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Old 02-14-2018, 01:19 PM   #1
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AIS or DSC in an emergency?

I just posted about a battery backup system for my electronics but it got me to thinking, in the case of emergency whats the importance of DSC versus having AIS. I know DSC transmits GPS data in case of an emergency but does it transmit at full power (25w), how would it compare to my FA170 (Class A AIS). The reason I am asking is I'm coming up with a list of critical systems in case of an emergency and am unclear if AIS should be on there? Any info about the two systems use in cases of emergency or SAR would be helpful as I am rather novice in that area.
BTW this isn't support to be about other types of emergency systems, I have two epirbs, etc.

Thanks
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Old 02-14-2018, 01:39 PM   #2
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I don't think they are the same thing. DSC (the emergency button on your marine radio) sends out an emergency call with your position. AIS just shows where you are (if you have a transceiver, not just a receiver). It doesn't transmit an emergency signal.

I have made an assumption that you have a MMSI number, that you have programmed it into your radio and that you have connected some sort of GPS device to your radio and they are all powered up.

Which brings up this question: I have had my chart plotter connected to my radio for several years and in theory, if I press the distress button on the radio, it will send out my position, provided the chart plotter is on and has been on long enough to acquire a position.

I wonder if I would be better off connecting the recently installed AIS transceiver to the radio for a position as the AIS is on 24/7.

Thoughts?
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Old 02-14-2018, 01:55 PM   #3
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Not being familiar with a Class A AIS, what does it take to send an emergency signal, what does it contain and who gets it if you don’t mind me asking?
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Old 02-14-2018, 02:04 PM   #4
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Yep, you are correct, I get how they work, I think the question is if the DSC signal is actually used and if its broadcast at a similar power/reliability to AIS.

In the case of an emergency you can set your AIS status code 2 and there is a status code 14 which says SAR or EPIRB but I don't know if it would be appropriate to set your code to that if you are in an emergency situation. Code 2 is "Not Under Command". Either way I don't think thats the intent of those codes but may help. I know that Vessel Traffic closely monitors status codes if you are class A and in heavily travelled areas... said by the guy who once forgot to change status and was quickly contacted

My core question is in the case of emergency does DSC actually work, I think in theory its suppose to show the vessel in distress on my TZT and TimeZero screens, but have never seen that work and I know most rescues are performed by other boaters not the coast guard. If thats the case would you consider AIS to be as critical a system as DSC in a radio. Since they share MMSI's they should be seen as the same thing to the USGC, etc.

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Old 02-14-2018, 02:08 PM   #5
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Quote:
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Not being familiar with a Class A AIS, what does it take to send an emergency signal, what does it contain and who gets it if you don’t mind me asking?
Class A sends at a much higher wattage, also can't easily and isn't usually filtered out by commercial traffic where they see class b as noise. Also the class A AIS sends more info and at a higher frequency. The other thing I have found with it is I get pinged by Vessel Traffic occasionally asking for updates on fog/weather or alerting of other class A traffic which I should be aware of. Victoria BC tends to be really good about this but you do have to listen in on their channel.

It doesn't really send out an emergency signal but in the case of an emergency may be a much more effective way of ensuring you can be found easily by vessels not specifically equipt for SAR.

Thats at the core of my question having never seen someone activate DSC in an emergency I just don't know how easily it will show up on my plotters, etc versus using AIS to find them.
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Old 02-14-2018, 02:10 PM   #6
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Class A AIS can send a wide variety of messages, but DSC is really the correct tool for signaling distress within the distance range of VHF. It also has a relay capability which can further extend the range. Your VHF also will transmit at 25 watts where class A AIS is 12W, as I recall, and class B is lower.

But AIS helps too, because anyone within range can see exactly where you are, as opposed to where you were when you activated the DSC alarm. So as help approaches, they can come right to you rather than have to hunt around.

As pointed out, both are dependent on having a valid GPS input, and this is exactly why all AIS devices include their own GPS. Most VHFs needs to be connected to an external GPS, and this discussion is an excellent example of why it's preferable to have an always-on GPS on the boat. That way your DSC is always available, and not dependent on a chart plotter, etc. Other things like anchor alarms can also benefit from an always-on GPS. Most AIS devices can transmit their GPS data to other devices, so that is indeed a viable source, as long as your AIS is on.
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Old 02-14-2018, 02:19 PM   #7
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Great point TwistedTree, the GPS call out is a really important one. As I'm re-wiring my GP32 to an upgraded GP33 I am specifically doing an always on NMEA2000 network and GPS due to the importance of that information.

You are correct on AIS, its 12.5w on the FA170

Has anyone actually seen/participated in a DSC emergency? Did the boat in distress actually show up on your plotter, etc?
Id test but the USCG might now like that
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Old 02-14-2018, 02:28 PM   #8
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Here's a list of coverage maps for "Rescue 21". ( It is supposed to be able to work 20 miles offshore with a 1 watt transmitter 2 meters above the water)

This does not show any coverage for Canadian waters....that may just be because its a USCG website.
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Old 02-14-2018, 02:59 PM   #9
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I would start with a Mayday on 16, then a VHF DSC, then if I wasn't busy putting out a fire or bailing or something, I would also send an AIS distress message. Then, if I really had time on my hands, I would get on the SSB. As a last resort, I would try to figure out who to call on the Sat phone.
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Old 02-14-2018, 03:04 PM   #10
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Yes, class A AIS transmits at 12.5 Watts, whereas your VHF radio in DSC distress mode will transmit at 25 Watts. When you select the distress function of your radio, regardless of how the power is set, it will automatically switch to high power; i confirmed this with iCom and i suspect all VHF radios do this.
Therefore, assuming your antennas have similar gain performance, your VHF will have a longer range.
I agree both are helpful in an emergency but if i was forced to choose between the two, I'd place the radio at higher priority (ensuring it has a GPS or position source).
Since you have a class A AIS, perhaps you know it transmits more frequently and it is advisable for the AIS to have its own antenna.
The other factors that help range on both systems is height of antennas and gain.
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Old 02-14-2018, 03:08 PM   #11
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True, although I guess I see SSB and VHF is the same, if you are more than 20 miles offshore id start with SSB (I think its 6215), the M802 has a DSC button as well.
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Old 02-14-2018, 03:09 PM   #12
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Quote:
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Yes, class A AIS transmits at 12.5 Watts, whereas your VHF radio in DSC distress mode will transmit at 25 Watts. When you select the distress function of your radio, regardless of how the power is set, it will automatically switch to high power; i confirmed this with iCom and i suspect all VHF radios do this.
Therefore, assuming your antennas have similar gain performance, your VHF will have a longer range.
I agree both are helpful in an emergency but if i was forced to choose between the two, I'd place the radio at higher priority (ensuring it has a GPS or position source).
Since you have a class A AIS, perhaps you know it transmits more frequently and it is advisable for the AIS to have its own antenna.
The other factors that help range on both systems is height of antennas and gain.
Correct, I have a totally dedicated VHF system for AIS (similar antenna quality/height to VHF), it uses the GP33 for GPS but has an internal GPS (which is actually externally mounted) as backup.
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Old 02-14-2018, 03:11 PM   #13
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I have received multiple DSC emergency calls, some tests, and some real. But I can't say whether they showed on the chart plotter or not.

And I have sent two DSC distress calls, though neither was intentional. Both were when the boat was brand new. An alert went off and was received by one radio, so out of curiosity, I checked it out. I was docked, and soon there were all sorts of boats coming over to us. Then I realized the MMSI was ours! I wasn't even near the radio at the time, so didn't set it off by accident.

The about a week later were were heading north passing LA and it happened again. I called the CG to let them know I was having equipment trouble and there response was, Oh, you again.

Long story short, it turned out one of my remote mics was spontaneously triggering DSC alarms.
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Old 02-14-2018, 03:15 PM   #14
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The VHF radio, the gps that feeds position data to the VHF/DSC radio and the NMEA interface need robust power supply. AIS is nice to have.
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Old 02-14-2018, 03:27 PM   #15
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Quote:
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True, although I guess I see SSB and VHF is the same, if you are more than 20 miles offshore id start with SSB (I think its 6215), the M802 has a DSC button as well.
Our Hf radio, an M802, has the DSC function hooked up with its own antenna. I researched the need 10 years ago when I installed it. HF DSC has a huge number of false alarms and I mean by more than 50% according to the reference I had at the time. I still hooked it since who knows but Ive never used or heard a distress call. When were off shore, the VHF and the EPIRB are our 2 devises for emergencies. If the situation calls for activating one or both, I need to be dealing with the emergency. Close to shore, the VHF, cell phone and EPIRB would be the three.
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Old 02-14-2018, 03:41 PM   #16
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Quote:
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Our Hf radio, an M802, has the DSC function hooked up with its own antenna. I researched the need 10 years ago when I installed it. HF DSC has a huge number of false alarms and I mean by more than 50% according to the reference I had at the time. I still hooked it since who knows but Ive never used or heard a distress call. When were off shore, the VHF and the EPIRB are our 2 devises for emergencies. If the situation calls for activating one or both, I need to be dealing with the emergency. Close to shore, the VHF, cell phone and EPIRB would be the three.
Hi Larry, just to be clear do you mean you would rely on the VHF for offshore or did you mean HF?
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Old 02-14-2018, 03:52 PM   #17
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Hi Larry, just to be clear do you mean you would rely on the VHF for offshore or did you mean HF?
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The VHF to see if there are other vessels that can here me, ie Mayday, after that, the EPIRB gets activated. In my opinion, if I have to activate the DSC on either radio its a real emergency. When we did our circumnavigation, the EPIRB and HF radio voice were the tools. In 12 years we never heard of a HF radio DSC initiated rescue.
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Old 02-14-2018, 03:53 PM   #18
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Got it, thank you! as someone who has lucky never had to be in that situation its super useful insight.

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Old 02-14-2018, 06:04 PM   #19
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At this point in time, there are many more vessels equipped with VHF than AIS. So for coastal cruising) DSC is going to be the the preferred "first alert", followed quickly by EPIRB next (MUCH wider range, but not boat to boat), then AIS.
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Old 02-14-2018, 07:18 PM   #20
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Agree....true, scary emergency.....DSC Distress Button and EPIRB/PLB as soon as possible.

As soon as the EPIRB/PLB info is received by the USCG, they start VHF briadcasts for you.

Everything beyond that should be down the list after personally being involved in the emergency.

This assumes you are not in a mass of boats someplace where a decent VHF call wouldnt get someone there in minutes.
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