Doppler Speed log works on the measurement of Doppler effect.
What is principle of Doppler?
- In 1842, Mr. Doppler in Vienna observed that a star moving towards us appears to have a different color than when it is moving away.
- In 1943, Buy Ballot proved that sound seems to have higher frequency when the source is approaching the observer.
- Whenever there is a relative movement between a transmitting source and a receiver, there will be a ‘Doppler shift’ in the frequency received. This shift is termed as Doppler effect named after the Austrian physicist Christian Doppler & will be as follows:
fr= ft (c+v)/(c-v)
Where
- fr = Received Frequency
- ft = Transmitted frequency
- c =Velocity of sound in Sea Water (1500m/sec)
- v = Velocity of the vessel
How Doppler log works?
- A transducer emits continuous a high frequency sound pulse in the forward direction at an angle of 60° to the keel.
- Higher the sound frequency, smaller the transducer, narrower the beam and higher the accuracy.
- The beam bounces back from the sea bottom.
- The frequency of the bottom echo will be higher when the ship is moving ahead or lower if she is moving astern.
- The Doppler equation is solved to obtain ship’s speed.
- When signal is bounced off the sea bed, (called Bottom Track), the speed displayed the “Speed over the ground (SOG)”
What are advantages of Doppler Log?
Advantages of Doppler Log:
- Most accurate
- Can measure ahead, astern & athwartship movements
- Can be used for ocean navigation as well as berthing and maneuvering in close waters.
- Can measure very low speeds
- This log is most prevalent in today’s marine world.
What are the factors that affects Doppler Log?
Factors affecting Doppler Log performance:
- The performance (accuracy) may be reduced or even lost under following conditions:
- Rough weather (may be sea state 6 or severer)
- Improper location of sensor, e.g., too close to the propeller, thrusters, drain tubes, echo, sounder transducer
- Depth under the keel if less than 3 m.
- Aeration in the proximity of propellers while coming along side.
Dear Vikash Kumar. Would you be so kind as to disabuse me of information overload and misinterpretations. Until recently I was sure I know what is meant by SOG versus STW, all these explanations resonate with me, e.g. if adverse current to the bow is 5 knots and we are propelling at 5 knots against the current our STW is 5 knots and SOG is zero since we are not moving relative to the land. Conversely if we are drifting down the river with engines off our SOG is 5 knots and STW is zero as we are completely surrendered to the environmental forces and are passively moving controlled by the flow. Same logic applies to favorable current which speeds up the vsl or slows it down. It was clear until I dived into the operating principles of speed measuring equipments… namely: EM speed Log, Pitometer, Doppler log etc. Before I came across your article actually I thought Doppler log is used only for measuring STW and doesn’t propagate its ultrasonic soundwaves deeper than appr. 2 meters where it normally bounce back off plankton and other microorganisms (e.g. DS 800) . What really concerns me is, why all these equipments measuring STW not affected by current as they all operate depending on the vsl’s motion, i.e. EM log requires seawater flow to produce electromagnetic induction, Doppler log needs vsl to move to calculate differences in frequences, Pitometer is relied on dynamic/static pressure caused by facing water flow etc. Physical motion is involved, not the RPMs of the propeller, which might be moving with the same speed regardless of current and other influences. If all these devices were interfaced with the prop rpms (it may sound silly I know) representing engine’s speed, then it would be crystal clear why STW is not considering current,tide,wind etc. As opposed to SOG which is affected by set and drift. Now to top it all off… I have found out that actually Doppler log is not only used to estimate STW but SOG too propagating beams as far as the seabed. I just cant get all these pieces together. Thanks for reading this. I appreciate your time.
Dear VL,
A very valid and albeit logical query! Well, in my 25 years career at sea, handling various sonars and other sound wave spectrum based devices, the explanation is quite reasonably simple. A EM log and Pitometer log had their own limitation, primarily, EM wave stability and diffraction/losses and similarly, reduced efficiency of pitot of the Pitometer based on dissolved particulate matter in water medium. To overcome, the Doppler log, which primarily uses the copper shift was adopted. A Doppler log transducer can be considered as a “portable” sonar beam, with restricted hydrophone principal. In a nut shell, the Doppler log transducer’s emissions can digitally be traced/transmitted at a required magnitude/strength to measure STW or SOG. In case of STW mode, the transducer emits the required strength of emissions, primarily to bounce off the immediate varying interphase layer of depth of water, which is calibrated on principles of depth gradient (I wont go much in details about bathytherm and similar profiles).
Having said that, when the Doppler log is set to STW mode, then the system is designed to scale the transducer emissions in relative motion of water flowing under the hull/transducer (at depth > 3 m). Hence, we can still measure relative speed of water flowing past with respect to vessel. However, as vessel’s speed is “zero” (can be anchored or docked) then the relative speed becomes true speed of water flowing past the vessel. In this mode, therefore the doppler waves are bounced off the water interphase flowing in reference to the speed log transducer.
In case of SOG settings, the Doppler log’s transducer logic understands the need to have a relative speed. That is, the vessel has to move over the ground to generate a relative shift of the doppler wave coming off the bottom/sea bed. In this mode, the doppler log logic/program understands to identify the reflected signal strength/amplitude reflected off the sea bed as opposed to intermediate water layers. As, if the wave reflects off the sea floor/bed, the coefficient of absorption of the waves will be in a given spectrum when compared to corresponding final losses due to coefficient of absorption from intermediate water layers. And here you go, your Doppler log will now give you speed over ground – SOG, and will discard the intermediate waves coming off the water layers.
Hope, this helps you understand better. However, if you still have queries, feel free, there is always a sea and depth of information on this subject.
All the best!
Capt. Sanjeev