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Thread: AIS Receiver Information

  1. #1
    Join Date
    Sep 2007
    Posts
    99

    Default AIS Receiver Information

    Hi All,

    I just typed an email responding to a friend about my AIS setup and my experiences with AIS on a recent 1000 nm trip down the coast, half of which was singlehanded and thought some of people thinking of participating in the 08 transpac might also find it interesting. I couldn't include the screenshots I mention in the email, but would be happy to send them separately to anybody that would like them. They are about 250K each. I hope this is an appropriate use of the forum and some find it interesting.

    ================

    Hi Chris,

    I can't tell you a great deal about AIS, but here's what I do know. I copied the rest of the crew of Encore in case they might find this interesting too.

    AIS (Automatic Identification System) is required on all commercial vessels over 30 gross tons. It is simply a short text string transmitted on one of two discreet VHF frequencies. It contains (if the vessel has filled in everything) the name of the vessel, MMSI number, position, course, speed, cargo, destination, length, width, draft, rate of turn and several other fields that I am not thinking of. More than half of the vessels that I encountered had MMSI number, position, course and speed and nothing else. This however is the collision avoidance information.

    I have a Nobeltec receiver (I am sure they don't make it) that cost about $400 (this is a huge rip-off, but also a supply and demand thing so prices will come down dramatically as this catches on). It listens to one frequency for a while and then listens to the other for a while and goes back and forth. Nobeltec also sells one for about $900 that monitors both frequencies all the time. Having used the one I have, it is adequate for most things. I did notice that it takes longer to get all the information on a vessel and sometimes the vessel will disappear because the string hasn't updated frequently enough because the receiver is changing frequencies. It always comes back, and if this disappearing causes you a problem, you are *way* too close. It usually only disappears when it's a ways away too and so you have weak signal to contend with there also. I have my dedicated VHF antenna on the stern rail, so even seas can block the signal sometimes. As with all antennas, the higher the better, but since you are not concerned about vessels 100 miles away, it doesn't really matter and might only add clutter, not useable information. In LA harbor, for instance, you might have trouble getting a full string from any ship because there are so many transmitting at the same time, they walk on each other?? My receiver, or any receiver for that matter, will only receive and decode the string. The output then has to "go somewhere." I am using the Nobeltec VNS (Visual navigation Suite) chart plotting software on the laptop. Sooo, I have the output from a Garmin GPS receiver and the output from the AIS receiver feeding into the laptop. The software is OK as a chart plotter, although very expensive, but interfaces with the AIS receiver very nicely. I have included screen shots of the display from my "close encounter" and Santa Barbara Channel where I had ships in front of and behind me.

    I think that only now are the built in chart plotters starting to include the capability to interpret and display the output of an AIS receiver. I wouldn't buy a chart plotting program or built in chart plotter without that capability!!!!!! There is a stand alone unit available that has the AIS receiver inside a "radar" display and requires only power and a GPS input. It just shows you in the center and the targets around you. I don't think it displays any of the other AIS information and I don't think it has CPA (Closest Point of Approach) and TCPA (Time to Closest Point of Approach) and I don't know if it has an alarm function.

    Things to look for: I think the things that are Critical are that it go to a chart display with you in the center, it have CPA and TCPA with a settable alarm function. It is also nice to have the ship information, specifically course and speed. The name of the vessel (if they have filled this information in) is also very handy in case you want to hail them on VHF, you can call them by name. I have mine set for CPA of 2 miles and TCPA of 15 minutes triggers the alarm. This is pretty conservative, but since I am using it so I can sleep singlehanded (I set my alarm clock to go off every two hours for a look about when coastal cruising singlehanded also), I want time to clear the "fuzzies" and watch it a little and still have time to call or change course as appropriate.

    The big disadvantage of the laptop is the power consumption and the fact that if you lose your laptop, you lose AIS capability. The drawbacks to the Nobeltec VNS software are that it is expensive, requires a Dongle in the USB port to operate (with international charts anyway) and I find using it for navigation a little cumbersome, not bad but....... The AIS portion of the software is generally great. My only complaint here is the alarm. It gives two funny little blasts and then stops. I would like to be able to set it so the alarm goes off until it's reset manually from the keyboard or mouse. When I am trying to sleep, I have some external amplified speakers attached and this helps. I want to be sure the alarm wakes me since it only goes off once. I am going to email Nobeltec and suggest they incorporate this in future revisions to the software. Just FYI, I am not particularly interested in a stand alone chart plotter. I don't want or need it on all the time, especially open ocean. I turn on the handheld a couple of times a day to check position and that's all that's needed. If I were cruising the intercoastal waterway or going way up the Sacramento River routinely, my feelings would be different. The only time I leave the chartplotting software on is when I'm in the fog or when I'm trying to sleep.

    As a side note, I think that in the fog, watching the AIS display is far more likely to help you avoid a collision than standing a conventional watch. In my "close encounter" with NYK Athena 100 miles offshore (one of the attached pics shown after I altered course 90 degrees port), I would have never seen it until it ran me over at 24.5 Kts. Before I altered course, I saw the closest point of approach as 55 feet at one point. Athena was 985 feet long and 131 feet wide, this could have been ugly (for me anyway). I would have heard the fog horn but couldn't have told where it was coming from until it was tooooo late. With coastal cruising, you probably need a conventional watch because of smaller vessels such as yourself that could be out tooo?

    This might be more information than you wanted, but I am now a *real* believer in AIS and I think that all cruising vessels who have it feel the same way. BTW - I got the software and AIS receiver at the Strictly Sail Show this year. They had a show package where if you bought the software and the AIS receiver you got the chart area of your choice free. I chose the chart package that goes from Pt. Arena to the Panama Canal.

    John

  2. #2
    Join Date
    Sep 2007
    Location
    Cascade Lakes Highway
    Posts
    6

    Default

    AIS receiver prices have dropped considerably. So has the navigation software that supports them.

    Some AIS resources..

    http://www.navcen.uscg.gov/enav/ais/default.htm

    http://www.milltechmarine.com/products.htm

    http://www.sping.com/seaclear/

    http://www.gpsnavx.com/

  3. #3
    Join Date
    Sep 2007
    Posts
    59

    Default

    On the LongPac, I used the NASA "radar" display AIS receiver. This is a freestanding unit (needs no laptop, just a VHF antenna and a GPS) that's marketed under the Sy-Tec name in the US. It costs about $450. I was very happy with it. It's small, uses little juice, and is easy to install (I was able to install it myself), and works very well. Only complaints are that it must be mounted below (a problem when coming in past the Farallones to the Gate on the last night, when there was thick fog and I had to close the companionway due to heavy weather) and it takes a minute or two for the data to show up on a target. Both of these issues probably arise with most or all AIS receivers.

    Christian Humann on the Carole E has the same unit and I believe he is pleased with it.

    - Tom Kirschbaum, Feral

  4. #4
    Join Date
    Sep 2007
    Posts
    16

    Default Alarm?

    Thanks to all of you for the great info on AIS. Tom, does that NASA unit have an alarm?

    Tony

  5. #5
    Join Date
    Sep 2007
    Posts
    134

    Default

    I used the SR162 unit coupled with Nobeltec on the laptop for the LongPac, and it worked great. As has been mentioned before, the alarm is pretty worthless, it beeps a couple of times and then quits on its own. I'm thinking about also installing the Sitec standalone unit as an alternative to the laptop and as a backup, and because I've heard the alarm is great on that unit - nice and loud, and stays on until you shut it off. Unfortunately, I don't think it computes CPA or TCPA, which is one of the most important functions of an AIS unit I would think! Instead, the alarm triggers on simple proximity, which is better than nothing but could have been so much more useful if it worked on CPA instead.

    - Mark

  6. #6
    Join Date
    Sep 2007
    Posts
    59

    Default NASA/Si-tech AIS

    Yes, the NASA/Si-Tech AIS unit has an alarm that works well. It got me up a few times during the LongPac when ships were around. Also, the alarm goes off if the AIS unit becomes disconnected from the GPS -- an important feature if the thing was personally wired up by somebody with all thumbs (like me!) - Tom Kirschbaum, Feral

  7. #7
    Join Date
    Sep 2007
    Location
    Vancouver British Columbia, Canada
    Posts
    132

    Default

    I agree that having CPA & TCPA is a major factor in selection of an AIS unit, which as Mark points out the NASA unit does not. I wrote them a couple months ago to see if they would be doing an upgrade to the existing unit or releasing a newer model with this function......such a move is not in their plan.

    The only stand-alone I can find that has this feature is the Simrad A150, which is also a Class 'B' transponder - so others can 'see' you as well. Had discussions with a number of the big-boat skippers at work about AIS and they agree that seeing is only part of the safety equation - being seen, is the other half.

    Not approved by FCC yet so will get a friend in either Australia or England to send one over for me. Not inexpensive at 850 pounds sterling. but if it saves you once, it is a bargain at twice the price!! Can also rationalize that it will get a lot more use than a liferaft or EBIRB ever will and they cost just as much or more.

    By the way....you can turn off the transmit fuction if you wish.

    Jim/Haulback

  8. #8
    Join Date
    Sep 2007
    Posts
    3,026

    Default

    NASA's "AIS Radar" is still the ticket at the simple/low cost/low power end of things. I didn't miss having CPA, the alarm was plenty loud and although I had a laptop I was glad not to have to rely on it for the AIS to work, not to mention the power savings. (The laptop and its converter were one of the biggest power draws on the boat.)

    The AllGadgets guys will send a unit via DHL and you'll have it in 3-4 days:

    http://www.allgadgets.co.uk:80/ag/pr...pf%5Fid=AG3928

    You might want their cradle bracket too, unless you make your own.
    Last edited by BobJ; 06-03-2008 at 11:41 PM. Reason: Cost/exchange rate data screwed up

  9. #9
    Join Date
    Sep 2007
    Location
    Danville, CA
    Posts
    2

    Default MillTech SR-161 AIS -> Raymarine

    I've had the Milltech unit hooked up to my Raymarine chartplotter and it works great. I did a quick and dirty install with the unit behind the chartplotter at the nav station. I'm currently using a 6" stick antenna mounted on the unit. It works fine in the Bay and out the gate. "Visibility" would be improved with a better antenna and location, but this was cheap, easy and quick -- less than 1 hr to mount and wire up. I will "upgrade" the antenna with an external antenna that will also function as a backup for my masthead antenna.

    On advantage of the SR-161 for my setup is that it will multiplex a NMEA input with it's own data, effectively sharing the one NMEA input I have on my Raymarine.

    My setup is -

    VHF (DSC output data) -> AIS -> Chartplotter [all NMEA 0183 @ 4800 baud)
    GPS [SeaTalk] -> Chartplotter [NMEA 0183 @ 4800 baud] -> VHF (DSC GPS Data)

    Paul Koenig
    S/V Bear Boat
    - Paul
    S/V Bear Boat
    Jeanneau 40.3
    Go Bears !

  10. #10
    Join Date
    Sep 2007
    Location
    Capitola,CA
    Posts
    2,136

    Default

    I was skeptical at first. But after testing, found there is little or no difference in AIS reception when using a stern rail mount antenna versus a masthead antenna. The stern rail antenna alerts to passing commercial shipping 32 miles offshore of Santa Cruz, which is the max range of the NASA unit.

    A little FYI: when sailing offshore and plotting AIS targeted shipping on a paper chart, it became apparent the unit gives the target's course (COG) in True, not Magnetic degrees.

    For all you potential SHTP'ers, encourage you to make the Sail Seminar. Dave Hodges has good info, and does shorthanded sailing himself on TIMBERWOLF.

    ~sleddog

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