YSI SonTek ADVField, Technical Documentation Manual

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SonTek/YSI
Acoustic Doppler Velocimeter Principles of Operation (September 1, 2001) 3
Figure 3 shows the geometry of a 2D ADV. The axis of each re-
ceiver is slanted 30
°
relative to the transmitter. The bistatic axis
for each receiver is in the middle of the transmitter and receiver
axes (slanted 15
°
off the transmitter axis). The receivers are fo-
cused on a volume located 5, 10, or 18 cm below the transmitter
(the distance varies depending on probe configuration). For 3D
probes, each receiver is slanted 30
°
off the transmitter and the re-
ceivers are rotated at 120
°
relative azimuth angles.
When analyzing ADV velocity data, it helps to understand the ef-
fect of probe geometry. We define the vertical direction as the axis
of the transmitter, and the horizontal direction as perpendicular to this axis. The ADV measures
bistatic velocities, each along axes 15
°
off the vertical axis, and converts these to Cartesian ve-
locities using probe geometry. Bistatic velocities are more sensitive to vertical motion than hori-
zontal motion by roughly a factor of four (tan(15
°
) = 0.27). Thus, noise in horizontal measure-
ments is 4 times larger than in vertical measurements, and the maximum horizontal velocity that
can be measured by the ADV is 4 times larger than the maximum vertical velocity (see §6.1.1).
The velocity measured by each receiver is referred to as the bistatic velocity, and is the projection
of the 3D velocity vector onto the bistatic axis of the acoustic receiver. Bistatic velocities are out-
put directly by the ADV only in specialized applications; normally they are converted to Carte-
sian (XYZ) velocities using probe geometry. Cartesian velocities give the 3D velocity field rela-
tive to the orientation of the ADV probe. As it is not always possible to control instrument orien-
tation, the ADV can be equipped with an internal compass and tilt sensor. Knowing instrument
orientation allows the ADV to report velocity data in an Earth (East-North-Up or ENU) coordi-
nate system, independent of probe orientation.
4. Sampling Volume Definition
The size of the ADV sampling volume is determined by four factors: transmit beam pattern, re-
ceive beam pattern, pulse length, and the receive window (the period of time over which the re-
turn signal is sampled). Figure 4 shows a 2D illustration of the effect of each of these factors.
The ADV sampling volume does
not have clearly defined horizontal
boundaries. These boundaries are
determined by the intersection of
the transmit and receive beam pat-
terns. The precise definition of the
beam pattern intersection is not
easily modeled; for our purposes,
it is approximated from the trans-
mit beam pattern as a cylinder the
size of the transmit ceramic (6-mm
dia. for 10-MHz ADVs; 12-mm for
5-MHz ADVOceans). While this is a simplification, the results are reasonably accurate especially
since the vertical limits of the sampling volume are typically most important. It is important to
remember that the horizontal boundaries of the sampling volume are not precisely defined and
that objects near these boundaries can potentially interfere with velocity measurements.
Figure 3 – ADV Probe
Geometry
Figure 4 – ADV Sampling Volume Definition
30
o
30
o
5, 10,
or 18 cm