SonTek/YSI Argonaut-XR Technical Manual - HydroScientific West

SonTek/YSI
interference to an aspect ratio of 4-5 in most conditions, and in some situations (with smooth
boundaries and high scattering levels) to an aspect ratio of 8-10. For example, if installed at midwater
depth in 3 m of water, the SL can accurately measure over a range of at least 6 m and
perhaps as much as 15 m. The SL includes diagnostic software (ARGCHECK) that helps you
determine the maximum effective range for a particular installation.
6.3. Near-Boundary Data Collection – Argonaut XR and SL
The XR and SL are designed for shallow-water operation and provide excellent performance near
boundaries (surface, bottom, or underwater structures). Argonaut performance near boundaries is
limited by three factors – direct reflection from the boundary, side lobe interference, and
variations in the boundary range.
The definition of the XR and SL measurement volume is discussed in Section 4. The true end of
the measurement volume extends 0.25 m beyond the specified limit. This area is not normally
considered part of the measurement volume since its relative weighting is quite small. However,
if the boundary falls into this region, it can have a significant impact on velocity data. The
user-specified end of the measurement volume must be placed a minimum of 0.25 m from any
boundary to avoid interference from this portion of the measurement volume.
Although Argonaut transducers concentrate most of the acoustic energy in a narrow beam, some
energy is transmitted in all directions. A portion of this energy will take a direct path to the
boundary and the reflection will return while the main beam is still some distance from the
boundary. This is known as side lobe energy and the reflections are called side lobe interference.
Although side lobe energy levels are very small, the reflection from the boundary is much
stronger than the return from the water and can potentially affect velocity measurements. The
potential for side lobe interference exists in the last 10% of the measurement range. To avoid
potential interference, the end of the measurement volume should be placed no closer than 10%
of the total distance to the boundary (e.g., the end of the volume should be at 9 m if the boundary
is at 10 m). The distance from the cell end to the boundary should be the greater of the potential
side lobe interference (10% of range) and the 0.25 m mentioned above.
One final consideration for near-boundary measurements is allowance for any possible variations
in boundary range (typically variations in water level). These variations can be long-term (tide or
river stage) or short-term (waves). When using the XR with dynamic boundary adjustment, these
variations and any potential boundary interference are accounted for automatically (see §6.1). If
the measurement volume limits are controlled by the user, they should be set to avoid boundary
interference at all times during a deployment.
6.4. Non-Directional Wave Parameters - Argonaut XR and SL
Argonaut XR and SL systems that have a pressure sensor can be configured to collect and record
estimates of wave frequency spectra. The spectra are estimated from the 1-Hz pressure time
series collected over the averaging interval. The estimation is done using standard methods
appropriate to simple linear theory including: segmentation of the data in 256-point segments
with at least 128-point overlap between consecutive segments; application of Hanning window to
each segment with constant energy correction; and correction for sensor/water depth using
general first order dispersion relationship for surface waves.
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Argonaut Principles of Operation (March 1, 2001)