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Old 17-07-2021, 14:17   #1
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Why Does AIS Transceiver Require Dedicated GPS Antenna

The B&G (Simrad) NAIS 500 Class B AIS transceiver has a dedicated GPS antenna. The transceiver is on the NMEA 2000 network, where there's a chart plotter with its own GPS. Why can't the NAIS 500 get its coordinates from the chart plotter's GPS instead of having a dedicated antenna? Thanks!
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Old 17-07-2021, 14:23   #2
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Re: Why Does AIS Transceiver Require Dedicated GPS Antenna

It's a regulation to prevent feeding incorrect GPS data to the AIS to conceal criminal activity. It would be very simple to feed in a NMEA signal that had the boat in some other location. By requiring the AIS use its internal GPS, that becomes much harder to do.
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Old 17-07-2021, 19:19   #3
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Re: Why Does AIS Transceiver Require Dedicated GPS Antenna

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Originally Posted by wholybee View Post
It's a regulation to prevent feeding incorrect GPS data to the AIS to conceal criminal activity. It would be very simple to feed in a NMEA signal that had the boat in some other location. By requiring the AIS use its internal GPS, that becomes much harder to do.
Makes sense. Thanks.
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Old 18-07-2021, 07:34   #4
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Re: Why Does AIS Transceiver Require Dedicated GPS Antenna

That's silly. Really? Anyone who knows how to manipulate a data stream can falsify one's location by simply providing false GPS data. So that blow the theory of preventing criminal activity right out of the water. (pun intended)

Now if one would have guessed that the purpose of a dedicated GPS signal versus a GPS signal on a cluttered network would be logical. Or, that the interface for the AIS to GPS connection is cost effective versus a network interface.

But to curve criminal activity?

JMHO
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Old 18-07-2021, 07:56   #5
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Why Does AIS Transceiver Require Dedicated GPS Antenna

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Originally Posted by Brian.D View Post
That's silly. Really? Anyone who knows how to manipulate a data stream can falsify one's location by simply providing false GPS data. So that blow the theory of preventing criminal activity right out of the water. (pun intended)

Now if one would have guessed that the purpose of a dedicated GPS signal versus a GPS signal on a cluttered network would be logical. Or, that the interface for the AIS to GPS connection is cost effective versus a network interface.

But to curve criminal activity?

JMHO


Itís not at all easy to generate spurious gps signals over any length of time and such emmisions can be detected. Whereas anyone with a nmea0183 feed could simply mock up false gps messages which the ais would use if it didnít have its own gps

Thatís why they do have their own gps and not really for any other reason.
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Old 18-07-2021, 08:41   #6
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Re: Why Does AIS Transceiver Require Dedicated GPS Antenna

Sorry, but are you saying that it is not easy to fabricate GPS sentences with data that is not correct? Sure it is. Get the data from an NMEA0183, modify it, then loop it back into the AIS. Now, granted there are some systems that might have better security than others, but it can be done.

This reminds me of the time people were saying that you could not be tracked with a cell phone. LOL We have always been able to be tracked as long as three towers can hear your signal.

Nothing electronic(s) is fool-proof. Nothing. Again, JMHO
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Old 18-07-2021, 08:55   #7
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Re: Why Does AIS Transceiver Require Dedicated GPS Antenna

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Sorry, but are you saying that it is not easy to fabricate GPS sentences with data that is not correct? Sure it is. Get the data from an NMEA0183, modify it, then loop it back into the AIS. Now, granted there are some systems that might have better security than others, but it can be done.

This reminds me of the time people were saying that you could not be tracked with a cell phone. LOL We have always been able to be tracked as long as three towers can hear your signal.

Nothing electronic(s) is fool-proof. Nothing. Again, JMHO
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Old 18-07-2021, 08:58   #8
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Re: Why Does AIS Transceiver Require Dedicated GPS Antenna

AIS transponders also use GPS to establish slot timing which needs to be much more precise than possible via serial date on NMEA.
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Old 18-07-2021, 09:14   #9
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Re: Why Does AIS Transceiver Require Dedicated GPS Antenna

I donít think AIS GPS antennas feed NMEA sentences to the AIS transceiver, do they? I believe the raw GPS signals come into the transceiver where the time and position data is derived for use by the transceiver. So the AIS transceiver is not using NMEA sentences itself for its transmitted data, even if it outputs sentences to an NMEA network.
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Old 18-07-2021, 10:40   #10
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Re: Why Does AIS Transceiver Require Dedicated GPS Antenna

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Originally Posted by Brian.D View Post
Sorry, but are you saying that it is not easy to fabricate GPS sentences with data that is not correct? Sure it is. Get the data from an NMEA0183, modify it, then loop it back into the AIS. Now, granted there are some systems that might have better security than others, but it can be done.

This reminds me of the time people were saying that you could not be tracked with a cell phone. LOL We have always been able to be tracked as long as three towers can hear your signal.

Nothing electronic(s) is fool-proof. Nothing. Again, JMHO


You need to read what I said. Itís precisely because you can fabricate nmea position data that AIs mandates an internal gps. Itís not so easy to spoof gps signals ( thatís what I said )
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Old 18-07-2021, 10:42   #11
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Re: Why Does AIS Transceiver Require Dedicated GPS Antenna

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Originally Posted by SoonerSailor View Post
I donít think AIS GPS antennas feed NMEA sentences to the AIS transceiver, do they? I believe the raw GPS signals come into the transceiver where the time and position data is derived for use by the transceiver. So the AIS transceiver is not using NMEA sentences itself for its transmitted data, even if it outputs sentences to an NMEA network.


Correct Ais certification requires an internal independent position feed , in fact itís not supposed under a change in 2013 to output that data to other devices on a net.
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Old 18-07-2021, 10:54   #12
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Re: Why Does AIS Transceiver Require Dedicated GPS Antenna

As OP, I hadn't intended on opening such a controversial topic. Sorry for that, but it sure was informational. Thanks, all!
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Old 18-07-2021, 12:46   #13
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Re: Why Does AIS Transceiver Require Dedicated GPS Antenna

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Originally Posted by goboatingnow View Post
It’s not at all easy to generate spurious gps signals over any length of time and such emmisions can be detected. Whereas anyone with a nmea0183 feed could simply mock up false gps messages which the ais would use if it didn’t have its own gps

That’s why they do have their own gps and not really for any other reason.

the older class A's (IE saab R4 , haven't played with any newer ones) have their own gps antenna and nmea 183 input. they default to the nmea 183 input when both are present. I used to put false signals in to test them. it was funny to watch ghost ships traveling through land. this was 15 years ago when only a few boats had AIS. I would not do that now. but was easy to do with a computer, serial port, and nmea 183 test program
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Old 18-07-2021, 13:15   #14
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Re: Why Does AIS Transceiver Require Dedicated GPS Antenna

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Originally Posted by Brian.D View Post
Sorry, but are you saying that it is not easy to fabricate GPS sentences with data that is not correct? Sure it is. Get the data from an NMEA0183, modify it, then loop it back into the AIS. Now, granted there are some systems that might have better security than others, but it can be done.

This reminds me of the time people were saying that you could not be tracked with a cell phone. LOL We have always been able to be tracked as long as three towers can hear your signal.

Nothing electronic(s) is fool-proof. Nothing. Again, JMHO
Yes, it *would* be that easy if the GPS accepted NMEA data. But it doesn't, it MUST use it's own GPS. So you are not just simulating NMEA data, but the GPS RF signal from multiple satellites, containing all the clock and timing data. Not simple "sentences" like NMEA uses. That is *VERY* difficult to do. The military has that capability, and presumably some foreign governments have figured it out. But even a well funded criminal organization isn't going to pull it off.
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Old 18-07-2021, 13:42   #15
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Re: Why Does AIS Transceiver Require Dedicated GPS Antenna

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Originally Posted by goboatingnow View Post
You need to read what I said. Itís precisely because you can fabricate nmea position data that AIs mandates an internal gps. Itís not so easy to spoof gps signals ( thatís what I said )
Then I apologize. I misunderstood you comment.
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