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Old 15-07-2013, 22:32   #961
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Re: Freighters vs. Sailboats

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I learned early that keeping a constant eye on targets helps you keep track of specific targets. How many times have you been unable to monitor all the targets? Most operators only sporadically watch the radar, or electronic data. It is REALLY easy to lose track of and confuse targets.

Maybe this is part of the dilemma?
Absolutely. The radar operator on my boat also minds the helm, grinds the winches, washes the dishes, weaves the splices, monitors the VHF, records the log, and maintains a proper watch. Every now and again we give him a minute to go pee. That, inevitably, is when he loses track of a radar target.
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Old 16-07-2013, 00:48   #962
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They are going a mile in a couple of seconds?

OR, what is "marine traffic for fast vessels?
For fast vessels the internet feed can show them a mile or more behind where they really are, or where ais says they are which is pretty close at those speeds (30Kn on the thames)

http://www.marinetraffic.com/ais/
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Old 16-07-2013, 02:28   #963
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Re: Freighters vs. Sailboats

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This story of VTS, TSS and all reminds me of a story.

I was about 50 miles south of Long Island, off the NY coast.

About 0300 I hear a frantic call to the CG about: "This big ship almost hit me"

The end of the story was, when he relayed his position to the CG, (which he was hanging onto a fishing gear buoy) I plotted it. He was directly IN the middle of the Ambrose to Nantucket Safety Fairway Separation Scheme. He had NO idea not just IF he was in a TSS, But he was clueless as to his position, where he was, and where he was on a chart. The only thing he was certain of was: This ship almost ran me over.
Yet you were able to plot his location, with certainty. Something smells rotten.
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Old 16-07-2013, 02:47   #964
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Re: Freighters vs. Sailboats

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I am unsure why...eta would matter to yachters,
T/R/D. Several scenarios come to mind. F''rinstance, if I knew a freighter had x time from the approach to the sea buoy to the dock (or vice versa!) I could better expect his actions. In fact, a ship making way from the dock may come as a surprise given an unexpected rate of speed.
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Old 16-07-2013, 04:16   #965
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Re: Freighters vs. Sailboats

I've watched plenty of AIS targets overlayed on the radar display. The position error is negligible when we are talking about crossing situations. The update rates, even for Class-B are perfectly adequate.

AIS via the internet (MarineTraffic.com, etc)? This is completely inappropriate for real-time piloting. There are delays, and in the interest of reducing internet bandwidth, not all data is sent.

Nasa AIS receiver? If this is the early design, it is a single-channel receiver and only gets half of the messages sent (AIS transponders send their data on two different frequencies, alternating back and forth). While a single-channel receiver is useful, a dual-channel receiver gives you more frequent position updates. No ship carrying a transponder (A or B) uses a single-channel receiver, all transponders include a dual-channel receiver. If you choose to carry a single-channel receiver the main issue you will have is that it may take 12 minutes or more before you see the ship's name (etc).

Class-B AIS transmit power is 2 Watts, and the range typically exceeds six miles. With bad antenna placement at both the transmitter and receiver you might cut out beyond three miles.

Class-B transponders do not sent navstat (moored, underway, etc) information. Class-A transponders do, but the crew has to manually change the status, as well as other static information. None of this makes any difference in collision avoidance.

The AIS problems described in the USCG document are in most cases ancient history, and have been largely cleaned up by now. I wouldn't base the Class-A vs Class-B decision on this.

GPS errors are typically much less than 65 ft (?!?) Even if they were that bad, would a 130 ft error really make a difference in your decisions during a crossing situation?

I really don't know how this relates to the original topic, I'm just correcting and commenting on some of what has been said.
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Old 16-07-2013, 04:44   #966
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Re: Freighters vs. Sailboats

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Yet you were able to plot his location, with certainty. Something smells rotten.
It IS amazing what one can do when one actually maintains a radio, and bridge watch! And, unlike you this is done on a two person bridge, with one licensed watchstander! I WISH I had a military bridge watch manning complement. One guy on the radio, one on the RADAR, one plotting, one on the helm, one on port lookout, one on bow lookout, one on stbd lookout, one more on security, and on and on and on. Where is the reality in military? Care to jump to civilian marine work? Ulp!!!!!

How so? The CG asked for his position, I quickly wrote it down. then when the cg asked for it again, I wrote it down again. You have to learn to smell the roses, in between the manure!
As the topic depends on, he actually HAD a GPS, and was too lazy (or incompetent) to actually plot a position on a paper chart. (like he actually had them aboard!!) to see where he really was.

I remember one of his laments to the CG. He said: "I am 34' long! I had my deck lights on. The ship HAD to see me." In his mind he was invinceable. Too bad no one was awake while they were tied off to a fishing buoy, in the middle of a TSS lane !
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Old 16-07-2013, 05:05   #967
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Re: Freighters vs. Sailboats

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T/R/D. Several scenarios come to mind. F''rinstance, if I knew a freighter had x time from the approach to the sea buoy to the dock (or vice versa!) I could better expect his actions. In fact, a ship making way from the dock may come as a surprise given an unexpected rate of speed.
The ETAs are not real time. they were likely inputted at the start of a voyage. The ETA may be to seabuoy, pilot station, dock, or anticipated docking time. Nevermind that in the US most people don't put UTC time (as is supposed to be) they put local. The destination and ETA would let one know where the vessel is generally heading, not its specific course and speed (which is needed to assess risk of collision.

ETA is an acronym. It stands for "Estimated Time of Arrival." How much more wishy washy info does one want to use for collision avoidance?
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Old 16-07-2013, 05:13   #968
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Re: Freighters vs. Sailboats

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Class-B AIS transmit power is 2 Watts, and the range typically exceeds six miles. With bad antenna placement at both the transmitter and receiver you might cut out beyond three miles.
Paul or anyone,

At what range do you normally pick-up Class A targets? Where is your antenna mounted?

Does anyone know when a Cl B unit displays a targets CPA etc, does it base that on the last transmitted ownship data, or does it continuously update? For instance if you make a large course alteration, does it show within a few seconds or does it lag until the next transmission?
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Old 16-07-2013, 06:35   #969
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Old 16-07-2013, 06:50   #970
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Re: Freighters vs. Sailboats

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Paul or anyone,

At what range do you normally pick-up Class A targets? Where is your antenna mounted?

Does anyone know when a Cl B unit displays a targets CPA etc, does it base that on the last transmitted ownship data, or does it continuously update? For instance if you make a large course alteration, does it show within a few seconds or does it lag until the next transmission?
There seems to be no 'minimum' or 'maximum'. Having seen ships 80 or 90 miles out, and seen ships within sight NOT on the display.

I will have to look at the manual, on my "A" but I don't recall it being updated via own ship course, speed, heading change. It only updates when it gets another 'ping' of data received.

From experience, I have worked in rivers and in terrain where mountains often makes AIS signal come and go. The RADAR (at least mine) has the ability to keep targets in memory, ( But, with an ANNOYING 'lost target alarm',) until you silence the alarm. So the last ais target reception holds the last info, crs, spd, cpa, tcpa,

One thing that Paul didn't write in his post, is that Class As transmit at 12mw. That is 6 times the power of the B.


Antenna location seems to be a hit or miss proposition. I know several of the tugs have played musical mount to get AIS to properly communicate with others. Trial and error.
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Old 16-07-2013, 12:34   #971
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Re: Freighters vs. Sailboats

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Originally Posted by cappy208 View Post
There seems to be no 'minimum' or 'maximum'. Having seen ships 80 or 90 miles out, and seen ships within sight NOT on the display.

I will have to look at the manual, on my "A" but I don't recall it being updated via own ship course, speed, heading change. It only updates when it gets another 'ping' of data received.

From experience, I have worked in rivers and in terrain where mountains often makes AIS signal come and go. The RADAR (at least mine) has the ability to keep targets in memory, ( But, with an ANNOYING 'lost target alarm',) until you silence the alarm. So the last ais target reception holds the last info, crs, spd, cpa, tcpa,

One thing that Paul didn't write in his post, is that Class As transmit at 12mw. That is 6 times the power of the B.


Antenna location seems to be a hit or miss proposition. I know several of the tugs have played musical mount to get AIS to properly communicate with others. Trial and error.
That's 12W, not 12mW (Watts vs milliwatts). But yes, Class-A transmits a 6x stronger signal. And more often in most cases. I wouldn't put a Class-B transponder on a high-speed vessel, but on my sailboat it's really OK. We just don't move fast enough for the 3-second updates to be a problem.

I agree with cappy re antenna placement and range. I've seen Class-A ships beyond 100 miles, but this is due to freak radio propagation, not a reliable range. Class-A: 10-20 miles, Class-B: 6-20 miles is what I typically see. My AIS antenna is on the upper spreader, perhaps 3 ft from the mast. It seems to work just fine, but I haven't done a pattern analysis.

Some programs (and plotters?) will do dead reckoning on an AIS target. It's an estimate of current position based on last received signal, course, and speed. It's not exact, but it is usually better than no DR at all. On my NavMonPc program, DR (extrapolation) is an option. I believe that CPA/TCPA are updated based on your vessel's current GPS data (usually updated once a second). I know that's how NavMonPc does it.

But these are details that in most cases make no practical difference. The position accuracy and update rate are good enough for collision avoidance at reasonable distances. If you are close enough that 100 feet will be an issue you shouldn't be looking at the AIS display anyway.
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Old 16-07-2013, 14:37   #972
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Re: Freighters vs. Sailboats

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For fast vessels the internet feed can show them a mile or more behind where they really are, or where ais says they are which is pretty close at those speeds (30Kn on the thames)

Live Ships Map - AIS - Vessel Traffic and Positions
Then it would seem to be worthless.
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Old 16-07-2013, 15:29   #973
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Re: Freighters vs. Sailboats

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Then it would seem to be worthless.
The internet feeds are NOT to be used for navigation information, just a general overview. They are sometimes right near realtime. BUT, often are really behind. This is said numerous times ON the MT website.
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Old 16-07-2013, 16:19   #974
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Re: Freighters vs. Sailboats

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The internet feeds are NOT to be used for navigation information, just a general overview. They are sometimes right near realtime. BUT, often are really behind. This is said numerous times ON the MT website.
I did not realize someone threw in internet feeds.

I guess I got lost in the details of the AIS workings and someone tossed in something else. From the internet.
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Old 16-07-2013, 17:06   #975
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Re: Freighters vs. Sailboats

Marine Traffic is COMPLETELY dependent on public input of the raw data. This is how MT works. The actual AIS info you and I receive is ONLY what YOUR and MINE ais actually receives. Not everyone receives the same identical info. your reception depends on how well your individual AIS is working. (thus the comments about antenna height, and user settings.)

On my particular tug, I had the original laptop hooked up to the AIS data plug, and the internet, which allowed me to transmit the vessels nearby me to MT. It was neat seeing the 'cloud' of coverage following me around in areas that have poor base station coverage. If you look on the MT website, it shows you how to download their raw data program, which will transmit it live, as long as you have internet coverage.

I had to stop, because the company replaced our laptops with NON serial port ones, and I can't figure out how to hook up a USB port to the NEMA plug on my AIS.
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