal is sampled by the acoustic receivers. The ADV6600
processor measures the change in frequency for each receiver. The Doppler shift is proportional to
the velocity of the particles along the bistatic axis of the receiver and transmitter. The bistatic axis
is located halfway between the transmit and receive axes. Knowing the relative orientation of the
bistatic axes of all receivers allows the ADV6600 to calculate 3D water velocity in the sampling
volume.
Profile of signal strength versus time for the ADV6600. The horizontal axis shows time after the
transmit pulse. The vertical axis shows the returned signal strength measured by one receiver. As
the transmit pulse travels through the water, sound is reflected in all directions. Immediately
following the transmit pulse, reflections come from outside the receiver beam; the receiver measures
only the ambient noise level. As
the pulse propagates along the
transmit axis, it moves closer to
the receiver beam; the receiver
sees an increase in signal
strength. Signal strength reaches
a maximum at the intersection of
the transmit and receive beams.
By sampling the return signal at
its peak, the ADV6600 makes
measurements in the sampling
volume defined by the intersection of transmit and receive beams.
9-2. Velocity Data Coordinate System
The ADV6600 supports two coordinate systems for velocity data:
• ENU (East-North-Up, or Earth coordinates)
• XYZ
9-2.1. ENU
The primary advantage of using the internal compass/tilt sensor is the ability to report velocity data
in ENU (East/North/Up or Earth) coordinates – independent of the rotational orientation of the
ADV6600. An ADV6600 mounted with an unknown orientation will provide velocity data
consistent with the direction and speed of the water current itself.
Velocity data in each sample are averaged using continually updated compass data. If the
ADV6600 changes orientation during a sample, velocity data will be averaged in Earth coordinates
and will not be contaminated by instrument motion. Using any other coordinate system for the
ADV6600 (beam or XYZ) eliminates the Earth coordinate system vector averaging and can cause
contamination of the data if the instrument moves during the course of an averaging interval.