USCG Rescue 21 range

[Jackdaw]

The VHF MAYDAY call from the stricken Catalina in Lake Superior got me thinking. That is a VERY remote place in the world... Basically the middle of Lake Superior. How was he heard? The USCG provides the chart of the Rescue 21 network on Lake Superior. The boat was at the red dot. I'll put the technical details below.

This is VERY impressive. He was mostly likely heard by the station in Marquette, 55nm miles away! Note that the much closer station in Eagle Harbor is blocked from LOS (Line Of Sight) by the Brockway mountain, a large (by Midwest standards) ridge that runs along the upper shore the the Keweenaw. That would expain why the MAYDAW was uncorrelated, without another station triangulation would be impossible.

The 55nm distance alone creates a 101dB Free Space Path Loss on a VHF signal with expected antennas on both ends. Someones got some good radios at the Regional Fixed Facilities!

Also note that sailboats will typically do much better than this, with mast-top antennas and 25 watt radios. The chart is based on worst-case; a powerboater with a handheld.


Tech details on how the maps are created:
Charts showing predicted areas of VHF National Distress Coverage. Plots assume a mobile transmitter power of 1 watt or greater at sea level over water.

Predicted charts were created by C3CEN Remote Missions Systems Product Line using open source software RF Signal Propagation, Loss, and Terrain (RF SPLAT) analysis tool. The software calculates path loss based on the NTIA Longley-Rice Irregular Terrain Model (ITM) as well as the new Irregular Terrain with Obstructions Model (ITWOM v3.0). Terrain data was modeled based on the high resolution 1 arc-sec obtained from NASA’s Shuttle Radar Topography Mission (SRTM) flown in 2000. The coverage plots are displaying predicted regional coverage area based on line of sight between the Regional Fixed Facility VHF receive antenna and an antenna six feet above water level.

 

[dlochner]

That would expain why the MAYDAW was uncorrelated, without another station triangulation would be impossible.

And this is a good reason to upgrade the VHF to one with an MMSI number and GPS. The panic button will transmit the GPS coordinates and the MMSI, which should yield a faster response time.

 

[Jackdaw]

And this is a good reason to upgrade the VHF to one with an MMSI number and GPS. The panic button will transmit the GPS coordinates and the MMSI, which should yield a faster response time.

Indeed. And due to the digital nature of the DSC signal with its built in error correction and redundancy, is much more like to get through clearly in marginal conditions.

 

[Parsons]

...Also note that sailboats will typically do much better than this, with mast-top antennas and 25 watt radios. The chart is based on worst-case; a powerboater with a handheld. ...

I think that mast height and power are the difference. I've been in the middle of the lake and heard clear calls from sailboats that I knew were over 40 nm away. As a result, I'm very careful of when I transmit at 25 watts because they can hear you in Canada, fer god's sake! I wish that ship's radio's were made with some middle power setting between one and 25, like the 5/6 that are on our handhelds.

 

[Jackdaw]

I think that mast height and power are the difference. I've been in the middle of the lake and heard clear calls from sailboats that I knew were over 40 nm away. As a result, I'm very careful of when I transmit at 25 watts because they can hear you in Canada, fer god's sake! I wish that ship's radio's were made with some middle power setting between one and 25, like the 5/6 that are on our handhelds.

LOS really is the name of the game. A GPS satellite broadcasts at 35 watts, and we 'hear' it 12,000 miles away.

 

[dlochner]

I think that mast height and power are the difference. I've been in the middle of the lake and heard clear calls from sailboats that I knew were over 40 nm away. As a result, I'm very careful of when I transmit at 25 watts because they can hear you in Canada, fer god's sake! I wish that ship's radio's were made with some middle power setting between one and 25, like the 5/6 that are on our handhelds.

Antenna height of both the transmitting and receiving antennas makes the most difference. Power allows the transmitter to "blast" through any noise on the frequency and improves signal clarity.

Another phenomena occurs when a very cold layer of air lays over the lake, the RF signal bends and follows the cold air, this allows for longer transmitting differences, but is unpredictable.

 

[Scott B]

I think that mast height and power are the difference. I've been in the middle of the lake and heard clear calls from sailboats that I knew were over 40 nm away. As a result, I'm very careful of when I transmit at 25 watts because they can hear you in Canada, fer god's sake! I wish that ship's radio's were made with some middle power setting between one and 25, like the 5/6 that are on our handhelds.

Of course we can hear you in Canada...... we've got you covered.

 

[Jackdaw]

Of course we can hear you in Canada...... we've got you covered.

I like how this shows if you climb up the aforementioned Brockway Mountain, Thunder Bay will hear you!

 

[Parsons]

Now if I can convince Sarnia Coast Guard to turn their gain down a bit ... we hear them TOO well in Saginaw Bay. I appreciate knowing they're over there when I'm crossing though, if the need ever presents itself.

 

[rgranger]

The VHF MAYDAY call from the stricken Catalina in Lake Superior got me thinking.....

Is this the same boat from your "On the Bricks" post?

 

[Roland5048]

Another phenomena occurs when a very cold layer of air lays over the lake, the RF signal bends and follows the cold air, this allows for longer transmitting differences, but is unpredictable.

Very true. But, it doesn't necessarily have to be very cold layers of air. Maybe relatively colder air. I have picked up transmission from as far away as Key West, 225 nm south of here. And those were not the CG transmissions. I can regularly pick them up at 50 - 100 nm.

 

[justsomeguy]

Is this the same boat from your "On the Bricks" post?

Yes.

 

[Gunni]

I have received USCG Rescue21 transmissions WAY off the southeast coast as they were looking for a mayday transmission. Maybe 75 miles offshore. It is remarkable, but I have little confidence they could have received me if I had anything to report. My fixed radio at the nav station has a thingy-dingie that can be connected to my GPS antenna and turn it into a DSC radio but it sits unused. None of my handhelds have DSC. I guess I could upgrade all these radios. But my solution is and has been a ship's sat EPIRB and individual PLBs. Once the kimchee hits the fan and I am stepping off I don't want to be at the mercy of some Coastie watchstander, or the range of my handheld. I want to light up SARSAT real good!

 

[Scott B]

Now if I can convince Sarnia Coast Guard to turn their gain down a bit ... we hear them TOO well in Saginaw Bay. I appreciate knowing they're over there when I'm crossing though, if the need ever presents itself.

CCG Radio Station in Thunder Bay was closed down couple years back. Sarnia is now handling Superior, Huron, Georgian Bay, Erie so have to turn up the volume to be heard....

 

[SG]

In furtherance to other comments above, and in some cases in question to some comments...
Digital vs. Analog

• I don't believe that digital signals are not easier to decode or reliable in marginal situations than analog ones
  • Take your TV signals, a digital signal just craps out vs. an analog that is partially available a much longer range. Analog cell phones have much greater range than when the signal becomes digital. (of course the power drain may be greater when your cell phone is using "analog" mode; and, I'm not sure all phones even have an analog option these days?)
  • You have parity checks that block the partial digital decoding -- you get the whole message or "nothing". (A somewhat garbled message is better than none if you're in a desperate way.)
  • Once an "alarm" signal comes through, on a DSC, it's clearly alarming!!!

• A DSC with an automatic alarm mode function does have the huge advantages because
  • Fisrt, the DSC alarm triggers a "relay" function that immediately using every DSC radio a repeater; and,
  • Secondly, it repeatedly send the signal out so you have a greater likelihood that the call might be heard.
• I wouldn't depend on just the digital signal -- but I think one problem is that once you activate it, I believe you have to go through a "cancel" rig-a-ma-role to use the analog function. I haven't hit the emergency function on our Icom DSC radio (and hope not to have to) -- maybe I should bone-up on that.

High antenna location and power

• Height is paramount.
  • It's not only your height, but the height of the "other" antenna on the other end.
  • They may hear you, but you might not hear them.
• I'm not sure about the comments about high power being less relevant. I agree at moderate distances in "normal" weather; however, I don't think that's so at longer distances or in rain/fog/etc.
• Gain on the antenna and whether your mast is "vertical" vs. heeled over is also a factor. It does make a difference, or everyone would have broader range antennas.

Handhelds

• Range is pitiful at best unless you're in ideal circumstance.
• The attitude of the antenna does make a difference
• Save your battery in an emergency if you can. (Obviously, if you're in a dingy being blown out to sea, you need to call while someone might still hear you. But even then, you should be mindful that your battery only will last so long.)

We too have heard odd situations where "skipping" has allowed us to here VHF signals well over a hundred nautical miles away. (Coast Guard in Long Island Sound distinctly in Chesapeake Bay -- maybe closer to 200 nms than 100 nm.) I've had digital alarms from far away -- but it's tough to tell whether they were just relayed by a bunch of boats.

The AIS, which uses VHF signals (albeit on a different frequency with less interference), has picked-up data over 50 nm away on cruise ships and freighters. The AIS uses a low power VHF signal. Ours is multiplexed off of our masthead VHF antenna.

I've routinely communicated with such vessels at 25-30 nms offshore when there aren't others "stepping on my signals". The top of our antenna is about 75' above the water (when we're not heeling).

With a handheld, I find communication spotty at most distances over a mile away to larger vessels -- to other boats, it depends. DON'T grab handheld if you can use a base station except for close-in work or in an emergency.

I've had boats that were calling for help, and we were trying to render assistance where the person calling wouldn't speak on the fixed station VHF and could -- they used their handheld. If you can, don't use "a pistol, when you have a rifle available" in an emergency. I routinely will ask someone if they're using a handheld at the first sign that we can't communicate. Also, if Channel 16 has traffic, in an emergency you either need to "CLEAR" the channel by using a "MAYDAY", or just switch to a clear frequency if people are "stepping" on your signal.

 

[JohnShannon]

Low data rate digital signals with error correction and fancy stuff like that can transmit incredible distances on low power. Radio hams can go coast to coast on a few watts, on Morse code.

 

[Brian D]

I can hit a repeater at 68mi from my home on my handheld with 2 watts. Yeah, height of the antenna is paramount, not power. Although power has some advantage, but I think mainly in how "clear" one's signal is received. However, gain trumps power any day of the week. If I were to take my antenna and add a reflector or a director to make it a two-element antenna then I have just increased my gain. OK, got it, hard to do on a boat, but not on a fixed land station.

Just to add, a 36' Catalina was headed to Hawaii and had issues 200nm out. Their mayday was picked up in San Diego and Coast Guard was dispatched. But my point is, don't underestimate VHF communications.
http://www.equipped.org/0698rescue.htm

By the way, I also use VHF to talk to LEO satellites (1200mi) with a whip antenna.

 

[Jackdaw]

In furtherance to other comments above, and in some cases in question to some comments...
Digital vs. Analog

• I don't believe that digital signals are not easier to decode or reliable in marginal situations than analog ones
  • Take your TV signals, a digital signal just craps out vs. an analog that is partially available a much longer range. Analog cell phones have much greater range than when the signal becomes digital. (of course the power drain may be greater when your cell phone is using "analog" mode; and, I'm not sure all phones even have an analog option these days?)
  • You have parity checks that block the partial digital decoding -- you get the whole message or "nothing". (A somewhat garbled message is better than none if you're in a desperate way.)
  • Once an "alarm" signal comes through, on a DSC, it's clearly alarming!!!

This is not a great comparison.

First, the TV example does not factor that you are comparing 10x the data in the digital stream in less bandwidth than the analog uses. As a non-mission critical use, they simply prioritize capacity over reliability.

High-reliability digital formats can include error detection and forward error correction. This makes the signal MUCH more reliable than analog, and in addition, you can be sure that if it checksums correctly, you have the right information. Its also machine understandable, a huge advance. There really is no comparison vs analog in this regard.

 

[plenny7]

You have parity checks that block the partial digital decoding -- you get the whole message or "nothing". (A somewhat garbled message is better than none if you're in a desperate way.)

I have to disagree with this statement. A garbled message can leave out important words, which completely change the message:

"I am not in need of immediate rescue" is not the same as "I am <garble> in need of immediate rescue"​

 

[Brian D]

@plenny7, most good digital communications require an <ACK> or have a checksum at the end of the packet. Not saying it is Packet (AX.25 protocol) but as in a data packet. So the chance of a message getting that garbled is minimal, but not impossible. I understand the checksum can be damages, or there is ACK spoofing going on at the transmitter end.

Too many variables involved. I personally have not looked at the protocols involved in Marine Digital communications, but no doubt it is very standard.