THE DOPPLER SPEED LOG – DOPPLER EFFECT

• THE DOPPLER EFFECT - A CHANGE IN OBSERVED SOUND PITCH RESULTING FROM RELATIVE
MOTION BETWEEN THE SOUND SOURCE AND RECEIVER.
➢ AN EXAMPLE IS THE SOUND OF A TRAIN WHISTLE AS IT APPROACHES A STATIONARY OBSERVER
AND PASSES BY. THE PITCH INCREASES AS THE TRAIN APPROACHES AND REDUCES AS THE TRAIN
PASSES AND MOVES AWAY. THIS OCCURS EVEN THOUGH THE FREQUENCY OF THE TRAIN
WHISTLE DOES NOT CHANGE.
➢ THE CHANGE IN PITCH IS DIRECTLY
PROPORTIONAL TO THE RELATIVE
SPEED BETWEEN THE SOUND SOURCE
AND THE RECEIVER.
➢ THUS, IF WE CAN MEASURE THE
CHANGE IN PITCH AND THE AMOUNT
OF CHANGE FROM THE SOURCE
FREQUENCY, WE CAN GAUGE THE
RELATIVE SPEED.

PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 1
 THE DOPPLER SPEED LOG – DOPPLER EFFECT
f=1/t, f=v/λ
f -frequency
WHY DOES THE “APPARENT” FREQUENCY CHANGE?
T -time
• IMAGINE EACH CIRCLE IN THE ADJACENT FIGURE v -velocity of
IS A WAVEFRONT THAT COINCIDES WITH THE sound
PEAKS OF THE SOUND FREQUENCY. λ - wavelength
• BECAUSE THE SOUND IS COMING FROM THE
CENTRE OF THE INNERMOST CIRCLE AND THE
SOURCE OF SOUND [TRAIN] IS MOVING FROM REDUCED INCREASED
RIGHT TO LEFT, HENCE…… WAVELENGTH, WAVELENGTH,
INCREASED DECREASED
• THE WAVEFRONTS REACH QUICKER AND GET FREQUENCY FREQUENCY
SQUEEZED TOGETHER AS THE TRAIN MOVES
TOWARD THE BOY WHILE THEY REACH LATER
AND ARE STRETCHED FURTHER APART AS THE
TRAIN MOVES AWAY FROM THE GIRL.
• THIS MEANS THE WAVELENGTH OF THE SOUND
IS SHORTER GOING TOWARD THE BOY AND
LONGER GOING AWAY FROM THE GIRL.
• SINCE f=v/λ, THE SHORTER THE WAVELENGTH
THE HIGHER THE FREQUENCY AND VICE VERSA. ORIGINAL WHISTLE
FREQUENCY
DOPPLER EFFECT by CAPT. S.F. GOMEZ

PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 2
 THE DOPPLER SPEED LOG – PRINCIPLE
• IF A SIGNAL BEAMED IN THE FORWARD DIRECTION REFLECTS OFF AN OBJECT AND IS RETURNED
TO THE VESSEL, IT WILL SUFFER AN APPARENT FREQUENCY SHIFT WHICH IS PROPORTIONATE
TO THE VESSELS MOVEMENT. THE VESSELS SPEED CAN THUS BE CALCULATED BASED ON THE
DIFFERENCE IN FREQUENCY BETWEEN TRANSMITTED AND RECEIVED SIGNALS.
• HOWEVER, SINCE THERE IS GENERALLY NO REFLECTING SURFACE AHEAD OF A SHIP AT SEA, WE
USE THE SEA BED TO HELP GAUGE THE SHIPS SPEED.
• THE DOPPLER TRANSDUCER IS MOUNTED ON THE KEEL AND PRODUCES A CONCENTRATED,
CONICAL BEAM OF ENERGY.
➢ IF THIS MOUNT FACES STRAIGHT DOWN, THERE WOULD BE NO RELATIVE MOVEMENT
BETWEEN THE TRANSMITTER AND RECEIVER AND SO NO DOPPLER SHIFT.
➢ THE TRANSDUCER BEAM IS THEREFORE ANGLED FORWARD AND DOWNWARD SO THAT THE
SHIP APPROACHES OR MOVES AWAY FROM THE SIGNAL REFLECTED OFF THE SEA BED AND SO
CAUSES DOPPLER SHIFT OF THE RECEIVED SIGNAL.
➢ AN ANGLE OF 60˚ TO THE KEEL HAS BEEN FOUND TO BE SUITABLE AND WILL REFLECT A SIGNAL
OF SUFFICIENT STRENGTH BACK TO THE RECEIVING TRANSDUCER.
• THE AMOUNT OF FREQUENCY SHIFT DEPENDS ON :
➢ THE TRANSMITTED FREQUENCY
➢ THE VELOCITY OF THE TRANSMITTED SIGNAL
➢ THE VELOCITY OF THE TRANSMITTER / RECEIVER [SHIP]

PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 3
 THE DOPPLER SPEED LOG – DOPPLER EFFECT
IF BOTH TRANSMITTER AND RECEIVER ARE MOVING TOWARD A IF TX AND RX MOVES AWAY
REFLECTING SURFACE, THEN THE RECEIVED FREQUENCY IS FROM A REFLECTING
GIVEN BY SURFACE
𝑐+𝑣 𝑐−𝑣
𝑓𝑟 = 𝑓𝑡 ∗ 𝑓𝑟 = 𝑓𝑡 ∗
𝑐−𝑣 𝑐+𝑣

AND THE DOPPLER FREQUENCY SHIFT 𝑓𝑑 IS, THEN, DOPPLER SHIFT IS

𝑓𝑑 = 𝑓𝑡 − 𝑓𝑟 𝑓𝑑 = 𝑓𝑡 − 𝑓𝑟
AND AND
2𝑣𝑓𝑡 2𝑣𝑓𝑡
𝑓𝑑 = 𝑓𝑑 =
𝑐−𝑣 𝑐+𝑣
AND SINCE c IS MUCH, MUCH LARGER THAN v IN THE CASE OF
SHIPS, THE EQUATION CAN BE SIMPLIFIED TO COMMON FORMULAE
2𝑣𝑓𝑡
𝑓𝑑 = f [frequency] = c/λ = 1/T
𝑐
λ [wavelength] = c/f = c*T
T [time] = λ/c = 1/f
C [spd of sound] = λ/f

PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 4
THE DOPPLER SPEED LOG – DOPPLER SHIFT & DOPPLER SPEED
SINCE THE TRANSDUCER IS ANGLED AT 60˚ TO
Doppler Log – Principle
THE KEEL THE MEASURED SHIP SPEED 𝑣 IS By CAPT. S.F. GOMEZ

THE SPEED OF THE TRANSDUCER/SHIP

TOWARD THE POINT OF REFLECTION “R”.
BUT WE REQUIRE SPEED IN THE FORWARD
DIRECTION AND THIS IS EQUAL TO 𝑣𝐶𝑜𝑠𝛼 vCosα
WHERE 𝛼 IS 60o. (Speed)
Adj

2𝑣𝑓 α
𝑓𝑑 = 𝑐 𝑡 WILL THEREFORE BECOME,
vSiinα Hyp [v]
2𝑣𝐶𝑜𝑠𝛼𝑓𝑡
𝑓𝑑 = (Depth) 𝑎𝑑𝑗
𝑐
𝑐𝑜𝑠α =
ℎ𝑦𝑝
ALSO, 𝑣𝑆𝑖𝑛𝛼 WILL GIVE US THE DEPTH 𝑎𝑑𝑗 = 𝑣𝑐𝑜𝑠𝛼
BELOW THE TRANSDUCER OR THE UKC AS R
LONG THE DOPPLER LOG IS GROUND OR
BOTTOM TRACKING. DEPTH IS NOT
AVAILABLE WHEN WATER TRACKING.
WHEN THE VESSEL PITCHES/HEAVES THE VERTICAL
COMPONENT 𝑣𝑆𝑖𝑛𝛼 WILL BE IN ERROR. THIS IS
RESOLVED BY USING A JANUS TRANSDUCER.

PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 5
 THE DOPPLER SPEED LOG – JANUS CONFIGURATION
Doppler Log – Janus Configuration
THE JANUS CONFIGURATION USES TWO By CAPT. S.F. GOMEZ
TRANSDUCERS, ONE EACH FACING FORE AND
AFT. THIS ELIMINATES THE ERROR DUE TO THE
VERTICAL COMPONENT 𝑣𝑆𝑖𝑛𝛼.
𝑜𝑝𝑝
vCosα -vCosα 𝑠𝑖𝑛α =
ℎ𝑦𝑝
(Speed) (Speed)
Adj 𝑜𝑝𝑝 = 𝑣𝑠𝑖𝑛𝛼

α α

vSinα vSinα
𝑣𝑆𝑖𝑛𝛼 FOR BOTH FORWARD AND AFT [Depth]
[Depth] Hyp [v]
TRANSDUCERS WILL EITHER BOTH BE
+ OR – SINCE BOTH MOVE VERTICALLY
UP/DOWN TOGETHER. THUS,
2𝑣𝑐𝑜𝑠𝛼𝑓𝑡 2𝑣𝑠𝑖𝑛𝛼𝑓𝑡 −2𝑣𝑐𝑜𝑠𝛼𝑓𝑡 2𝑣𝑠𝑖𝑛𝛼𝑓𝑡 4𝑣𝑐𝑜𝑠𝛼 𝑓𝑡
𝑓𝑑𝐹 − 𝑓𝑑𝐴 = + − + =
𝑐 𝑐 𝑐 𝑐 𝑐
AND SINCE THE VERTICAL COMPONENTS CANCEL OUT, SPEED IS GIVEN BY
𝑐(𝑓𝑑𝐹 − 𝑓𝑑𝐴 )
𝑣=
4𝑐𝑜𝑠𝛼 𝑓𝑡
NOTE : JANUS – IS A TWO-FACED GOD FROM GREEK MYTHOLOGY – SEES THE FUTURE AND THE
PAST.
PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 6
 THE DOPPLER SPEED LOG – WORKING
• THE JANUS CONFIGURATION WITH FORE AND AFT TRANSDUCERS, ELIMINATES ERRORS DUE TO
HEAVING AS WELL AS REDUCES ERROR DUE TO PITCHING.
• TWO ADDITIONAL ATHWARTSHIPS TRANSDUCERS WILL PROVIDE ATHWARTSHIPS SPEED,
ELIMINATE ERROR DUE TO HEAVING AND REDUCE ERROR DUE TO ROLLING.

• THE TRANSMITTED PULSE USUALLY HAS JUST ENOUGH POWER TO

RETURN GOOD ECHOES FROM DEPTHS OF AROUND 200m. THIS IS
CALLED “BOTTOM TRACKING” AND IS NOT AFFECTED BY SET AND
DRIFT SINCE IT IS “SPEED OVER THE GROUND”.
• WHEN THE DEPTH IS GREATER THAN 200m THE LOG SWITCHES TO
“WATER TRACKING” OR “SPEED THROUGH THE WATER” MODE.
➢ IN THIS MODE THE LOG RECEIVES ECHOES FROM THE STILL WATER
LAYER THAT LIES SOME 10 TO 30m BELOW THE KEEL.
➢ THE RETURNING ECHO FROM THIS LAYER WILL BE STRONGER THAN
THAT FROM THE BOTTOM DEPTHS GREATER THAN 200m. Teledyne RD – Janus Txdr
➢ NOTE THAT SET AND DRIFT MUST BE ACCOUNTED FOR WHEN IN
WATER TRACK MODE.
➢ MOST LOGS WILL AUTO-SELECT THE CORRECT TRACKING MODE BUT
MANUAL OVERRIDE IS ALSO AVAILABLE.

PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 7
 THE DOPPLER SPEED LOG – ERROR DUE TO TRIM
• THE JANUS CONFIGURATION ELIMINATES ERRORS DUE TO HEAVING, PITCHING AND ROLLING,
BUT IT DOES NOT CANCEL ERRORS DUE TO TRIM AND LIST.
• THIS IS BECAUSE, WITH A TRIM / LIST, THE REST POSITION ITSELF IS NOT HORIZONTAL.
• WHEN TRIMMED BY THE STERN BY AN ANGLE 𝛽 THE
TX ANGLE OF THE FORE AND AFT TXDR’S BECOME Doppler Log – TRIM
By CAPT. S.F. GOMEZ
𝛼 − 𝛽 AND 𝛼 + 𝛽 RESPECTIVELY. THUS,
𝑐(𝑓𝑑𝐹 − 𝑓𝑑𝐴 )
𝑣=
4𝑐𝑜𝑠α 𝑐𝑜𝑠𝛽𝑓𝑡
• THIS IS DIFFERENT FROM 𝑣, WHEN HORIZONTAL, β
BY THE FACTOR 1/cosβ. α β
• MAX PERMITTED TRIM [MARPOL ANNEX I REG 13] α
IS 1.5% OF SHIPS LENGTH.
• SO, β = tan-1 1.5/100 = 0.86˚. THUS,
𝑐𝑜𝑠𝛽 = 𝑐𝑜𝑠 0.86˚ = 0.9998874.
• SO, FOR INDICATED SPEED 20𝑘𝑡𝑠, ACTUAL SPEED
WOULD BE 20/0.999874 = 20.00225𝑘𝑡𝑠.
• THIS IS A VERY SMALL DIFFERENCE AND CAN
SAFELY BE IGNORED.

PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 8
 THE DOPPLER SPEED LOG – ERROR DUE TO TRIM
• WHEN TRIMMED BY THE BOW BY AN ANGLE 𝛽 THE
TX ANGLE OF THE FORE AND AFT TXDR’S BECOME Doppler Log – TRIM
By CAPT. S.F. GOMEZ
𝛼 + 𝛽 AND 𝛼 − 𝛽 RESPECTIVELY.

AND,
𝑐(𝑓𝑑𝐹 − 𝑓𝑑𝐴 )
𝑣= β
4𝑐𝑜𝑠α 𝑐𝑜𝑠𝛽𝑓𝑡
α β
α
• AGAIN INDICATED SPEED 𝑣, WHEN HORIZONTAL,
IS LESS BY THE FACTOR 1/cosβ.
• THE EFFECT OF LIST ON TRANSVERSE SPEED IS
SIMILAR AND INDICATED SPEED IS LESS BY THE
FACTOR 1/cosβ.
• SINCE THE ANGLE OF HEEL MAY BE MORE THAN
THAT OF TRIM, ERROR IN THE TRANSVERSE IS
HIGHER THAN IN THE F-A DIRECTION.
• HOWEVER, THE DIFFERENCES BEING SMALL,
THEY CAN SAFELY BE NEGLECTED.

PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 9
 THE DOPPLER SPEED LOG – ALPHA TRANSDUCERS
• THE VELOCITY OF THE ACOUSTIC WAVE HAS THE GREATEST EFFECT ON ACCURACY, SALINITY
AND TEMPERATURE BEING MAJOR INFLUENCERS.
• THE ALPHA TRANSDUCER ASSEMBLY EFFECTLY COUNTERACTS THESE EFFECTS BY THE USE OF A
PHASED BEAM.
➢ IF A 3 ELEMENT TXDR IS FED WITH IN-PHASE
A/C THE BEAM FORMED WILL BE
PERPENDICULAR TO THE TXDR FACE.
➢ IF THE SIGNAL PHASE DIFFERENCE AT EACH
ELEMENT WAS SHIFTED CLOCKWISE 120˚, THE
BEAM FORMED IS ANGLED AT ABOUT 50˚.
➢ IN A JANUS CONFIG THE ELEMENTS ARE FED
ALTERNATELY CLOCKWISE/ANTICLOCKWISE.
THIS ELIMINATES ERROR DUE TO HEAVING.
➢ THE ANGLE OF PROPAGATION IS A FUNCTION
OF WAVELENGTH AND THE NUMBER AND
SPACING OF ELEMENTS. n = number of elements
𝜆 𝑐 a = element spacing
𝑐𝑜𝑠𝜃 = =
𝑛𝑎 𝑛𝑎𝑓𝑡
2𝑣𝑐𝑜𝑠𝜃 𝑓𝑡 2𝑣𝑐𝑓𝑡 2𝑣
𝑓𝑑 = = =
𝑐 𝑐𝑛𝑎𝑓𝑡 𝑛𝑎 ELECTRONIC NAV AIDS by TETLEY & CALCUT
THUS, EFFECT DUE TO 𝑐 AND cos𝜃 ARE NEGATED.
PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 10
 THE DOPPLER SPEED LOG – GENERAL
• THE TRANSDUCER IS USUALLY MOUNTED AT THE KEEL, TOWARD THE BOW, WHERE
DISTURBANCES DUE TO ENGINE, PROPELLOR NOISE AND AERATION EFFECTS ARE LEAST.
• THE SAME TRANSDUCER SERVES AS BOTH TRANSMITTER AND RECEIVER. RECEPTION STARTS
JUST AFTER THE TRANSMITTED PULSE HAS ENDED.
• TRANSDUCERS USED ARE OF THE ELECTROSTRICTIVE TYPE.
• SINCE TRANSDUCER FACES ARE ANGLED, THE TXDR PROTRUDES A LITTLE BELOW THE KEEL.
HOWEVER, ALPHA TRANSDUCERS REMAIN FLUSH WITH THE HULL.
• THOUGH THE TRANSMITTED SIGNALS ARE CONCENTRATED BEAMS, THE CONICAL SHAPE OF
THE BEAMS MEANS THAT THE FURTHER THE BEAM TRAVELS, THE MORE IT SPREADS. THIS
WEAKENS THE SIGNAL AND LIMITS THE DEPTH FOR BOTTOM TRACKING TO ABOUT 200m.
• THE FREQUENCIES USED FOR DOPPLER SPEED LOG SYSTEMS VARY WIDELY AND MAY RANGE
FROM 100 kHz TO 1 MHz. HIGHER FREQUENCIES REQUIRE SMALLER TRANSDUCERS BUT SUFFER
GREATLY FROM ATTENUATION.
• MODERN DOPPLER LOG SYSTEMS USUALLY FUNCTION AS A DEPTH SOUNDER AS WELL.
• LOGS WITH BOTH LONGITUDINAL AND TRANSVERSE SPEED FUNCTION MAY ALSO HAVE DIGITAL
DISPLAYS AND CONNECTIONS TO GPS ALLOWING FOR A BERTHING MODE.
• DIGITAL SIGNALS MAY BE FED TO VARIOUS OTHER EQUIPMENT LIKE AUTO-PILOT, RADAR ETC.

PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 11
 THE DOPPLER SPEED LOG – ERRORS
• THE VELOCITY OF THE ACOUSTIC SIGNAL THROUGH WATER IS INFLUENCED BY THE SALINITY
AND TEMPERATURE OF THE WATER. SOME COMPENSATION IS MADE BY HAVING A
THERMOMETER EMBEDDED IN THE TXDR. ALPHA TRANSDUCERS ELIMINATE THIS ERROR.
• THE CONICAL PATTERN CAUSES SPREADING OF THE TRANSMITTED SIGNAL
AND A LARGE NUMBER OF BACK-SCATTERED ECHOES ARE RECEIVED BY THE
TRANSDUCER. STATISTICAL METHODS ESTIMATE THE CENTRE FREQUENCY
OF THE SPECTRUM OF RECEIVED SIGNALS THUS MINIMISING THE
POSSIBILITY OF ERRORS FROM SPURIOUS SIGNALS.
• INCORRECT ORIENTATION OF THE TRANSDUCER AND DEFORMATION OF HULL PLATING NEAR
THE TRANSDUCER WILL LEAD TO ERROR IN COMPUTED SPEED.
• DRIFT IN OSCILLATOR FREQUENCY – NOT NORMALLY AN ISSUE IN MODERN SYSTEMS. VERY
STABLE TEMPERATURE CONTROLLED OSCILLATORS AR USED.
• THE SHIPS MOTION DUE TO ROLL & PITCH – THE ANGLE AT TRANSMISSION AND RECEPTION
MAY BE MARGINALLY DIFFERENT. A TWO DEGREE DIFFERENCE GIVES ABOUT 0.1% ERROR.
HEAVY ROLL & PITCH ALSO LEADS TO ERROR. EG AT 10˚ PITCH, INDICATED SPEED OF 15kts AS
AGAINST ACTUAL SPEED 15/cos 10˚, WHICH IS 15.23kts.
• AN AERATED WATER LAYER WILL REFLECT SIGNALS GIVING RISE TO ERRORS.
• CHANGE IN VESSELS NORMAL TRIM AND LIST WILL AFFECT SPEED READING. THE JANUS
CONFIGURATION CANNOT ELIMINATE BUT REDUCES THIS ERROR TO NEGLIGIBLE AMOUNT.

PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 12
 THE DOPPLER SPEED LOG – ERRORS
• ON A SHALLOW SLOPING BOTTOM SIDELOBE ECHO CAUSES SPURIOUS ECHOES.
• IN WATER TRACK MODE, MOVEMENT OF THE WATER LAYERS MAY CAUSE
ERROR [SET MUST BE APPLIED TO CORRECT FOR THIS].
• INCORRECTLY CALIBRATED EQUIPMENT WILL HAVE A FIXED ERROR.

• IT IS NECESSARY THAT THE DOPPLER LOG BE CORRECTLY CALIBRATED AT INSTALLATION.

• THE UNIT MUST BE FIRST CORRECTLY ZEROED. THIS CAN BE DONE WHILE ALONGSIDE OR AT
ANCHOR WHEN SPEED OVER GROUND IS ZERO.
• AT SEA TRIAL, A MEASURED MILE IS STEAMED AND THE SPEED CALCULATED. ANY DIFFERENCE
BETWEEN CALCULATED AND DISPLAYED SPEED IS ADJUSTED.

ERROR DUE TO CHANGE IN VELOCITY OF SOUND DUE TO :

• 1 DEG INCR IN TEMP = 3m/s INCR IN VELOCITY
• 100M INCR IN DEPTH [INCR.IN PRESSURE] = 1.7m/s INCR IN VELOCITY
• 1ppt INCR IN SALINITY = 1.3m/s INCR IN VELOCITY
SINCE, FOR NAVIGATION PURPOSES, THE MAX DEPTH THAT THE DOPPLER LOG WORKS AT IS 200m,
TEMPERATURE CHANGE HAS THE GREATEST AFFECT.

PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 13
 THE DOPPLER SPEED LOG – ADVANTAGES & DISADVANTAGES
• ADVANTAGES
• HIGH ACCURACY
• CAN MEASURE VERY LOW SPEEDS
• CAN MEASURE AHEAD, ASTERN & ATHWARTSHIP MOVEMENTS
• CAN BE USED FOR OCEAN NAVIGATION IN WATER TRACK MODE [SET AND DRIFT MUST BE
ACCOUNTED FOR IN THIS MODE]
• USEFUL IN BERTHING AND MANOEUVRING IN GROUND TRACK MODE
• MAY PROVIDE A DOCKING MODE DISPLAY FOR EASY ASSESSMENT OF VESSELS MOVEMENT
WHEN BERTHING
• IN GROUND TRACK MODE, DEPTH INDICATION IS ALSO POSSIBLE [WORKS AS A DEPTH SOUNDER AS
WELL]

• DISADVANTAGES
• FAIRLY COMPLEX DEVICE
• JANUS TRANSDUCER PROTRUDES A LITTLE BELOW THE HULL
• DAMAGE TO THE HULL NEAR THE TRANSDUCER MAY CAUSE DEFLECTION OF THE TRANSDUCER
MOUNTING LEADING TO ERRORS
• ACCURACY MAY BE REDUCED OR EVEN LOST IN ROUGH WEATHER [BF 6 AND UP]
• AERATION IN THE WATER AS WELL AS SAND/MUD IN SUSPENSION IN SHALLOW, DISTURBED WATER
WILL LEAD TO ERRORS
• AERATION IN THE PROXIMITY OF PROPELLERS/BOW THRUSTERS WHILE COMING ALONGSIDE CAUSES
ERRORS

PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 14
 DOPPLER
SPEED
LOG

wind

drift

current

PREP. BY. CAPT.S.F. GOMEZ DATE: 12JUL20 REV. 1 OF 22JUL20 PH2, SM, NCV 15