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Should pitch/roll angle be applied to the depth?
Many surveyors assume that sonar is like a laser being shone through the water and that the distance measured
should be assumed is a hypotenuse measurement to be trigonometrically corrected by any pitch/roll angle that
has been measured.
The best physical analogy of the echo sounder beam would be a torch light beam that is shone over an area,
within that area there is a small piece of mirror that reflects the light at that point, sonar is very similar except that
the reflected point is normally the closest point within the beam.
What is a narrow beam transducer?
The properties of a transducer are normally a function of its physical size/shape and its resonant frequency.
The beam pattern of a given transducer is normally presented as a radial distribution pattern versus output
power applied. This generally means that the spread (beamwidth) of the transmitted ultrasound increases with
amplitude. Most modern echosounders use a digital signal processing (DSP) technique to reduce the power/gain
of its transmitted signal and thus maintain the minimum beam width for a given transducer.
The advantage of a narrow beamwidth survey transducer is the ability to “see” into narrow valley shapes thus gain
a clearer representative definition of the bottom surface being surveyed. This is contrary to the navigation use
of an echosounder, which has a reasonably wide beamwidth, where the returned signal within the beam is the
‘’shoalest’’ or shallowest point within the beam, obviously of more interest for hull clearance requirements.
Is my fishfinder okay for survey use?
Modern fishfinder type navigation echo sounders use quite sophisticated DSP techniques to show the bottom
surface, bottom type, and fish in the water column. As with the beamwidth discussion the electronics of a ‘leisure’
fishfinders are not designed for a survey application. In particular, the depth values are either heavily averaged
to show a smooth transition in numeric depth values or are optimized to show the shallowest depth seen in
the particular beam area. Similarly, survey echosounders make efforts to remove anomalies such as fish swim
bladder reflections from the measured data.
Do I require more pings per second?
Assuming the pings all return good values, the advantage of more pings per second is that the survey boat can
travel faster and therefore cover much larger areas in a shorter time. The assumption that more pings provide
higher quality dense survey data is not so. Density is a function of boat speed and is constrained by the limitations
of transducer beamwidth as discussed above. More pings can also create more noise with the high degree of
insonaration in the water, particularly in shallow water situations with multipath reflections.
What is latency?
Latency is the time difference between a position and depth being recorded. The topic is extremely complex and
can included some of the following sources of error:
• GPS position correction source time differences
•
Time of flight of ultrasound in water
• Serial transmission of data to, from and through computer systems
• Physical mounting of antennas and transducers In general the survey software attempts to minimize the error
by recording a precise timestamp on each piece of data recorded, the sum of all latency sources can then
be calculated by post- process adjustment of the data using a ‘’patch test’’ algorithm. The degree of latency is
dynamic so is always a function directly related to speed of the boat when data acquired, faster the boat then
more potential latency.
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SonarMite MILSpec/M8 User Manual | Rev 07/22/2016