YSI SonTek ADVField, Technical Documentation Manual

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SonTek/YSI
Acoustic Doppler Velocimeter Principles of Operation (September 1, 2001) 11
There is a second, less obvious, source of boundary interference for ADV measurements. The
ADV sends two pulses for each velocity measurement (see Section 5). Thus it is possible that the
reflection of the first pulse from the boundary may arrive at the same time as the return of the
second pulse from the sampling volume. This illustrated in Figure 8 below.
Figure 8 shows three possible cases for ADV operation near boundaries. In Case 1, the distance
between the two pulses (known as the pulse lag) is shorter than the distance from the sampling
volume to the boundary. In Case 2, the pulse lag is greater than the distance to the boundary. In
either of these cases, the ADV is able to make velocity measurements without interference. In
Case 3, the pulse lag matches the distance to the boundary, so the ADV will be receiving the re-
flection of the first pulse from the boundary when it is sampling the second pulse. This contami-
nation appears as very high noise levels, typically making it impossible to interpret velocity data.
The ADV has been designed to look for the interference shown in Case 3. Before starting data
collection, the instrument looks for a boundary and, if present, measures the distance to the
boundary. If that distance matches the pulse lag (determined by the velocity range setting), the
ADV will automatically adjust the pulse lag to avoid interference. When possible, it will adjust
the lag to a higher velocity range. However, when working very near the boundary it may not be
Figure 7 – ADV Signal Strength Profile with Boundary Return
Figure 8 – ADV Multiple Pulse Boundary Considerations
Time after transmit pulse
R
eceive
d s
igna
l s
tr
ength
Sampling
Volume
Boundary
Reflection
1
T
∆
T
∆
T
∆
2
3
CASE 1 CASE 2 CASE 3