September
2017
Sentinel V SC and RT Operation Manual
Page 216
EAR-Controlled Technology Subject to Restrictions Contained on the Cover Page.
Beam Coordinate Systems
The Sentinel V Real-Time can produce velocity measurements in any of the following four sets of coordi-
. Except for the first, they are all right-handed orthogonal systems.
The user operational requirements dictate the best coordinate system to be used.
Earth Axis, also known as Geographic or Geodetic Coordinates
. (E, N, U) Earth Axis are selected (default set-
ting) with command EX11xxx. These axes are named east, north, and up. Strictly speaking, these terms
refer to true orientations, although magnetic orientations are often used instead. This is the most com-
monly used coordinate system because it provides a stable reference frame for ensemble averaging.
Radial Beam Coordinates
. (BM1, BM2, BM3, BM4) Radial Beam Coordinates are selected by the EX00xxx
command. These are the “raw” velocity measurements measured independently by each transducer, in
units of millimeters per second. The sense is positive when the motion is towards the transducer. These
axes are not orthogonal.
Instrument Coordinates
. (X, Y, Z) Instrument Coordinates are selected by the EX01xxx command. This set
of axes is always oriented the same relative to the transducer head. Looking at the end view of the hous-
ing, the transducers are labeled clockwise in the order 3-2-4-1 (Figure 97). When you look at the face of
the transducer head, the transducers are labeled clockwise in the order 3-1-4-2 (Figure 98). The X-axis
lies in the direction from transducer Beam 1 towards transducer Beam 2 and the Y-axis lies in the direc-
tion from transducer Beam 4 towards transducer Beam 3. The Z-axis lies along the axes of symmetry of
the four beams, pointing away from the water towards the housing. The internal compass is mounted so
that when the X-Y plane is level, the compass measures the orientation of the Y-axis relative to magnetic
north.
Figure 97.
X, Y, and Z Velocities
The PD0 Bottom Track output data format assumes
that the instrument is stationary and the bottom is
moving.
•
If Beam 3 is going forward, then the Y veloc-
ity is negative.
•
If Beam 2 is going forward, then X velocity is
negative.
•
If the bottom is going towards the face of a
down facing ADCP, then Z is positive.
Ship Coordinates (or Righted Instrument Coordinates)
. (S, F, M) Ship Coordinates are selected by the
EX10xxx command. TRDI uses the names Starboard, Forward, and Mast, although these axes are more
commonly called the pitch, roll, and yaw-axes, respectively. Assuming that Beam 3 is aligned with the keel
on the forward side of the ADCP, for the downward-looking orientation, these axes are identical to the in-
strument axes:
S = X, F = Y, M = Z
