The purpose of a calibration run is to measure the following two parameters:
AWAC VM (VMCP) installation angle
VMCP accuracy correction
The installation angle represents the total angular offsets required to correct from VMCP coordinates to earth coordinates. The main contributions are VMCP mounting angle offset, offset in gyro or heading sensor, and unaccounted magnetic deviation.
The VMCP accuracy correction is the difference between the average speed measured by the VMCP and by the GPS system. It is given as a fractional error in the VMCP data and includes the errors in the VMCP transducer orientation and in the speed of sound entered by the operator in the software.
To run a calibration, the effect of the local currents must be eliminated. We do this by running the ship in opposite directions along the same track. You must make the two runs quickly to minimize changes in the flow from one run to the next. Better yet, make your runs in an area with weak currents.
Find an area with low currents and decide on a straight track that will take at least 2–3 minutes to run. In areas with strong currents the calibration tracks should be into or with the currents to minimize the effect of crabbing, i.e. large differences between track course and ship heading.
To calibrate, do as follows:
Give the heading sensor or gyro and the DGPS system time to stabilize by letting the ship go for 30 seconds on a steady course before reaching the starting point.
Keep the ship on a steady track during data collection. The ship speed should be comparable or slightly exceeding the speed you will use in an actual survey.
Start data collection at the beginning of your course and stop data collection at the end. The procedure is not extremely sensitive to the start and stop time of the data collection.
Again, let ship heading stabilize before you rerun the in the opposite direction. Run the ship at the same speed you used in the first run and record data the same way, into a new file
In the SurveyVM/SurveyVM2 software: Click Tools > Calibration... to open the Calibration menu.
Load the two data files you just collected.
Select the current depth range you are interested in.
Click Compute and note the results a few seconds later.
Note particularly the VM scaling! It should be between 0.9 and 1.1; normally be close to 1.0. Be sure to save this calibration result with your configuration files (.svm). The resulting calibration parameters will then be used automatically for all subsequent data collection surveys.
If you record data, keep track of the calibration parameters you use because the parameters are not stored with the data files. The best way to do this is to record new configuration files every time you change configuration or recalibrate, and to use the same name for both data and configuration files.
Particular care should be taken to ensure the erroneous readings are not included in the calibration data set. If the calibration is performed in shallow water, select a corresponding depth range in the Configuration... dialog. The user can also choose which cells to use in the calibration such that any shears can be removed. If the VM scaling is outside of 2%, ie 0.98 or 1.02, reducing the number of cells can often give a better result. In general the best result is the correct one.
It is hard to say exactly how often the calibration routine should be repeated because this will depend on the stability of the navigation equipment as well as the local conditions. Nortek therefore recommends that you perform the calibration routine more frequently in the beginning until you become confident in the stability of the results. However, there are some scenarios when you should always calibrate:
Every time the VMCP is installed or reinstalled.
Every time the navigation equipment has been changed, serviced, or upgraded.
In addition, it is prudent to repeat the calibration whenever the equipment has been sitting idle for a while of if the operator is unsure of when the last calibration was performed. The calibration should be stable over time (within 0.5% in the VMCP correction and 0.5° in the angular correction) if the user is providing the correct salinity. If it is not stable, it could be a sign that there is a systematic error in the procedure, the gyro is unstable, the temperature sensor in the VMCP is malfunctioning or the VMCP mount is not fixed.