2-5
Depth lock mode
ransmit pulse
Echo signal
Sampling
Depth lock
1
2
3
4
5
6
7
8
9
1
Speed measurement
If the edge comes too late, the pulse duration is lengthened, if it comes too early, the pulse
duration is shortened.
During cycle 5 and 6 a second pulse is transmitted. This pulse is equal to the time the signal
travels from the transducer to the sea-bed and back again. During cycle 7 and 8 the echo is used
for speed measurement.
1.3 Speed measurement
The receiving sensor elements will see a speckle pattern. First we discuss this pattern, and then
correlation technique to estimate the speed.
1.3.1 Speckle pattern
A coherent beam is transmitted towards the sea-bed. This means that the signal has both
intensity and phase information. When the signal is reflected back, the signal will be modulated
both in amplitude and phase. Each reflector will give a phase and amplitude modulation of the
signal. If the bottom has a depth variation larger than the wavelength (10 mm) it will give a
random phase modulation. All reflectors will sum up in one position depending on the distance
to that position, see figure below.
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