DSC Basics

[Rustic Charm]

Originally Posted by Rusty007

It seems that most vessels broadcasting there MMSI number with their AIS transmitter do not have the MMSI number programed into there radio. If this is the case you can call some from the vessel list but they will not here the call.

ahh, good point.. I hadn't thought of that being the problem.

[ve7mdl]

Hello,

There seem to be a few misunderstandings about how VHF DSC Distress works, so let me try and clarify:

1. When the red button is pushed a DSC radio will transmit a distress call addressed to All Stations (actually, the radios I have tested will send several calls in a row) and wait for an acknowledgement. It should then go to channel 16 for a distress call in voice.

2. Most likely your radio will not allow you to acknowledge the distress using DSC, but if it does, DON'T DO IT. Only the Coast Guard or a Rescue Coordination Centre (RCC) should acknowledge using DSC. Class A DSC radios can acknowledge, but still should not, except in rare circumstances where a coast station does not hear the call.

3. If the DSC distress call is not acknowledged, the DSC radio will wait about 4 minutes and send the call again. It will wait another 4 minues, etc...

4. A DSC radio that receives a DSC call will sound the distress alarm for the crew to hear. Usually, it starts at a medium volume and then gets louder. The alarm does not depend on the radio's volume setting. It is stopped by the operator pushing a button (which one depends on manufacturer) and the radio should then be set to channel 16 and listen for voice follow-up.

5. Distress calls are not automatically relayed, but it is possible for a Class A DSC radio (and coast stations) to issue a Distress Relay DSC message. Most of the information from the original call must be entered manually. However, a Distress Relay function is not avaialble on a Class D DSC radio.

6. I am often asked: why issue a DSC distress when it still has to be sent by voice? There are two reasons:
a. People that have their radios on another channel or have the volume turned down will still get the DSC distress call.
b. In some cases you may not have time to issue a voice call. The DSC distress call takes less than 5 seconds to send off - and most of that time is just pushing the red button.

All this only works if a GPS is connected to the radio and the MMSI has been programmed in.

There are a lot more uses for DSC. See my book "Marine AIS & DSC Handbook" (ISBN 978-0-9881525-2-6)

Cheers, ....Erik.
m/v B.C. Girl

[Rustic Charm]

Originally Posted by ve7mdl

Hello,

There seem to be a few misunderstandings about how VHF DSC Distress works, so let me try and clarify:

1. When the red button is pushed a DSC radio will transmit a distress call addressed to All Stations (actually, the radios I have tested will send several calls in a row) and wait for an acknowledgement. It should then go to channel 16 for a distress call in voice.

I don't understand this. How does the radio go to channel 16 for a distress call in voice? IN Australia people putting out a mayday are advised to put the DSC button and then go to channel 16 for a voice message But you seem to be saying the DSC will do a voice message as well???

2. Most likely your radio will not allow you to acknowledge the distress using DSC, but if it does, DON'T DO IT. Only the Coast Guard or a Rescue Coordination Centre (RCC) should acknowledge using DSC. Class A DSC radios can acknowledge, but still should not, except in rare circumstances where a coast station does not hear the call.

This advice is contrary to AMSA advice for Australians. There are NO shore stations set up with DSC and our co ordination centre does not answer radios. For Australian's, look at the AMSA sight and you wills see that 'If' your in a position to help and can, then you are asked to acknowledge the mayday on channel 16, after giving it approximately 5 minutes to see if someone else can acknowledge it. But don't wait around for a Coast Guard or Rescue co ordination centre to acknoweldge it, as it won't happen.

3. If the DSC distress call is not acknowledged, the DSC radio will wait about 4 minutes and send the call again. It will wait another 4 minues, etc...

4. A DSC radio that receives a DSC call will sound the distress alarm for the crew to hear. Usually, it starts at a medium volume and then gets louder. The alarm does not depend on the radio's volume setting. It is stopped by the operator pushing a button (which one depends on manufacturer) and the radio should then be set to channel 16 and listen for voice follow-up.

Great to know.. I've not heard one yet.

5. Distress calls are not automatically relayed, but it is possible for a Class A DSC radio (and coast stations) to issue a Distress Relay DSC message. Most of the information from the original call must be entered manually. However, a Distress Relay function is not avaialble on a Class D DSC radio.

Don't understand this.. My understanding is that A DSC being picked up by another vessel, should relay the mayday after five minutes of not hearing anyone else responding to it.

6. I am often asked: why issue a DSC distress when it still has to be sent by voice? There are two reasons:
a. People that have their radios on another channel or have the volume turned down will still get the DSC distress call.
b. In some cases you may not have time to issue a voice call. The DSC distress call takes less than 5 seconds to send off - and most of that time is just pushing the red button.

Another reason, is because a digital mayday, can get through when a voice message can't. Bad Weather, poor reception, noise, disablement, can all stop a 'voice' message from being sent. A push of a button gets the main urgent message and location going.. This is the main benefit given for DSC in Australia.

All this only works if a GPS is connected to the radio and the MMSI has been programmed in. too true

There are a lot more uses for DSC. See my book "Marine AIS & DSC Handbook" (ISBN 978-0-9881525-2-6)

Cheers, ....Erik.
m/v B.C. Girl

first, let me advise I don't pretend to know much about DSC. It's very new technology down this way and there are not even shore stations set up in Australia. Though some of our volunteer organizations have gone and purchased equipment with DSC. DSC in Australia is mainly a ship to ship system of distress. I've some questions and comments in red above.

[Rustic Charm]

Originally Posted by Sandero

If you have two VHF.... or more.... do they each have a separate unique MMSI #?

IN Australia, mobile units will be issued with their own MMSI but it will be noted what ship/vessel they belong to. Authorities emphasizes that like an epirb, they should be advised when changing hands or going to a different vessel.

[conachair]

Originally Posted by Rustic Charm

IN Australia, mobile units will be issued with their own MMSI but it will be noted what ship/vessel they belong to.

The mobile DSC VHF GMDSS implementation seems a bit messy, UK registered units aren't to be used outside UK waters.
Plus even with fixed sets the US is even messier, with their own internal MMSI numbering scheme in addition to the "real" global GMDSS MMSI numbers.

http://www.navcen.uscg.gov/index.php?pageName=mtMmsi
Hand held VHF DSC FAQs | Ofcom

[Rustic Charm]

Originally Posted by conachair

The mobile DSC VHF GMDSS implementation seems a bit messy, UK registered units aren't to be used outside UK waters.
Plus even with fixed sets the US is even messier, with their own internal MMSI numbering scheme in addition to the "real" global GMDSS MMSI numbers.

http://www.navcen.uscg.gov/index.php?pageName=mtMmsi
Hand held VHF DSC FAQs | Ofcom

Yeah, it would be too difficult to have one international emergency system with everyone agreeing to the same system hey

As if it's not bad enough that a third of the world switch the colours around on navigation lights and another half drive on the wrong side of the road

[nigel1]

What Erik said is correct, that is how the system should work. Boats and ships should not acknowledge DSC Distress Alerts, by doing so they can cancel the alert by the vessel in distress.
This is particularly important with MF/HF DSC, a DSC Alert transmitted on SSB could be picked up thousands of miles away, if the operator on the receiving set acknowledges, and that acknowledgement is picked up by the distressed vessel, then the Alert is cancelled. Only under very exceptional circumstances should another vessel acknowledge a DSC distress Alert.
When DSC was first implemented, it was utter chaos on the MF/HF bands, a distress alert would be made, and literally dozens of ships would acknowledge, purely through lack of training and radio discipline.

The correct procedure is if your vessel is in a position to offer assistance, that you respond to the alert by radio telephony, Ch 16 for VHF, 2182 for MF.

If you have not done so already, it's good to have some flow charts to hand which outline the correct procedure for receiving a DSC Distress Alert





If you are using DSC Radio's, most countries would require that you are licensed to use that equipment. Undergoing a short DSC VHF course would benefit everyone.

[Rustic Charm]

Originally Posted by nigel1

What Erik said is correct, that is how the system should work. Boats and ships should not acknowledge DSC Distress calls, by doing so they can cancel the call by the vessel in distress.
This is particularly important with MF/HF DSC, a DSC call transmitted on SSB could be picked up thousands of miles away, if the operator on the receiving set acknowledges, and that acknowledgement is picked up by the distressed vessel, then the call is cancelled. Only under very exceptional circumstances should another vessel acknowledge a distress call.
When DSC was first implemented, it was utter chaos on the MF/HF bands, a distress call would be made, and literally dozens of ships would acknowledge, purely through lack of training and radio discipline.

The correct procedure is if your vessel is in a position to offer assistance, that you respond to the alert by radio telephony, Ch 16 for VHF, 2182 for MF.

If you have not done so already, it's good to have some flow charts to hand which outline the correct procedure for receiving a DSC Distress Alert





If you are using DSC Radio's, most countries would require that you are licensed to use that equipment. Undergoing a short DSC VHF course would benefit everyone.

I don't have an ss band don't know anything about them.

The picture you just posted seems to contradict what you and Erik have said about 'not' acknowledging a distress call.

How does acknowledging a distress call 'cancel' it?

[nigel1]

RC, there seems to be some confusion with terminolgy.
The DSC will send a Distress ALERT.
A distress call is made by voice, in the case of VHF, on Ch 16.
I see I am guilty to of mixing the terms Call and Alert, I will amedn my post, thanks for picking up on that.

If a DSC Alert is received, and DSC is used to acknowledge the Alert, and that acknowledgement is received by the originating radio, the DSC Alert transmission will stop.

Its the same if you receive a Routine DSC Alert, when you acknowledge/accept the alert, the alert is stopped at the transmitting station.

Thanks for picking up on my error

[Rustic Charm]

Originally Posted by nigel1

RC, there seems to be some confusion with terminolgy.
The DSC will send a Distress ALERT.
A distress call is made by voice, in the case of VHF, on Ch 16.
I see I am guilty to of mixing the terms Call and Alert, I will amedn my post, thanks for picking up on that.

If a DSC Alert is received, and DSC is used to acknowledge the Alert, and that acknowledgement is received by the originating radio, the DSC Alert transmission will stop.

Its the same if you receive a Routine DSC Alert, when you acknowledge/accept the alert, the alert is stopped at the transmitting station.

Thanks for picking up on my error

Your welcome ()

How does acknowledging an alert (sent by DSC) cancel the alert?

[nigel1]

As far as I am aware, it's just done by the DSC signalling code.

[Rustic Charm]

Originally Posted by nigel1

As far as I am aware, it's just done by the DSC signalling code.

Ok, I'm more than a little confused. Which I'm sure is due to my ignorance of DSC.

When I'm talking about 'acknowledging' the dcs alert, I'm talking about getting on 16 and verbally acknowledging I've heard their DSC alert. Is that what your talking about?

[nigel1]

If you had a Class A DSC VHF radio, it has a function to Acknowledge a DSC Distress Alert. If you used that, the DSC Alert is cancelled.

Most boaters have Class D sets, you dont get that function.

Responding to a DSC Alert by Voice on Ch 16 will not cancel the Alert (All DSC Traffic is on Ch 70).

If you hear a DSC Distress Alert, the normal procedure is to listen on Ch 16. If you do not here a response from a Coast Station, then if you are in a position to assit, then call the Distressed station on Ch 16, or, if needed, Relay the Distress message.

[Sandero]

I am sorry to say that those who have attempted to describe the features, functionality and use of VHF-DSC radios have done a poor job. The USGC video was much better.

It might be clearer if typical use examples were given... and what is the difference between practices of standard VHF, DSC -class A and DSC class D. There is also a high seas DSC as well to make things even more complicated or complete... take your pick.

[nigel1]

Sandero, I apologise for such a poor job.

RC, this attachment may help, its taken from the Australian GMDSS site, so it must be correct