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Old 04-07-2015, 13:10   #61
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Re: AIS Range

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Originally Posted by CarlF View Post
Electrical noise can reduce AIS range. My Vesper range jumps from 15NM to 45NM if I remove the fuse from the NMEA 2000 backbone power tap. Drops back to 15NM if I put the fuse back in.

Obviously there's some noise creeping in somewhere but it's the devil to find.
Yes, a high local noise floor will put a stop to weak signal reception. I spent many years chasing down sources of noise in my neighborhood when I was interested in working weak-signal Amateur Radio DX. Every time I removed a noise source, I opened up a new layer of signals that I could suddenly hear.

The good thing about both VOICE and AIS is they are not amplitude-modulation systems, but rather frequency or phase modulation systems, which offers some immunity to noise. But strong local noise can blanket even a good FM system and limit its sensitivity.

Re your NMEA-2000 network noise, perhaps you could try disconnecting individual drop cables to individual devices to attempt to isolate the device causing the noise.
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Old 04-07-2015, 13:57   #62
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Re: AIS Range

I have a DigitalYacht IAIS and antenna mounted at the top of my mast, 58 ft. I use a splitter and have a connection at the base of my mast. My typical Kentucky lake range is around 20 to 30 miles, but I see tows on the Ohio from just above Kentucky dam at 49-44 miles all the time. At Indian Key a year and a half ago, I saw three ships - class A near the Dry Tortugas, at ranges of 155 miles. They were all three in that range and were real targets.

When on the Tenn-Tom coming home with the mast down, I mounted a Shakespeare antenna to the coupling at the bottom off the mast and extended on two 5 foot sticks of 3/4 PVC. I was around 15 to 16 feet off the water. My typical distance was around 8 miles.

I am sitting in Lighthouse Landing Marina and my farthest target is on the Ohio at Paducah and is 17.69 miles. I routinely see Class A targets at Metropolis, IL from here. That's around 5 miles further. None are there today. On the Tennessee River it depends a lot on terrain and the character of the river. There is plenty of time when I see one as generally the closure rate is 10-12 knots.


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Old 04-07-2015, 17:38   #63
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Re: AIS Range

Nobody is contesting the ability to see extremely far targets at times, only the claim of "routinely" seeing targets at 100nm+.

One thing to consider is that our Vesper keeps a large list of previously seen targets so that it can claim it has a fast acquisition time - if the target appears on power-up, it fully populates the info fields. I have noticed that it often treats these stored "targets" as real when activated and it takes a while before it finally admits they are not there.

Other than transient atmospheric conditions, repeaters, or extremely tall antennas, I am at a loss when it comes to explaining routine 100nm+ reception.

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Old 04-07-2015, 18:22   #64
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Re: AIS Range

"One thing to consider is that our Vesper keeps a large list of previously seen targets so that it can claim it has a fast acquisition time"

That was a feature of SOB software that I really liked. It kept a list of the static information based on MMSI. When a target shows up with the same MMSI then some of the static information (ship name and call sign) is filled in for the target rather than waiting for the static data transmission. That feature provided the comm info required for ship contact immediately after the target showed up.
Unfortunately for me, Digiboat, the publisher of SOB appears to have gone belly up. Now my go-to AIS application is OpenCPN., but it does not appear to have this feature.

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Old 06-07-2015, 00:20   #65
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Re: AIS Range

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Originally Posted by colemj View Post
One thing to consider is that our Vesper keeps a large list of previously seen targets so that it can claim it has a fast acquisition time - if the target appears on power-up, it fully populates the info fields. I have noticed that it often treats these stored "targets" as real when activated and it takes a while before it finally admits they are not there.
It keeps the static data for targets it has encountered so you don't have to wait for that data to be received in the future as it can take 6 minutes or so.

But this data is only used when it later receives a position report. For example, you encountered a target yesterday and it received the static data. It remembers that. Today you encounter that same target again but this time you don't have to wait to receive it's static data. However, what you are seeing is a "real" target for which it has received a position report. It has only filled in the static data from yesterday's encounter. Eventually it will stop receiving position reports and the target will be dropped off the app's display but the stored static data continues to persist for the next time you encounter it.

The "fast acquisition time" is a little different though. Apps, plotters, software, etc can only plot the targets it has received over time. So if your transponder has been running for awhile and then you fire up an app, you need to wait while the targets appear over time as each new position report and static data is received. But our apps are able to acquire all the active targets, positions, and static data at once. So when you fire up our app the whole scene appears immediately rather than over time.
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Old 06-07-2015, 05:57   #66
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Re: AIS Range

Thanks for making that clear Jeff. I guess those times that I have seen a long distance ship appear and then drop off was just transient ducting or similar, and not the AIS incorrectly thinking a ship still exists.

We have a Vesper transceiver, antenna 60' high wired with LMR400 and good connectors. However, we do not regularly receive ship signals from 100nm+. When we do, it is either a temporary thing or I can correlate it with known repeaters.

We do regularly see AIS signals from further away than we can make VHF voice contact, but voice is dependent on many other variables (different radios on both boats, different antenna systems on both, different sound fidelities, etc).

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Old 06-07-2015, 06:51   #67
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Re: AIS Range

ill add my 2c. we have the raymarine 650 through a splitter to masthead antenna at 68'
all works well and ranges are probably around 10M for yachts and 25M for ships in general. We are anchored so our current range is effected by the mountains, but you can see from the image below the ships went out of range at around 20M (the range ring) and the yachts to the South are about 10M before dropping out of range.
I like the Raymarine unit because we can USB it to the laptop to display targets on openCPN, which has a lot of useful features and information not available on the plotter. We can save AIS tracks, keep lost targets and their tracks for hours and save their tracks. This can be interesting to compare courses as well as various yacht performances. The saved tracks have full data including SOG. In the second picture we were anchored and saved a couple of the tracks to compare with our own. I was interested if other yachts were tacking upwind and upcurrent (2-3kn) at about the same speed and angle as us. Most of the yachts were motoring the rhumb line but I did see a couple of others over a couple of days that actually sailed and saved their tracks. Ours is in red.
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