speed measurement
TRANSCRIPT
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Presented by:
MA. THERESA C. JUSTO
MARICAR T. SORIANO
MEASUREMENT
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HISTORY
In the most ancient times, speed at sea was
measured by dropping a piece of driftwood or a
small log off of the stern of the moving ship. As
the ship moved away from the wood, an
approximate speed could be guessed. This was
remedied by attaching a length of light twine or
line to the log; the same log could then be
retrieved and used repeatedly. Marks were
added to the line to allow for a more accurate
speed reading.
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SPEED MEASUREMENT
has always been of the utmost
importance to the navigator
the speed of any object must bemeasured relative to some other point
at sea, speed may be measured relative
to either the seabed (ground reference
speed) or to the water flowing past the
hull (water reference speed)
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SPEED LOG
It is a marine electronic deviceused to measure the speed of a
moving vessel The speed of a ship is measured
in KNOTS.
1 knot = 1.51miles/hr
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CHIP LOG
It is also called as COMMON LOG or SHIP LOG.
It is a navigation tool used by Mariners in the old times to estimate the speed of a
vessel through water.
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METHODS OF SPEED
MEASUREMENT
1. Speed Measurement using WATER PRESSURE
- measures W/T speed only
2. Speed measurement using ELECTROMAGNETIC INDUCTION
- measures W/T speed only
3. Speed Measurement using ACOUSTIC CORRELATION
TECHNIQUES- measures both W/T and G/T speed
4. Speed Measurement using DOPPLER PRINCIPLE
- measures both W/T and G/T speed
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WATER
PRESSURE LOG
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SPEED MEASUREMENT USING
WATER PRESSURE Pitometer logs (also known as pit logs) are devices
used to measure a ship's speed relative to the water.
They are used on both surface ships andsubmarines.
Data from the pitometer log is usually fed directly
into the ship's navigation system.
The pitometer log was patented in 1899 by Edward
Smith Cole.
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SPEED MEASUREMENT USING
WATER PRESSURE
Speed Log operates on Pitotmeter Principle based on the
pressure developed when an open-ended tube is exposed to
water movement due to a vessels speed. The difference of
head pressure of a Static tube and a Pitot tube is comparedin a pressure box, being applied to opposite sides of a flexible
diaphragm.
A mechanical arrangement employing the servo principle
converts the movements of the diaphragm and a synchrotransmitter coupled to a drive motor shaft transmits the
vessels speed electrically to remote synchro receivers to
drive display units of speed and distance.
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SPEED MEASUREMENT USING
WATER PRESSURE
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SPEED MEASUREMENT USING
WATER PRESSURE
Bernoulli's Principle states that as the speed of a moving
fluid increases, the pressure within the fluid decreases.
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SPEED MEASUREMENT USING
WATER PRESSURE
An expression can be derived for the velocity of waterimpacting the ship as a function of the difference indynamic and static water pressure using Bernoulli's
principle. The total pressure of the water in the tubewith moving seawater can be described by the equation:
pTotal = pstatic+ pdynamic
where:
pTotal is the total fluid pressure.
pStatic is the static pressure, which strictly depends on depth.
pDynamic is the fluid pressure caused by fluid motion.
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SPEED MEASUREMENT USING
WATER PRESSURE
PITOTS LAW state that pressure is
proportional to the square of the ships
speed v multiplied by the coefficient K .
= K x v 2
where t he constant K is deri v ed from t he v essels
tonnage, shape of hull, speed of t he ship and t he
lengt h of t he protruding part of t he Pitot tube
(distance d).
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SPEED MEASUREMENT USING
WATER PRESSURE
Note:
To ensure that the dynamic pressure
reading, and thus speed, is accurate, the
effect of static pressure must be
eliminated.
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ELECTROMAGNETIC
SPEED LOG
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SPEED MEASUREMENT USING
ELECTROMAGNETIC
INDUCTIONELECTROMAGNETIC SPEED LOG
- this type of log uses Michael Fardays
well-documented principle of measuring
the flow of a fluid past a sensor by means
of electromagnetic induction
- the operation relies upon the principlethat any conductor which is moved across a
magnetic field will have induced into it a
small electromotive force (e.m.f.).
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SPEED MEASUREMENT USING
ELECTROMAGNETIC
INDUCTIONTypical Electromagnetic System consists of a:
Sensor unit
Preamplifier
Digital Display Unit
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SPEED MEASUREMENT USING
ELECTROMAGNETIC
INDUCTION
- Alternatively, the e.m.f. will also be
induced if the conductor remains stationaryand the magnetic field is moved withrespect to it.
-
Assuming that the magnetic field remainsconstant, the amplitude of the inducede.m.f. will be directly proportional to thespeed of movement.
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SPEED MEASUREMENT USING
ELECTROMAGNETIC
INDUCTION
EM Speed Log Translating System
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SPEED MEASUREMENT USING
ELECTROMAGNETIC
INDUCTION
The flow sensor is projected out into the sea
water through the hull. The sea water acts
as the conductor and generates an emf
and hence a current flows in the coil which
is transmitted to the bridge panel via a
master unit which amplifies the signal.
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SPEED MEASUREMENT USING
ELECTROMAGNETIC
INDUCTIONTo reduce the effects of electrolysis and make
amplification of the induced e.m.f. simpler, a.c. is
used to generate the magnetic field. The magnetic
field strength H now becomes Hmsint and induced e.m.f. = Hmlvsint. If t he strengt h of t he magnetic
field and t he lengt h of t he conductor bot h remain
constant then,
e.m.f. = velocity where:
l = t he lengt h of t he conductor
v = t he v elocity of t he conductor.
Hmsint = magnetic field strength
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ACOUSTICCORRELATION
TECHNIQUE
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SPEED MEASUREMENT USING
ACOUSTIC CORRELATION
TECHNIQUES
The Single- Axis Speed Log (SAL-ICCOR log)
measures the speed with respect to the
seabed or to a suspended water mass.
The log derives the vessels speed by the use
of signal acoustic correlation. Simply, this is a
way of combining the properties of sonicwaves in seawater with a correlation
technique.
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SPEED MEASUREMENT USING
ACOUSTIC CORRELATION
TECHNIQUES
Speed measurement is achieved bybottom-tracking to a maximum depth
of 200 m. If the bottom echobecomes weak or the depth exceeds200 m, the system automaticallyswitches to water-mass tracking and
will record the vessels speed withrespect to a water massapproximately 12 m below the keel.
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SPEED MEASUREMENT USING
ACOUSTIC CORRELATION
TECHNIQUES
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SPEED MEASUREMENT USING
ACOUSTIC CORRELATION
TECHNIQUES The transducer transmits pulses of energy at a
frequency of 150 kHz from two active piezoceramic
elements that are arranged in the fore and aft line
of the vessel. Each element transmits in a wide lobe
perpendicular to the seabed. As with an echo
sounder, the transducer elements are switched to
the receive mode after transmission has taken
place.
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SPEED MEASUREMENT USING
ACOUSTIC CORRELATION
TECHNIQUESThe reflected signals possess a time delay (T)
dependent upon the contour of the
seabed. Thus, the received echo is uniquely
a function of the instantaneous position of
each sensor element plus the ships speed.
T = 0 .5 x sv where:
T time delay in seconds
s distance bet w een t he recei v ing elements
v ships v elocity
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SPEED MEASUREMENT USING
ACOUSTIC CORRELATION
TECHNIQUES
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SPEED MEASUREMENT USING
ACOUSTIC CORRELATION
TECHNIQUESThese are the factors that will not affect
the calculation of the speed of the
vessel: Temperature
Salinity of the seawater
Variables of sound velocity inthe seawater
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SPEED MEASUREMENT USING
ACOUSTIC CORRELATION
TECHNIQUESIt is also possible to use t he time delay (T)
bet w een t he transmission and reception tocalculate dept h.
where:
d dept h in meters
C v elocity of sonic energy in seaw ater (1500ms-1 )
T time delay in seconds
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DOPPLER PRINCIPLE
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Doppler Principle/Doppler Effect- named after Christian Andreas Doppler, isthe apparent change in frequency andwavelength of a wave that is perceived byan observer moving relative to the sourceof the waves.
-states that Doppler shift f (Hz) is proportional to bot h t he flow v elocity,v
(cm/s) and t he transmission frequency of t he ultrasound f (MHz)
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
A source of waves moving to the left. The frequency is
higher on the left, and lower on the right.
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Examples of Doppler Effect:
Doppler phenomenon with sound and relati v emov ement
The whistle from a moving train: as the trainapproaches a stationary listener, the pitch(frequency) of the whistle sounds higher thanwhen the train passes by (recedes), at whichtime the pitch sound the same as if the trainwere stationary. As the train recedes from thelistener, the pitch decreases.
The car horn (noise from a car) exhibits the samephenomenon
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Principles of Doppler Speed Radar/Log
Traffic Radar:
Radar (wave transmitter and receiver) is stationary,target car is moving
Radar (wave transmitter and receiver) is moving,target car is moving
Ships Doppler Speed Log:
Wave transmitter and receiver are installed onboard the ship whose velocities are measured.
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Illustration of Doppler Effect
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Doppler Effect Doppler Shift
Frequency of Transmit Signal: ft (Hz)
Velocity of Transmit Signal: c (m/s2)
Frequency of Reflection Signal fr (Hz)
Velocity of Target: v
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Approaching (moving towards) target:
fr = ft + fd
Receding (moving away) target:
fr = ft fd
where fd is Doppler shift t he difference bet w een t he frequency of transmit signal and reflection frequency of reflection signal (echo)
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Both the source and target are moving:
Source and Target moving in the OPPOSITEdirection
Source and Target moving in the SAME direction
where:
v is velocity of moving target,
vs is velocity of moving source
vr is the relative velocity between the source and the target
+ opposite direction
- same direction
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Applications of Doppler effect
Speed logging systems (ship, police radar)
Depth logging systems
Astronomy
Medical imaging
Temperature measurement
Laser Doppler velocimeters
Acoustic Doppler velocimeters
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Principles of Speed Measurement
using the Doppler Effect
The phenomenon of Doppler frequency shift is often used to
measure the speed of a moving object carrying a transmitter.Modern speed logs use this principle to measure the vesselsspeed, with respect to the seabed, with an accuracyapproaching 0.1%. If a sonar beam is transmitted ahead of avessel, the reflected energy wave will have suffered afrequency shift, the amount of which depends upon:
- t he transmitted frequency
- t he v elocity of t he sonar energy w av e
- t he v elocity of t he transmitter (t he ship)
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Speed: Doppler Speed Log
Illustration of the change of wavelength that occurs when an acoustic wave crosses a
water mass
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
It follows that if the angle changes, the speed
calculated will be in error because the angle of
propagation has been applied to the speed
calculation formula in this way.
If the vessel is not in correct trim (or pitching in
heavy weather) the longitudinal parameters will
change and the speed indicated will be in error.
To counteract this effect to some extent, twoacoustic beams are transmitted, one ahead and
one astern.
JANUS configuration it is the transducer
assembly used for this type of transmission
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
(a) Derivation of longitudinal speed using trigonometry. (b) The effect of pitching on a
Janus transducer configuration.
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
The figure shows the
advantage of having a
Janus configuration over
a single transducer
arrangement. It can be
seen that a 3° change of
trim on a vessel in a
forward pointing Doppler
system will produce a 5%
velocity error. With aJanus configuration
transducer system, the
error is reduced to 0.2%
but is not fully
eliminated. Graphs of Speed Error caused by Variations of the Vessels Trim
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Speed V ectors during aSt arboard T urn
-A dual axis Doppler speed log
measures longitudinal andtransverse speed, at the location of the transducers. If transducers aremounted in the bow and stern of avessel, the rate of turn can becomputed and displayed. Thisfacility is obviously invaluable to the
navigator during difficultmanoeuvres.
- Starboard - Starboard, or the right side of a boat, comes from the word steorbord,an old English term meaning side on whicha boat is steered.
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
CHOICE OF TRANSMISSION
MODE:
Continuous Wav e Mode (CW)
Transmission
Pulse Mode Operation
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
C ontinuous W ave Mode ( C W) T r ansmission
-Two transducers are used in each of the Janus positions. A continuous
wave of acoustic energy is transmitted by one element and
received by the second element.
-Received energy will have been reflected either from the seabed, or, if the depth exceeds a predetermined figure (20 m is typical), from a
water mass below the keel.
-Problems can arise with CW operation particularly in deep water when
the transmitted beam is caused to scatter by an increasing number
of particles in the water. Energy due to scattering will be returned
to the transducer in addition to the energy returned from thewater mass.
-The receiver is likely to become confused as the returned energy from
the water mass becomes weaker due to the increasing effects of
scattering. The speed indication is now very erratic and may fall to
zero. CW systems are rarely used for this reason.
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
P ulse Mode O per ation
-To overcome the problems of the CW system, a pulse mode
operation is used.
-This is virtually identical to that described previously for depth
sounding where a high energy pulse is transmitted with the
receiver off.
-The returned acoustic energy is received by the same
transducer element that has been switched to the receive
mode.
- In addition to overcoming the signal loss problem, caused by
scattering in the CW system, the pulse mode system has the
big advantage that only half the number of transducers is
required.
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
C om parison of the P ulse and the C W systems
Pulse systems are able to operate in the ground reference mode at depths
up to 300 m (depending upon the carrier frequency used) and in the water
track mode in any depth of water, whereas the CW systems are limited todepths of less than 60 m. However, CW systems are superior in very
shallow water, where the pulse system is limited by the pulse repetition
frequency (PRF) of the operating cycle.
The pulse system requires only one transducer (two for the Janus
configuration) whereas separate elements are needed for CW operation.
CW systems are limited by noise due to air bubbles from the vessels ownpropeller, particularly when going astern.
Pulse system accuracy, although slightly inferior to the CW system, is
constant for all operating depths of water, whereas the accuracy of the
CW system is better in shallow water but rapidly reduces as depth
increases.
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Environmental Factors Affecting the Accuracy of Speed Logs
Unfortunately environmental factors can introduce errors and/or produce sporadic
indications in any system that relies for its operation on the transmission and
reception of acoustic waves in salt water.
W ater cl arity. In exceptional cases t he purity of t he seaw ater may lead to
insufficient scattering of the acoustic energy and prevent an adequate signal
return. It is not likely to be a significant factor because most seawater holds
the suspended particles and micro-organisms that adequately scatter an
acoustic beam.
Aer ation. Aerated w ater bubbles beneat h t he transducer face may reflect
acoustic energy of sufficient strength to be interpreted erroneously as seabottom returns producing inaccurate depth indications and reduced speed
accuracy. Proper siting of the transducer, away from bow thrusters, for
instance, will reduce this error factor.
V essel trim and list. A change in t he v essels trim from t he calibrated normal
w ill affect fore/aft speed indication and an excessive list will affect athwartship
speed. A Janus configuration transducer reduces this error.
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Environmental factors affecting the accuracy of speed logs
Ocean current profile. T his effect is prev alent in areas w it h strong tides
or ocean currents. In t he water track mode, a speed log measures
velocity relative to multiple thermocline layers several feet down in the
water. If these layers are moving in opposite directions to the surfacewater, an error may be introduced.
Ocean eddy currents. W hilst most ocean currents produce eddies t heir
effect is minimal. T his problem is more likely to be found in restricted
waters with big tidal changes or in river mouths.
Sea st ate. Follow ing seas may result in a change in t he speed indication
in t he fore/aft and/or port/ starboard line depending upon the vectorsum of the approaching sea relative to the ships axis.
Temperature profile. T he temperature of t he seaw ater affects t he
v elocity of t he propagated acoustic. Temperature sensors are included in
the transducer to produce corrective data that is interfaced with the
electronics unit.
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Major Components:
Transducer The transducer assembly transmits
sonic energy into the water and receives back the
Doppler shifted echoes.
Electronics Unit The electronics unit houses the
majority portion of the electronics for the system
speed and distance processing.
Master Display Unit The master display unit
indicates vessel speed and distance traveled and
contains switching that controls all system power
and operations.
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Dist ance dis pl ay . This shows the distance run innautical miles or km.
-Depending upon the selected mode and depth, the
display indicates over-the-bottom distance or, whenthe unit is water tracking, the distance travelledthrough the water.
- If the ALT characters are showing, the system tracksboth bottom and water simultaneously and provides
both outputs to external devices.
-This display also provides a numerical indicationwhich, when used in conjunction with the systemmanual, provides clues to any system malfunction.
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SPEED MEASUREMENT USING
DOPPLER PRINCIPLE
Different DI SPLAY U nit
Speed dis pl ay. This shows the vessels fore/aft
speed in knots, m/s (metres per second) or ft/s
(feet per second).
P ort/st arboard dis pl ay. This indicates
athwartship speed in knots, m/s or ft/s.
De pth/time dis pl ay. This indicates water depth
to the seabed, in fathoms, metres or feet, when
in either water or bottom-tracking mode,
providing the depth is within 200 m. The depth
indication circuitry also includes a depth alarm.
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DEFINITION OF TERMS
Aeration - The formation of bubbles on the transducer face causing
errors in the system.
Beamwidth - The width of the transmitted acoustic pulsed wave. The
beam spreads the further it travels away from a transducer.
CW Mode (Continuous Wave Transmission) - Both the transmitter and
receiver are active the whole time. Requires two transducers.
Distance integrator - The section of a speed log that produces an
indication of distance travelled from speed and time data.
Doppler principle - A well-documented natural phenomenon enabling
velocity to be calculated from a frequency shift detected between
transmission and reception of a radio signal.
E.M. log - An electronic logging system relying on the induction of
electromagnetic energy in seawater to produce an indication of velocity.
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DEFINITION OF TERMS
G/T - Ground-tracking or ground referenced speed.
Pitot log - An electromechanical speed logging system using
changing water pressure to indicate velocity.
Pulse Mode - Acoustic energy is transmitted in the form of pulses similar to an echo sounding device or RADAR
Transducer - The transmitter/receiver part of a logging
system that is in contact with the water. Similar to an
antenna in a communications system. Translating system - The electronic section of a logging
system that produces the speed indication from a variety of
data.
W/T - Water-tracking or water referenced speed.
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DEFINITION OF TERMS
Starboard - Starboard, or the right side of a boat, comesfrom the word steorbord, an old English term meaning sideon which a boat is steered.
Port - The nautical term "port" refers to the left side of aboat when facing the front, or bow. Another version of theterm "port" is larboard, which dates back to the 16th centuryand was used up until the 1800s to describe the left side of the boat. The port side of a boat is marked with a red light inthe front.
Bow (also known as fore) The "bow" is a term for the
front of a ship. Structurally, the bow curves to a point that iscentered on the ship itself.
Stern (also known as aft) The stern is a term for theback of a ship.
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THE END
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