An instrument function test must always be performed before every deployment to verify that the instrument works as intended. The tilt and magnetometer sensor response, reporting pitch, roll, and heading respectively, should be checked. These are also important concepts to examine when assessing data quality. All three variables tell how the instrument is oriented at the time of data collection and give an indication of the instrument's movement. Together they state an instrument's orientation in three planes. In the process of coordinate transformation to ENU, information from all three is used. Pitch and roll are also important for bin mapping.
Pitch and roll, collectively referred to as tilt, show how instruments are orientated and represent the position in two orthogonal directions. As Figure 1 visualizes, roll is rotation around the X-axis and pitch is rotation around the Y-axis. In the figure, you can also see which direction of rotation is defined as positive and negative. Both pitch and roll are given in degrees, where zero degrees means that there is no tilt and the instrument head is looking straight upwards or straight downwards (along the Z-axis). There is one exception, and that is when the instrument has an AHRS (which can be integrated into all instruments from the Signature series). For these instruments roll will be 0 degrees when looking upwards, and +/-180 degrees when looking downwards. Tilt data is important in order to know how much the instrument orientation deviates from a leveled position when measuring. There are various types of tilt sensors in Nortek instruments, with some different characteristics and limitations. The three main types are liquid level, solid state accelerometer, and AHRS. The two latter can measure full 3D, while the liquid level sensor (embedded in Midlife instruments) has angular limitations due to its design. All types are nevertheless only calibrated to 30 degrees. However, this constraint is of little importance as such an excessive tilt provides bad data and should in general be discarded. Check out the technical specification for your specific instrument to view the accuracy and precision that applies to its tilt sensor.
Pitch and roll data are also used to transform the magnetic vector to the horizontal plane before the heading is found.
Figure 1: Definition of positive and negative direction of pitch and roll shown on an AWAC.
The heading indicates an instrument’s orientation relative to the magnetic north pole. In practice, it works like a traditional mechanical compass with the X-axis as the magnetic needle. The value of the heading is given in degrees and corresponds to the readings on a compass. 0 degrees is north, 90 degrees is east, and so on. Due to the phenomenon of gimbal lock, heading readings will not be readable if the X-axis (defined to be the main horizontal axis) is aligned with the vertical axis (Z-axis if no tilt is present). For profiling instruments, this will rarely be a problem as we almost always want to measure consistently pointing vertically or sideways. The instruments must be severely tilted for this to happen, and at that point, the data is most likely so degraded that it must be discarded anyway. A compass calibration should always be performed before every deployment to correct for any magnetic disturbances on the rig.