SonTek/YSI Argonaut-XR Technical Manual - HydroScientific West

SonTek/YSI
5.3. Standard Deviation
Each velocity sample recorded by the Argonaut is the average of a number of pings. In addition
to mean velocity, the Argonaut records the standard deviation of these samples as a direct
measure of the accuracy of the velocity data. The measured standard deviation includes
instrument-generated noise and real variations of the water velocity.
Instrument-generated noise in velocity data can be estimated for a given set of operating
parameters. This is useful in planning deployments, particularly for determining the operating
parameters required for a desired accuracy. It can also be useful to compare the measured
standard deviation to predicted values when analyzing data. Under normal operating conditions,
these values will agree to within about 20%.
The MD pings once per second; standard deviation decreases with the square root of the number
of samples averaged. Increased averaging times are used to improve the precision of velocity
measurements. Table 1 shows predicted standard deviation for the MD. The predicted standard
deviation for other averaging times can be calculated by scaling with the square root of the
number of samples.
The standard XR and SL operate at an acoustic frequency of 1500 kHz. The expected standard
deviation for each system is a function of the size of the measurement volume and the number of
samples averaged. Like the MD, the XR and SL ping once per second and the number of samples
averaged is equal to the averaging time in seconds.
The formula below can be used to predict the standard deviation of XR and SL velocity data (in
cm/s) based on cell size (CS, in meters) and the number of samples averaged (N). Cell size is the
extent of the measurement volume and is equal to the difference between cell begin and cell end.
Table 2 shows standard deviation values for the Argonaut XR and SL at different cell sizes and
averaging times.
8
Table 1. Argonaut-MD Predicted Velocity Precision (Standard Deviation)
Single Ping 30 second average 120 second average
7.7 cm/s 1.4 cm/s 0.7 cm/s
σ
=
20
N CS
Table 2. Argonaut-XR/SL Predicted Velocity Precision (Standard Deviation)
1 m Cell Size 3 m Cell Size 10 m Cell Size
Single ping 20 cm/s 11.5 cm/s 6.3 cm/s
30-s avg. 3.7 cm/s 2.1 cm/s 1.2 cm/s
120-s avg. 1.8 cm/s 1.1 cm/s 0.6 cm/s
For the MD, velocity performance is identical between horizontal (XY) and vertical (Z) data. For
the XR, the values shown in Table 2 reflect the performance of horizontal (XY) velocity data;
vertical (Z) velocity data will have lower noise levels by about a factor of two. For the SL, the
values reflect standard deviation in the cross-range direction (X) direction; along-range (Y) data
will have lower noise levels by about a factor of two.
Argonaut Principles of Operation (March 1, 2001)