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Old 02-04-2014, 06:29   #16
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Re: DSC Distress Message Test

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Originally Posted by continuouswave View Post
Could you point to the page of the manual for the Standard-Horizon GX1500S radio, a Class-D DSC radio made by Standard-Horizon, where the DSC functions can be completely disabled. I reviewed the owner's manual carefully trying to find that option, but I could not. Other than not assigning an MMSI to the radio, I don't see where you completely shut off the DSC functions.
Cant say all SH radios can do it, Mine can. As a leisure fit radio, Im not required to have DSC operational


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Old 02-04-2014, 06:37   #17
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Re: DSC Distress Message Test

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Yes, the DSC packet is the same, but the Class-D radio then sends enhanced position data, so the outcome is completely different. I am more interested in discussing the outcome than in discussing the minutia of the protocols. If I send a position report or a distress message with an SC-101 radio it only has a resolution of one nautical mile. If I send it with a Class-D radio it can have a resolution of six-inches. That is a significant difference. I really don't care too much if the radio has to send an extra packet to get the different outcome.

Do you?
Im confused I understand that you tested SC101 and picked it up on the class D, I don't see where you tested the class D GPS resolution transmission

I don't see in the ITU specs m where seconds degrees are transmitted.!

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Old 02-04-2014, 07:44   #18
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Re: DSC Distress Message Test

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I'm confused[.] I understand that you tested SC101 and picked it up on the class D...I don't see in the ITU specs m where seconds degrees are transmitted.
Position in units of seconds is never sent. I am confused where you think I said that. I think you made the presumption that position was send in units of seconds. It is not.

The DSC protocol permits interoperation between radios that use position resolution only to one whole minute and radios that use position resolution to one ten-thousandths of a minute. The position is first sent to a resolution of one whole minute. This is according to

http://www.itu.int/dms_pubrec/itu-r/...0-I!!PDF-E.pdf

in Section 8.2.3, which says:

"8.2.3 Message 2 is the “distress coordinates” message, consisting of ten digits indicating the location of the vessel in distress, coded on the principles described in Table 2, in pairs starting from the first and second digits (see Note 1 to § 5.3):

"The first digit indicates the quadrant in which the incident has occurred...The next four figures indicate the latitude in degrees and minutes. The next five figures indicate the longitude in degrees and minutes."

The enhanced position is provide in a different recommendation, ITU M.821.

M.821*:*Optional expansion of the digital selective-calling system for use in the maritime mobile service

This specification allows for an optional message to be sent in which the position is resolved to higher resolution:

"2.1.2.1 The data for the enhanced position resolution message field consist of either a command character taken from Table 3 or four characters (eight digits). These digits represent the tenths, hundredths, thousandths and ten thousandths of minutes for both latitude and longitude. The first two characters are the extra resolution for latitude and the second two for longitude. Unused digits are filled with zeros."

The Class-D radios send this position following the initial position sent to whole minute resolution. By sending the position first in the whole-minutes form and then adding more resolution in a second message, the DSC system allows for the interoperation of both types of radios.

The result is that when a Class-D radio sends its position to an accuracy of one ten-thousandths of a minute, and that transmission is received by a SC-101 radio, which does not understand the enhanced position message, the SC-101 radio only knows the position of the Class-D radio to one whole minute.

This is actually a good system. If the specifications were changed and the Class-D radio sent its position in a new message form that was completely unknown to the older SC-101 radios, then the SC-101 radios that received the transmission would not know anything about the position.

I am sure this was all worked out by a lot of very smart people sitting in a room and figuring out the best method.

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I don't see where you tested the class D GPS resolution transmission.
You don't see it because the article I wrote is about testing an SC-101 radio.
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Old 02-04-2014, 07:54   #19
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Re: DSC Distress Message Test

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Originally Posted by goboatingnow View Post
Cant say all SH radios can do it.
As far as I can find, my Standard-Horizon Class D radio cannot have the DSC receiver shut off.

It seems rather odd that a regulatory agency, like the FCC, would require all radios sold in the USA to have a certain feature (DSC), and then the radio would permit the operator to completely remove the feature.
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Old 02-04-2014, 12:49   #20
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Re: DSC Distress Message Test

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Originally Posted by continuouswave View Post
Position in units of seconds is never sent. I am confused where you think I said that. I think you made the presumption that position was send in units of seconds. It is not.

The DSC protocol permits interoperation between radios that use position resolution only to one whole minute and radios that use position resolution to one ten-thousandths of a minute. The position is first sent to a resolution of one whole minute. This is according to

http://www.itu.int/dms_pubrec/itu-r/...0-I!!PDF-E.pdf

in Section 8.2.3, which says:

"8.2.3Message 2 is the “distress coordinates” message, consisting of ten digits indicating the location of the vessel in distress, coded on the principles described in Table 2, in pairs starting from the first and second digits (see Note 1 to § 5.3):

"The first digit indicates the quadrant in which the incident has occurred...The next four figures indicate the latitude in degrees and minutes. The next five figures indicate the longitude in degrees and minutes."

The enhanced position is provide in a different recommendation, ITU M.821.

M.821*:*Optional expansion of the digital selective-calling system for use in the maritime mobile service

This specification allows for an optional message to be sent in which the position is resolved to higher resolution:

"2.1.2.1 The data for the enhanced position resolution message field consist of either a command character taken from Table 3 or four characters (eight digits). These digits represent the tenths, hundredths, thousandths and ten thousandths of minutes for both latitude and longitude. The first two characters are the extra resolution for latitude and the second two for longitude. Unused digits are filled with zeros."

The Class-D radios send this position following the initial position sent to whole minute resolution. By sending the position first in the whole-minutes form and then adding more resolution in a second message, the DSC system allows for the interoperation of both types of radios.

The result is that when a Class-D radio sends its position to an accuracy of one ten-thousandths of a minute, and that transmission is received by a SC-101 radio, which does not understand the enhanced position message, the SC-101 radio only knows the position of the Class-D radio to one whole minute.

This is actually a good system. If the specifications were changed and the Class-D radio sent its position in a new message form that was completely unknown to the older SC-101 radios, then the SC-101 radios that received the transmission would not know anything about the position.

I am sure this was all worked out by a lot of very smart people sitting in a room and figuring out the best method.



You don't see it because the article I wrote is about testing an SC-101 radio.


I think you have made a process error. What you compared was a specific radio. But I don't think you can use that to make a sc101 versus class d comparison

Furthermore you mentioned you did compare in this thread the GPS resolution , but by your own admission not based on any test.

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Old 02-04-2014, 13:07   #21
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Re: DSC Distress Message Test

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I think you have made a process error.
I did make on process error. I did not realize I would have to defend anything said as it were part of a thesis being reviewed by an academic committee that might award me a degree.

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Originally Posted by goboatingnow View Post
What you compared was a specific radio. But I don't think you can use that to make a sc101 versus class d comparison
Yes, I tested one radio. It is a radio sold in the during a period of time when any radio sold had to conform to at least RTCM SC-101. I think it is a reasonable assumption that:

--the radio I tested conforms to RTCM SC-101

--the radio is representative of radios sold in that era

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Originally Posted by goboatingnow View Post
Furthermore you mentioned you did compare in this thread the GPS resolution , but by your own admission not based on any test.
You have invented something new. I never stated (or "admitted" as you phrased it) that I did not test of the resolution of the Class-D radio. You simply inferred that. Actually, I have tested the position resolution of Class-D radios. I did not include information about those tests in the article, because the article was about a test of a SC-101 radio.
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Old 02-04-2014, 13:22   #22
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DSC Distress Message Test

I'm sorry of your are getting defensive

You said
Quote:
Yes, the DSC packet is the same, but the Class-D radio then sends enhanced position data, so the outcome is completely different.
I don't beleive this is the case and you keep defending this, while there is an ITU recommendation I beleive this post dates class D definitions

My basic point was that sc101 was a quite an acceptable standard, even if some radios made a poor implementation of it.

Of course I could be wrong



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Old 02-04-2014, 13:49   #23
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Re: DSC Distress Message Test

Thanks, this discussion was very enlightening. Since I mostly sail inland lakes and find the boaters usually only use cell phones to communicate, I only use the VHF to listen to the weather reports. Thanks all the same.
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Old 02-04-2014, 14:23   #24
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Re: DSC Distress Message Test

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My basic point was that sc101 was a quite an acceptable standard, even if some radios made a poor implementation of it.
I agree. I was actually quite surprised that the old RTCM SC-101 radio I tested proved many more digital selective calling features than I originally thought it had. Unfortunately, the features were rather hard to utilize due to poor user interface design.

The biggest handicap of these older radios seems to be the lack of enhanced position resolution.

I really did not intend to become an historian of the ITU, RTCM, and FCC recommendations and regulations.
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Old 02-04-2014, 14:43   #25
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Re: DSC Distress Message Test

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...while there is an ITU recommendation I beleive this post dates class D definitions...
I am going to study this more thoroughly. I'll let you know what I find.
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Old 02-04-2014, 15:28   #26
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Re: DSC Distress Message Test

Re the requirement to transmit an enhanced position in a distress message:

The ITU recommendation M.493-13 describes the distress alert message process in Section 11. There it says:

Quote:
11.4 Immediately following a distress alert a DSC expansion message giving enhanced position resolution according to Recommendation ITU-R M.821 should be transmitted in the following manner...(goes on to explain the manner depending on how many frequencies the messages is sent on).
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Old 02-04-2014, 18:23   #27
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DSC Distress Message Test

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Re the requirement to transmit an enhanced position in a distress message:



The ITU recommendation M.493-13 describes the distress alert message process in Section 11. There it says:

Yes but I believe that post dates class D. Ie as far as I can see this is an optional requirement.

The only difference between sc101 and class D is that class requires a dedicated ch70 DSC receiver so that distress alerts can still be detected during voice receive operations. sc101 can not do that and hence was never accepted by the ITU. In Europe ETSI , our equivalent to the US FCC, refused to certify such radios so we had class D from the get go.


Icom in early radios actually mixed up class D and class A features. For example they allowed a class D radio to issues a DSC distress acknowledge message, which is exclusively a class A feature.

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Old 02-04-2014, 19:28   #28
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Re: DSC Distress Message Test

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Yes but I believe that post dates class D. Ie as far as I can see this is an optional requirement.
Just to confirm your stance, do you agree that the present day standard does mandate the enhanced position message? Or do you still think it is something optional.

If you can send me (a link to or a copy of) a PDF of some prior version of the ITU recommendation (M-493) that contains a description of a Class-D radio and makes no mention of transmitting an enhanced position message following the distress alert, then I would be willing to consider that there might have been some moment in the history of digital selective calling for marine radio when a Class-D qualified radio did not send enhanced position.

When I look at the current ITU recommendation, ITU-Rec. M-493-13, I see that it mandates transmission of the enhanced position message following the distress alert message. That is how a Class-D radio sold today in the USA will work. As I demonstrated in my test, that is NOT how an SC-101 radio from c.2001 worked.
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Old 02-04-2014, 19:30   #29
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Re: DSC Distress Message Test

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Icom in early radios actually mixed up class D and class A features. For example they allowed a class D radio to issues a DSC distress acknowledge message, which is exclusively a class A feature.
I have an early ICOM DSC radio. I will see if it can send a distress acknowledge message.
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Old 02-04-2014, 21:03   #30
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Re: DSC Distress Message Test

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Re the requirement to transmit an enhanced position in a distress message:

The ITU recommendation M.493-13 describes the distress alert message process in Section 11. There it says:
Is there some significance to the use of the word "should" as opposed to "shall" in ITU parlance? This particular recommendation uses mainly "should" to outline requirements with very few "shall"s.
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