All Nortek instruments define the direction of measured current velocities using three primary coordinate systems:

• Beam coordinate system
• Cartesian coordinate system (XYZ)
• East-North-Up coordinate system (ENU) 

Beam coordinate system

In beam coordinates, each beam makes up one axis and velocities are measured as a vector in the direction of the transducer beam. A positive velocity measurement is directed in the same direction as the beam points. The transducers/beams are numbered, and the axes are named according to the numbering (as shown in the Figure 1). All raw velocity measurements are taken in beam coordinate system initially, and then converted to either XYZ or ENU. Since the transducers are fixed, the beam directions and hence the direction of the beam axes, depends on the orientation of the instrument head.

Figure 1: Beam coordinates on a Signature instrument. 

Cartesian coordinate system (XYZ)

The Cartesian coordinate system is defined by an origin point and three perpendicular axes X, Y, and Z. On most Nortek instruments, the X-axis is in the same direction as beam 1. However, the X-axis does not have the same angle as the Beam 1-axis, rather it points orthogonal to the instruments with positive direction defined outwards. The Y-axis follows the same convention. The XY-plane is also orthogonal to the instrument, which means that the Z-axis points straight upwards when the instrument is looking up and straight downwards when the instrument is looking down. Two independent vertical estimates (Z1 and Z2) are provided from instruments with four slanted beams, where Z1 and Z2 are associated with beam 1 & 3 and beam 2 & 4 respectively. The axis directions can be found using the right-hand-rule with the first (index) finger pointing in the positive X-direction, the second (middle) finger in positive Y-direction and the thumb in the positive Z-direction. It is important to note that the XYZ coordinates are in reference to the instrument head so the measured velocity directions are dependent on the instrument orientation. In the XY-plane, Y is reported as a direction of 0° and X is reported as the direction of 90°.

Figure 2: XYZ coordinates on a Signature instrument. 

East-North-Up coordinate system (ENU)

ENU stands for "East", "North" and "Up" and similarly to the Cartesian coordinate system is also defined by three perpendicular axes. The first axis (x-axis) points towards magnetic East, the second axis (y-axis) points towards magnetic North and the last (z-axis) points vertically upwards orthogonal to the East-North plane. As with the XYZ coordinates, the right-hand-rule can be used to find the axis directions, with the index finger indicating East, the middle finger as North, and the thumb representing Up-direction. This coordinate system is relative the earth's magnetic field and not the instrument head like the others, meaning that measured velocity directions do not depend on instrument orientation. The directions East and North will always be towards magnetic East and magnetic North, while Up will always points towards the sea surface (for a submerged upward facing instrument). Instruments with four slanted beams will provide two independent Up estimates, where Up1 is associated with beam 1 & 3 and Up2 is associated with beam 2 & 4. 

Built-in compass and tilt sensors make the determination of this coordinate system possible. Although tilt corrections are made during the ENU velocity conversion, these corrections are separate from the optional corrections made when remapping cells using the "remove tilt effects" or "bin mapping" feature in Nortek software. Additionally, just like a compass, North is reported as 0° and East as 90°. It is important to note that only the slanted beams can be represented in ENU coordinates. If collecting current data from the vertical center beam, this will always be given in BEAM coordinates. 

Figure 3: ENU coordinates on a Signature instrument.