DSC Distress Alert Call Confusion

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jimh
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DSC Distress Alert Call Confusion

Postby jimh » Mon Aug 13, 2018 11:40 am

Recently I read a first-hand account of a rather odd occurrence of a DSC DISTRESS ALERT transmission and subsequent radio communication about it. (It was posted to a private email exchange list, so I will paraphrase the writer's account of the events.) The situation was as follows:

In a port on the coast of the USA on an ocean there is a lot of close-by activity from several U.S. warships, and a fellow was monitoring VHF Marine Band Channel-16 with a DSC radio. Around 2 p.m. his DSC radio receives a DISTRESS ALERT call and sounds an appropriate alert signal. The on-shore radio operator had not previously received a DSC DISTRESS ALERT call, and it took him about half-a-minute to figure out how to silence the alarm and to save the position of the vessel in distress as a waypoint on the radio's built-in mini-chartplotter.

Shortly after that, one of the U.S. warships makes a voice transmission on Channel-16, apparently addressed generally to the vessel in distress. Then another vessel--let's call it BOAT-1, replies to the warship's call. The observer describes this call in reply from BOAT-1 as containing this information:

    "Sorry, it was me, my radio started blaring an alarm and acting weird, so I turned it off and went to a second radio to respond, sorry I don't know what happened, it was an accident."

To this reply the U.S. warship responds that it acknowledges the activation of the distress call was accidental, and according to the fellow recounting this even, that ended their involvement.

Shortly afterwards, a U.S. Coast Guard station begins to call BOAT-1, the vessel that responded to the general call "to the vessel in distress," trying to contact them on Channel 16. Several calls were made, and eventually, after about five minutes passes, BOAT-1 replies to the USCG call. The USCG and BOAT-1 then communicate on Channel 16. The USCG wants to confirm that BOAT-1 was actually the vessel that initiated the DSC DISTRESS ALERT CALL, so it asks the radio operator aboard BOAT-1 to furnish BOAT-1's maritime mobile service identity (MMSI).

BOAT-1 does not know what its MMSI actually is. Next the USCG asks them for their vessel name and a description. BOAT-1 one replies. The USCG then asks BOAT-1 to standby.

According to the listener, about five more minutes pass, until the USCG returns and contacts BOAT-1, informing them that "your vessel name and description do not match what is registered to the MMSI that sent the distress." Next the USCG asks BOAT-1 for its position. Apparently BOAT-1 does know their position.

Five more minutes elapse, and the USCG comes back on Channel-16 calling BOAT-1. They inform BOAT-1 that the position they gave by voice does not match the position that was sent in the DSC DISTRESS ALERT. The USCG has more questions for BOAT-1: "did you press the red button," they ask. BOAT-1 replies that, no, they did not press a red button, and that their radio was "just acting weird and alarming."

Now the situation is getting even stranger. The USCG tells BOAT-1 that apparently they just received a DSC DISTRESS ALERT, they did not send the alert, and by responding to the general call to the vessel in distress on Channel-16 they have thrown a huge diversion into the attempt to contact the real vessel in distress and render some assistance to them. According to the observer, this whole process took about 30-minutes to reach this point of awareness by the USCG, the rescue agency monitoring for distress alert calls.

The observer did not continue to monitor Channel-16 after this point in the saga, so the ultimate outcome is unknown. The situation as described sounds like a ball of confusion, greatly augmented by the knucklehead who announced he was the vessel in distress. Complete unfamiliarity with DSC radio operation is probably rather common. The notion that anyone can buy, install, and operate a DSC radio without any sort of training or certification is bound to produce outcomes like this. Since just about EVERY fixed-mount radio available today IS a DSC radio, the general recreational boat population probably needs some further training in DSC radio operation.

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Dutchman
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Re: DSC Distress Alert Call Confusion

Postby Dutchman » Mon Aug 13, 2018 11:50 am

That is a bad situation as the boat in distress might have gotten help much sooner.
This again shows that it is important to get some kind of training to operate a boat.
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jimh
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Re: DSC Distress Alert Call Confusion

Postby jimh » Mon Aug 13, 2018 1:13 pm

There are more oddities to the event as described. If a vessel initiates a DSC Distress Alert call, the DSC radio will continue to re-transmit the DSC Distress Alert every four or five minutes automatically, unless the operator of the DSC radio cancels the retransmission. Since the duration of the event described is said to be 30-minutes, there is an expectation that the distress alert would have been repeated by the vessel in distress. The observer does not mention receiving any follow-on distress alerts during the time he was monitoring with his DSC radio.

A CLASS-D DSC radio cannot create a DISTRESS ALERT ACKNOWLEDGEMENT transmission, but a CLASS-A DSC radio can. To know if either the warship or the USCG shore station sent a distress alert acknowledgement to the radio that sent the original distress alert would be interesting.

I am not sure what a Class-D DSC radio will do when, if it has initiated a distress alert call, it receives an acknowledgement. I will have to look that up. I suspect that if the distress has been acknowledged, then perhaps the automatic retransmission would stop.

The observer also mentioned that the position transmitted in the DSC distress alert contained only the latitude and longitude to a whole minute, no decimal minutes were sent. This behavior suggests that the radio that send the distress alert was probably an older radio qualified only to RTCM SC-101 recommendations. Those early DSC recommendations allowed for the position to be sent only to the whole minute, a rather coarse resolution. Or, that the observer's radio receiving the distress alert was older, as, again, an RTCM SC-101 radio will only receive the distress alert position to truncated whole minutes. (I think the observer's radio was a Class-D radio, so the most likely inference is the actual vessel in distress was using an obsolete radio to transmit the distress alert.)

As for the guy on BOAT-1 who interpreted the behavior of his DSC radio when receiving a distress alert--sounding an alarm and waiting for operator input to cancel the alarm and react to the call--as being indicative of his radio having SENT a distress alert, that is actually not that hard to understand. It is typical for a DSC radio that receives a distress alert to make a lot of noise to alert the operator something VERY important has just happened and to call his attention to the radio. Without any prior experience or training, the operator on BOAT-1 could have easily misinterpreted what happened, and when he returned to Channel-16 to find a general call being made to the "vessel in distress," he concluded that his boat must have accidently set off some sort of alarm because his radio went into an alarm state. Yes, it is quite naive to think like that, but given an untrained boater it seems quite possible.

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Re: DSC Distress Alert Call Confusion

Postby jimh » Tue Aug 14, 2018 10:09 am

Here are some further oddities in the situation described above:

The procedure recommended by the USCG for distress alerting consists of the following steps:

Transmission of DSC Distress Alert

A distress alert should be transmitted if, in the opinion of the Master, the ship or a person is in distress and requires immediate assistance.

A DSC distress alert should as far as possible include the ship's last known position and the time (in UTC) when it was valid. The position and the time may be included automatically by the ship's navigational equipment or may be inserted manually.

The DSC distress alert is transmitted as follows:

    --tune the transmitter to the DSC distress channel (2187.5 kHz on MF, channel 70 on VHF)
    --if time permits, key in or select on the DSC equipment keyboard the nature of distress, the ship's last known position (latitude and longitude), the time (in UTC) the position was valid, type of subsequent distress communication (telephony), in accordance with the DSC equipment manufacturer's instructions;
    --transmit the DSC distress alert
    --prepare for the subsequent distress traffic by tuning the transmitter and the radiotelephony receiver to the distress traffic channel in the same band, i.e. 2 182 kHz on MF, channel 16 on VHF, while waiting for the DSC distress acknowledgment.

Distress Traffic

On receipt of a DSC distress acknowledgment the ship in distress should commence the distress traffic by radiotelephony on the distress traffic frequency (2182 kHz on MF, channel 16 on VHF) as follows:

    --"MAYDAY",
    --"this is",
    --the 9-digit identity and the call sign or other identification of the ship,
    --the ship's position in latitude and longitude or other reference to a known geographical location,
    --the nature of distress and assistance wanted,
    --any other information which might facilitate the rescue.

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Re: DSC Distress Alert Call Confusion

Postby jimh » Wed Aug 15, 2018 10:13 am

The recommended behavior for a DSC radio includes the following behavior:

15 Position Request operation for Class D, E and H
The Position Request Acknowledgement function should be capable of being deactivated by the user in order to ensure privacy. However, after transmission of a distress alert, the position request acknowledgment of that particular radio should be activated automatically and then stay active until reset by the user. The Position Request Acknowledgement should be sent automatically by the equipment if requested. This would ensure that search and rescue entities are able to request the position of the vessel in distress even after a Distress Acknowledgement has been received by the equipment.

This is a well-designed feature. To repeat, a Class-D DSC radio which had initiated a distress alert should then automatically place itself into an mode in which the radio will reply to a position request without any need for operator intervention. This allows the rescuers to update the position of the vessel in distress without requiring any operator intervention on the vessel in distress.

Cf.: ITU-R M493-14, "Digital selective-calling system for use in the maritime mobile service"

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Re: DSC Distress Alert Call Confusion

Postby jimh » Wed Aug 22, 2018 5:24 pm

I mentioned earlier in the thread that knowledge about any DSC DISTRESS ALERT ACKNOWLEDGEMENT call sent to the vessel in distress by a rescue agency would be interesting. I investigated the recommended behavior of a VHF DSC CLASS-D radio that has made a DISTRESS ALERT transmission and then subsequently receives an ACKNOWLEDGEMENT of that transmission. I couldn't find an explicit mention of this in the ITU recommendations, so I emailed the USCG technical liaison. I got this reply:

Once the distress alert has been properly acknowledged the radio will stop sending the alert. The distress alert is sent five times if not acknowledged and would have to be manually initiated again