Question

Topic 4: Echo Sounder and Speed Logs:

According to your Knowledge, answer the following questions:

a. In Echo sounder systems some devices has one transducer (transmitting and receiving transducer

in one unit) and the other devices has two transducers which of them is better and why?

b. The factors that affect the Velocity of Sonic & ultrasonic Wave in Seawater are Salinity, Pressure and Temperature; explain with aid of drawing and equation their effect on the measurement depth indicated by an echo sounder?

c. Explain how to cancel out the effects of pitching and rolling on the speed indicated by Doppler Speed Log (Janus Configuration); Support your answer by drawing and equations?

d. The using of DGPS offers improve fix accuracy, over what range and frequency would you expect to receive local area DGPS (LAD) data?

Answer 1

A) A transducer can be described as an energy converter; during transmission its input is electrical and its output is acoustic; for perception the input is acoustic and the output electrical. It is normal to use only one transducer for both transmission and reception. But the effectiveness is higher for two transducers one for receiving and other for transmitting. This will help in avoiding disturbance while sending or receiving signals. If we are using one transduced, there is a change of disturbance or interference. Using multiple transducers can rectify the problem

B) Salinity

A change of salinity will cause a small corresponding change in density with a resulting change in bulk modulus, causing variation of sound speed. A change in salinity of one part per thousand will result in a change in the sound speed of approximately 1.3 meters per second.

Pressure

Pressure in most circumstances is more important than salinity, but in the sea its change is constant and thus predictable. It also causes a change in bulk modulus and density, and the result is an increase in the sound speed of 0.017 m/sec for every meter of depth increase. This slight change, which is important when the temperature remains constant, causes a sound beam to bend upward at great depths as will be discussed later.

Temperature

Temperature, the foremost factor affecting sound speed, usually decreases with depth, and this leads to an accompanying decrease in sound speed at the rate of approximately 3 m/sec per degree Celsius. Below a depth of about 1,000 m, however, the temperature is fairly constant, and the predominant factor affecting sound speed becomes pressure. As the temperature of a medium decreases, bulk modulus decreases while density increases. It should be noted that the effect of temperature is relatively large compared to the other factors. It takes a depth change of about 165 meters to cause the same change in sound speed as a one-degree temperature change.

C) Changing the speed transmissions will help to reduce the effect of pitching and rolling.

D) The differential global positioning system (DGPS) is the most representative augmentation
system. DGPS services have been established worldwide. They can be divided into so-called local-area
DGPS (LADGPS) services for small areas, such as a country, and wide-area DGPS (WADGPS) services
for larger areas such as an entire continent or even worldwide. A single reference station (RS)
in a LADGPS at a known location can estimate a range of error corrections for each GPS satellite in view. These error corrections are then broadcasted to users nearby an RS. By applying these corrections to the received signals, a user can typically improve the accuracy up to a range of 1–3 m. However,as the distance between the user and the RS increases, the range decorrelation increases, thus reducing the accuracy. The positioning accuracy reduces largely because of the spatial decorrelation such as ephemeris error and ionospheric and tropospheric delays. Furthermore, if a user moves away from the RS of the LADGPS while receiving the corrections, the error due to the spatial decorrelation cannot be ignored. In this case, it is possible to receive corrections from another RS that is getting closer while moving, but the discontinuity of the position can occur.