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Old 04-07-2015, 07:59   #46
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Re: AIS Range

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Originally Posted by FamilyVan View Post
It seems the 2200 maxes out at 10 miles. Also the acquisition range is limited to 30 miles.

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That's too bad. I wonder why they changed it from the GX2150's 15/50 ranges?

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Old 04-07-2015, 09:08   #47
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Re: AIS Range

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Originally Posted by MYTraveler View Post
The extreme ranges experienced by myself and others are Class A signals. In that context, and since voice is 25 watts and Class A is 12.5, isn't that only about a 3db advantage, giving AIS a net 9 DB advantage? If so, that would give AIS a significant advantage, especially given the digital nature of the signal, as noted above by colemj?
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Old 04-07-2015, 09:21   #48
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Re: AIS Range

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Originally Posted by TxCoastSailor View Post

I didn't notice a VSWR reading in ProAIS. I may have an older version and I need to update it.
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The VSWR is on the diagnostics page. See bottom right in the Dr appt photo below.
the ProAIS is a free download so check yours is up to date
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Old 04-07-2015, 09:23   #49
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Re: AIS Range

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Originally Posted by TxCoastSailor View Post
Mike,

Antenna is on the mast head about 58ft above the water. The antenna on a ship must be close to 100ft off the water. My distance to the horizon is about 9 mi and the ships distance to the horizon is about 12 miles so a large ship should be visible out to about 20 miles. My distance might come down a bit when heeled over. Not sure how folks are seeing targets at 100 miles or more. I thought VHF was strictly line of sight.

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The short answer is that differences in temperature, humidity and density between air masses can cause refraction or bending of radio signals, allowing them to be picked up at greater distances than line of sight. Here's the long answer:

https://en.wikipedia.org/wiki/Tropospheric_propagation
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Old 04-07-2015, 09:32   #50
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Re: AIS Range

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Originally Posted by MarkJ View Post
The VSWR is on the diagnostics page. See bottom right in the Dr appt photo below.
the ProAIS is a free download so check yours is up to date
You must have the Australian version of that app.

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Old 04-07-2015, 09:50   #51
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Re: AIS Range

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Originally Posted by colemj View Post
I agree with all of that except for one thing - AIS is a digital signal, so will be able to pull intelligent data out beyond analog voice (which will be variable depending on how well one hears and understands the transmission, as well as the fidelity of the output).

Earlier, I stated that AIS would have a very slight increase in range over VHF for this reason. However, it is just a gut feeling.

Mark
A voice signal with a signal-to-noise ratio of 12-dB is generally quite a good signal. The ability of a human operator to have clear copy of a voice transmission is subject to a lot of variation. One significant variation is the modulation level of the transmission. There are many VHF Marine Band radio transmission in which the modulation level is anemic, and the recovered audio is far lower that it could be if the transmitter were properly modulated. That problem does not exist in digital transmission.
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Old 04-07-2015, 10:09   #52
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Re: AIS Range

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Originally Posted by MYTraveler View Post
The extreme ranges experienced by myself and others are Class A signals. In that context, and since voice is 25 watts and Class A is 12.5, isn't that only about a 3db advantage, giving AIS a net 9 DB advantage? If so, that would give AIS a significant advantage, especially given the digital nature of the signal, as noted above by colemj?
Your addition is in the wrong direction. Comparing a CLASS-A AIS signal (12.5-watts) to a VOICE signal (25-Watts) means the AIS signal level will be 3-dB weaker. Since the receiver sensitivity for AIS is rated for 12-dB less sensitivity, the net difference between VOICE and CLASS-A AIS would be a 15-dB advantage to VOICE.
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Old 04-07-2015, 10:19   #53
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Re: AIS Range

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Originally Posted by jstevens View Post
...When a digital processor receives a signal, the data is validated by various integrity checks. If the checks are not 100% validated, the signal is thrown on the floor. Your AIS receiver may receive a transmission from 100nm, but most times it will not pass the checks and is discarded. The VHF voice receiver makes very few quality checks (e.g. signal below squelch level) before it sends the transmission to the speaker. The human ear and brain can often make sense from even a very garbled transmission.

So it makes sense that, all other things being equal, the effective range of a VHF voice receiver would be greater than that of an AIS receiver.

John
My understanding is the AIS signal is transmitted with a cyclical redundancy check encoding (CRC) that permits the detection of errors. There are some receiver systems that have been proposed that might be able to leverage the CRC into some sort of forward error correction system. But, I believe, as John mentions, that an a typical AIS receiver would discard data that appeared to be corrupt due to the CRC being in error.
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Old 04-07-2015, 10:26   #54
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Re: AIS Range

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Originally Posted by colemj View Post
There aren't range settings for the AIS itself - rather many AIS display units have range settings for how far targets will be displayed. For example, our AIS/VHF radio has a maximum display range of 15nm and our Furuno chart plotter 24nm. The AIS signals are receive further than those values, and are displayed on other devices that do not have limits.

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Originally Posted by carstenb View Post
yes, I meant that whatever your display instrument has a range setting for when it will show the AIS traffic. My furuno is set for 20nm

Got it, thanks guys, wasn't looking in the right place for the right thing. And now I know our MFD is set to ignore targets beyond 24nm.

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Old 04-07-2015, 10:32   #55
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Re: AIS Range

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Originally Posted by mikereed100 View Post
The short answer is that differences in temperature, humidity and density between air masses can cause refraction or bending of radio signals, allowing them to be picked up at greater distances than line of sight. Here's the long answer:

https://en.wikipedia.org/wiki/Tropospheric_propagation
Mike,

Thanks, I had a quick look at the Wikipedia article. I knew HF transmissions (Ham and marine SSB) achieved over the horizon coms via reflections from the ionsphere but I wasn't aware of the refraction of VHF and UHF transmissions in the troposphere. Good to know.

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Old 04-07-2015, 10:36   #56
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Re: AIS Range

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Originally Posted by continuouswave View Post
Your addition is in the wrong direction. Comparing a CLASS-A AIS signal (12.5-watts) to a VOICE signal (25-Watts) means the AIS signal level will be 3-dB weaker. Since the receiver sensitivity for AIS is rated for 12-dB less sensitivity, the net difference between VOICE and CLASS-A AIS would be a 15-dB advantage to VOICE.
I am sorry but I vehemently disagree that voice goes further or father that AIS. It just does not. You may think you have some argument behind you but its just wrong in the real world. Just utterly wrong.
you may have good intentions but the content you are producing is so wrong its dangerous.

Please check you facts and go do some experiments.

Mark
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Old 04-07-2015, 10:40   #57
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Re: AIS Range

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Originally Posted by Ann T. Cate View Post
Approx. 66 ft. antenna height; ships 100+ mi.; sailboats, varies, depends on how high their transmitting antenna is.
Agree:

Ours is Vesper Wartchmate 850; mast 80 feet. We see the Milwaukee to Muskegon ferry in Milwaukee - 70 miles, class A. Class B - not so far. We also see the large freighters & Coast Guard 20 to 60 milse depending on antenna height.
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Old 04-07-2015, 11:57   #58
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Re: AIS Range

To the original question. 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.

Vesper, great VSWR, LMR-400 coax, Vesper not connected to NMEA 2000 network, no other electronics turned on. Same behavior when I rigged a temporary antenna on a different piece of coax. Can't find a ground loop. Fun puzzle
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Old 04-07-2015, 12:54   #59
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Re: AIS Range

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I am sorry but I vehemently disagree that voice goes further or father that AIS. It just does not. You may think you have some argument behind you but its just wrong in the real world. Just utterly wrong.
you may have good intentions but the content you are producing is so wrong its dangerous.

Please check you facts and go do some experiments.

Mark
My only "content" is to look at signal levels and receiver sensitivity, and perform some addition. Could you please explain the basis for your determination that there is danger in this?

My "facts" come from:

--the specified sensitivity of the receivers per their manufacturers

--the ratio of transmitter powers between VOICE (25-Watt) and AIS (12.5 or 2-Watt)

What "facts" are you offering?

Also, please explain the basis for your assertions that AIS transmission have greater range than VOICE.
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Old 04-07-2015, 13:02   #60
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Re: AIS Range

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Originally Posted by TxCoastSailor View Post
...I knew HF transmissions (Ham and marine SSB) achieved over the horizon coms via reflections from the ionsphere but I wasn't aware of the refraction of VHF and UHF transmissions in the troposphere...
Radio waves are refracted in the atmosphere at low altitudes. You don't need to depend on the troposphere to get refraction of radio waves.

Radio waves, having much longer wavelength than light waves, are refracted more in the atmosphere than light waves, with the result that the distance to the radio horizon is almost always greater than the distance to the optical horizon. Many boaters use the term "line of sight" to describe the propagation of radio waves at VHF frequencies, but the truth is that radio signals at VHF frequencies travel beyond the visible horizon all the time.

Refraction in the atmosphere at VHF frequencies can routinely be enhanced by atmosphere that is stratified into different temperature layers. This occurs often when air is over water and there is no breeze.
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