Speed logs Flashcards
What is the doppler effect?
The apparent change in the frequency of a wave caused by relative motion between the source of the wave and the observer
Whats the difference between doppler log and electromagnetic?
1.Can read speed over the ground
2. If twin axis can also read athwartships speed
3. Accurate in shallow water
Explain the Janus configuration with respect to a Doppler log
One transducer faces in the opposite direction to the other so that any error due to change in trim or pitching can be eliminated
Describe how a doppler log calculates speed (doppler shift principle)
The log transmits an acoustic signal from a transducer angled at 60 degrees to the horizontal. The signal which is reflected back will have an increasingly higher frequency the faster the vessel is moving forward and an increasingly lower frequency the faster it goes astern. This ‘doppler shift’ can be measured and converted to a speed read out.
Explain the dangers of using a Doppler log as a speed input to a radar
If the radar is being sea stabilized by the log, it must be water tracking. If the seabed comes within range, the log may change to ground tracking automatically. The OOW may not be aware and the radar would then be ground stabilized. The correct aspect of targets may not be displayed for collision avoidance.
Also, if the log is being used for sea stabilization of the radar, it is not certain which water layer is being used, therefore accurate aspect of targets will be affected.
Describe how an electromagnetic log calculates vessel speed
The transducer houses a coil through which an electrical current is passed
This sets up a magnetic field around it. As the vessel moves through the water an electro-motive force (e.m.f.) is created. Sensors pick up this e.m.f. which increases with increase in vessel speed. The log measures the voltage which it converts to a speed and distance read out
Name some doppler log errors
- Error in transducer orientation
- Error in oscillator frequency (frequency must be accurate and constant)
- Errors due to the effect of rolling and pitching
- Aeration
- Error due to side lobes
- Parallax error
- Speed propagation error
What is beat frequency? (Doppler shift principle)
Difference between emitted frequency and returned frequency
What angle is a doppler log installed at?
60°
Who is required to have a doppler log?
Vessels over 300GT
What is sea tracking mode?
Reflected of a thermocline = STW
What is bottom tracking mode?
Reflected of the seabed = SOG
Most modern doppler logs will have what?
A GPS input, so we can have a continuous SOG input
How can the doppler transducer differentiate from BTM to STM?
Because it will know what the beat frequency should be from the two (on an average)
Advantages of Doppler log? (4 factors)
Very accurate
Can give speed in multiple axis
Gives both SOG & STW
No moving parts
