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Author Topic:   Some Distress in Distress
jimh posted 02-08-2013 11:01 AM ET (US)   Profile for jimh   Send Email to jimh  
Distress Calling

Since the invention of wireless and going back to the time of the TITANIC, vessels in distress have been using radios to send out distress broadcasts. Over the decades—actually over more than century—the mode and frequency of the radio transmissions have changed, from spark telegraphy at low frequencies to digitally encoded messages at very high frequency. From 1912 to 2012 there has been one-hundred years of technical evolution. Boaters now have access to the most sophisticated shipboard radios ever available, and the USCG has the most sophisticated network of shore stations ever available to listen for those distress calls.

Beginning about one year ago, regulations in the United States of America required that every fixed-mount marine radiotelephone for the VHF Marine Band must provide the feature of and adhere to the provisions of the Global Maritime Safety and Distress System (GMSDSS) Class-D standard of digital selective calling (DSC). This standard provides for the sending of a distress call with the exact location of the vessel in distress encoded into the transmission. A vessel in distress with a modern DSC radio can send a distress message with their position just by pushing a button. In addition to the vessel location, the DSC distress call can also transmit the vessel identity, which will give rescuers the ability to look up details of the vessel, such as size, type, appearance, and contact information for the vessel owner from a database of registered vessels—if the vessel owner has registered his vessel and obtained a proper marine mobile service identity (MMSI). On the shoreline a network of hundreds of newly-installed linked radio receiver sites stands ready to receive the message and route it to a United States Coast Guard (USCG) radio-watchstander for response. The cost of creating these facilities has been around a $300-million for the Coast Guard's radio system (called RESCUE 21), and probably a few million for the radio manufacturers world wide to develop radios that comply. It seems like a great plan, and the money seems reasonably well spent, but is it working?

About two years ago (in February 2011) the United States Coast Guard issued an amazing statistic. Of the distress calls it had received using the new digital selective calling system, nine out of ten of the calls did not transmit the vessel position. The new digital selective calling distress system gave no more information than a conventional voice distress call. (It might have given less information: because the digital distress call is a very short transmission, it might not been as effective in allowing radio direction finding equipment to locate the source as a conventional voice MAYDAY call of longer duration.) The nine-out-of-ten statistic came out in a letter sent to an industry trade association, the National Marine Electronics Association (NMEA), urging them to help. The underlying problem of the failure of the DSC distress call system was the lack of an electronic data connection between the DSC radio and another electronic source of vessel position information. Marine electronic devices like radios, chart plotters, and global positioning system receivers are typically interconnected using a interface protocol promulgated by NMEA, the NMEA-0183 standard. On recreational vessels, the interconnection of devices using NMEA-0183 is typically left to the boat owner to perform. The USCG urged NMEA to help, to make changes in the standard to permit the interconnection to be done more easily by the average boater.

More recently (in February 2013) a USCG statistic was cited that is astounding: of more than 23,000 rescue calls (including telephone calls) to the Coast Guard in 2010, only 263 were DSC radio calls. These statistics are not specific to just recreational boats. The DSC calls could have been made on medium-frequency or high-frequency commercial ship radios. On further inquiry with the USCG, I found that in 2010 the RESCUE 21 system—the new nationwide radio system most likely now to receive a distress call from a recreational boat via VHF Marine Band radio—received about 3,000 voice calls for distress and only about 60 DSC calls for distress: only one distress call in 50 was made with DSC via RESCUE 21. (I spoke directly to a USCG officer in the Search and Rescue division who gave me this data.)

Statistics can be misleading. There is a reasonable chance there is some skew to these numbers. There very well may be a correlation between needing to make a distress call and not having your vessel equipped with a DSC radio that is properly interfaced. In other words, perhaps there are significant numbers of recreational boats that are properly equipped for DSC distress calling, but these boats make distress calls very infrequently. There could be a large fleet of boats with DSC radios, properly interfaced to their position locating devices, with properly registered MMSI information encoded in the radio, and these boats just don't make many distress calls. In contrast, boats that don't have DSC radios might be more likely to need to make a distress call. If these assumptions were true, it would create a skew in the data regarding the ratio of distress calls made by voice compared to DSC.

Also, in 2010 the RESCUE 21 system was not completely deployed. Not all areas of the USA were being covered by the RESCUE 21 system. And in 2010 the percentage of all recreational boats equipped with a DSC radio may have been small, perhaps as proportionally small as the statics suggest, that is, perhaps only one in fifty boats.

The federal government thinks that the investment in RESCUE 21 has returned the expected benefits. They rate RESCUE 21 as a five out of a possible five on their "investment dashboard."

Cellular Telephone

There has been a suggestion that the Coast Guard ought to provide some special integration of their radio-watchstanding service with the cellular telephone service, in order that mariners could contact the Coast Guard by using a cellular telephone. One of the arguments made in support of this concept is the widespread use of cellular telephones, creating a reasonable assumption that on most recreational boats there will be someone with a cellular telephone. The Coast Guard explains in detail why they do not advocate for mariners to use cellular telephones as a substitute for a properly installed VHF Marine Band radio that is compliant with Class-D DSC requirements (that is, the minimum fixed-mount radio that can be legally installed now in the USA). The USCG is correct. A cellular telephone is not a good replacement for a proper radio installation.

NMEA Interconnections

The National Marine Electronics Association promulgated their 0183 standard for interconnecting marine electronics many years ago. In response to the recent urging of the United States Coast Guard, NMEA has collaborated with equipment manufacturers and the Coast Guard (and others), and is in the process of revising their NMEA-0183 standard to include recommendations for a consistent color coding of conductors carrying NMEA-0183 data in a new NMEA-0183 Version 4.10. These recommendations are only voluntary, and do not carry any force of law. Compliance is expected to be high, but it may take some time to be achieved. For the technically curious, the new color code standard is as follows:

NMEA 0183 Color Code
Name Color Description
Talker A White Data-H
Talker B Brown Data-L
Listener A Yellow Data-H
Listener B Green Data-L
HS C Black Ground
Shield Bare (See standard for special handling)

While NMEA-0183 Version 4.10 will specify conductor color codes, it will not specify a standard connector. NMEA anticipates that more and more the interconnection of devices will be moving to the newer NMEA-2000 standard, which already provides for a standard connector and wiring arrangement. This has greatly simplified interconnection of devices for the average boater. In 2013 several new VHF Marine Band radios have been announced which will provide NMEA-2000 interfaces and will sell for around US-$200. These radios will provide NMEA-2000 connectivity at a much lower price than anything previously available. Interconnection of a radio and position sensor (like a chart plotter with GPS receiver) using NMEA-2000 is extremely simple. This should greatly improve the ability of the average boater to interconnect a modern Class-D DSC radio to their chart plotter and GPS receiver.

Boaters Need to Act

The RESCUE 21 system and the requirement of compliance with DSC Class-D for VHF Marine Band radios are two great ideas. The federal government has done its part in creating the regulations and building the radio network. The radio manufacturers have complied with the regulations and created modern and very sophisticated radios that sell for extremely modest prices—less than $200. It seems that the final link in this system is now in the hands of recreational boaters: boaters need to join the program. Recreational boaters need to review their VHF Marine Band radios to see if they are ready for the twenty-first century.

dfmcintyre posted 02-08-2013 02:19 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
Jim--regarding the use of cellular. Yeah, I understand the CG, but there's theory and then there's reality. And reality is always going to mean that people either in danger or watching it unfold will call using a cell phone. And, probably not to the Coast Guard, but they will simply hit 911.

Since a significant number of cell phones currently in use are phase 2 phones, a reasonably advanced 911 dispatch center should have equipment and software to capture the Lat/Lon.

Second thought - What does a GPS chip cost, and why do they still produce fixed mount VHF's without a model with one built in, thereby eliminating the need for any sort of external connection between devices? A number of handheld's have had that feature for, oh, gosh, years?

I don't remember where or when (will check my regional center soon) but I had heard at one time, an incoming 911 call cannot be transferred out of the center, say to the local or regional USCG center.

That wouldn't preclude making a three way call, and giving the CG center the L/L from the readout.

Will investigate further.


conch posted 02-08-2013 03:05 PM ET (US)     Profile for conch  Send Email to conch     
Several years ago we could call the Coast Guard by cellphone with dialing only three characters ,like a call to 911. But dialing [mentions the obsolete dialing shortcut] is not used any longer in the Keys to my knowledge. I thought it was a good backup to the radio.


jimh posted 02-08-2013 03:06 PM ET (US)     Profile for jimh  Send Email to jimh     
Don--You don't need to convince me; you need to convince the Coast Guard. However, I agree with them. It is better to use VHF Marine Band radio and the RESCUE 21 system as the primary method for distress signaling.

There has been a recent revision in global standards regarding DSC calling with handheld radios with their own GPS receivers. This should lead to more handheld radios being available with DSC and their own position-finding source.

For any boat that might venture into waters where the Coast Guard is on alert for distress calls, the primary method of alerting the Coast Guard and other nearby vessels should be using DSC and VHF radio.

jimh posted 02-08-2013 03:10 PM ET (US)     Profile for jimh  Send Email to jimh     
That cellular telephone dialing shortcut is mentioned in the Coast Guard literature I linked to. It is explained as being a feature that is carrier-dependent and not available in all areas. This is a good example of why using a cellular telephone as a primary means of distress alerting is not a good idea.

Distress alerting using DSC and VHF Marine Band radio is a global system that works everywhere and is highly coordinated among all vessels.

jimh posted 02-08-2013 04:42 PM ET (US)     Profile for jimh  Send Email to jimh     
I just tried the obsolete dialing shortcut mentioned above which has been mentioned as a way to dial the United States Coast Guard using a cellular telephone. Using my cellular telephone and my carrier, my call was answered by an automatic announcement reporting that my call could not be completed as dialed. That is not particularly comforting in a vessel distress situation. If I were 25-miles offshore in Lake Huron, I probably would not even have cellular telephone service. And if I did, I would probably get the same message: call cannot be completed. Who wants to count on that system as a distress signaling method? Not me. Give me a good radio, DSC, and RESCUE-21 instead.
6992WHALER posted 02-08-2013 05:18 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
The number one reason cell phones are a poor choice for a "mayday" call is that even if you get the Coast Guard on the line, they are the only people who know you are in trouble.

A DSC distress call lets every boat with a DSC marine radio that is in range know that you are in trouble and your location.Even if they have not hooked their [chart plotter] to their radio, the radio will still display the caller's latitude and longitude.

Cell phones tell one person, VHF radios tell the world.

jimh posted 02-13-2013 12:16 AM ET (US)     Profile for jimh  Send Email to jimh     
According to a Summary Report of the GMDSS Task Force, the commercial towing company Sea Tow recently surveyed 11,850 registrants of their service to ask for a response from those who had a DSC radio with a GPS receiver connected. Sea Tow reported only 33 affirmative responses. Certainly there is probably some generally low rate of response to the survey, but it seems astonishing that only 33 responses were received indicating possession of a boat with a DSC radio that is connected to a GPS. That is a response rate of only 1 in 360 boats.

It should be recalled that the notion of a global distress alerting system was first begun in 1979, some 34 years ago. To be 34 years into the creation of the system and have such extremely low rates of compliance by recreational boaters with the necessary equipment and registration seems hard to comprehend. In particular, boaters who have affiliated with Sea Tow are probably boaters who are cognizant of the possibility of a distress situation arising when boating. One would think that among a sub-group of all boaters like that, there would be higher compliance.

The United States Coast Guard is likely to suggest that responding to distress calls not made with modern DSC radios using GNSS position data will be, in most instances, a more expensive response compared to responding to a distress alert that gives a good position for the vessel in distress and background information about the vessel from a registry. It would not surprise me to discover that there could be some consideration of making carriage of proper GMDSS equipment on recreational boats mandatory. Considering the low cost of a modern DSC radio with built-in GNSS receiver, the USCG could probably make a good argument about mandatory carriage.

jimh posted 02-13-2013 09:00 AM ET (US)     Profile for jimh  Send Email to jimh     
The new NMEA-0183 revision mentioned above that enforces a standard color code of conductors in a NMEA-0183 interface will be a big help to the average boater when trying to interconnect devices. At least the color code will become consistent, assuming there is wide compliance with the voluntary standard. The revised NMEA-0183 standard does not go as far as proposing a standardized connector. This is understandable if one considers the notion of how two devices would interconnect. If they each had a standard connector, the connector would have to be some type of genderless connector so it could mate with itself. There is some prior discussion on this approach:

NMEA-0183 Interface: Four-pole Genderless Connectors

There was also some investigation into low-cost two-pole male and female connector pairs, suitable for a marine environment. In this method, each NMEA-0183 interface would be terminated in two connectors, one of each gender:

NMEA-0183 Interface: Paired Two-pole Male, Female Connectors

I developed my own method for interfacing, which resulted in a universal system, permitted interconnection of older NMEA-0183 single-ended interfaces with newer NMEA-0183 differential interfaces in any sort of combination. In my method all devices are terminated in a connector which then mates with a specially wired backplane connector to accomplish the interconnections:

Universal NMEA-1083 Interface

Regarding the connection of individual conductors of small gauge, as typically seen in the cables used for NMEA-0183, we had this discussion:

Connecting ridiculously small wires

One of the reasons for preferring a connector on NMEA-0183 devices used on small boats is the typical practice of removing the devices from the boat when not in use. It is a common practice to remove expensive electronic equipment like a chart plotter or radio from a small boat when the boat will not be in use on the water. This is typically done as a way to protect the electronics from exposure to weather; on a small boat the electronics may be mounted in the open. Or it is done to avoid the risk of the electronics being stolen. It seems that having a connector on all cables associated with marine electronics used on a small boat is a generally good idea.

bluewaterpirate posted 02-13-2013 09:03 AM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
There are many reasons boaters don't have their DSC VHF's connected to a chartplotter. The two more prevalent reasons I hear are:

--the dealer they purchased their boat and electronics from didn't connect the the two components because they didn't see the need for it or they didn't know how to. In eastern NC many boat dealers don't have professional electronic installers in house. And,

--if connected correctly, the boat owner doesn't understand how to use the functionality and the immediate benefits of it.

As I moved out of the installation business and into speaking at club functions I have run into some interesting discussions with weekend boaters. The average boater is basically clueless when it comes to truly understanding their electronics and the embedded fuctionality. Ninety percent of weekend boaters I speak to would grab their cell phones in an on water emergency because they feel more secure contacting the police than the Coast Guard. Even when I explain Rescue 21 features and capabilities they still insist the cell phone would be quicker. I think the primary reason for this is the more local police and wildlife boats on the water than USCG boats.

I even had a boater stand up in a meeting and say he didn't connect the wires because he didn't want others to know where he was fishing. My response was when hailing the Coast Guard using your VHF would you not have to provide a position via voice. His response not as many boaters would take the time to write down the coordinates.

The crowd took care of the rest of the response. Interesting discussion.


jimh posted 02-13-2013 09:14 AM ET (US)     Profile for jimh  Send Email to jimh     
As mentioned above, the interconnection of NMEA-0183 devices has been made more complicated by the use of two types of electrical interfaces on the ports. In the early days of the NMEA-0183 the standard allowed interfaces to be single-ended interfaces, that is, a configuration in which a common ground was used as a reference level, and the data signal was conveyed on a single conductor whose voltage changed in reference to ground. In later versions of NMEA-0183, the standard required use of differential interfaces. In a differential interface there are two wires, and the signal is conveyed by the difference in voltage between the two wires, not in reference to ground.

The present standard continues to call for differential interfaces. Unfortunately it is very common to find a mix of interfaces in the equipment available, and sometimes a device with use both methods simultaneously. This has led to even more confusion on the proper interfacing of NMEA-0183 devices. If marine electronic manufacturers begin to adhere to the recommendations of the new standard and provide only differential interfaces, a great improvement will be made.

The history of NMEA-0183 is believed to be as follows:

Prior to NMEA-0183 Version 2.0, use of single-ended interfaces was allowed. In NMEA-0183 Version 2.0 and subsequent revision, use of differential interfaces was recommended. There has also been a revision of the allowed baud rate. Most interfaces operate at 4800-baud; a higher speed option of 38400-baud was introduced.

The NMEA organization is often not the best source of information about its own standards if you are looking for publicly available information. A good source for information about NMEA-0183 interfaces is available from the United States Coast Guard. See

jimh posted 02-13-2013 09:26 AM ET (US)     Profile for jimh  Send Email to jimh     
I have to observe:

Until the USCG declares a formal GMDSS Sea Area 1 designation, there should not be any complaints about lack of use of DSC distress calling. Once RESCUE 21 is in place and the USCG formally says to the international maritime authorities that it is keeping watch for DSC calls in the defined areas, then we will have a situation where greater compliance with GMDSS and DSC calling should develop among all boaters

Using a cellular telephone may be a workable method if close to shore. I don't think cellular coverage extends to the reach of RESCUE 21. Where we do a lot of boating there is seldom even good cellular coverage ashore in the marina.

K Albus posted 02-14-2013 03:45 PM ET (US)     Profile for K Albus  Send Email to K Albus     
In addition to connecting your DSC-enabled radio to a GPS source and programming in your MMSI number, you should also carefully review the distress calling instructions in your radio's owners manual. There is more to making a DSC distress call than simply pushing the "distress" button.

I recently reviewed the DSC distress calling instructions in the owners manual for my Standard Horizon GX2150 VHF radio, and I must say that the instructions are not altogether clear, nor do I believe they are complete. The following are my notes regarding some of the instructions.

The first two instructions are:

1. Lift the red spring loaded DISTRESS cover and press the [DISTRESS] key. The "DISTRESS ALERT" menu will appear on the display.

2. Press and hold the [DISTRESS] key. The radio display will count down (3-2-1) and then transmit the Distress Alert. The backlight of the display and keypad flashes while the radios display is counting down.

As these two instructions show, it actually takes two pushes of the distress key to send a distress alert, including holding the button down for several seconds on the second push. This may not be apparent to a novice user. I will make a point of explaining this to future guests on my boat.

Instruction #3 indicates that after the distress alert is sent, the radio will monitor channels 70 and 16.

Instructions #4 through #6 provide apparently conflicting information.

4. If an acknowledgement is received, select channel 16 and advise your distress situation.

5. If no acknowledgement is received, the distress call is repeated in 4 minute intervals until a DSC acknowledgement is received.

6. When a DSC Distress acknowledgement is received, a distress alarm sounds and channel 16 is automatically selected. The display shows the MMSI of the ship responding to your distress.

The first point of confusion is that instruction #4 states that you should manually change the radio to channel 16 after an acknowledgement is received, whereas instruction #6 states that the channel will be automatically changed by the radio.

The second point of confusion relates to instruction #5. If no acknowledgement is received, are you supposed to just sit there and wait while the distress alert it rebroadcast every four minutes? Or should you try making a Mayday call on channel 16 in the meantime? (I know what I would be doing.)

Instruction #7 provides the following instructions for after your distress alert has been acknowledged and your radio has been switched to channel 16:

7. Press the PTT button and state your name, vessel, number of persons on board and the distress situation, then say over, wait for a reply from a reply from the acknowledging ship.

So, you don't use "Mayday" when you begin your transmission on channel 16? Couldn't that lead to confusion for anybody monitoring channel 16 with a non-DSC radio?

The instructions for making a DSC distress call also do not provide any guidance for what happens to your DSC distress call after it has been acknowledged. Does it continue to transmit at four-minute intervals until cancelled, or does it stop transmitting after it has been acknowledged.

On a related noted, it's probably a good idea to read and become familiar with the instructions for cancelling a DSC distress call.

Unfortunately, there's no way to practice sending DSC distress calls. All you can do is read your manual and hope that you remember the procedures when an emergency arises. If you're the one having a heart attack, you better hope somebody else on the boat knows how to send a DSC distress call for you.

6992WHALER posted 02-14-2013 05:28 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
My Standard Horizon GX2100 has same as you procedure except:

Step 6, ....can be set to pause re-transmitting the distress call every 4 minutes by pressing the PAUSE soft key.

Step 9, To cancel the DSC distress alarm signal from the speaker press any key.

I think it is easier to remember the procedure if you learn the "Transmitting a DSC Distress Alert with Nature of distress" procedure. Using that procedure the first push of the distress button makes sense.

My Standard Horizon GX1700 procedure is the same as yours except it omits your step 4.

jimh posted 02-14-2013 05:30 PM ET (US)     Profile for jimh  Send Email to jimh     
Kevin makes a very good observation. I checked my radio. The procedure is even more complex. After the first push of the DISTRESS button, there is an option to select the nature of the distress from a list of choices. Once the nature of the distress has been selected, then the DISTRESS button can he held down. The countdown is 5-4-3-2-1.

Once the DSC distress message has been sent, a alarm will sound. I bet that will be comforting. There is no indication of the duration of the alarm. Later in the procedure the operator is advised that the alarm can be cancelled by pushing any key.

One of the requirement of Class-D devices is that their operation is simple and can be taught in ten minutes or less.

K Albus posted 02-14-2013 10:50 PM ET (US)     Profile for K Albus  Send Email to K Albus     
After reading the instructions, the procedure seems simple enough. I would guess, however, that most strangers to the system would think that all that is necessary to make a distress call is to simply push the distress button once and wait for help to come. It does not appear that pushing the distress button just once will do anything to help a boater in distress.

One more interesting note - the "Warning" sticker which came with my radio reads as follows:

WARNING - This equipment is designed to generate a digital maritime distress and safety signal to facilitate search and rescue. To be effective as a safety device, this equipment must be used only within communication range of a shore-based VHF marine channel 70 distress and safety watch system. The range of the signal may vary but under normal conditions should be approximately 20 nautical miles

The stickers seems to give the impression that radio should not be used if you are more than 20 nautical miles from shore, which of course is nonsense. The sticker is required for FCC compliance. It certainly appears that a better sticker could be designed, perhaps one with instructions for making a DSC distress call.

Tom W Clark posted 02-25-2013 10:50 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
The details are still coming in and we do not yet know their fate, but this story of a missing sailboat of the coast of California may be a cautionary tale: 2020428948_apusmissingboatersseasearch.html

[UPDATE: Call may have been a hoax.]

bluewaterpirate posted 02-25-2013 01:12 PM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
Mother nature is unforgiving enough without helping her out. Nuff said.
jimh posted 02-25-2013 04:36 PM ET (US)     Profile for jimh  Send Email to jimh     
The signal strength of the voice transmission in the recording sounds good. It suggests that if a VHF Marine Band radio with DSC and integration with a GNSS receiver had been aboard, the signal would have been strong enough to deliver a good signal for data, sending the boat position to the United States Coast Guard. (It is usually considered that the range of a particular radio for DSC transmission is probably greater than for reliable voice communication.) If they had a DSC radio they'd have had a Helo overhead in an hour.
jimh posted 02-26-2013 03:03 PM ET (US)     Profile for jimh  Send Email to jimh     
Following up to Kevin's comment about the warning sticker, its inclusion and its wording is required by federal law. Compare at 47 CFR 80.255 (b)

bluewaterpirate posted 02-26-2013 06:17 PM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
New twist to the [distress call] story: Call may have been a hoax.To see where this ends will be interesting.--Tom
jimh posted 02-26-2013 06:30 PM ET (US)     Profile for jimh  Send Email to jimh     
There is much higher rate of false alarms in voice distress calls than in DSC distress calls. This is another reason to switch to DSC for distress signaling.
Tom W Clark posted 02-26-2013 10:33 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark 2020434228_apusmissingboatersseasearch.html

The possibility of that distress call being a hoax occurred to me yesterday, not because they have not been found, not because the voice transmissions were (reportedly) calm, but because there is nobody missing a family and nobody knows a boat named Charmblow, Charm Blow, Charm Glow or Charmglow.

This take us off-topic a bit, but the problem with those transmissions being a hoax is that the Coast Guard (reportedly) used RADAR and radio direction finding equipment to place the source of the transmissions at about 60 miles off the coast, west of Monterey.

What are the limitations of radio direction finding equipment?

jimh posted 02-26-2013 11:22 PM ET (US)     Profile for jimh  Send Email to jimh     
If a transmission is received at only a single station, only a line of position can be determined. The accuracy of the bearing depends on the equipment. In theory, a bearing with accuracy of 2-degrees is possible even with a minimal Adcock Antenna, which has only four elements. The RESCUE 21 direction-finding antennas appear to have many more elements, more than a dozen. I believe that this may allow them to get a bearing faster, but I don't know if it allows them to get a bearing more accurately.

If a transmission is received by two stations, they each have a line of position. The lines of position cross, giving an approximate position for the radio call.

The range of a particular shore station is determined by its antenna height and the antenna height on the vessel calling. On a sailboat, it is typical to have a mast whose height is at least as tall as the boat's length. On a 30-foot sailboat you'd expect a 30-foot mast. If the radio antenna is mounted at the mast top, it would be at least 30-feet above sea level. The boat's radio horizon would be about 8-miles. If a shore station antenna is at 300-feet, their radio horizon would be about 25-miles. Communication should be reasonably reliable between two stations like that to a range of over 30-miles, and possibly farther under optimum conditions. Could you reliably get 60-mile range? I don't want to bet my life on that. But it could be possible under favorable circumstances.

andygere posted 03-01-2013 03:24 PM ET (US)     Profile for andygere  Send Email to andygere     
VHF radios are generally fairly simple devices, that provide a long service life. I think that there are a very large number of boaters out there that have a non-DSC radio installed that is functional and has plenty of service life remaining. It's a tough sell to say that it's not the latest technology, and you should throw it away and buy a new one that has one additional feature that most boaters don't think they will ever need.

I have three GPS chartplotters on my boat, and one fixed VHF radio that is not DSC enabled. I bring a handheld VHF radio as a back up. Despite good arguments presented here for installing a DSC enabled radio, I haven't done it. I suppose I'm counting on the fact that if I get into trouble, I'll be able to relay my position manually. I also suppose that in a real emergency, the PLB that I keep on my person will provide continuous locational information, and provides the distress call automatically as well. I think similar arguments could be made for requiring a PLB on all boats.

jimh posted 03-13-2013 02:15 PM ET (US)     Profile for jimh  Send Email to jimh     
The USCG has a library online of distress call recordings. Here is a one of the many distress call recordings: g2_itemId=575818

If you listen to the recording, the vessel in distress sends its position in the initial transmission, but the reception is slightly mutilated and the USCG has trouble copying. A few minutes later the USCG apparently must have listened to a recording of the transmission a few times, and they come back with a broadcast of the position of the vessel in distress. After the first two transmissions from the vessel in distress, it is no longer heard.

The position reported by the vessel is deduced by the USCG to be

29° 21.029' N
093° 48.907' W

The vessel reported they were sinking and were tied to "a rig," presumably some sort of oil rig platform.

Checking on my NOAA charts I see a structure WESTPORT-160-1 charted in position

29° 21.010' N
093° 48.886' W

which is very close to the distress position. It seems reasonable to assume that might be the rig they vessel in distress has tied itself to. This call occured in 2009.

The position of the vessel in distress is about 21-miles offshore from Sabine Pass entrance light. The USCG radio installation in that area is not specified, but it is reasonable to assume it is probably at "Johnson's Bayou", based on the present location of a RESCUE21 site at that location. The distance from the oil rig position to the Johnson's Bayou area is about 30-miles. This give us an idea of the range of the VHF Marine Band radio coverage. The signal from the vessel in distress is copyable, but it is not extremely solid or clear.

If you listen to the recording (linked above) you will hear transmissions from the USCG station are very confused and difficult to copy because the USCG station is back-monitoring itself by listening to some remote receiver which delivers the signal with considerable delay. The result is an awful echo and reverberation of the USCG transmission that, at times, makes it extremely confusing to copy. Toward the end of the recording the USCG station gives a position for the vessel in distress that is practically unintelligible as the initial numbers of the position echo back and are repeated with clarity equal to the initial voicing of the position. I found it very hard to copy the USCG operator because of this.

It is hard to believe that the USCG operator wasn't also hearing this echo, as he must have been talking into his microphone in the presence of a loudspeaker that was letting him hear what his transmission sounded like. This is extremely poor radio operating practice. You'd think a radio watchstander would be more skilled, and know to turn down the speaker when transmitting, when the back monitored signal was echoing with such a long delay.

jimh posted 03-13-2013 02:20 PM ET (US)     Profile for jimh  Send Email to jimh     
The library of audio recordings of distress calls is located at

I found it instructive to listen to some of these calls with an ear to hear if the vessel in distress gave enough details on the initial transmission. In some of the calls the vessel in distress is never heard again after the initial MAYDAY or distress call, and practically no useful information was included in the initial call.

The short window in which these vessels in distress had to make a MAYDAY radio call before they apparently sank, the radio became disabled, or the master abandoned ship, shows how important it is to be able to give information effectively in the initial call. You might not have more opportunity than the initial call to get your message sent.

Hoosier posted 03-14-2013 09:02 AM ET (US)     Profile for Hoosier  Send Email to Hoosier     
Interesting discussion. I would think that Rescue 21 would be able to automatically get a DF fix on the first detection of a voice MAYDAY given today's voice recognition technology. They might even be able to get a one station fix using some sophisticated multipath detection techniques.
jimh posted 03-14-2013 10:14 AM ET (US)     Profile for jimh  Send Email to jimh     
To speculate about a distress situation like the one presented in the audio recording (see above for link) in the context of present-day VHF Marine Band radios, digital selective calling (DSC), and the latest Coast Guard radio system, RESCUE 21, will be an interesting discussion. Let's consider what might be different in 2013 from 2009.

The biggest change in distress signaling is the readiness of the United States Coast Guard to receive and respond to distress calls via digital selective calling. If the boat in distress were equipped with a modern Class-D DSC radio, the distress call could be sent by activating the distress call feature on the radio. This would send the vessel position and distress alert to all stations listening for DSC calls. The USCG would receive the call in their RESCUE 21 system. They would immediately have the vessel position. It would probably be displayed on a chart plot in front of the radio watch stander. The radio-direction-finding capability of RESCUE 21 should also indicate a line of position or bearing to the source of the transmission.

In the recording of the distress, at least two other boats hear the initial distress call. One of the boats makes a transmission giving the USCG a partial fill on the missing position data. A second boat hails the boat in distress and asks them to give the number of "the rig" they're tied to. This is evidence that the distress call was received by at least two other boats in the vicinity. With DSC distress alerting, both of those boats would now also have received the exact position of the vessel in distress in their DSC radios. If the DSC radios on those boats were integrated with the chart plotter, then those boats would be looking at a plot of the position of the distress vessel on their chart plotters, too.

Toward the end of the recording, a good samaritan vessel and the USCG radio operator discuss the distress vessel's position relative to the good samaritan vessel. The good samaritan does not know his relative position. He tells the USCG, "...I'll have to get my pencil out, write 'em down [i.e., the position coordinates] and plot a course."

Today, there is a good chance that a good samaritan vessel that received the DSC distress alert and has a well-integrated chart plotter could very quickly know the distance and bearing from his present position to the distress vessel. The exact manner in which this range and bearing might be calculated will vary with the particular chart plotter. (I cannot say with certainty how my own chart plotter might present a distress call position, because I have never received a DSC distress call. By the way, this is not unusual. A USCG officer told me during 2010 they only received 60 distress calls via DSC and RESCUE 21. That is an amazingly low number. I am going to investigate a bit to see if I can simulate the reception of a distress call on my radio. I will use two radios, and connect the simulated distress radio to a dummy load so that the distress call is not actually broadcast on the VHF Marine Band to other stations.) I suspect that once a distress call is received, it should not be very difficult or time consuming to compute a course and bearing to the vessel in distress. I suspect my chart plotter might treat the distress alert position as a new WAYPOINT. If it does, there will be a "GO TO" button option available. Hitting GO TO will immediately calculate the course and distance to the waypoint. Assuming that is correct, then I would know in a few seconds what my position was relative to the vessel in distress. I won't need to get out my pencil, write 'em down, and plot a course. It will all be done by my chart plotter through its integration with the DSC radio.

The USCG has been bemoaning the lack of vessels having their radio connected to their GNSS receiver, citing statistics of only one in ten vessels in distress having that facility. That is step one in getting prepared for distress alerting. Step two is to integrate the radio to the chart plotter, to allow plotting on your chart plotter of the position of the vessel sending the distress alert.

jimh posted 03-14-2013 01:39 PM ET (US)     Profile for jimh  Send Email to jimh     
ASIDE: In the recording the vessel in distress is asked by a good samaritan vessel to identity "the rig" they're tied to by number. I noticed that on the electronic NOAA raster chart the many oil platforms in that area are shown but are not identified by name or number. On the electronic NOAA vector charts the oil rigs are shown and identified with various sorts of designators, like WESTPORT-160-1. The number of platforms shown on the chart is quite amazing. It is hard for me to imagine that someone would be able to know where all of them were by number or name unless they had a really good chart that identified them. The electronic NOAA raster chart does not identify them, and I assume that means the paper NOAA chart does not either.
Hoosier posted 03-18-2013 10:27 PM ET (US)     Profile for Hoosier  Send Email to Hoosier     
Not to muddy the waters, but what about muddy waters, the USACE jurisdictions? What use is DSC on the nation's extensive river system? Does the Army Corps of Engineers have some equivalent to Rescue 21, or even the capability to monitor for DSC calls? Given the massive amount of commercial traffic on the rivers there has to be some sort of vessel control/monitoring system in place.
jimh posted 03-19-2013 11:50 AM ET (US)     Profile for jimh  Send Email to jimh     
There was some prior discussion of how RESCUE 21's capabilities might be expanded to include coverage of the so-called Western Rivers. See

for the links. I do not think the elaborate direction-finding array antennas will be deployed for coverage of the river systems. In a river system a boat has to be somewhere on the river, which is a rather well defined location. This is unlike open water, where a boat could be anywhere, and use of radio-direction finding antennas at the Coast Guard radio sites is extremely helpful. Most of the traffic on the river probably knows what mile marker they just passed, so if in distress they can refer to their location by that designator.

I would expect that any new installation of radios by the Coast Guard or other federal agency would employ radios with digital selective calling. Since it is required by FCC rules that any fixed mount radio for the VHF Marine Band that is to be sold in the USA must be at least Class-D rated for DSC, I would hope that includes radios sold to the federal government.

jimh posted 03-19-2013 11:57 AM ET (US)     Profile for jimh  Send Email to jimh     
Dave's speculation about commercial trafic on rivers brings to mind the situation in Europe. There, on certain canals and in certain areas, radios must comply with the ATIS requirements. ATIS requires that the radio transmit a unique identifier code with every transmission. This was mentioned in a prior article, but let me repeat a bit here:

Many new VHF Marine Band radios are fitted for compliance with the European ATIS or automatic transmitter identification system. ATIS operation requires its own ATIS call sign, somewhat like a vessel's MMSI, which is generated from the vessel's radio call sign by a numerical encoding. It is used on inland water ways in Europe in certain countries. At the end of every voice transmission from the radio, the ATIS numerical call sign is appended and transmitted in a short burst of frequency-shift keying. This positively identifies the source of the transmission.

The ATIS is an outgrowth of the Regional Arrangement on the Radio-communication Service for Inland Waterways or RAINWAT. See

for more details.

As far as I know, commercial vessels in the USA on navigable rivers are not required to conform to this sort of identification system.

Vessels that are required by regulations to equip with VHF Marine Band radios are also subject to meeting certain GMDSS requirements. Not being familiar with operation of a commercial vessel that has mandatory carriage of GMDSS, I don't know exactly what gear is needed. I would presume it would be at least a Class-D DSC radio, if not something more sophisticated.

6992WHALER posted 10-13-2014 11:41 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
Back in 2013 Keven said

"As these two instructions show, it actually takes two pushes of the distress key to send a distress alert, including holding the button down for several seconds on the second push. This may not be apparent to a novice user. I will make a point of explaining this to future guests on my boat."

Not true, tonight I just pushed the emergency button on my Standard Horizon HX851. Even though my owners manual states it takes two pushes to make the emergency call when I pushed once and held it down the radio started to count down.

If you push it and let go you can set the emergency type, and then push and hold for the count down. If you push it and just hold it it makes the emergency call with just one push.

It is easy to test, I just flip up the cover and pushed and held the emergency button in making sure to let go right away after it started to count down.

jimh posted 10-14-2014 06:19 AM ET (US)     Profile for jimh  Send Email to jimh     
I have also tested the DISTRESS button on two or three VHF Marine Band radios with digital selective calling (DSC), and I report the results in


I think it a good idea for boaters to become familiar with the DISTRESS button on their DSC radios in case they ever actually need to use it. They might be surprised by the reaction that occurs when pushing the button and exactly what needs to be done to initiate the distress alert call.

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