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Old 08-07-2016, 11:23   #1
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DSC (Digital Selective Calling) Explanation

Hello to all,
My intent here is just to provide some easy-to-understand info about Marine DSC....not trying to convince anyone of anything!


DSC, or Digital Selective Calling, has gotten a rather bad rep amongst some "sailing folks", in my opinion due to two primary reasons....

a) lack of easy-to-understand information about what DSC is, how it works, and how to use it...

b) the DSC designers (the ITU, IMO and the GMDSS designers) being overly swayed by the consumer electronics manufacturers to allow some variations in actual "routine" (non-Distress) DSC radio operations...


While I cannot do anything about some odd menu-driven features in some models of radios....fact is if you stick to Icom and Standard Horizon (at least their higher end units), you won't be frustrated!

And, I can do something about providing some easy-to-understand info about DSC...

~~~~~~~~~~~~~~~


So, here goes...

a) DSC (whether VHF-DSC or MF/HF-DSC) is sort of like "text messaging", except it doesn't use any external infrastructure (no mobile phone towers, nor internet needed!), just your radio and anyone else's DSC radio...



ALL DSC messages will include your vessel's MMSI# (Maritime Mobile Service Identification number), which is like your radio's "phone number"....(you cannot send any DSC messages, unless your radio has a proper MMSI# programmed into it...only need to do it once, when you install the radio, and only takes one minute or less...)

AND also included with ALL DSC messages will be your exact GPS position!!! (assuming you have your radio connected to your GPS on-board, or you have a DSC radio with a built-in GPS)


b) Maritime DSC has much greater range than your cell phones / mobile devices...
(VHF-DSC has a slightly greater range that Marine VHF-FM Voice, typically 30 - 40 miles 24/7/365, for ship-to-ship, and up to 75 miles, or more, ship-to-shore....
And HF-DSC has worldwide range)


c) You the user, can use your DSC Radios to send these "text messages" to either:
all other vessels,
a specific group,
or just one individual vessel...


d) These DSC "text" messages are limited to just 5 simple messages...
1 --- A "test" message (just confirming that your radio's DSC systems are working).

2 --- A "routine" message (just as it sounds, this is just a friendly, routine greeting)

3 --- A "Safety" message ("SÚcuritÚ")

4 --- An "Urgency" message ("Pan Pan")

5 --- A "Distress" message ("Mayday")


e) Maritime DSC, both VHF-DSC and MF/HF-DSC have been with us since 1992, and mandatory for all SOLAS vessels and all signatory nations, since Jan 1999...

{take note that in the mid-1990's, Australia elected to use ONLY HF-DSC (and INMARSAT) to cover all of their waters (including all the int'l waters they have responsibility for), due to their vast undeveloped coastline...hence they are the only GMDSS signatory nation that didn't implement VHF-DSC (nor MF-DSC) into their "coastal" GMDSS plan...but VHF-DSC radios will still signal other vessels in range, no matter whose waters you're sailing in!}


f) The DSC "Distress" signaling functions are the same for ALL DSC radios...

ALL VHF-DSC radios and ALL MF/HF-DSC radios need the same "push 'n hold, for a few seconds" of the Red Distress Button, to send out a DSC Distress Alert!!

ALL VHF-DSC radios send out their DSC Distress messages the same way...

ALL MF/HF-DSC radios send out their DSC Distress messages the same way...


There aren't any variations, between different manufacturers / models of radios, in how DSC Distress messaging is done...


g) The unfortunate stupidity of letting the economic clout of the consumer marine electronics manufactures sway regulators (especially the US FCC and USCG), allowed the dreaded RTCM standard/class radios to be sold....and with them, no real standardization....

But, Now even with worldwide acceptance of "Class D" VHF-DSC radios, there are still a few operational differences in how you would send a "routine" call or "group" call...

And, while this is unfortunate....the good news is two-fold...
--- Most sailors will never use these "routine" or "group" features...but, are most interested in Distress alerting...
--- Icom and Standard Horizon have good user interfaces and are easy-to-use....and as luck would have it, make up a significant percentage of VHF-DSC radios on "offshore cruising boats"...



I think I hit the high points of what DSC is, what it does, etc...
BUT....
No more mumbo-jumbo!!
How about some simple, easy-to-understand videos, designed to help the layperson sailor???

Please have a look at these two Youtube playlists...one specifically about VHF-DSC and one generally about HF-DSC, but that describes the entire maritime DSC system...

Here ya' go...

VHF-DSC
https://www.youtube.com/playlist?lis...J6QugtO2epizxF
(please note that while the first video here shows the M-802 MF/HF-DSC-SSB radio, most of the information in this first video is about all DSC, vhf and mf/hf...so, please do NOT skip past the first video...)


HF-DSC (including, LIVE real-world use), and the overall maritime DSC system
https://www.youtube.com/playlist?lis...ga2zYuPozhUXZX


Please watch these videos and tell me if I made it easy-to-understand...
Thanks and Fair Winds to all!!!


John
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Old 08-07-2016, 12:15   #2
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Re: DSC (Digital Selective Calling) Explanation

Great contribution, John.

One thing I would like to add to the excellent first video -- you go into some detail explaining the difference between "Safety", "Urgency", and "Distress" calls, but I'm not sure (although maybe I missed it) whether you explained that these categories are formally and rigidly connected to "Securite", "Pan Pan", and "Mayday" calls by voice, and in fact, a "Safety" DSC call must be followed by "Securite" by voice, an "Urgency" by "Pan Pan", and so forth.

Hope that is helpful.


Here's a link to a good summary of procedure:

http://www.maricom.de/dscbetri.htm
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Old 08-07-2016, 12:20   #3
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Re: DSC (Digital Selective Calling) Explanation

Im glad you are taking a crack at explaining this as it is not only confusing to understand the concept but difficult to understand the usual SSB manuals w their options and double option keys and nested menus and the like.

From your list, can you also explain a bit more about the following, including how people can efficiently edit and also any limitations?


1 --- A "test" message (just confirming that your radio's DSC systems are working).

2 --- A "routine" message (just as it sounds, this is just a friendly, routine greeting)

as this is a feature people might want to use day to day.
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Old 08-07-2016, 12:31   #4
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Re: DSC (Digital Selective Calling) Explanation

Hi John,

Thanks for the great post, I cna't watch the videos at work but will for sure whe nI get home, your stuff is always very informative. I have a icom 504vhf and 802hf, one thing I have yet to figure out from the manual on the hf radio is the dedicated DSC antenna port in the radio? Do you know the proper setup for this? The radio is set up with a tuner and long wire antenna. Both are setup with the vessels MMSI number and have a dedicated GPS antenna.

Thanks
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Old 08-07-2016, 15:51   #5
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Re: DSC (Digital Selective Calling) Explanation

Dockhead,
Thanks for the tips!
I think I will edit-in some narration, referring to the correlation of "safety" to "securtie" and "urgency" to "pan pan"...



Salty,
Please watch the videos....as much of the confusion you speak of is cleared up there!



Not sure what you are asking about here??
What do you wish to "edit"??

As for "limitations", the limitations are these are the only 5 messages that can be sent....and except for the "test" message, all the others include a channel number to change to, for further communications via Voice!
Quote:
Originally Posted by SaltyMonkey View Post
From your list, can you also explain a bit more about the following, including how people can efficiently edit and also any limitations?


1 --- A "test" message (just confirming that your radio's DSC systems are working).
This is exactly what it says....just a two-way digital connection between the two radios, which tests the connection / DSC parts of your radios...
That is it....nothing else!

2 --- A "routine" message (just as it sounds, this is just a friendly, routine greeting)
This could be a message sent to anyone, for any reason...such as requesting their position, polling them to see if they are within range, wishing the other vessel to talk to you on a specific channel, asking them for a fishing report, inquiring about dinner plans, etc. etc. etc...
But, the "reason" is NOT part of the DSC message, it is just sent as a "routine" message...

as this is a feature people might want to use day to day.
Yes, these are features that many sailors could find VERY useful!!!
Problem is that some are sailing with OLD non-DSC radios...
And, many with DSC radios don't even have an MMSI# (which means the radio will NOT send any DSC messages)....and even some with MMSI#'s don't have a GPS connected to the radio, thereby reducing the effectiveness of the DSC system!!
I hope this helps...
But, again....much of this is covered in the videos...

Fair winds..

John
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Old 08-07-2016, 16:12   #6
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Re: DSC (Digital Selective Calling) Explanation

Thank you. I'm going to put on my project list (My whole life is a project list) connecting one of many GPS antennas to my DSC VHF. Got the international MMSI number programmed. At least they know who I am, now it would be good that they know where I am.
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Old 08-07-2016, 16:49   #7
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Re: DSC (Digital Selective Calling) Explanation

Jake, et al,
The M-802 manual is pretty poor in many regards....
But, in brief, its "DSC antenna" is the only way the M-802 can receive any DSC messages from the GMDSS DSC frequencies, not having this connected will still allow you to send DSC messages, but not receive any, nor replies to your own DSC Distress message!!
Quote:
Originally Posted by Cruisingscotts View Post
I have a icom 504vhf and 802hf, one thing I have yet to figure out from the manual on the hf radio is the dedicated DSC antenna port in the radio? Do you know the proper setup for this? The radio is set up with a tuner and long wire antenna. Both are setup with the vessels MMSI number and have a dedicated GPS antenna.
Although I use a 22' long piece of insulated rigging (one of my aft lower shrouds), as my HF-DSC receive antenna, and it works GREAT!!

The virtue of the narrow-band, extremely robust HF-DSC transmissions (which are narrow-band FSK), allow significant signal-to-noise ratio improvement versus "SSB Voice", which means you can actually have some effective DSC comms, using a smaller, less-efficient antenna...

So, many sailors use the small 54" HF whip antenna manufactured by Metz....designed for HF WeFax and HF-DSC receiving...
Metz Communication General Coverage/WeatherFax Antenna
Many have had good results....
(I used this for the first couple months of my M-802 use in 2004....before changing to the insulated shroud...and the Metz was "okay", but not as good as my shroud antenna!)


Also, take note that the stations that would be responding to your "DSC distress" are going to have large transmit antennas and much higher power (4000 watts on average), so the chances are that you'd not need much more than the Metz for these signals...
(where it falls short is for "sailboat-to-sailboat" DSC comms)


Whatever DSC antenna you choose, feed it with coax, and keep it out in the clear as best you can...but, no need for it to be up high....stern-rail mounting is fine...
Do NOT try using a wire inside your boat!! (as has been unfortunately recommended by some on-line)


I hope this helps...

fair winds...

John
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Old 08-07-2016, 17:23   #8
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Re: DSC (Digital Selective Calling) Explanation

Quote:
Originally Posted by ka4wja View Post
Jake, et al,
The M-802 manual is pretty poor in many regards....
But, in brief, its "DSC antenna" is the only way the M-802 can receive any DSC messages from the GMDSS DSC frequencies, not having this connected will still allow you to send DSC messages, but not receive any, nor replies to your own DSC Distress message!!

Although I use a 22' long piece of insulated rigging (one of my aft lower shrouds), as my HF-DSC receive antenna, and it works GREAT!!

The virtue of the narrow-band, extremely robust HF-DSC transmissions (which are narrow-band FSK), allow significant signal-to-noise ratio improvement versus "SSB Voice", which means you can actually have some effective DSC comms, using a smaller, less-efficient antenna...

So, many sailors use the small 54" HF whip antenna manufactured by Metz....designed for HF WeFax and HF-DSC receiving...
Metz Communication General Coverage/WeatherFax Antenna
Many have had good results....
(I used this for the first couple months of my M-802 use in 2004....before changing to the insulated shroud...and the Metz was "okay", but not as good as my shroud antenna!)


Also, take note that the stations that would be responding to your "DSC distress" are going to have large transmit antennas and much higher power (4000 watts on average), so the chances are that you'd not need much more than the Metz for these signals...
(where it falls short is for "sailboat-to-sailboat" DSC comms)


Whatever DSC antenna you choose, feed it with coax, and keep it out in the clear as best you can...but, no need for it to be up high....stern-rail mounting is fine...
Do NOT try using a wire inside your boat!! (as has been unfortunately recommended by some on-line)


I hope this helps...

fair winds...

John
Hi John

Thank you for the reply, that makes much more sense. I'm studying away at hamtestonline but I seem to autopurge after a few neglegent days so... couple more questions. I do have insolators for my backstay which was my original long wire plan however it would have made some strange wire runs due to rig configuration, radio and ground plane location, so I opted for a dedicated insulated wire directly above the ground plane and tuner to the mast head. SInce there is no ground plane required for the DSC (correct?) I would guess the insulated backstay would work, is that correct? How does one go about making the connection via coax to the long wire? Did you set up the stay on a specific length incluing the coax run? Tuner involved etc?

Thanks

Jake
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Old 08-07-2016, 17:29   #9
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Re: DSC (Digital Selective Calling) Explanation

John thanks for your input, enjoyed.
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Old 08-07-2016, 18:33   #10
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Re: DSC (Digital Selective Calling) Explanation

Jake,
The minute details of a specific install are well beyond the gist of this thread...
So, best to start a new one, with lots of description and/or photos of your install...and we can discuss the details there!
Quote:
Originally Posted by Cruisingscotts View Post
I do have insolators for my backstay which was my original long wire plan however it would have made some strange wire runs due to rig configuration, radio and ground plane location, so I opted for a dedicated insulated wire directly above the ground plane and tuner to the mast head. SInce there is no ground plane required for the DSC (correct?) I would guess the insulated backstay would work, is that correct? How does one go about making the connection via coax to the long wire? Did you set up the stay on a specific length incluing the coax run? Tuner involved etc?
But, until then, some brief answers...
I'm not to keen on having the two antennas that close!!
And, until I saw the design details, I'd not recommend this approach...

No tuner is needed...

No ground plane is "needed", but it might help reduce received RFI, so this is another example of each install being unique!!

No specific length is needed, but I recommend anything from 15' to 35'....

At first I just attached the center conductor to the shroud chainplate, and have just an upper insulator...
Later, I added a balun at the feedpoint, attached one terminal to the shroud chainplate and the other to a ground wire, running to a keel bolt.....no changes were ever noticed, so I used the balun for a vertical dipole....and disconnected the ground wire...



Again, the details of marine SSB installation, especially DSC antennas, is best to do in a new thread...

John
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Old 08-07-2016, 19:59   #11
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Re: DSC (Digital Selective Calling) Explanation

Quote:
Originally Posted by Dockhead View Post

Here's a link to a good summary of procedure:

DSC Operational Procedures for Ships
Nice link but I think that for a crib sheet to be any use to the crew in an emergency situation it should be far simpler than that. Simply dealing with 'how to send an undesignated distress and what to do when someone responds ' kept next to the set.

I received my HX870E DSC VHF two days ago ( purchased on your recommendation) and am quite impressed.
The manual is light years away from what we had in the day job when GMDSS was first introduced ( Skanti kit... instructions written by non english speakers who had never been to sea..).

I don't think I shall be buying an MF/HF DSC but am now convinced that the VHF was a worthwhile purchase being portable and all.

Moving right along...

I don't often just have a chat with people on VHF, the last time was either November or December 2008. However a major impediment to making a ship to ship call by DSC VHF would be knowing the MMSI of your friend's yacht. Esp as in an anchorage the simplest source of that knowledge - his AIS transmission - is probably not available as his AIS transceiver is most likely switched off if indeed he has one.

OK, offshore if you want to speak to a big ship bridge you will have their MMSI if you have AIS.

If you have access to the internet its not a problem with other yachts either... you just go and have a looksee here....Particulars of Ship stations

( its probably not a bad idea to make sure your own details are up to date on that site)

Then... as long as their vhf is switched on you are in business.
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Old 08-07-2016, 20:37   #12
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Re: DSC (Digital Selective Calling) Explanation

Quote:
Originally Posted by ka4wja View Post




b) Maritime DSC has much greater range than your cell phones / mobile devices...
(VHF-DSC has a slightly greater range that Marine VHF-FM Voice, typically 30 - 40 miles 24/7/365, for ship-to-ship, and up to 75 miles, or more, ship-to-shore....
And HF-DSC has worldwide range)

John

How does sending a digital hail on channel 70 ( DSC ) have more range than a voice message on channel 16?
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Old 08-07-2016, 20:51   #13
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Re: DSC (Digital Selective Calling) Explanation

Quote:
Originally Posted by Dockhead View Post


Here's a link to a good summary of procedure:

DSC Operational Procedures for Ships
Just to show I do read stuff...

How did telex ever get involved with GMDSS ? from the link ...

'The following procedures shall be used in cases where the distress traffic on MF/HF is carried out by radiotelex:

The forward error correcting (FEC) mode shall be used unless specifically requested to do otherwise;
all messages shall be preceded by:
at least one carriage return,
line feed,
one letter shift,
the distress signal "MAYDAY";
The ship in distress should commence the distress telex traffic on the appropriate distress telex traffic channel as follows:

carriage return, line feed, letter shift,
the distress signal "MAYDAY",
"this is",...'

My last day job, built 1999, had std C and telex as well as hf dsc.. the telex was a total waste of space..only fitted to save the coy the $$$ of fitting a second Std C.
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Old 08-07-2016, 23:12   #14
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Re: DSC (Digital Selective Calling) Explanation

I forget which of the growing number of DSC threads had the comment re confusion caused by having two radios ( fixed and portable VHF) sharing the same MMSI.

If under UK jurisdiction you must get a second MMSI for the portable...

lookee here Hand held VHF DSC FAQs | Ofcom
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Old 09-07-2016, 07:04   #15
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Re: DSC (Digital Selective Calling) Explanation

@John

Your explanation satisfactory expands the difference between #1 and #2 type messages and their usages.

Thanks.

#2 is what people might want to use everyday and therefore DSC becomes useful. Using SSB gives WW range.

Quote:
Originally Posted by ka4wja View Post
What do you wish to "edit"??
John
text messages...

How do people normally perform this efficiently? What is the common software and hardware? Is there a means to convert DSC => SMS so you can contact others via phone?

and are there any limitations on character length?

for example.
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