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Electronic Navigation

William Ackerman May 27, 2017 03:55PM EDT

I have a couple of questions regarding this section:
1. Question 13 in the workbook: What should you apply to a Fathometer reading to determine the depth of water?
A. Subtract the draft of the vessel
B. Add the draft of the vessel
C. Subtract the seawater correction
D. Add the seawater correction.
Aside from the fact that the term "Fathometer" is not used at all in either the workbook or online lecture, the correct answer is stated as "B". This appears to contradict the statement in the workbook on page 164, under "Transducer", that the distance from the transducer to the lowest point of the hull must be subtracted. It appears "adding the draft" would give you MORE water under the hull than you actually have.
2. Question 15 in the workbook: When operated over a muddy bottom, a Fathometer may indicate:
A. A shallow depth reading
B. A zero depth reading
C. No depth reading
D. Two depth readings.
The answer is D, which I can't find in any of the readings, either in the book or online.
3. In the online quiz question 6: With regard to GPS, a civilian receiver will have the same accuracy as a military receiver if:
A. Selective availability is set to zero.
B. The satellites are all below 15 degrees in elevation
C. Your vessel is equipped with a Doppler receiver
D. Horizontal dilution of precision is high
The answer is A, which I cannot find in any of the readings.

Any clarification on these questions greatly appreciated!

Bill

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MLS Staff Instructor May 29, 2017 07:54AM EDT Mariners Learning System Agent

1) When setting up an electronic depth sounder (Fathometer) you would add the draft of the vessel to determine the actual depth of the water. The heading of that topic in your online classroom is: Electronic Depth Indicating Equipment (Fathometers/Depth Sounder)

2) When operating over a muddy bottom your depth sounder may indicate two depth readings. This is caused by the sound signal first hitting the top layer and then penetrating the soft seabed “echoing” back two nearly simultaneous signals to the transducer.

3) When the system was first created, timing errors were inserted into GPS transmissions to limit the accuracy on non-military GPS receivers to about 100 meters. This part of GPS operations, called Selective Availability, was eliminated in May 2000.

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William Ackerman May 31, 2017 05:25PM EDT
Thanks for the response; however, the heading in my workbook and in the online lecture is only "Depth Indicating Equipment." Again, the word "Fathometer" is completely missing, unless I am opening the wrong course somehow. Thanks.

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