AIS 2009, forward in all directions!

Ben Ellison

Ben Ellison

Panbo editor, publisher & chief bottlewasher from 4/2005 until 8/2018, and now excited to have Ben Stein as very able publisher, webmaster, and editing colleague. Panbo is going to the next level in 2019 and beyond.

10 Responses

  1. Richard D says:

    Proposed satellite AIS receivers will ignore Class B AIS transmission to avoid overload. At least that is the proposal based on simulation by the EU. The proposal goes further stating that a 3rd AIS frequency (one of the DSC CH 70 gardbands), which is allocated worldwide be used with an abbreviated message as a Class A uplink to satellites until shore-based AIS basestation transmissions are in range at which point the Class A AIS would cease transmissions intended for satellite reception.
    I haven’t seen any specifications for the Orbcomm AIS receivers in order to see if they conform to the recommendations regarding ignoring Class B but they may will. Since there is no agreement on using the 3rd AIS frequency I imagine the Orbcomm satellites receive existing Class A transmissions on the existing frequencies, one of which is allocated worldwide.
    Cheers and Happy New Year to all.
    Richard D.
    s/v RED

  2. Dan (b393capt) says:

    I had a bad experience with a SART.
    I bought a SART for my sailboat back in 2006, and using the integrated test feature (go 40 yards away and push switch to test mode) did not see it on my Raymarine radar. After similiar results with a replacement unit, the vendor told me that it’s possible that new Raymarine radars are filtering it out (They had only tested with Furuno).
    After four months of demanding a refund (they tried to argue that if I needed help I was better off that a Furuno (commercial) user responded rather than a Raymarine (recreational) user), I got a refund.

  3. Dan (b393capt) says:

    My original intention of using the SART (with one button press my crew can summon the coast guard), is now meet with DSC and Rescue 21 … but this AIS SART would seem useful for those of us that are outside the current Rescue 21 deployments.

  4. Michel Capel says:

    As far as I know, 121 mhz ans 243 mhz are no longer supported bij GMDSS conforming EPIRBs.

  5. Ben Ellison Ben Ellison says:

    It’s confusing, Michel. The old EPIRBs that used 121 MHz to send distress signals to low orbit satellites are obsolete. But all current 406 MHz EPIRBs also broadcast a low power 121 MHz homing signal. That’s because the basic 406 technology (without the GPS option) is only accurate to a mile or so, and because many SAR resources, and some yachts, are equipped to home in on 121 MHz.

  6. Tony says:

    As I was weaving my way through a bunch of crab pots in the San Juan Islands recently, I was wishing they all had AIS transponders on them. They would not need much power, a mile warning would be great.

  7. bcl says:

    Ignoring Class B will not prevent overload. It uses the same frequencies as Class A messages. The problem with Satellites is that their ‘view’ is much wider than a ship, resulting in collisions.

  8. Ben Ellison Ben Ellison says:

    Regarding the confusion about 121.5 MHz, and the demise of EPIRBS using that and 243 as primary distress frequencies: The USCG has created some neat widgets to remind all of the Feb. 1 deadline:

  9. Olsonist says:

    Class A uses SOTDMA whereas Class B uses CSTDMA. Basically that means that Class A, the big guys, randomly broadcast on one of the 4500 slots per minute. (GPS provides the distributed time synchronization for slotting.) Class B on the other hand *listens* first and then broadcasts (every 30s under 14 knots).
    So yes, the big guys will always get through, avoiding overload for them. And the little guys will eventually get through, avoiding overload for them.

  10. bcl says:

    Slight correction — there are 2250 slots per minute. 2 channels. But IIRC part of the Class A algorithm says not to transmit on the other frequency if the slot is already in use. Also, Class B uses the same slots, it just waits a bit to detect use before transmitting. There is no way for a receiver to ignore one transmission type or another. It has to receive it first 🙂

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