What is the channel spacing used for VHF marine radio?

25 kHz.

Though the smaller the channel spacing the more channels, the ITU agreement calls for 25 kHz. This governs the VHF ship to shore or ship number of channels.

Reference from International Telecommunications Union, pdf of Rec. ITU-R M.489-2 TECHNICAL CHARACTERISTICS OF VHF RADIOTELEPHONE EQUIPMENT OPERATING IN THE MARITIME MOBILE SERVICE IN CHANNELS SPACED BY 25 kHz

For details, please see Wikipedia's article on Marine VHF radio

Also, comprehensive info on Continuous Wave site, article VHF Marine Band Channels

Reference: VHF specifications and use are listed in the Code of Federal Regulations, Title 47 Chapter I Subchapter D
Part 80 - STATIONS IN THE MARITIME SERVICES

Since applicable regulations are in a number of places, please use CTRL-F and search on the word "VHF" to look up applicable regulations.

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What VHF channel is assigned for distress and calling?

16

From wp2ahg:

Channel 16 is reserved as the distress and calling channel.
It occupies the 156.8 MHz frequency.

Channel 70 - - Digital Selective Calling (DSC).
Channel 21A -US Coast Guard use only.
Channel 68 - -for non-commercial use.

For practical info regarding VHF radio use, please see Madden Marine Time site, pdf called Marine VHF Radio Use.

For detailed info, please see US Coast Guard site, article U.S. VHF Channel Information

Reference: VHF specifications and use are listed in the Code of Federal Regulations, Title 47 Chapter I Subchapter D
Part 80 - STATIONS IN THE MARITIME SERVICES

Since applicable regulations are in a number of places, please use CTRL-F and search on the word "VHF" to look up applicable regulations.

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What VHF Channel is used for Digital Selective Calling and acknowledgement?

70

From wp2ahg:

Channel 70 - - Digital Selective Calling (DSC).
Channel 16 -- Distress and calling channel.
Channel 21A -US Coast Guard use only.
Channel 68 - -for non-commercial use.

Channel 70 is at 156.525 MHz
Channel 16 is at 156.800 MHz
Channel 21A is at 157.050 MHz Channel 68 is at 156.425 MHz

For practical info regarding VHF radio use, please see Madden Marine Time site, pdf called Marine VHF Radio Use.

Also, a quick plan guide is at DPD Productions site, Marine Frequencies

For detailed info, please see US Coast Guard site, article U.S. VHF Channel Information

Reference: VHF specifications and use are listed in the Code of Federal Regulations, Title 47 Chapter I Subchapter D
Part 80 - STATIONS IN THE MARITIME SERVICES

Since applicable regulations are in a number of places, please use CTRL-F and search on the word "VHF" to look up applicable regulations.

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Maximum allowable frequency deviation for VHF marine radios is:

+/- 5 kHz.

This is for VHF marine wide-band. Others use a more narrow band.

See the regulation that covers this on Code of Federal Regulations, Title 47 Chapter I Subchapter C Part 74 Subpart D
§ 74.462 Authorized bandwidth and emissions.

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What is the reason for the USA-INT control or function?

It changes some channels that are normally duplex channels into simplex channels.

Duplex works kind of like talking on the phone, with receiving on one frequency and transmitting +/- offset on a close frequency. Simplex is basically a single channel.

For detailed info, please see The Coastal Passage site for the article on What every VHF user should already know

The goal is to inform all vessels to be able to receive maneuvering orders.

In US waters, change all VHF channels from Duplex to Simplex while in U.S. waters.

In International waters, reverse. Change all VTS frequencies to Duplex so all vessels can hear the orders.

See Wikipedia's article on Marine VHF radio

For comprehensive regulations, see Code of Federal Regulations, Title 47 Chapter I Subchapter D Part 80 Subpart H - Frequencies

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How might an installer verify correct GPS sentence to marine DSC VHF radio?

Look for latitude and longitude, plus speed, on VHF display.

GPS transmits data to DSC VHF radio to be displayed.

For practical info on installation, please see
West Marine pdf called VHF 580 DSC Marine Manual.

From wp2ahg:

If the latitude/longitude is correctly displayed on a marine DSC VHF radio, then the radio is interpreting the GPS sentence correctly.

As for (B): You would not press and hold the red distress button as that would initiate a distress signal.

As for (C): Unsure of what a 'GPS confirmation readout' is.

As for (D): A VHF radio check position report would confirm a VHF radio's transmit/receive capabilities, but would not confirm the GPS system is working.

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