EPIRB or PLB Assisted Rescue: First-person Accounts

[jimh]

The marine electronics manufacturer ACR Electronics, Inc., has an interesting website devoted to customers who have used one of their many 406-MHz beacon products to summon help in a distress situation. They call this their "Survivors Club." The website offers first-person accounts from customers that describe how they were rescued from a distress situation by use of an ACR product. One such first-account describes a recent boating trip. I summarize the events below:

On May 28th, 2017, two adults and two children (ages seven and nine) departed Jetty Park near Port Canaveral, Florida, in an undescribed outboard engine boat, and headed offshore to do some recreational angling for dolphin. The sea state was predicted to be waves of one to two feet at eight second intervals, and the intended destination was an area known locally as "The Cones." On the way to their destination, the sea state was worse than predicted, so they stopped short, and began trolling. When angling for about an hour was unproduction, the boat got underway again, and resumed a course offshore for the original destination.

At this point the helmsman noticed the boat seemed to be heavier and trimmed down by the stern, but this was attributed to the crew weight distribution and the higher seas. Upon arrival at "The Cones" the sea state was now three to four feet. Angling resumed. When the helmsman left the console and went to the stern, he observed that the engine splash well was taking water over the transom from the waves, and the water level was high enough to submerge the boat's batteries. Only about one-inch of freeboard remained at the transom. An electric bilge pump was started. In an interval described as "within minutes" water was coming over over the transom and downflooding the cockpit

At this point the VHF Marine Band radio was used to make a voice distress alert broadcast (MAYDAY) on 156.800-MHz (CH-16), and PFD's were put on by the crew. After several broadcasts of MAYDAY, a reply was received and an attempt was made to communicate by voice the position of the vessel in latitude and longitude. Several repetitions were necessary to communicate the position of the vessel to the other station. (The account does not identify the other station.)

In anticipation of the boat sinking, all crew moved toward the bow. The boat had poor stability at this point, and the movement caused the starboard gunwale to submerge, accelerating the downflooding. Preparations to abandon ship were made. One adult went into the water first, then the two children joined him, and the second adult prepared to leave the boat, collecting a flare gun and a Personal Locator Beacon (PLB) with GPS receiver.

There were boats in the vicinity, and three flares were fired. (The account does not describe the type of flare.) Then the last adult went into the water. One more flare (their last) was fired, and the PLB activated.

In the next approximately 45-minutes the first-hand account describes that four to five other "fishing vessels" (probably other recreational boats engaged in angling) passed nearby without stopping to render assistance, causing concern that no organized search effort was underway. After about one-hour in the water, the children began to show signs of hypothermia. Finally, after about two hours in the water, a USCG HC-144 aircraft was sighted. The account of the rescue ends there.

In the aftermath of the rescue, the USCG is said to have never received their VHF Marine Band voice MAYDAY distress alert broadcast on CH-16 or any sort of relay of it from other boaters. The presence of the USCG on the scene is attributed entirely to the reception of the 406-MHz distress beacon transmitter that was activated when they abandoned ship.

(The original first hand account appears on the ACR website.)

More first-person accounts of rescues attributed to use of 406-MHz distress alerting beacons, either EPIRB or PLB devices, can be found at

https://www.acrartex.com/survivors/

[jimh]

In modern form, both a Personal Locator Beacon (PLB) and an Emergency Position-Indicating Radio Beacon (EPIRB) transmit their distress alert on the internationally coordinated 406-MHz frequencies. The difference between PLB and EPIRB devices is the nature of their registration. A PLB is registered to an individual person; an EPIRB is registered to a vessel.

PLB devices are usually smaller and are intended to be worn or carried by an individual. EPIRB devices are usually larger and can be mounted on a vessel in a manner that permits self-deployment if the ship sinks, and the EPRIB can be automatically triggered to begin transmitting by immersion in water.

The distress monitoring system is a global system, and it works just about equally well in any area due to coverage by satellites. The COPAS-SAR system was described recently in another article. See:

EPIRB and SARSAT: Next Generation MEOSAR
http://continuouswave.com/forum/viewtopic.php?f=9&t=1703&p=10187

In the account of a boating rescue described above, the ACR device used was their model ResQLink™+ PLB. This device retails for about $370 and is described as a buoyant, GPS-enabled rescue beacon. To use the device requires the operator to perform two steps: deploy the antenna; activate the device. Because this PLB contains its own GNSS receiver, the device can transmit its position to the rescue system in its distress alert message.

[jimh]

Regarding the distance offshore in the first-hand account mentioned above, there is no clear statement provided, other than the boat was in the vicinity of a fishing destination known locally as "The Cones." [The position is later deduced from some aerial photo images; see additional comments in follow-on postings below--jimh] This destination appears to be part of a reef formation known as the Oculina Reef. The general description of the reef system is that it's located about 20-miles offshore. If we assume the vessel in distress that is described in the initial article was 20-miles offshore, one would have anticipated that it was within the range of coverage of the Coast Guard's RESCUE 21 radio system. The RESCUE 21 system is designed to cover 90-percent of the sea area within 20-miles of shore, and to be able to detect a transmission of one second duration from a transmitter of 1-watt with an antenna height of 2-meters above the sea. Many portions of the RESCUE 21 system extend their coverage well beyond the 20-mile range specified. Transmissions of longer duration and with higher power will proportionally improve the likelihood of reception by the RESCUE 21 system. The system also monitors for DSC distress alert broadcasts.

If a reader has better knowledge of where the fishing site called "The Cones" is located, please pass it along.

Another troubling aspect of the narrative: the apparent failure of other boats in the vicinity to have been monitoring 156.800-MHz with their VHF Marine Band radios. The boat in distress made several MAYDAY broadcasts. Yet they report that four or five other vessels passed in sight of them while they were in the water without apparently being aware of their distress. This affirms, again, the importance for ALL boats to maintain a radio watch on 156.800-MHz when underway.

A further problem is the apparently lack of a DSC radio on the boat in distress. Transmission of a distress alert using DSC would have alerted any other boaters or rescue authorities in radio range of their position and their distress. This affirms, again, the wisdom of equipping with a DSC radio and with proper registration of the boat radio with an maritime mobile service identity (MMSI) number.

To the great fortune of the children and the two adults, there was a working PLB aboard, it was activated, and a rescue coordination center dispatched an aircraft to search for them. It was also very fortunate that the PLB used contained its own GPS receiver. This likely facilitated the finding of the boaters. Without a GPS-equipped PLB, the position of the distress signal origination would have to be computed by methods using Doppler Shift signal analysis. This takes time (as more than one pass may be needed) and has much less accuracy.

[jimh]

Here is the USCG Press Release about this incident. It confirms that the PLB was the device that alerted the Coast Guard to the distress situation:

News Release
May 28, 2017
U.S. Coast Guard 7th District PA Detachment Jacksonville
Contact: Coast Guard PA Detachment Jacksonville
Office: (904) 714-7606/7607
After Hours: (305) 318-1864
Coast Guard rescues 4 boaters 25 miles east of Port Canaveral

JACKSONVILLE, Fla. — The Coast Guard rescued four boaters Sunday 25 miles east of Port Canaveral.

Coast Guard 7th District Command Center watchstanders received a distress alert from a personal locating beacon at 10:30 a.m.

A Coast Guard Air Station Miami HC-144 Ocean Sentry airplane crew in the area diverted and located an abandoned vessel at 12:30 p.m. The aircrew expanded their search pattern and located four boaters in the water at 12:35 p.m. The aircrew directed a Coast Guard Station Port Canaveral 45-foot Response Boat-Medium crew to the boaters.

Two adults and two children were rescued and taken to Station Port Canaveral. All boaters were wearing life jackets.

There were no reported injuries.

The cause of the incident is under investigation.

The interesting aspect is the aircraft that found the abandoned boat and the four swimmers in the water was already in flight on another mission. This may have been crucial in the rapid response. The distress alert was received at 10:30 a.m., and two hours later the aircraft located the boat and the swimmers.

The Coast Guard also posted images of the boat, which was still afloat and in a normal orientation, although down by the stern.

boat.jpg (61.68 KiB) Viewed 6960 times

The camera was in position

Lat = 28° 27.853' N
Lon = 80° 06.050' W

The time stamp [which I have cropped out in the image to reduce the width] indicates 16:04:37L. If "L" is for local time, that would be 4:04 p.m. The Az/El data appears to be relative to the aircraft, and probably not a compass bearing.

Another image shows the 45-foot Response Boat on scene with the swimmers.

swimmers.jpg (58.43 KiB) Viewed 6957 times

The camera position is

Lat = 28° 29.723' N
Lon = 80° 07.550' W

and the time stamp is 16:31:34L, which perhaps would be 4:31 p.m.

The images were taken from an aircraft overhead and may be long telephoto shots, giving the position of the aircraft. But we can compute the distance between the two locations of the aircraft: 2.63-miles--quite a lot of drift away from the boat.

Since the PLB was apparently with one of the swimmers, the PLB should have been giving the position of the swimmers, not the boat. On the other hand, when the PLB was first activated, it was close to the boat. Again, because this PLB had a GPS receiver, the position of the PLB would be updated by the GPS receiver in the device, presumably in a subsequent transmission. I believe that once the PLB is activated, it transmits continually, so the position would be continually updated. (See more about the position update intervals in a follow-on article, below.)

As for the time, the local time in Florida in May would have been GMT-4 due to Daylight Savings Time being in effect. If the time stamps are interpreted as GMT time, then the local time would have been 12:04 p.m. (boat image) and 12:31 p.m. (swimmer image).

Since the narrative describes the PLB activation as 10:30 a.m. local and the first sighting of the aircraft by the swimmers as about two hours later or about 12:30 p.m., perhaps the time stamps should be interpreted as GMT time. If the time stamps are local time, the swimmers would have been in the water for six hours, which seems unlikely to have produced such a good outcome.

[Don SSDD]

I wonder if the submersion of the batteries interfered with their radio transmissions due to reduced or lack of power? Or maybe a short antenna limited the range of the radio?

Those are four very lucky people.

ASIDE [on a non-electronic topic]: I [took] a marine course which included a video by USCG. A link below is to part of it. These people were in much better physical shape than I am and than most of us. One thing most people have trouble doing is putting a PFD on in the water.

https://www.youtube.com/watch?v=J1xohI3B4Uc

[Note: the narrative explicitly mentions the crew of the boat in this distress put on PFD's before abandoning ship.--jimh]

[jimh]

I wonder if the submersion of the batteries ...or a short antenna limited the range of the radio?

The range of any radio communication is not affected by antenna length; antenna height is the most important factor. The image shows what appears to be a radio antenna of about 8-foot length mounted to the T-Top, which is probably at least 8-feet above the water. That is actually quite a good radio antenna installation.

But the power output of the radio transmitter would be reduced if the battery voltage was reduced. However, the narrative describes the radio as working well enough to communicate with some other (never identified) station, and being able to confirm successful communication of their position.

The real questions are:

--who was the other station
--what did the other station do to assist in the rescue

[jimh]

In the narrative the crew abandoned the boat because they believed it was unstable and could capsize, possibly trapping them under it. Since the boat was still afloat two hours after they abandoned it, they may have underestimated its buoyancy. This gives rise to another question: should the swimmers have tried to stay close to the boat while it was still afloat?

[jimh]

Using the position of the aircraft that took the image of the swimmers, we can find the distance offshore:

--almost 25-miles from nearest land

--almost 29-miles from the Port Canaveral Inlet

The USCG press release says the 45-foot Response Boat departed from Port Canaveral. Assuming the base is within a mile of the inlet, that suggests they had to make a 30-mile run out to sea. The sea state is described as three to four-foot high waves. Wikipedia says the cruising speed of the USCG 45-foot response boat is 35-MPH, and the top speed is 48.9-MPH. it seems reasonable to assume that the USCG 45-footer could have reached the location in an hour, and perhaps less if they ran at their top speed.

[jimh]

The position of the boat in distress as documented by the aerial photo is roughly

Lat = 28° 27.853' N
Lon = 80° 06.050' W

Now we compare with the RESCUE 21 radio coverage map for that area. Here is a portion of the low-resolution coverage maps provided by the USCG for that area:

RESCUE21Coverage.jpg (19.29 KiB) Viewed 6943 times

I have drawn a black horizontal line from shore using the 20-mile scale of the original image. We see that the coverage contour stops short of longitude 80-W, but does extend beyond the nominal system goal of 20-miles to seaward in that area. That the boat was in a position further than shown in the coverage map--the boat was just west of 80-degrees longitude--does not mean that the boat was necessarily beyond the range of the system. The described contour is for a signal of 1-Watt from an antenna at 6-feet, and for a duration of only 1-second. The narrative describes the boat as making several long transmissions on 156.800-MHz as MAYDAY broadcasts, and communicating with some other (never identified) station. The aerial image shows a boat radio antenna whose base is at least 8-feet above the water, and the transmitter is assumed to be a conventional fixed-mount 25-Watt model. With those parameters, you would expect that RESCUE 21 would have received the broadcast, at least on the installation at SAINT CLOUD. A single radio direction bearing from the SAINT CLOUD facility would not have give a position for the boat, just a bearing line. But the narrative describes the boater passing his position coordinates to another station. Someone had their position.

[jimh]

To go a bit further into the aspects of the locating beacon, the particular model in use was an ACR Model PLB-375, called the ResQLink + PLB. This device contains two transmitters:

--a 5-Watt 406-MHz transmitter, and

--a 0.050-Watt 121.5-MHz

The 406-MHz transmitter is designed to send a signal to the COPAS-SARSAT satellite receivers. There are two types of satellites in the system: geostationary earth orbit satellites (GEO) and low earth orbit (LEO) satellites. The GEO satellites give very good coverage, particularly at lower latitudes.

Since this particular distress situation was occurring at approximately latitude 29-North, a GEO satellite would be able to provide excellent coverage. For the approximate longitude 80-West, the GOES-13 satellite would be the closest GEO satellite to provide coverage.

The LEO satellites in the system are in polar orbits, but their relatively low altitude results in a limited area of coverage. Their orbital period is about 102-minutes. There is no way (that I can find) to know if a SARSAT LEO might have been in range of the beacon transmission we are discussing.

When a GEO satellite receives a 406-MHz distress alert transmission, there generally is no apparent motion between the transmitter and the receiver, so signal analysis using Dopper Shift methods cannot be used to deduce a location for the transmitter. But in this particular instance, the beacon transmitter was a modern design and interfaced with the devices internal GPS receiver, so the distress alert transmission included the position of the transmitter.

The 406-MHz signal is transmitted in short bursts of duration 0.25-seconds. For modern devices, once the device is activated it will transmit a series of six burst transmission at five second intervals. For the next 30-minutes the transmissions occur at 30-second intervals.

Beacons with GPS devices transmit what is described as "a rough (rounded)" position that gives the beacon's current position.

The 121.5-MHz transmitter is a low-power (50-milliWatt) signal that is transmitted continuously upon beacon activation. This signal is intended to be received by aircraft or other searchers using radio-direction-finding receivers. The range is, or course, quite limited.

Several years ago I visited a local USCG base that has a 45-foot Response Boat. We were given a tour of the boat. I do not recall seeing a 121.5-MHz receiver. I would expect that a USCG aircraft, even if not designed especially for search and rescue, would have a 121.5-MHz receiver. However, I found a seemingly authoritative website that mention the USCG Response Boat, Medium is equipped with a TAIYO TD-L1550 VHF direction-finding receiver. This particular receiver can receive the 121.5-MHz homing signal from an EPIRB or PLB. On that basis it is likely a good assumption that the locating of the the swimmers in the water may have been aided by the 121.5-MHz homing signal being transmitted from their PLB device.

The present transmission protocol being used for the 406-MHz beacon apparently only allows for a limited amount of position data to be transmitted in the message. One organization reviewing this protocol commented as follows:

COSPAS-SARSAT should revise or provide an alternative to the existing location protocol long message format to allow for transmission of location data resolution to at least 1 second. The current rounding of the location data deprives the search and rescue system of improved location resolution that already inherently exists within the GPS capability, but which cannot be taken advantage of due to the artificial limitations of the existing protocols. Improved resolution can only serve to improve chances for a successful rescue.

The COSPAS-SARSAT standard for location accuracy should be revised to require a greater degree of accuracy. Considering the current state of the art in GPS technology, and other similar technological options for location information existent or planned, the standard is unnecessarily and counterproductively low, as even very inexpensive GPS receivers provide data which is orders of magnitude better than the current 5 kilometers standard.

Cf.: http://www.equipped.org/406_beacon_test_summary.htm

The crux of this protocol limitation is that, even with a GPS-equipped PLB or EPIRB, the exact position of the transmitter is not being sent to the SARSAT system. There is quite a thorough review of the protocol and the problems with GPS reception when the GPS receiver is co-located in a small device that also has two transmitters operating. For more technical background see

GPS and GPS Limitations
http://www.equipped.org/406_beacon_test_background.htm#GPS

in the excellent article

406 MHz GPS Enabled
Emergency Beacon Evaluation
Background Information
http://www.equipped.org/406_beacon_test_background.htm

According to the cited source above, the position data transmitted is only updated at intervals of 20-minutes, and then only for the first four hours of activation of the beacon.

Another problem (discussed in the cited article) is the acquisition time of the GPS to first position fix. If the GPS receiver is operating from a completely cold-start condition, as would be assumed for a beacon with an internal GPS receiver that has just been powered on, the time to first position fix could easily be several minutes, and possibly longer, if the GPS receiver was not able to maintain continuous reception of the satellites in view. Without a current ephemeris loaded into the receiver, the receiver will have to undertake a hunt for satellites without being able to make a preliminary guess about which ones are in view. Also, the GPS satellite broadcast of the satellite almanac takes about 12.5-minutes to accomplish, and the GPS receiver must have a continuous copy of the message in order to receive it. For a receiver that has no stored copy of the almanac in memory, the time to first fix could be about 15-minutes, but this time interval is affected by the number of simultaneous channels the GPS receiver can track; more channels lead to faster times to first fix.

[jimh]

Regarding the problem I raised earlier about the crew of the boat going into the water while the boat remained afloat and subsequently becoming separated from the boat by more than two miles, the previously mentioned article on 406-MHz beacons gives some further details about the general search methods used by Search And Rescue (SAR) responders, and about the usefulness of updated position reports received by a beacon with a GPS. I excerpt a paragraph or two from that fine article:

The value of the ability to update location under current search and rescue protocols used in the U.S. SAR community is limited, even in the maritime environment where movement due to wind, current, and waves is the norm. Current protocol is to provide the initial location to SAR forces who launch on that information. In the vast majority of circumstances in response to a GPS-enabled 406 MHz alert, SAR resources will arrive on scene within an hour or two. Unless a new location is significantly distant from the original, they will not be provided with it enroute. Typically, they find the survivor(s) within viewing distance of the original location as drift is generally slow enough that they will not have moved a significant distance in the interval, or SAR on scene can quickly determine the direction and speed of drift and can thereby locate the survivor(s).

If the survivor(s) is a person in the water in a PFD, rather than an easier-to-detect vessel or life raft, then the probability of detection is much lower and the value of updated location information becomes greater. If the SAR resource that first arrives on the scene does not promptly locate the survivors, they may contact their operations control and should receive an updated position at that time. Movement due to drift is more significant a problem in blue water conditions far from land, where the time to arrive on scene may be measured in hours or even days. Extreme conditions can also increase the rate of drift to a sometimes surprising degree. In such instances, updated location has the potential to be much more valuable.

The article also comments on the use of the 121.5-MHz low-power beacon, as follows:

It is also expected that the on-scene SAR resource will use their 121.5 MHz homing capability to locate the survivors upon arrival, if necessary. In some instances crews do not even turn on direction-finding equipment unless they fail to locate survivor(s) initially. The instances when this tool is useful with 406 MHz location protocol alerts are relatively few because the overall location accuracy is so good. However, in those cases where it is needed, there are a number of potential problems with this strategy, not the least of which being both the poor overall performance of 121.5 homing in some conditions, and the poor performance of some aircrews in use of existing direction-finding equipment to quickly locate 121.5 MHz transmission sources.

Source: http://www.equipped.org/406_beacon_test_background.htm