The Nortek Vector ADV can be purchased with an Inertial Motion Unit (IMU) that measures the ADV motion. These measurements can be used to remove motion from ADV velocity measurements when the ADV is mounted on a moving platform (e.g. a mooring). This approach has been found to be effective for removing high-frequency motion from ADV measurements, but cannot remove low-frequency (≲ 0.03Hz) motion because of bias-drift inherent in IMU accelerometer sensors that contaminates motion estimates at those frequencies.
This documentation is designed to document the methods for performing motion correction of ADV-IMU measurements. The accuracy and applicability of these measurements is beyond the scope of this documentation (see [Harding_etal_2017], [Kilcher_etal_2017]).
Nortek's Signature ADCP's are now also available with an Altitude and Heading Reference System (AHRS), but does not yet support motion correction of ADCP data.
In order to perform motion correction the ADV-IMU must be assembled and configured correctly:
- The ADV head must be rigidly connected to the ADV pressure case.
- The ADV software must be configured properly. In the 'Deployment Planning' frame of the Vector Nortek Software, be sure that:
- The IMU sensor is enabled (checkbox) and set to record 'dAng dVel Orient'.
- The 'Coordinate system' must be set to 'XYZ'.
- It is recommended to set the ADV velocity range to ± 4 m/s, or larger.
- For cable-head ADVs be sure to record the position and orientation of the ADV head relative to the ADV pressure case 'inst' coordinate system (Figure 1). This information is specified in terms of the following variables:
- inst2head_rotmat
The rotation matrix (a 3-by-3 array) that rotates vectors in the 'inst' coordinate system, to the ADV 'head' coordinate system. For fixed-head ADVs this is the identity matrix, but for cable-head ADVs it is an arbitrary unimodular (determinant of 1) matrix. This property must be in
dat.data_vars
in order to do motion correction.- inst2head_vec
The 3-element vector that specifies the position of the ADV head in the inst coordinate system (Figure 1). This property must be in
dat.attrs
in order to do motion correction.- These variables are set in either the userdata.json file (prior to calling
dolfyn.read
), or by setting them explicitly after the data file has been read:dat.velds.set_inst2head_rotmat(<3x3 rotation matrix>) dat.attrs['inst2head_vec'] = np.array([3-element vector])
inst2head_vec
vector $\vec{\ell}_{head}^*$ . The perspective slightly distorts the fact that x̂head ∥ − ẑ* , ŷhead ∥ − ŷ* , and ẑhead ∥ − x̂* .
The values in dat.attrs
can also be set in a json file, <data_filename>.userdata.json
, containing a single json-object. For example, the contents of these files should look something like:
{"inst2head_rotmat": "identity",
"inst2head_vec": [-1.0, 0.5, 0.2],
"motion accel_filtfreq Hz": 0.03,
"declination": 8.28,
"latlon": [39.9402, -105.2283]
}
Prior to reading a binary data file my_data.VEC
, you can create a my_data.userdata.json
file. Then when you do dolfyn.read('my_data.VEC')
, will read the contents of my_data.userdata.json
and include that information in the dat.attrs
attribute of the returned data object. This feature is provided so that meta-data can live alongside your binary data files.
The 'userdata.json' file corresponding to the ADV sounding weight in Figure 2 looks like:
{"inst2head_rotmat": [[ 0, 0, 1],
[ 0, 1, 0],
[-1, 0, 0]],
"inst2head_vec": [0.20, 0, 0.04],
"motion accel_filtfreq Hz": 0.03,
}
After making ADV-IMU measurements, the package can perform motion correction processing steps on the ADV data. Assuming you have created a vector_data_imu.userdata.json
file (to go with your vector_data_imu.vec
data file), motion correction is fairly simple. You can either:
Utilize the API to perform motion-correction explicitly in Python:
import dolfyn.adv.api as avm
Load your data file, for example:
dat = avm.read('vector_data_imu01.vec')
Then perform motion correction:
avm.correct_motion(dat, accel_filtfreq=0.1) # specify the filter frequency in Hz.
For users who want to perform motion correction with minimal Python scripting, the
motcorrect_vector.py<tree/master/scripts/motcorrect_vector.py>
script can be used. So long as has beeninstalled properly<install>
, you can use this script from the command line in a directory which contains your data files:$ python motcorrect_vector.py vector_data_imu01.vec
By default this will write a Matlab file containing your motion-corrected ADV data in ENU coordinates. Note that for fixed-head ADVs (no cable b/t head and battery case), the standard values for
inst2head_rotmat
andinst2head_vec
can be specified by using the--fixed-head
command-line parameter:$ python motcorrect_vector.py --fixed-head vector_data_imu01.vec
Otherwise, these parameters should be specified in the
.userdata.json
file, as described above.The motcorrect_vector.py script also allows the user to specify the
accel_filtfreq
using the-f
flag. Therefore, to use a filter frequency of 0.1Hz (as opposed to the default 0.033Hz), you could do:$ python motcorrect_vector.py -f 0.1 vector_data_imu01.vec
It is also possible to do motion correction of multiple data files at once, for example:
$ python motcorrect_vector.py vector_data_imu01.vec vector_data_imu02.vec
In all of these cases the script will perform motion correction on the specified file and save the data in ENU coordinates, in Matlab format. Happy motion-correcting!
After following one of these paths, your data will be motion corrected and its .u
, .v
and .w
attributes are in an East, North and Up (ENU) coordinate system, respectively. In fact, all vector quantities in dat
are now in this ENU coordinate system. See the documentation of the ~dolfyn.adv.motion.correct_motion
function for more information.
A key input parameter of motion-correction is the high-pass filter frequency that removes low-frequency bias drift from the IMU accelerometer signal (the default value is 0.03 Hz, a ~30 second period). For more details on choosing the appropriate value for a particular application, please see [Kilcher_etal_2016].
The two following examples depict the standard workflow for analyzing ADV-IMU data using .
./examples/adv_motion_correction1.py
./examples/adv_motion_correction2.py
- Harding_etal_2017
Harding, S., Kilcher, L., Thomson, J. (2017). Turbulence Measurements from Compliant Moorings. Part I: Motion Characterization. Journal of Atmospheric and Oceanic Technology, 34(6), 1235-1247. doi: 10.1175/JTECH-D-16-0189.1
- Kilcher_etal_2016
Kilcher, L.; Thomson, J.; Talbert, J.; DeKlerk, A.; 2016, "Measuring Turbulence from Moored Acoustic Doppler Velocimeters" National Renewable Energy Lab, Report Number 62979.
- Kilcher_etal_2017
Kilcher, L., Thomson, J., Harding, S., & Nylund, S. (2017). Turbulence Measurements from Compliant Moorings. Part II: Motion Correction. Journal of Atmospheric and Oceanic Technology, 34(6), 1249-1266. doi: 10.1175/JTECH-D-16-0213.1