Usage - Specific Cases¶
Motion Correcting ADV-IMU measurements¶
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 (\(\lesssim\) 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 (journal articles are forthcoming).
Pre-Deployment Requirements¶
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 body (pressure case, which contains the IMU sensor).
- For fixed-head (fixed-stem) ADVs the body is rigidly connected to the body.
- For cable-head (cabled-probe) ADVs, a support structure must be constructed that rigidly connects the ADV body to the head.
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 body (Figure 1). It is recommended that this information be stored in a
<vec_filename>.userdata.json
file (e.g., for a data filevector_data_imu01.VEC
, the userdata file should have the namevector_data_imu01.userdata.json
). json is a simple text-based data format. DOLfYN expects the format of this file to be a single dict-like container (i.e., the file should start with{
and end with}
, that contains'name': value
pairs, for example:{"body2head_rotmat": [[ 0, 0, 1], [ 0,-1, 0], [ 1, 0, 0]], "body2head_vec": [0.13, 0.04, 1.3], "declination": 14.3, "lat": 43.2, "lon": -123, "depth": 90, "inds_range": [1000, 5000] }
You can store any information you want to attribute with the binary .VEC file in this file. One advantage of creating the .userdata.json file is that this file is read when you read the .VEC file using theread_nortek()
function, and the name-value pairs are added to theprops
attribute (a dictionary) of the returnedADVraw
data object.
The other major advantage of using these userdata.json
files, is
that many property names are special in that they are used by
DOLfYN in later processing steps. The property names that are
recognized and used by DOLfYN to provide specific information are:
- body2head_rotmat
- The rotation matrix (a 3-by-3 array) that rotates vectors in the ADV’s pressure case coordinate system (i.e., the ‘body’), to the ADV head coordinate system. For fixed-head ADVs this is the identify matrix, but for cable-head ADVs it is an arbitrary unimodular (determinant of 1) matrix.
- body2head_vec
- The 3-element vector that specifies the position of the ADV head in
the ADV pressure case’s coordinate system (Figure 1). This property
must be in the
props
attribute of the data object in order to do motion correction, so you might as well store it in theuserdata.json
file. - declination
- This is the magnetic declination at the measurement site. If it is
in the
props
attribute, it will be incorporated into the orientation matrix wheninst2earth()
is called so that all rotations are to/from a ‘True’ north coordinate system, rather than a ‘magnetic’ north coordinate system.
Data processing¶
After making ADV-IMU measurements, the DOLfYN package can perform
motion correction processing steps on the ADV data. Assuming you have
created the .userdata.json
file and added the body2head_rotmat
and body2head_vec
attributes to it, motion correction is fairly
simple, you can either:
Utilize the DOLfYN api perform motion-correction processing explicitly in Python:
from dolfyn.adv import api as adv
Load your data file, for example:
dat = adv.read_nortek('vector_data_imu01.vec')
Then perform motion correction:
adv.motion.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_vectory.py script can be used. So long as DOLfYN has been installed properly, 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-stem ADVs (no cable-head), the standard values for
body2head_rotmat
andbody2head_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 it’s .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 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 0.033Hz, 30second period). By default, DOLfYN uses a value of 0.03 Hz. For more details on choosing the appropriate value for a particular application, please see [Kilcher_etal_2016].
[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. |