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Old 11-10-2014, 16:05   #61
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Re: Class B AIS

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Originally Posted by Wotname View Post
Off topic so I won't go into detail here but the are several different reasons why aviation is still using double sideband full carrier AM for VHF coms but as generalisation, FM is always better than AM. We can take this via PM if you wish .
Please keep it here.... I'd like to know why they use AM....
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Old 11-10-2014, 16:42   #62
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Re: Class B AIS

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Originally Posted by El Pinguino View Post
Please keep it here.... I'd like to know why they use AM....
Like SSB, which is is a form of amplitude modulation, AM signals will not disappear if a slightly stronger signal appears on the same frequency. I believe that a very weak AM signal will also remain somewhat readable after an FM signal of the same strength is gone. These consideration could have safety implications for aviation. I think those are the main rational reasons for aviation to use AM, but I have no doubt that considerations of old tradition and installed base of (very expensive, hard to certify, and rarely replaced) equipment also played a big or even the major role. If there's more to it than that, then do tell, of course.

Naturally an FM signal always sounds better -- as long as you're reading it.
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Old 11-10-2014, 16:56   #63
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Re: Class B AIS

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Originally Posted by maine50 View Post
Milltec Marine is currently selling the vesper 850 (transmitter & receive on independent display w/ anchor alarm, can feed plotter also) for $763. This is a short time discount due to boat show">Annapolis Boat Show Special.
Thanks to all who recommended Vesper. I just bought the XB-8000 for $740 at Landfall Navigation.

This sounds like a great unit. I was already planning to buy a NMEA 2000 to WiFi adapter ($500) and a wireless router. This unit has both built in. It can also take NMEA 1083. So now all my NMEA data will be available to my phones, tablets and laptops.

It allegedly comes with an external GPS antenna. Note that the specs say internal GPS "receiver" not antenna. Some people reviewing it were confused by that.
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Old 11-10-2014, 19:43   #64
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Re: Class B AIS

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Thanks to all who recommended Vesper. I just bought the XB-8000 for $740 at Landfall Navigation.

This sounds like a great unit. I was already planning to buy a NMEA 2000 to WiFi adapter ($500) and a wireless router. This unit has both built in. It can also take NMEA 1083. So now all my NMEA data will be available to my phones, tablets and laptops.

It allegedly comes with an external GPS antenna. Note that the specs say internal GPS "receiver" not antenna. Some people reviewing it were confused by that.
Yes, it does come with an external GPS antenna, which is necessary for operation.

Be aware that it does not presently transmit all NMEA2000 data over wifi. They seem to be adding more, but a lot of common PGN's are still missing. It does translate NMEA0183 pretty fully. Their website lists the N2K PGN's that they currently support over wifi.

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Old 12-10-2014, 07:49   #65
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Re: Class B AIS

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You are leaving out the all-important issue of how the signal is read.
No, I explicitly mention that.

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This argument is only slightly more valid than saying that I've got a good AF amplifier in this radio, and can get this signal up to the same strength as that one by cranking up the gain, therefore they are the same.
No, there are limitation on the useful gain in both cases.

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Surely you have experienced for yourself the enormous practical difference in readability of a binary CW signal compared to a phone emission all sent and received with the same power and using all the same equipment?
The ability of an operator to get solid copy on a weak CW signal is extremely variable with the operator. And the same applies to copying weak voice modulation signals.

The ability of an AIS receiver to copy is set out in its receiver specification. At -107 dBm the AIS receiver has a packet error rate of 20-percent. This is a specified level of signal and of readability. I don't quite understand how you can insist that the AIS receiver is going to work a great deal better that it is specified.

The ability of a VOICE receiver is also set out in its specification. At -107 dBm it will produce a recovered modulation with 12-dB SINAD. A good operator, experienced in listening to communication radios, well get quite good readability of a voice signal with 12-dB SINAD. I don't understand how you can insist that a human operator can't copy much at 12-dB SINAD. The desired signal is more than ten times stronger than the noise. Good operators can copy voice transmission that are actually below the noise.

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Furthermore, we are comparing AIS not to AM or SSB, but to FM...
AIS is FM modulation. It is just not voice.

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Your username implies you are a fan of CW and you are obviously a knowledgeable ham.
I have been licensed for 50-years. My callsign is K8SS. I enjoy radiotelegraphy.
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Old 12-10-2014, 07:53   #66
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Re: Class B AIS

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Originally Posted by Dockhead View Post
Like SSB, which is is a form of amplitude modulation, AM signals will not disappear if a slightly stronger signal appears on the same frequency.
Full-carrier double-sideband AM, as used in Aviation, is not very good when two signals are competing. The carriers heterodyne, and there is a lot of distortion of the modulation. AM is not at all like SSB in that regard.

Aviation went to voice communication much earlier than maritime. Ship communication was by telegraphy when aircraft were using voice. FM was not a practical choice for voice until much much later.
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Old 12-10-2014, 08:21   #67
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Re: Class B AIS

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Originally Posted by continuouswave View Post
Regarding the range of an AIS Class B transponder:

For an AIS receiver, the sensitivity is typically specified as better than -107dBm for 20-percent packet error rate (PER).

A typical FM VHF voice receiver is rated for a sensitivity of 0.25-microVolt for 12dB-SINAD. Converting this voltage to dBm for a 50-Ohm antenna system results in -107dBm
Our specific VHF and AIS specs are 0.3uV 12db-SINAD for the VHF and -124dBm (real-time measurement for the AIS, which typically ranges from -120 go -130).

So I guess our AIS is just a bit more sensitive than the VHF, but the AIS also has a 12dB gain before the receiver.

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Old 12-10-2014, 13:57   #68
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Re: Class B AIS

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Our specific VHF and AIS specs,,, -124dBm (real-time measurement for the AIS, which typically ranges from -120 go -130)
You have to specify the Packet Error Rate. Otherwise you just specify a minimum discernible signal.
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Old 12-10-2014, 15:31   #69
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Re: Class B AIS

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Originally Posted by continuouswave View Post
AIS is FM modulation. It is just not voice.
I think not! AIS uses FSK -- Frequency Shift Keying -- and GMSK -- Gaussian Minimum Shift Keying. FM is an analogue modulation scheme and has nothing to do with it.
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Old 12-10-2014, 15:46   #70
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Re: Class B AIS

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I thought you would have read the myriad of AIS threads?

I can't find any Class B AIS thats available that has Bluetooth at all. One has it but wont work on iPad.

I dont think any iPad aps use Bluetooth... I have iNavx, Garmin and Navamatics and none use AIS as far as I can see.

Spitters cost $250

A seperate antenna costs $70 plus cable $30.

A seperate antenna gives you redundancy.

I have Raymarine (Defender $729) as its USB, and, well, Raymarine. There are lots of others out there but, interestingly, I dont think theres any threads where people slag any particular brand. Once you get one and its got the features you want they seem to work.

You want a unit that can be used in as many of the things as possible. Mine will do a chart plotter (if I had one) at the same time as my PC and VHF.

Once you have one installed and working and go for a sail you will never want to be without one again. Gone are the days of seeing a ship and wondering where its going.

Beware: There are many nay-sayers who do not have AIS that for some bizarre reason don't want you to have AIS. No, they don't work on the moon and the ships of the desert dont have AIS and they will not stop you getting drunk, losing your virginity or healing Ebola.




Mark
Yup - AIS only tracks you if you want it to, right? So, used as a receiver, it'll let you see who's about you, but not give up your own position. It's the latter that's the bug-a-boo some people worry about.

It's akin to the fear of the GPS receivers that some people avoid because they see them as position trackers. GPS units can track you only if they're connected to something that can communicate bidirectionally - like a cell phone or tablet. The little receivers themselves are benign.
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Old 12-10-2014, 18:09   #71
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Re: Class B AIS

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I think not! AIS uses FSK -- Frequency Shift Keying -- and GMSK -- Gaussian Minimum Shift Keying. FM is an analogue modulation scheme and has nothing to do with it.
Dockhead,

There is precious little difference between FSK/GMSK and FM. All are frequency modulation schemes with constant envelope power. The difference is that the receiver knows in the case of FSK or GMSK what the possible frequencies will be in advance. Therefore, the receiver can do a better job of decoding the data than a human can do listening to "random" frequencies that represent a voice signal. But the theory and math of all 3 modulation types is the same.

Technical minutiae ahead so skip if uninterested...

GMSK is really an "analogue" system at its heart. The digital ones and zeros are first filtered in such a way that the digital data becomes an analog wave that in turn frequency modulates the carrier. The modulator part of a GMSK AIS transmitter is exactly the same as the modulator part of a VHF FM transmitter.

FSK is only slightly different in that the digital data is not filtered and so directly modulates the frequency. Therefore the transmitter only transmits on specific frequencies. Other than that FM/FSK/GMSK are virtually identical as far as the transmitter is concerned.
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Old 12-10-2014, 20:36   #72
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Re: Class B AIS

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Originally Posted by Dockhead View Post
I think not! AIS uses FSK -- Frequency Shift Keying -- and GMSK -- Gaussian Minimum Shift Keying. FM is an analogue modulation scheme and has nothing to do with it.
Frequency shift keying is a form of frequency modulation, just like on-off telegraphy is a form of amplitude modulation.

You are mixing up two attributes: the nature of the modulation technique and the nature of the modulating signal. There is nothing about frequency modulation that limits its use to analogue or continuously varying signals. Indeed, frequency shift keying is a good example of frequency modulation by a digital signal.
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Old 13-10-2014, 08:01   #73
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Re: Class B AIS

Responding to my comments about range of AIS, Mark said:

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Then I put it to you that you do not have an AIS.
Mark--I am not sure how my ownership of an AIS Class-B transponder would affect the range of AIS Class-B transponders or my knowledge about that topic. First of all, let us assume I have an AIS transponder. How will I gain information about its range? My AIS transponder only transmits. I don't have any way to know how far its signal is being received by others. I could go about transmitting with my AIS transponder for years and have no idea how far its signal was being received. Thus, I don't see any sort of tie-in between knowledge of AIS range and ownership of an AIS Class-B transponder.

I have spent a lot of time observing RECEPTION of AIS transponders. I believe that experience in reception of AIS signals is likely to produce more information about the range of AIS than just transmitting them will. On rare occasions I have received some AIS Class-A transponder signals at extreme range, more than 50-miles out. On the other hand, I sometimes get FM Voice signals at extreme ranges, over 120-miles out. I attribute these occurrences to favorable propagation.

In order to draw some reasonable inference from the experience of listening to both AIS and VOICE signals, it would be necessary to have more meaningful data. For example, having a source of both AIS and VOICE signals from the same vessel and testing over time for a range. I am afraid I don't have any data like that. But it seems to me that experience in reception of AIS signals is likely to produce more knowledge about their range than experience in transmitting them will.
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Old 13-10-2014, 16:25   #74
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Re: Class B AIS

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How will I gain information about its range? My AIS transponder only transmits.
How about this:

Start in a city that you know has a receiver for MarineTraffic.com. Make sure you see your boat on the site.

Plot a course directly away from land. Estimate the theoretical range of your AIS using your height of antenna and a wild guess as to the height of the receiving antenna. Go twice that far. Come back. Check the web site again. See where your signal was lost.
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Old 13-10-2014, 16:57   #75
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Re: Class B AIS

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How about this:

Start in a city that you know has a receiver for MarineTraffic.com.
The ID of the station that is receiving you is displayed at the bottom of this screen as 'AIS source'.
A trap for young players is that multiple stations may be passing your position on to mission control...you may be getting handballed from one to the next.... there are at least 5 in the Auckland area frinstance....
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