Furuno CI-80 - Manual (Page 5)

1
PRINCIPLE OF MEASUREMENT
PRINCIPLE OF MEASUREMENT, FEATURES
When a moving vessel emits an acoustical pulse into the water
at an angle, a portion of emitted energy is reflected from the
seabed and other microscopic objects in the sound path, such
as plankton or air bubbles. The frequency of the received
signal is shifted from the transmitted frequency in proportion to
relative velocity between the vessel and underwater reflecting
objects. This is called Doppler Effect.
The CI-80 calculates and displays movements of ship and
currents at specific depths by measuring Doppler shifts
obtained from three separate directions.
Planktonic layer
Seabed
Frequency of reflected signal is
shifted in proportion to relative speed
between the vessel and reflecting
objects.
Figure 1
Ship’s Speed
(Here, speed is a vector value including velocity and direction.)
Depending on the base of measurement, ship’s speed is
expressed in two ways:
Ground tracking speed: Ship’s speed and course relative to
(Absolute speed) seabed (fixed base)
Water tracking speed: Ship’s speed and course relative to
(Relative speed) water layer just below the vessel
(floating base)
Nav-aided speed: Ship’s speed and course obtained
(Absolute speed) by external navigation equipment
(GPS)