Saturday, November 19, 2011

Doppler profiler: design considerations

With a pulse-to-pulse coherent Doppler profiler, we can pick a specific remote region where we want the measurement to be made. Here we plan to measure the average speed of water in a sample volume of 1 cm long situated at 1 meter from the transducer.

With a speed of sound through water of 1500 m/s, the time to a first echo from the beginning of the sample volume is 1333.3 μs. The time to a first echo from the end of the sample is 1346.7 μs.

To make a measurement, the 3 MHz transducer will send 2 pulses.

After sending the first pulse, the transducer will switch to receive mode and will sample the echo from exactly 1333.3 to 1346.7 μs after the beginning of the pulse. The microprocessor will digitize this echo with its internal 84 MHz ADC and store it in memory. Soon after, the transducer sends a 2nd pulse and the process is repeated.  The second echo is also digitized and stored. As the particles in the water have moved between the 2 samples, the echoes will be separated by a phase angle ϕ (the Doppler shift).  From the stored data, the microprocessor will then calculate the Doppler shift (90 deg in the following figure).

The Doppler shift is related to the component of the speed of the water in the axis of the ultrasonic beam. But there is a problem: more than one velocity value will produce the same Doppler shift.

To pick the right one, we need an external information. In our case, we will calculate an estimate of the lateral speed from the boat's K constant and the current values of boat speed and heel angle. We will then pick the profiler's measurement that is the closest to this estimate.


  1. Hello, I´ve been following your blog for a while and I´m very interested in what you do. I would like to contact with you by email. We are very interested to know more about you.

  2. I have added a contact e-mail at the bottom of the right column.

  3. Hi all this is so interesting...but is the project on hold? (3 months non news...)

  4. The design of the prototype transducer has been slowing down the project, but is still going on, and will be reported when available. Meanwhile, the transmitter part of the electronics is quite complete, and waiting for the transducer to be tested. I am also considering a simple ground-based test rig to help with the final development. Slowly but surely...

  5. Hi,
    if I'm not wrong, I think Doppler shift is in the firs sample echo signal (transmitted frequency is different from echoed back signal frequency). So by using this shift it's possible to calculate speed of moving particles. So second pulse would measure second sample of current?

    1. Tomas, what you describe is the principle of a doppler radar used to measure the speed of a moving target. A coherent doppler profiler uses a different technique, which is more sensitive to measurement of nearby objects at short distance.

      The following excerpts are from “Pure coherent Doppler systems – how far can we push it?” Lohrmann and Nylund, 2008.

      "Many physical processes in open waters occur at scales or velocities that make them undetectable with conventional Doppler sonars."

      "Doppler velocimetry measurement works by observing the changing distance to particles in the water, typically by transmitting two or more pulses and comparing the echoes. The time required for the echo to return gives an approximate distance to the particles, while phase changes provide a sensitive means to detect small changes in this distance."

    2. I was speaking from the perspective of the fixed horizontal doppler profiler (we do use such a device in the port). And as I remember from the training, this the the pricipe to calculate current. I will read your document, it's an interesting topic for me.

    3. Tomas, do you know the make and model of the horizontal doppler profiler you refer to?

    4. We are using RDI current profilers in our port: one horizontal:
      and three vertical ones:
      It's an interesting topoc for me as engineer in a port and also as a sailor :)

    5. Thanks, Tomas. It looks like these units also measure the phase change between successive echoes, which they call 'Broadband' to differentiate it from the standard doppler sonar. Their patented broadband technology refers to the signal processing approach that they use to calculate the phase change from the echoes.

      Their ADCP (Broadband) Practical Primer can be downloaded at the end of this page:

      One excerpt: "RDI engineers use phase to measure time dilation instead of measuring frequency changes because phase gives them a more precise Doppler measurement".