18
Rev. C • 10.2004
The transmit/receive beam pair
is sensitive to velocity in the
direction of the angular bisector
between the beams. The arrow
indicates a positive velocity.
Since the receive beams are
slanted at 30°, all three beam
pairs measure velocity that is
only 15° away from the transmit
beam.
30°
v
Since the receive beams are slanted at 30°, all three receivers measure the veloc-
ity that is slanted about 15° from the transmit beam.
This means that the Vectrino is more sensitive to the Z-velocity (the component
parallel to the transmit beam) than it is to the X- or Y-velocity. Consequently, the
Z-velocity component yields a lower measurement uncertainty.
Coordinate System. The Vectrino measures velocity components parallel to its
three beams, or in beam components. It reports data in Beam or XYZ coordinate
systems.
Defi ning the XYZ coordinates.
The arm with the marking
defi nes the X-direction. The
Z-direction is towards the
elctronics of the Vectrino.
View direction
X
Y
X-indicator
Z
X
Z
X
The XYZ coordinates are relative to the probe and independent of whether the
Vectrino points up or down.
In XYZ coordinates, a positive velocity in the X-direction goes in the direction
of the X-axis arrow.
Velocity Uncertainty. The Vectrino velocity is an average of many velocity es-
timates (called pings). The uncertainty of each ping is dominated by the short-
term error. We reduce the measurement uncertainty by averaging together many
pings. There is a limit to how much you can reduce your uncertainty. We call this
limit the long-term bias.
The long-term bias depends on internal signal processing, especially fi lters, and
CHAPTER 3
Technical Description
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