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Thread: AIS Class B Transponder Questions

  1. #11
    Join Date
    May 2012
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    79

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    Quote Originally Posted by cafemontaigne View Post
    Richard,

    Take my number with a grain of salt, it may have been as high as half an amp. I would get you an exact power consumption figure for the AIS, but unfortunately I've taken the unit off the boat (I'm selling the boat but like the AIS enough to want to keep it), but I think .3A is definitely in the ballpark. I remember being quite pleased with how little it drew, even while transmitting. I used my netbook running OpenCPN for a display. All in all, with AIS, netbook and instruments I was running at about 1.5 to 2A.

    I would be careful about using a splitter with an AIS transceiver. I would think having the antenna disconnected while it's sending a signal might be detrimental to the unit, and since they send every 30s it might be hard to prevent.

    Adrian
    I agree with Adrian's qualms on using a splitter: I tried several with my AIS Class B installation, and found that all introduced reception problems. You don't get something for nothing, and trying to use the same VHF antenna for both comms and AIS degrades both. If you read the warranty on your AIS transponder (transmitter/receiver), you will probably find that it's only valid when you use a dedicated antenna. There's a reason for that.

    Since you are unlikely to need line-of-sight to a 40 mile radius with your AIS antenna, mounting a separate antenna lower on deck dedicated to AIS will be adequate. Plus, the big ships you're really interested in seeing/seeing you have their antennas mounted high above the water, and adding height to your own antenna has little proportional benefit. Class A stations transmit every 2 seconds when underway, and if they disappear behind a swell, you'll reacquire them soon thereafter.

    A good quality splitter costs more than an additional antenna and coax. And one other factor against using a splitter is this: with a splitter, you can't transmit on your VHF radio and send AIS position reports at the same time. All the splitters I tested gave priority to the VHF radio. And when is the most critical time you'd be transmitting on your VHF? Probably while you're trying to negotiate a crossing with a conflicting vessel. And since a Class B AIS transponder only transmits your position once every 30 seconds, every position report will become quite precious during such a situation. You may end up "stepping on" your own position reports when you need them most.

    My antenna is mounted 12 feet above the water, and my solid recepition range for Class A targets is 20 miles. Since I'm posting reception reports to marinetraffic.com, you can see the stats on my receiver here: http://www.marinetraffic.com/ais/sta...station_id=595. My 24 hour reception area is here: http://www.marinetraffic.com/ais/def...39&oldmmsi=595. That area was severely degraded with every antenna splitter I tested.

    I have an ACR Class B transponder and it uses about 7 amp/hours per day. When under way, it transmits every 30 seconds and draws 2 amps when transmitting, but the transmit duty cycle is very low: about 100 milliseconds per transmission (a duty cycle of 1/300th). 2880 transmissions per day X 2 amps / 864,000 tenth-second segments per day = 0.07 amp/hours per day consumed by transmitting. That's a minuscule amount of extra power consumption.

    The Class A ships DO see Class B transmissions. However, you should memorize your MMSI because your "extended" data containing your vessel name is transmitted much less frequently (1/10th as often) as your raw MMSI and position. When hailing a ship where you need them to recognize your AIS position, you should include your MMSI in the hail because that may be the only identifying information they see on their navigation display. If there's a traffic conflict, I always hail like this: "Motor Vessel Behemoth, this is Sailing Vessel Ad Astra, MMSI 367 431 840, two points off your port bow, range one mile..."

    Note that without AIS I probably wouldn't know: 1) the vessel's name, 2) the exact range to the vessel, and in reduced visibility 3) my own position relative to the vessel's track -- and I certainly wouldn't know the vessel's MMSI for sending a DSC call (which can really get their attention when necessary). So having an AIS transponder gives me a wealth of information that I can convey to the other vessel in the initial hail. That's what makes AIS so powerful and in many cases much more useful than (power hungry) radar.

    I strongly endorse Vesper Marine's products for displaying AIS traffic. They are intuitive, consume very little power (a fraction of what a laptop consumes), and you can configure them to only warn you about vessels that pose a collision threat. You can see a demo vid here. You will probably grow very tired of being disturbed by warnings from a dumb chartplotter bleeping at you about anchored vessels and vessels moving away from you. I have the Vesper Watchmate on my vessel. I prefer a dedicated "appliance" approach to safety systems like AIS. Although the smartphone and latptop computer aps work fine for displaying AIS data, a dead smartphone battery or my PC crashing while I'm taking a nap gives a whole new ominous meaning to "blue screen of death." Part of my day job that pays for my sailing involves evaluating system reliabilities, and depending on any general purpose computer to perform safety-critical functions just invites a cascading failure where one unrelated buggy software application takes out other safety-critical functions. Plus, I'm able to use the alarm relay contacts on the Watchmate to sound a really loud alarm -- and the alarm only sounds when there is an actual collision hazard.

    -Patrick Bryant
    FCC licensed Global Maritime Distress and Safety System Maintainer
    Last edited by pbryant; 08-20-2013 at 05:10 PM.

  2. #12
    Join Date
    May 2009
    Location
    San Francisco
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    I upgraded from receive only AIS to a transponder several months back (because my wife refused to DH with me to Hawaii unless I added transmit). Yes I had to change the active splitter - the old one that worked with receive didn't support the new transponders transmit. $200 for the splitter... a lot when the transponder was less then $600 (with built in GPS and Bluetooth). I see the transponder (AMEC CAMINO-101 Class B) has gotten cheaper yet... $450+$50 more if you want bluetooth. I did a bunch of testing on the effect the splitter has on signal strength... not with instruments but just by calling a friend by VHF and asking for his subjective opinion on strength/clarity... the splitter did effect signal strength a bit (it was noticeable but not severe) - and putting the AIS in "silent mode" gave me full VHF signal strength back (no idea why that is... not obvious to me how that would effect the splitter).

    I do believe using a single mast head VHF antenna is a good thing... adding a second antenna somewhere near deck level doesn't seem all that reliable to me... and one less hole in the deck... and the mast head antenna means I can see those big ships (or another small boat like me)... and they can see me... long before either of us gets close enough to need to change course. I can simply slow down a wee bit as needed to let them pass.

    The integrated GPS is cool too... less wiring... less dependencies... higher reliability all around.

  3. #13
    Join Date
    May 2009
    Location
    San Francisco
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    68

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    On display/alarm for the AIS... I started with a Watchmate... which clearly has all the bells and whistles... several options on when the alarm will go off... but I find I no longer use it. Mostly because it does take a bit of thinking to set it up... and "relearn" it after not using it for a few months. And I have to run below to deal with it when the alarm goes off. Today I just have the AIS wired (NMEA 0183) into my on deck chart plotter (little 5 inch $600 Garmin)... and just use the Garmin's simple AIS proximity alarm. Its simple and the chart plotter is easier/clearer to see who the AIS targets are and what the targets are doing then the Watchmate was. Especially true for deliveries when I have less experienced crew keeping an eye on things (or when alone and I am overly tired and thinking slow). And the chart plotter is on deck where I can see it while I look for the targets with my real eyes.

  4. #14
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    Sep 2007
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    Quote Originally Posted by JimQuanci View Post
    I did a bunch of testing on the effect the splitter has on signal strength... not with instruments but just by calling a friend by VHF and asking for his subjective opinion on strength/clarity... the splitter did effect signal strength a bit (it was noticeable but not severe) - and putting the AIS in "silent mode" gave me full VHF signal strength back (no idea why that is... not obvious to me how that would effect the splitter).
    .
    Interesting, I looked at the spec on the MillTech splitter, it indicated <3db signal loss. <3db could be nearly 1/2 your transmitted power is lost. That seems surprising. While the single antenna notion is a good one, simplicity, wire,........ one gives up a lot of power to achieve that. Hopefully someone can do a more efficient splitter soon.

  5. #15
    Join Date
    Jan 2008
    Location
    Santa Rosa
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    576

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    I have a Standard Horizon GX2000 in the nav compartment (ex-head compartment) wired into my Garmin GPSMAP 478 with a Standard Horizon Ram 3 Remote Access Microphone in the cockpit. It's receive only of course. The Ram 3's screen is small, but it shows all the AIS information. It can also controls the GX2100 in the cabin. The Ram 3 was only about $100. The radio/AIS receiver uses the same masthead VHF antenna. It worked on the way to Hawaii in 2010 and it continues to work for SSS/OYRA races - and for those foggy days on the Bay.

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