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Old 11-11-2011, 17:25   #31
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Re: DSC Position Request from Handheld with Boat's MMSI

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Originally Posted by fairbank56 View Post
I'm afraid it's your ignorance and apparently some with the CG. Your statement, "With DSC, the signal repeats itself through other DSC radios in the area until it connects to somebody who actually cares" is incorrect. DSC radio's do not act as repeaters on a received distress or distress relay call, plain and simple. Operators of class A radio's can initiate a relay call but class D radio's can't even do that.

Eric
In fact becuase of massive over use of Distress relay and the need for land stations to individually acknowledge each DSC relay, recent proposed ITU changes will remove DSC Relay Messages from Class A and B sets.

Dave
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Old 11-11-2011, 17:44   #32
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Re: DSC Position Request from Handheld with Boat's MMSI

Apparently, rules are already in place regarding distress relays via VHF and MF. While relays are still possible via class A DSC equipment, they should not be used.

Annex 5
PROCEDURE FOR RESPONDING TO DSC DISTRESS ALERTS BY SHIPS
COMSAR/Circ.25
1 Introduction

The Sub-Committee on Radiocommunications and Search and Rescue (COMSAR), at its fourth session (12 to 16 July 1999), decided that digital selective calling (DSC) relays of distress alerts on all shipborne DSC equipment should be reduced and prepared a procedure for responding to VHF/MF and HF distress alerts, given in flow diagrams 1 and 2, recommending that it be displayed on the ship's bridge as A4 size posters. It also prepared the following guidance.
2 Distress relays

2.1 Radio personnel serving on ships should be made aware of the consequences of transmitting a distress relay call and of routeing a DSC distress relay alert to other than coast stations (CS).
2.2 The number of unintended activations of DSC distress alerts and DSC distress relay alerts creates extra work load and confusion to (M)RCCs and also causing delay in the response-time. The original distress alert from a ship in distress should not be disrupted by other ships, by transmitting a DSC distress relay alert.
2.3 Recommendation ITU-R M.541-8 on Operational procedures for the use of DSC equipment in the Maritime Mobile Service identifies only two situations in which a ship would transmit a distress relay call (distress relay alert):
.1on receiving a distress alert on a HF channel, which is not acknowledged by a coast station within 5 minutes. The distress relay call should be addressed to the appropriate coast station (Annex 1, paragraph 3.4.2 and Annex 3, paragraph 6.1.4); and
.2on knowing that another ship in distress is not itself able to transmit the distress alert and the Master of the ship considers that further help is necessary. The distress relay call should be addressed to "all ships" or to the appropriate coast station (Annex 3, paragraph 1.4).
2.4 In no case is a ship permitted to transmit a DSC distress relay call on receipt of a DSC distress alert on either VHF or MF channels.
2.5 Distress relay calls on HF channels should be initiated manually.
2.6 Compliance with operational and technical provisions above would prevent transmissions of inappropriate distress relay calls.
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Old 11-11-2011, 17:57   #33
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Re: DSC Position Request from Handheld with Boat's MMSI

The big question is, is where did this Coast Guard Lt. get his total BS information on how a DSC distress alert works and how many of his students and thousands of youtube users have been misinformed????

Amazing. Your government at work

Eric
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Old 11-11-2011, 18:07   #34
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Re: DSC Position Request from Handheld with Boat's MMSI

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It's good to see you remember your USCG course material, however, VHF is only line of sight. With DSC, the signal repeats itself through other DSC radios in the area until it connects to somebody who actually cares (USCG). This is why it's important to have your DSC-VHF radio turned on at all times (when at sea) as you won't know that you've been used as a repeater. Anyways, as for your position request, it probably went into etherland if you don't have it connected to your gps chartplotter. Another thing the radio needs to be set up (it doesn't come ready to go) see: page 52 in your manual and the vessel has to be in the polling list.
Sorry I did not reply sooner, but I've been away from the computer all afternoon. The radio was connected to the chart plotter and the GPS was powered up and had a stable fix. The radio was repeating the GPS information recieved from the GPS on its display. The Radio was setup for auto reply to a position request. I had previously entered my own MMSI into the directory. I sent the request to my MMSI which I selected from the directory. All was set up properly if the radio was going to report its own position in response to the request. As I suspected the radio cannot reply to its own position request because it cannot receive the position request. The receiver is disconnected from the antenna during transmission or the transmitter energy would travel into the receiver and fry it. Receivers are designed to deal with milliwatt or microwatt signals, not the 25watts from the transmitter.

It was noted in one post that the hand held would need a different MMSI if it was going to be used from land. In the US it is illegal to use marine VHF frequencies from land without a special license. I don't know under what circumstances on can obtain such a license but they seem to be limited to those with a business need for marine communication.
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Old 11-11-2011, 18:11   #35
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Re: DSC Position Request from Handheld with Boat's MMSI

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Originally Posted by fairbank56 View Post
Apparently, rules are already in place regarding distress relays via VHF and MF. While relays are still possible via class A DSC equipment, they should not be used.

.....
Yes but such DSC relays were always manually triggered. Even with manually triggered DSC Mayday Relay, The ITU has decided to remove that feature. Particulary in HF DSC alerts it caused ships thousands of miles away to manually generate DSC relay events, all which had to be singly cancelled by the coastguard

Dave
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Old 11-11-2011, 18:27   #36
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Re: DSC Position Request from Handheld with Boat's MMSI

I'm the holder of an FCC GMDSS Maintainer License and was going through some of my files and found some of the test questions on this matter.

7R-021C: Under what conditions would you relay a DSC Distress alert?
Correct answers are:

If the mobile unit in Distress is incapable of further Distress Alert communications.
If no Coast Station/Mobile Unit acknowledgement is observed.


7R-021D: The relay of DSC Distress Alerts:
All correct anwers:

Has completely overburdened the GMDSS system with improperly formatted or inappropriately relayed DSC calls.
Was originally an intended function of the GMDSS system.
Is no longer the preferred method for passing Distress message traffic to an RCC or Coast Station.
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Old 11-11-2011, 18:51   #37
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Re: DSC Position Request from Handheld with Boat's MMSI

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Originally Posted by Seahunter View Post
While DSC radios offer "Ship to Ship" communications, the "RED" button sends out and seeks a land based response. A ship who hears a mayday on DSC can communicate with the stricken boat, but this has nothing to do with the automated signal. I'm appalled at the ignorance here on the use of DSC and its operation.
USCG: Rescue 21
Digital Selective Calling
I do believe you went on about automated DSC relay, A function that doesn't exist and never has. I believe sir your ignorance is appalling.

Dave
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Old 12-11-2011, 06:50   #38
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Re: DSC Position Request from Handheld with Boat's MMSI

I watched the other videos in the CG DSC youtube series. This guy absolutely does not know what he is talking about. His presentation and answers to questions are full of misinformation. He even claims that radio's without an MMSI in them will do these automatic distress alert relays. These videos have been out there for over 2 years.

Theres a guy on another forum claiming that he keeps his DSC handheld clipped to his pfd so that if he falls overboard, he knows his fixed unit still onboard will relay his DSC distress call. I told him he was misinformed and that it is not going to happen. I also put out an FYI on the SailNet electronics forum. Unfortnately, thousands have now been misinformed about how DSC distress alerts work, thanks to the CG.

Eric
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Old 12-11-2011, 13:14   #39
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Re: DSC Position Request from Handheld with Boat's MMSI

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I'm looking at buying a hand held VHF with GPS capability. The advice given by the FCC is that they should be programmed with the same MMSI as the primary radio, unless they are to be used often on other boats. Under normal circumstances I would want the hand held to have the same MMSI as the primary, since I want it to be the backup in case of a failure of the primary. I would also like to use it on the dinghy. My question is what will happen if I send out a position request from the primary radio to the MMSI number that is programmed into the unit. Will the primary radio report its own position or will the hand held respond with its position? I can see that since reception is inhibited during transmission that if the primary radio just blindly sends out the request for a position from the inputed MMSI number the primary will never receive the request and the response will be from the hand held. However if the primary recognizes its own MMSI it may never send out the position request and simply report its own position. My experience is that marine electronics are not overly smart and it will probably just transmit the position request. I figure that someone on this forum must have tried this by now. So has anyone tried this and what was the result?
I have done it.

I have a two Standard Horizon radios, a handheld HX-851 and a fixed mount GX-2150, both programmed with the same MMSI designating my boat.

I use the HX to test the fixed GX and vice versa.

When I initiate a position report from the handheld, it reports the GPS position that the HX receives and computes from its GPS fixes. I send it to my own call sign/MMSI. Since it is transmitting on CH70, it's not receiving on Ch70, but the fixed unit IS. It accepts the PR from the handheld and then sends the position to my chartplotter and displays it as a waypoint with the handheld's MMSI (which is the same as the fixed unit's).

If you send a Position Report from the fixed unit to the handheld, the handheld will show the GPS fix from the chartplotter interface to your fixed mount radio.

If the fixed unit sends a Position Request, the handheld will respond with its own GPS position.

The fact that they share the same MMSI doesn't interfere with the handheld and fixed unit interaction, at least none that I've seen.

If I send a DSC Test message to, say, a Coast Guard MMSI, the acknowledge I get back will show up on both the handheld and on the fixed unit, assuming they are both in coverage. When the DSC Test addressee is far away, only the fixed unit will indicate receipt of the acknowledgement, since it has a lot more antenna gain that the little rubber ducky antenna on the HX.

I'm the guy Eric is referring to on the Hull Truth forum. I did think the fixed unit would relay a distress call from the handheld. If not, at least the handheld's GPS position can be shown on the chartplotter through a position report. I would also hope it would show through a distress alert, since distress alerts carry the position if the sender's got it set up right.

I'm disappointed to hear that the alert will not be relayed and will be digging in to understand that situation. Unfortunately, there is no way to test distress alerts that are not official distress alerts, so some things could be tested if that was allowed, but currently there is no provision to do that other than DSC Test.

I believe nearby boats will receive position info from the distress alert, since I have seen that myself on the only DSC distress alert I've ever received this past season. So while my handheld's distress alert appears that it won't be automagically forwarded, nearby boaters will receive the info and could relay it on CH16 by voice in a pinch.

Unfortunately, not many people know how to use DSC functionality, so that is far less reliable than auto-forwarding would be.
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Old 12-11-2011, 13:33   #40
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Re: DSC Position Request from Handheld with Boat's MMSI

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Sorry I did not reply sooner, but I've been away from the computer all afternoon. The radio was connected to the chart plotter and the GPS was powered up and had a stable fix. The radio was repeating the GPS information recieved from the GPS on its display. The Radio was setup for auto reply to a position request. I had previously entered my own MMSI into the directory. I sent the request to my MMSI which I selected from the directory. All was set up properly if the radio was going to report its own position in response to the request. As I suspected the radio cannot reply to its own position request because it cannot receive the position request.
You're right. Basically one has to view the MMSI as an old style telephone number. If you dialed your number, all you got was a "busy" signal.

Quote:
It was noted in one post that the hand held would need a different MMSI if it was going to be used from land. In the US it is illegal to use marine VHF frequencies from land without a special license.
That's what we've decided to do, even after we spent good money on 2m ship to shore handhelds. I found I was carrying 2 radios; the 2M and the VHF. It makes more sense to create a group (with your own radios) and communicate that way.
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Old 12-11-2011, 13:39   #41
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Re: DSC Position Request from Handheld with Boat's MMSI

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I like to keep my GX2150 on whenever I am on the boat however it doesn't like it when I dont have my gps on. It goes into alarm when it doesnt know where it is. My GPS is outside and when at anchor or the dock I take it down below, my vhf is down below and I have a RAM mic by the GPS at the helm.
Not to hijack but does anyone else have this problem? I think it is important to keep it on to be used as a repeater.....
The alarm on the 2150 is just indicating that it no longer has a live position updating from the chartplotter. It will still work and my guess is it will report the last GPS position it received, but I'm not sure. You just hit the "quit" softkey and the radio stops pestering.

Eric is adamant that repeating doesn't happen....I need to get to the bottom of that.
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Old 12-11-2011, 15:42   #42
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Re: DSC Position Request from Handheld with Boat's MMSI

Thanks Capecuddy, that's exactly the information I was looking for. I was hoping that would be the way it worked. I think I'll go forward with getting the HX.
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Old 12-11-2011, 17:06   #43
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Re: DSC Position Request from Handheld with Boat's MMSI

Interesting discussion. Thanks to Eric, et. al, for pointing out the erroneous information contained in the USCG videos.

You're right: I watched the video and it's clear the presenter, Lt. Parkhurst, hasn't a clue.

More to the point, most boaters don't have a clue as to the complexities and niceties of DSC. And, that's VHF DSC. When you begin to talk about HF/DSC as in the case of the Icom 802, it's even more complicated. Icom has 39 pages in its manual devoted to DSC operation.

Captn Bill: antenna height DOES matter. VHF distance potential is a function of line of sight from antenna to antenna. The height of BOTH the transmitting and receiving antennas is a major determinant of distance potential.

Bill
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Old 12-11-2011, 17:20   #44
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Re: DSC Position Request from Handheld with Boat's MMSI

I had a customer trying to tell me this summer that DSC equipped radios will "pass on" or relay a call. I explained it does not work that way, automatically, and he swore the USCG told him so. I brushed it off as strange but now I know the poor guy probably saw that video or spoke to a coastie who also believed that..
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Old 12-11-2011, 20:07   #45
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Re: DSC Position Request from Handheld with Boat's MMSI

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Captn Bill: antenna height DOES matter. VHF distance potential is a function of line of sight from antenna to antenna. The height of BOTH the transmitting and receiving antennas is a major determinant of distance potential.

Bill
The question I was answering was how far does the signal travel if the radio is floating, as the original radio I was discussing is a floating hand held. The radio waves will travel in a straight line until they are so weak they cannot be distinguished from background noise which is true of any VHF radio wave So how far the wave travels is no different if the radio is at 0 feet or 1000 feet or for that matter a million miles. As I stated however the reception distance is totally a function of the height of the receiving antenna because in the example I was responding to, the antenna height of the transmitting radio is effectively zero. That fact makes calculating the reception distance much simpler and only dependent on the radio horizon of the receiving antenna at least under normal circumstances. Under some strange atmospheric conditions I have listened to VHF conversations near Cape Hatteras from Long Island in the Bahamas, but I wouldn't count on that being a regular occurrence.
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