The most susceptible to fading or static interference is (B). HF SITOR (NBDP)

Sending text messages using HF band can be at times unreliable. The band occupies 3 MHz to 30 MHz, and that frequency range does bounce off the Earth's ionosphere. This provides a much greater range than sending radio waves in line-of-sight mode.

Once the waves hit ionosphere, they may become distorted, create an echo due to multipath bounces, and are affected by the weather condition of the ionosphere.

For additional info on HF propagation, see Classification of HF Signals

A great info on HF SITOR (NBDP) use by the National Weather Service, on
Marine Weather Broadcasts from USCG: USCG HF SITOR (NBDP)

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(C). MF/HF radiotelephony is the system most likely to be affected by atmospheric disturbances.

The HF frequency band waves bounce off the ionosphere and are affected by the changing condition of the ionosphere.

The MF frequency band waves are far less affected by the ionosphere. Also, propagation is best in salt water transmission. Though still a bit affected by ionosphere, MF performs well in salt water environments.

MF, the medium frequency band extends from 300 kHz to 3 MHz and the wavelengths range from 100 meters to 1 kilometer.

HF, the high frequency band extends from 3 MHz to 30 MHz, and the wavelengths range from 10 meters to 100 meters.

For more information, please see HF and Lower Frequency Radiation

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The least susceptible to fading or static interference is (D). Inmarsat

Of the choices presented, Inmarsat is the least likely to be affected by conditions leading to facing or static interferences.

The Inmarsat uses satellites to send and receive communications and is not affected by atmospheric conditions.

BUT it must have a line-of-sight connection, not a problem on open waters. Still class to any tall structure will affect connection with the 3 or more satellites it uses.

For more information, see the following sites:

Inmarsat Own site
RoadPosts Info on Inmarsat
The Basis of Inmarsat Communications

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The least susceptible to fading or static interference is (A). Inmarsat

Of the choices presented, Inmarsat is the least likely to be affected by conditions leading to facing or static interferences.

The Inmarsat uses satellites to send and receive communications and is not affected by atmospheric conditions.

BUT it must have a line-of-sight connection, not a problem on open waters. Still class to any tall structure will affect connection with the 3 or more satellites it uses.

For more information, see the following sites:

Inmarsat Own site
RoadPosts Info on Inmarsat
The Basis of Inmarsat Communications

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(D). The Medium Frequency (MF) band provides reliable communications between two stations that are 100 miles (160 km) apart.

The MF frequency band waves are far less affected by the ionosphere that other choice.

Also, propagation is best in salt water transmission. Though still a bit affected by ionosphere, MF performs well in salt water environments.

MF, the medium frequency band extends from 300 kHz to 3 MHz and the wavelengths range from 100 meters to 1 kilometer.

For more information, please see HF and Lower Frequency Radiation

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The least effective radius of operation has
(A) VHF DSC.

As for all VHS radios, the VHF-DSC, the VHF digital selective calling has an effective propagation range that ends at the horizon, that is at the end of the curvature of the Earth.

This can be somewhat compensated by increasing the height of the antenna, to increase the line-of-sight range. But, there is no ionospheric bounce, as with other frequency ranges.

More info from BoatBeat site
VHF Radio with Digital Selective Calling

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