Page 33 of 92
Chapter: 2. Operation
Principal Functions
Doppler Log Principle
The Doppler log works on the principle of detecting Doppler frequency shifts, i.e the phenomena of a
perceived frequency being dependent on the relative speed of the emitting and receiving systems.
Observed frequency shift ∆f.
Emitted frequency f.
Measured velocity, v.
Speed of sound in water, c.
A short burst of a known frequency is emitted into the water, and the frequency of the received echoes
are measured. The dierence between the emitted frequency and the received frequency makes it
possible to calculate the relative speed. If the reecting body is the bottom, the measured speed will be
relative to ground. By measuring the reections from the particles in a near water layer, a relative water
speed may be obtained.
Bottom Track Characteristics
DL850 will try to acquire a bottom track when the depth is between 2 and ca. 150 m, depending on the
bottom conditions. In this case, both bottom track and water track data will be acquired.
Water Track Characteristics
Water relative speed is measured at a depth of. 0,3-16 meter below transducer. (Depending on setting
and depth). Alternativly using NAUT setting ( See “NAUT rules” on page 89 ) the water track can be
xed to allways measure within 0,3-3 meter. Water track detected in depths less than 2m will include
interference from bottom track.“
Docking Function - (Only in Docking version of Display unit)
Docking function is a feature for navigators which have a need of the transversal speed aft the ship.
Instead of installing a second sensor aft, (which will normally cause problems due to turbulence under
the sensor) the movement of the ship is calculated using the forward sensor and the gyro compass’s
rotation signal. Provided the ROT (Rate of Turn) signals full the IMO standards (IMO Resolution
A.526 (13)) and the Doppler sensor is installed and operating correctly, the transversal speed aft of the
ship will be calculated to the required accuracy of 0.2 kn.
The longer the vessel, the more accurate the requirement of the Doppler sensor. The accuracy
of the sensor can be increased by increasing the averaging time of the system, or the size of the
measurement sample (cell). The software will calculate the optimum averaging and cell size, based
on the requirement spec of the system and set this as the default value. It is likely the averaging can
be reduced to give a faster response as most systems have an actual performance better than the
requirements. During installation, parameters for the ship length at waterline and distance between the
bow and sensor must be entered into the log. These parameters are used for calculating the aft speed.
Non-volatile Parameter Memory
The system contains memory to maintain installation and user parameters like unit of measurement,
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