Underwater Acoustics: Doppler shift (from 1957)


Doppler shift in sonar (from 1957)

In the early sonar installations, the observer’s hearing has always played an important role, especially when it comes to moving targets that showed a Doppler frequency shift. It was obvious to mimic the frequency-distinguishing characteristic of the human ear with a series of narrow-band filters. This could then be used in manifold in panoramic installations. The necessary filters were manufactured around 1957 with the aid of electron tubes at a later stage, transistors were used herein.

One of 25 filters of a filter bank receiver (version with electron tubes)
One of 25 filters of a filter bank receiver (version with electron tubes)

Incidentally, it was not only in analogue circuits that the transistor made its appearance. With transistors, it became feasible to manufacture digital circuits that would require far too much space with electron tubes, not to mention power consumption and heat generation. Thus, a ring core memory was manufactured which cooperated with the aforementioned filter elements. This memory made it possible to visualise the result of several sonar transmissions.

Filter bank with ring core memory
Filter bank with core memory

In such a memory, the Doppler echoes of four consecutive transmissions as a function of the distance is stored in memory. Only Doppler echoes that give results in all four transmissions are considered targets. In this way, false echoes are suppressed as much as possible. Submarines approaching or moving away give a frequency shift in their echoes due to the Doppler effect. This frequency shift is audible as a pitch difference. In the ADI, PAE and CWE sonars, this was indicated by the slope of the echo on the indicator screen. However, this reading was not very accurate. A major improvement was obtained by passing the echo signal through several electronic band filters with adjacent narrow frequency bands of each 10 Hz wide in the “filter bank receiver”.

The signals from these band filters were displayed on the indicator screen on either side of the central line. After the sonar signal has been transmitted underwater, a light spot passes from left to right across this screen. If an echo comes in, the dot will light up brightly. The location where that echo occurs then gives the distance of the submarine, while the extent to which the echo appears above or below that central line is a measure of the Doppler shift. The black registered signals in the figure are of a real purpose, the red signals are false detections. For a maximum Doppler shift of + and – 400 Hz, 80 filters with a bandwidth of 10 Hz are required. Subsequently, band filters were frequency relays, filter-type Q-multipliers using electron tubes, filter Q-multipliers using transistors, and magnetostrictive filters.

Sonar-Doppler indicator schematically
Sonar-doppler indication schematics

Multi Beam Doppler Indicator (MBDI)

A panoramic sonar can receive echoes from submarines from different directions simultaneously. To know not only the direction and distance of these echoes but also the Doppler shift (see the technique description above), a sonar receiver was developed in 1965 that could simultaneously reproduce echoes in different sound beams underwater. Divided over two screens, the echoes are shown on each screen in six adjacent beams, each 10 degrees wide. Together these beams cover a sector of 120 degrees. In each beam, the echoes are reproduced similar to the prototype Doppler Indicator used for the CWE-10, but now the dots of light run from bottom to top instead of left to right.

The electron (or vacuum) tubes were specially designed and manufactured by Sylvania, Syracuse, U.S.A. for this sonar visualisation device. Each electron tube has six cathodes. In addition, the MBDI contained a flat vacuum tube used for the “Doppler indicator”.

MBDI in development


Doppler indication on the flat electron tube
The processing cabinet


The processing cabinet
The processing cabinet


Multi Beam Doppler Indicator
Multi Beam Doppler Indicator (MBDI) in the museum