Here is All You Need to Know

About Doppler Log

Written by Capt Rajeev Jassal on February 27, 2018

I am going to be brutally honest with you.

To read about how Doppler log works is really a

boring task.
What makes it confusing also is that there is no clarity
in which this topic is usually explained.

And believe me, writing about it is equally boring too.

But then I received too many questions on Doppler log
that I thought of taking up the task of writing on this
topic.

So let us discuss about Doppler log but first let us look

at the regulation by which we need to have a speed log
on board.

Requirement for Doppler log

As per SOLAS Chapter V, Regulation 19.2.3.4,

All ships of 300 GRT and upwards and all passenger

ships irrespective of size shall be fitted with a speed
and distance measuring device, or other means , to
indicate speed and distance through water.
Doppler log is one of such equipment that measures
speed and distance through water.
EM log is another equipment that measures speed and
distance through water but Doppler log is what is
usually preferred by the owners.

Speed through water is required to be fed into the

RADAR and speed through water is what we need to use
for collision avoidance.
Briefly this is because, with speed through water we see
the aspect of the other vessel and we can correctly
judge which rule would apply to a collision situation.

I have covered this in detail in this blog and I would

refrain myself to discuss that here.

Doppler effect

Time to get our hands dirty. If we need to understand

Doppler log, we have to know few things about Doppler
effect.
But don’t worry.

I will serve only what we need to chew.

While trying to understand Doppler effect, one

mistake we make is to try to re-invent the wheel.
We try to think the way Christian Doppler (Inventor of
Doppler effect) might have thought.
Well, to understand Doppler effect (and Doppler log),
we don’t need to think that way.

Christian Doppler (and many scientists after him) have

already proved his theory and we simply need to believe
it and understand what it is.

And when we think that way, the theory is quite simple.

As per this theory

The Doppler effect (or the Doppler shift) is

the change in frequency or wavelength of
a wave for an observer who is moving relative to
the wave source.
Now don’t worry, hold your guns !!!

I will explain it in simple words.

Let us say you are standing on road and there is an

ambulance that is coming closer to you.

The frequency of its horn as heard by you will be more

than its actual frequency . Now when I say frequency,
many think of it as the loudness of the horn.
No, that is not what we are talking about here.

To make it simpler, let us say the sound of this

ambulance horn is Ti………Tu Ti…….Tu Ti……..Tu

If neither you or ambulance is moving, you will hear this

same sound pattern.

The sound will louder if the ambulance is closer but the

irrespective of the distance the pattern of the sound will
remain same.

Now if the ambulance is moving away from you at a

speed, you will hear it something like

Ti………………………Tu Ti………………………Tu
Ti………………………Tu

And if the ambulance is moving towards you, the horn

will seem sounding like

Ti…Tu Ti….Tu Ti…Tu

In other words, as the ambulance moving closer to or
away from the observer, one cycle of sound will take
different time to complete.
That is, the frequency of the horn will change if the
ambulance is moving with respect to the observer.

As Christian Doppler observed the change in frequency

depends upon the relative speed of the source of
frequency.

Now that was the most non-technical way of explaining

something like Doppler effect.

Technically, this graph is what Doppler effect is all

about.
When observer and source of frequency (for example
Ambulance horn in our example) are stationary (S0),
frequency received at the observer will be same as the
actual frequency (f0).

When the source of frequency is moving towards (or

Away from) the observer, the received frequency at the
observer will change (at speed S1, Frequency is F1 and
so on).
Christian Doppler gave the formula to calculate this
frequency shift.

The formula to calculate this change in frequency is

This formula and the Doppler effect has been used to

develop many equipment.

Did you know that even the speed meter used by traffic
police to measure the speed of the cars (to catch over-
speeding cars) is also based on Doppler effect?

And of course one of such equipment is Doppler log to

measure the speed of the ship through water.
Working principle of Doppler log

Doppler log uses the principle of Doppler shift to

calculate the speed through water.

A wave transmitter is installed at the bottom of the ship

which transmits waves at an angle (usually 60 degrees)
to the ship’s keel.

Let us say it transmits the wave at frequency f(o) and

the received frequency after reflection is f(o) + f(d).

Now in the case of Doppler log, the formula for Doppler

effect has been simplified as
Where V is the speed of the ship in the direction of the
transmitted wave.

As we would know the angle of the beam with respect to

the keel, a co-relation can be drawn between this speed
and actual speed of the ship in forward direction.

Transceiver will have both the frequencies f(d) and f(o)

known. We know the sound wave velocity in water.

The only unknown is the speed of the vessel (V) which

Doppler log can calculate easily with the above formula.

Speed through water or speed over

ground

This is one of the most common doubt seafarers have

about Doppler log. The Doppler log measures the speed
with respect to the surface reflecting the transmitted
wave.

In deep water the transmitted wave gets reflected by

water layer which gets denser as the depth increases.

A depth of 200 meters is perfect for the wave to get

reflected.

But does that mean in lesser depths, the Doppler log

can only measure speed though ground?

Not exactly.

The modern Doppler logs are advanced enough to have

the transmitted wave reflected from the water layer
with as less under keel depth as 3 meters.
 JRC Doppler log claim to measure the speed through
water even when depth below keel is as less as 2
meters.

In fact, that is the IMO performance standard required

for the speed logs fitted on ships (more on that later in
this blog).

So as long as the depth below the transducer is more

than 3 meters, the Doppler log can measure speed
through water.

Some Doppler logs will also have the functionality to

choose the tracking mode. The three tracking modes
available are

 Water (Measure speed through water)

 Ground (Measures speed over ground)

 Auto (Selects water mode or ground mode automatically
as per depth)

Well if you want to consider this as a limitation of

Doppler log, you can. But more than 3 meters depth
below the keel is what it requires to measure speed
through water.

If the mode is not changed to ground (or auto) when

the depth below keel is less than 3 meters, Doppler log
will show errors in the speed.

But the option to change modes are not present in all

makes and model of the Doppler log.

Some Doppler log only measures what it is supposed to

measure, which is speed through water.
In depths less than 2~3 meters below keel, these
Doppler logs displays the GPS speed for which a GPS
connection to the Doppler log is required.

Errors of Doppler log

Traffic police measures the speed of the moving vehicles

using the equipment that works on Doppler shift.

And these measurements are quite accurate.

But ship is a different place altogether. We do not have

ideal situations to have the equipment measure as
accurately as on land.

But all of these potential errors are taken into account

and corrected for in the Doppler logs.

Let us discuss these errors and how these are corrected.

1. Error due to ship’s motion

When the ship is moving, it may not move only in fore
and aft direction. The ship may yaw, roll or pitch.

And when ship does that the angle of the beam (based
on which the calculations are done in the processor)
changes.
In fact, this will also be the case in different trim and list
cases of the vessel.

This error is eliminated by collecting and averaging the

data from more than one beam. This configuration of
the beams is called Janus configuration.

By having this configuration, any positive error in data

of one beam is cancelled by the negative error from the
data of second beam.
 In fact, it is because of Janus configuration that Doppler
log is able to measure side speed (in athwart ship
direction) of the vessel which GPS does not measure.

GPS provides two speeds.

 Speed towards the vessel’s heading

 Speed towards vessel’s course made good

But when vessel is approaching a berth, we are also

interested in knowing the speed at which the ship will
touch the berth (0.3 knots or less is the ideal speed).
GPS does not provide this information. This is where
speed read out from the Doppler log helps. Doppler is
able to calculate this speed because of Janus
configuration.

2. Error due to reflections from the air bubbles

For the Doppler log to work accurately, the reflected
beam need to be the one that Doppler log thinks it is.

If there are false reflections, Doppler log will show

wrong readings.
One of the reason for the wrong reflections could be the
beams getting reflected from the air bubbles generated
because of ship’s motion.

This error is negated by carefully locating the transducer

to a location where the possibility of generation of
bubbles is minimum.

3. Error due to change in velocity of sound in

water
We know the velocity of sound in water but that is in
ideal conditions and at a particular temperature.

But as the sea water temperature changes the velocity

of the sound waves in water would change significantly.

If not corrected, this would bring an error in the Doppler

log readings.

This error is corrected by having a temperature sensor

fitted near to the transducer which measures the sea
water temperature.

The correction in speed of sound in water because of

temperature is then applied in the processing unit of the
Doppler log.

4. Other technical errors

 Apart from the errors listed so far, there can be few
errors related to the electronic equipment used in the
Doppler log.

For example, the frequency transmitted could be slightly

different from the one taken for calculation.

Any of these errors are eliminated during initial testing

of the Doppler log equipment and during sea trial.

IMO Requirements for Doppler log

IMO resolution A.824 (19) as amended by MSC

96(72) gives the details of the performance standards
for the Doppler logs fitted on ships.
The few of the main requirements as per this are

 The device measuring speed and distance through the

water should meet the performance standard in water of
depth greater than 3 m beneath the keel

 Error in the measured and indicated speed for a digital

display should not exceed 2% of the speed of the ship,
or 0.2 knots, which is greater. For analogue display the
error should not exceed 2.5% of the speed of the ship
or 0.25 knots whichever is greater.

 The performance of the equipment should be such that

it will meet the requirements of performance standards
when the ship is rolling up to 10 degrees and pitching
up to 5 degrees.

Conclusion

There is a lot of confusion about the purpose of having a

Doppler log on the wheel house.

Most of us know that Doppler log is required to measure

the speed through water and is required as per SOLAS
Chapter V.

But here is the main confusion.

How can a Doppler log measure speed through water in

shallow waters?

Well, the IMO performance standard requires that the

speed logs must be able to measure speed through
water with UKC of up to 3 meters.

Most of the speed logs (and Doppler logs) can measure

the speed through water even when the UKC is around
3 meters.

Apart from that Navigators must be aware of the errors

that a Doppler log can have and how these are
corrected.
To understand the Doppler log errors, we must briefly
know the principle of operation of Doppler log.