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Author Topic:   DSC and Position Request Replies
Chuck Tribolet posted 03-29-2007 11:32 PM ET (US)   Profile for Chuck Tribolet   Send Email to Chuck Tribolet  
The recent disappearance of my friend Kawika got me thinking
about [Digital Selective Calling] position request and what's required for your radio
to automatically respond to one.

When I talked to the [Coast Guard] early that Sunday morning, and gave
them a description of his boat, the one thing that seemed to
prick up their ears was his [Marine Mobile Service Identifier]. But Kawika had an
Icom radio (M-402 or M-502), and only the latest Icom radios
(M-304, M-422) support replying to a DSC position request
without manual intervention, and you have to change an option
to even allow for that. The older Icom radios require manual
intervention to reply to a position request. The I haven't
checked all of the Standard Horizon radios, but my older
SH Spectrum supports an automatic response, though there's
an option to disable it. So, unless you REALLY don't want
your fishing buddies to know where they're biting, if you
radio supports it, enable the automatic response, so when
the Coasties ask "where are you?" your radio can say "here!".


wywhaler posted 03-30-2007 12:13 AM ET (US)     Profile for wywhaler  Send Email to wywhaler     
I'm thinking the option you are referring to on the ICOM M422 is the one that gives you the choice of whether you want yourposition relayed automatically to a requesting station or not. We're not talking about the CH70 transmission, but rather somebody (anybody) that has your [Marine Mobile Service Identifier], can request your position. I'd rather have the option of sending the position only if I want. The request appears on the display, so if you want to send it you can. If you have a group thing going or something, then you just allow the position to go out on request. Go ICOM.


Chuck Tribolet posted 03-30-2007 07:34 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
That's your choice, but it means that if you can't reply,
it won't help anyone find you.

Me, I don't care who knows where I am.


jimh posted 03-30-2007 08:04 AM ET (US)     Profile for jimh  Send Email to jimh     
If someone already knows your vessel's [Marine Mobile Service Identifier], you would assume that you probably gave that information to them, and, on that basis, it seems unlikely that you would want to conceal your position from them. I do not believe the protocol allows people to just make a "broadcast" request for all boats in the area to report their positions. First of all, that would create too much traffic on the DSC channel. Imagine 100 boats all trying to transmit a reply to a position polling request. Their radio transmissions would all interfere with one another. The protocol only directs the position request to a specific vessel. Again, if someone has your [Marine Mobile Service Identifier], it is because you probably gave it to them. So if you don't want someone to know your position, don't give them your [Marine Mobile Service Identifier].

Standard-Horizon more or less invented the protocol for using DSC to relay vessel position and display it on their chart plotters. ICOM is just catching up on this feature for their radios. Vessel position polling is one of the reasons I plan to upgrade my vessel radio to a new Class-D DSC radio from Standard Horizon and to interface the radio with my old Standard-Horizon chart plotter (which I have been assured will support this feature).

wywhaler posted 03-30-2007 03:25 PM ET (US)     Profile for wywhaler  Send Email to wywhaler     
I'm wondering if there is, or will be, an [Marine Mobile Service Identifier] directory. Also if it will be a requirement to be listed or if you can opt out. Maybe the Coast Guard equip. accepts all [Marine Mobile Service Identifier]'s otherwise they might need a list of who's who.

I can't talk about other set-ups but my ICOM M422 hooks up DSC wise to my SITEX FF/PL and its GPS hook-up. Lat/Long scrolls across the display when requested. Menu allows selection for automatic replies or not.

I guess you're right about not caring who knows where you are but it could become important in the future.

As far as the Channel 70 transmission, it is digital and is a short burst. I'm guessing maybe a 1/4 of a second long. I could be wrong but I don't think frequency congestion would be an issue. Go ICOM/SITEX/GAM


David Pendleton posted 03-30-2007 03:34 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Of course there's [a Marine Mobile Service Identifier] directory. That's the whole point of the system.
Chuck Tribolet posted 03-30-2007 06:44 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
[As far as I know], the [Marine Mobile Service Identifier] directory is not publicly accessible.


David Pendleton posted 03-30-2007 09:23 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I think you're right. I don't think it's made public.

jimh posted 03-31-2007 08:22 AM ET (US)     Profile for jimh  Send Email to jimh     
Because all DSC transmission use the same VHF Marine Band radio channel, there is always a risk of congestion and interference.

For emergency transmissions, a DSC radio goes on the air when you hit the button. For non-emergency transmissions, the DSC radio is supposed to wait for the channel to be clear. Cf. IEC 62238, excerpt below:

5.3.3 Free channel transmission
The DSC equipment shall be provided with facilities which, except for distress calls, automatically delay the transmission of DSC until the calling channel 70 is free.

where2 posted 03-31-2007 08:47 PM ET (US)     Profile for where2  Send Email to where2     
Jimh's old SH CP150 (Chart Plotter) should work fine with his new Class-D radio. NOTE: one of the benefits of the Class D is that it has a dedicated receiver operating continuously on Ch70.

This brings up an interesting point which would be curious to test if I had another DSC radio. On a radio that does not have a dedicated "ear" listening to Ch70 (any radio that is only RTCM SC-101 compliant), what are the odds that it will not hear a position request because it is in the midst of scanning?

Obviously, if you own a RTCM SC-101 compliant radio and did not leave it in scan mode with Ch70 as one of the frequencies it is scanning, your DSC system is not going to hear a request and you are completely out of luck if your radio is queried in the event you are incapacitated.

Personally, I intend to leave my DSC set to automatically transmit my position in the event my MMSI is sent out asking for a position request. As noted, if I have given you my MMSI, I probably thought you might like to find me some day. I am surprised that the Coast Guard did not already check to see that Kawika had registered a MMSI radio. I thought part of the MMSI registration process included enough meaningful information that the coast guard would be able to look up my MMSI number without having to ask a good friend of mine who happened to know it?

wywhaler posted 03-31-2007 09:16 PM ET (US)     Profile for wywhaler  Send Email to wywhaler     
The Icom M422 has a Tri-watch function (not the scan/tag function)that monitors CH70, CH16 and a calling freq. at the same time. Is that what you are interested in?


wywhaler posted 03-31-2007 09:30 PM ET (US)     Profile for wywhaler  Send Email to wywhaler     
Forgot to mention that the ICOM M422 also has a "DSC WATCH" function to monitor CH70.


wywhaler posted 03-31-2007 09:39 PM ET (US)     Profile for wywhaler  Send Email to wywhaler     
The Icom M422 meets the RTCM SC101 requirement.


jimh posted 03-31-2007 10:41 PM ET (US)     Profile for jimh  Send Email to jimh     
The dual receiver function is the nice advantage of a Class-D radio. I am going to try to get up to speed on this DSC position polling stuff this summer--gotta find another boat to work with!

My view of the older RTCM SC101 radios is that they are just DSC emergency position transmitters, mainly. It was a way to get the ball rolling on DSC deployment in low-cost recreational devices. Now the Class-D standard has been moved to the minimum recommended level, and the price of a Class-D radio is very moderate. So why not get one and enjoy the greater capability?

wywhaler posted 04-01-2007 03:11 AM ET (US)     Profile for wywhaler  Send Email to wywhaler     
I guess the reason for the second receiver is to monitor while you are in the process of receiving on another channel. But wait a minute. The "DSC WATCH" function on the Icom M422 provides" DSC watch monitors Channel 70 while you are receiving another Channel. If a distress signal is received on Channel 70, the transciever monitors Channel 16 and 70 alternately until the distress signal disappears".

Is it possible that I'm getting Class "D" performance at a RTCM SC-101 price? I'm wondering if the Tri-Watch is something unique to Icom or something provided industry wide? I'm keeping the M422.


jimh posted 04-01-2007 08:40 AM ET (US)     Profile for jimh  Send Email to jimh     
See my article in the REFERENCE section on what makes a Class-D DSC radio different from a RTCM SC101 rated radio:

Digital Selective Calling: Class–D

wywhaler posted 04-01-2007 03:58 PM ET (US)     Profile for wywhaler  Send Email to wywhaler     
After reviewing your article on Class D, which I thank you for, I'm thinking a review of the M422 operating manual will show just how close the M422 is to Class D. Much of the new stuff is in there. I think the "DSC WATCH" function is significant, as is the "TRI-WATCH". Stuff like auto freq change, warning beeper limited to two minutes, display presentation during distress call, etc. are in there.

If going offshore regularly, I would get the Class D for the enhanced transmission capability, if nothing else. Here in the Rocky Mountains the M422 will do just fine.


David Pendleton posted 04-02-2007 09:58 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Interestingly enough, my Raymarine 53's (ca. 2001) are RTCM SC-101 radios, but have a configurable "DSC Watch" feature.

Enabling this feature watches channel 70 provided you are not transmitting, receiving or listening to a WX channel (there is only one receiver, after all) however, it does this independently of any other watch you have programmed and are monitoring at the time (or not).

Also Jim, I suspect you have seen this page already, but the features documented for SC-101 vs. Class D differ somewhat from your reference article.

I'm still wading through the ITU M.541 specification...

jimh posted 04-02-2007 10:02 PM ET (US)     Profile for jimh  Send Email to jimh     
Dave--My article comes right from the SOLAS documents re the differences. I have not made a careful comparison between what the USCG website says and what my article says. I can email you a copy of the official documents if you like.
David Pendleton posted 04-03-2007 11:41 AM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
Jim, like everything else it seems there is a lot of information out there and not all of it meshes perfectly.

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