Echo Sounder Operation & Controls PDF

# Electronic Navigation ## 2.10 Echo Sounder ### 2.10.1 Operation of the Echo Sounder A shipboard echo sounder consists of a display unit and a transducer. The display unit can either be a paper recording type or digital display type. Many instruments have a control unit integrated in the display...

# Electronic Navigation ## 2.10 Echo Sounder ### 2.10.1 Operation of the Echo Sounder A shipboard echo sounder consists of a display unit and a transducer. The display unit can either be a paper recording type or digital display type. Many instruments have a control unit integrated in the display unit that is used to generate electric pulses, receive echoes and measure the time taken by each pulse. Some manufacturers provide a completely separate control unit located away from the display unit, but the function is the same, regardless of their location. The display unit (an example of which is shown in Figure 2.23) is fitted on the navigation bridge. It is connected to the transducer by wiring. The transducer is located at the bottom of the ship in a tank known as the transducer compartment or tank. The pulse generator creates electric pulses that are sent to the transducer. At this time, an internal clock starts to measure the time each pulse takes to return. Electric pulses are then passed to the transducer, which converts them into sound waves (acoustic energy) at a frequency of 12 KHz to 200 KHz. These pulses are directed towards the seabed with the help of transducers, as shown in Figure 2.24. When the pulses are reflected from the seabed, the transducer converts the reflected sound waves to electric pulses. These are then passed through an amplifier to increase the strength of the received signal, and transmitted to the control unit. The clock notes the time when the pulse is returned and so the time taken is measured. The speed of sound waves in seawater is known, 1500 m/s, so the distance travelled by each pulse is calculated with this formula: **Distance = Speed of sound waves x time** However, depth will only be half the distance travelled by each pulse as the pulse travels to the seabed and is then reflected back to the transducer. To measure depth, the above formula can be modified as: **Depth = Speed of sound waves x time / 2** This depth is displayed on the digital display, as shown in Figure 2.25. In the case of paper type equipment, it is marked on thermal paper. On coloured digital display equipment, various colours can be used to display the nature of the bottom, which can be determined by the strength of the return echo. ### 2.10.2 Echo Sounder Controls Echo sounders are provided with many controls to adjust the performance of the equipment and to operate it at various depths. The controls may vary depending on the manufacturer, but the general controls are as below: | Control | Description | |---|---| | Auto | When switched on, this control adjusts the range scale and clutter level according to depth. | | Gain | Adjusts the sensitivity of the instrument. Care should be taken not to set it too high or too low. A setting with a 'speckled' background is considered satisfactory, too high a gain will blacken the display, too low a gain may not return enough echoes. In auto mode, gain adjustment is carried out automatically. | | Time Variable Gain | This control is similar to STC (Sensitivity | | TVG (Control) | Time Control) in radars. The objective of TVG control on an echo sounder is reduction of surface noise by compensation for attenuation to get a smooth display. To achieve smoothing at the time of transmission, sensitivity is reduced by the TVG control and then gradually increased. This ensures that objects at different depths are shown in the same colour or intensity on the display. TVG can work to a maximum depth of 150 m on echo sounders working on 200 kHz frequency. | | Echo Offset | This control may be used to adjust too weak or too strong echoes. | | Depth Alarm | The navigator sets a depth alarm to obtain a warning when the ship moves into a shallow area. The alarm can usually be silenced by a 'Mute' button. | | Draught | Some manufacturers may allow the present draught of the ship to be entered so that depth of water can be obtained and directly compared with soundings given on the chart. This facility allows navigators to apply only the 'height of tide' correction to compare measured depths with those given on the chart. In this case, the equipment will not show the UKC. | | Dimmer Control | This is used to control the brightness of the screen and keypad. | ### 2.10.3 Transducer Location The proper location of the transducer on the bottom of the ship is an important concern because if the sound waves are not transmitted in the right direction, or if they are reflected back from the bubbles caused by turbulent water around the bow or stern of the ship, depths may not be accurately measured. Bow waves generally create a stream of bubbles from a quarter of the ship's length from stem. The exact location of the bow wave depends on the shape of the bow (ie whether the ship has a bulbous bow trim), the speed and the draught of the ship. The best place to install a transducer is where there is the least possibility of aeration in the water under the ship and minimum interference from engine or propeller vibration. ## 2.10.4 Echo Sounder Errors Careful operation of the echo sounder is necessary, acknowledging the sources of any errors and the effect on its use as an aid to navigation. These errors can be divided into two types, 'instrument inaccuracies' and 'false echoes', as follows: ### 2.10.4.1 Instrument Inaccuracies #### Speed Error The speed of sound waves through the water is assumed to be constant, at 1500 m/s, for calculation of depth by an echo sounder. This speed varies due to changes in atmospheric pressure, temperature and the salinity of seawater. In these cases, the instrument may not be able to give accurate measurements. Generally, an increase in temperature, salinity or pressure results in an increase in speed and the depth indicated would be less than the actual depth. For example, in the Red Sea where the salinity and temperature of the seawater are higher, the depth indicated by an echo sounder may be about 5% less than the actual depth. Sound waves travel at a slower speed in fresh water than in seawater due to the difference in their densities. In other words, the speed of sound waves in water depends upon the density of the water. The density of water depends upon temperature, salinity and pressure, meaning sound waves take longer to travel to the seabed in water of lesser density (eg fresh water) than in water of greater density (seawater). Navigators must give extra allowance for UKC in areas where the atmospheric conditions are abnormal. #### Second Transmission Error At times when an echo sounder is used in greater depths, the pulse transmitted during one transmission may not return to the transducer. Instead, the pulse transmitted during the previous transmission may be received by the transducer, so the time interval measured by the echo sounder's internal clock will be for the second pulse transmitted. This depth will be shown as much less (almost half) than the actual depth. However, this occurs very rarely and navigators must verify any such occurrence by any other means available, such as comparing the depth obtained with the chart. In addition, the depth scale on an echo sounder should always be adjusted to match the depth. #### Refraction and Reflection of Sound Waves Sound waves transmitted from a transducer travel through seawater, which may consist of various layers at different salinity and temperature. Due to these variations, both reflection and refraction of sound waves may take place. This will cause unknown errors in the depth measured so navigators must use the measured depths with caution. #### Reflection from Sloping Seabed If the seabed is not horizontal, sound waves may be reflected in different directions and, in extreme cases, depths may not be measured at all. If the seabed has a rough surface, some of the sound waves may be diffused in different directions, resulting in weaker echoes being received by the transducer. ### 2.10.4.2 False Echoes #### Double Echo If the returned sound waves hit the hull of the ship they will be reflected back to the seabed and then received by the echo sounder. A similar reflection may occur from the surface of seawater. The echo received is known as a double echo. In these cases, the echo sounder will show two layers of the echo printed on the display. The weaker of the two is usually the double echo. A change in the selected depth, scale or sensitivity can eliminate it. #### Multiple Echoes Similar to a double echo but the reflection between the ship's hull and the seabed takes place a number of times. This can also be removed by a change in the selected depth, scale or an adjustment of sensitivity. #### Echoes from other objects in seawater Objects such as plankton, fish or seaweed may sometimes give strong echoes that are read as the seabed. During such an occurrence, navigators must double check the position of the ship and use alternate means to check the depth of water and UKC. #### Turbulence and Aeration Both these phenomena can cause the loss of an echo sounder's capability of depth measurement and may produce false results. This occurs when sound waves are reflected diffusely due to air pockets or bubbles caused by turbulence, particularly in rough weather or when the ship moves astern on engines. #### Pythagoras Error This error may be caused in echo sounders when the transmitter is located away from the receiver, as shown in Figure 2.26. In modern equipment, the difference in the length through which radio waves have to travel due to distant locations is accounted for in the equipment. #### Interference from other Echo Sounders In high density traffic areas, where other ships operate their echo sounders, sound waves transmitted from them may be received by another ship working on the same frequency. This occurrence can be seen by complete blockage of the display screen. An adjustment of gain control can eliminate the error. _Image Description:_ An image of a ship and two transducers sending sound waves to the seabed. The image illustrates the occurrence of the 'Pythagoras Error' when the distance between the transmitter and receiver is not taken into account. _Note:_ The image was not included in the markdown but described.