Why Does AIS Transceiver Require Dedicated GPS Antenna

[AedanC]

Originally Posted by goboatingnow

Correct Ais certification requires an internal independent position feed , in fact it’s not supposed under a change in 2013 to output that data to other devices on a net.

What are they trying to achieve by prohibiting an AIS transponder from outputting position data? That’s our main source of position for the other instruments.

[CarinaPDX]

A bit of misunderstanding here. AIS units use GPS modules, which do indeed put out location data in NMEA 0183 format to the AIS; many modules are capable of proprietary binary formats for output but they are documented and could be faked - but I doubt these are used anyway.

I believe that the AIS is required to have its own GPS for reliability purposes. I do appreciate the conspiracy theories but I seriously doubt avoiding hackers misusing the gear to spoof GPS info was a significant concern. Anyone wishing to do that can acquire a VHF radio, modem, and a bit of software and have at it.

Greg

[SoonerSailor]

I think it comes down to ease of spoofing. If all it took was to send spoofed position data into an AIS transceiver via NMEA, there would be many instances of this happening, perhaps just by “pranksters”. As it is, a certain amount of expertise would be required to do this now.

[C420sailor]

Also nice being able to broadcast AIS without having to turn your plotter on....

[goboatingnow]

Originally Posted by AedanC

What are they trying to achieve by prohibiting an AIS transponder from outputting position data? That’s our main source of position for the other instruments.

The idea is the gps function is internal to the AIS.

For example in a 2015 update , the raymarine Ais650 ( which is an SRT board) no longer outputs gps info on Seatalkng and the instructions now say the 0183 output is for diagnostics purposes

[goboatingnow]

Originally Posted by CarinaPDX

A bit of misunderstanding here. AIS units use GPS modules, which do indeed put out location data in NMEA 0183 format to the AIS; many modules are capable of proprietary binary formats for output but they are documented and could be faked - but I doubt these are used anyway.

I believe that the AIS is required to have its own GPS for reliability purposes. I do appreciate the conspiracy theories but I seriously doubt avoiding hackers misusing the gear to spoof GPS info was a significant concern. Anyone wishing to do that can acquire a VHF radio, modem, and a bit of software and have at it.

Greg

No the primary purpose is to deter spoofing the gps location, if it was reliability purposes then the ais would use network gps info of the internal gps failed. It doesn’t.

[goboatingnow]

Just a point ITU M1371-4 which is the applicable standard does not advance a reason as to why the internal GPS is required , merely that it is and that if it’s not working no position data should be transmitted

“3.3 Internal GNSS receiver for position reporting
The Class B “CS” AIS should have an internal GNSS receiver as source for position, COG, SOG.
The internal GNSS receiver may be capable of being differentially corrected, e.g. by evaluation of Message 17.
If the internal GNSS sensor is inoperative, the unit should not transmit Messages 18 and 24 unless interrogated by a base station”

[goboatingnow]

What is interesting by the way is when a class B ais is interrogated by a base station , it will report position ( and a failed gps flag ) to that base station irrespective of whether it’s internal gps is functional or not

[C420sailor]

Maybe in that case they felt that sending external pos data is acceptable as long as it is flagged as potentially inaccurate?

[goboatingnow]

Originally Posted by C420sailor

Maybe in that case they felt that sending external pos data is acceptable as long as it is flagged as potentially inaccurate?

It’s still not external data. Merely the last position recorded by the internal gps and flagged as faulty

[C420sailor]

Ahhhh makes sense.

[StoneCrab]

You might find these links interesting.

The Iranians spoofed the gps signal going to a USA Drone and captured it a number of years ago. GPS spoofing is possible and real.

A separate GPS antenna wouldn't make a difference if someone had a spoofing device. Alternately, truckers use a GPS jamming device to go off-book.

https://nakedsecurity.sophos.com/2012/07/02/drone-hackedwith-1000-spoofer/

https://www.schneier.com/blog/archiv...n_the_cap.html

I'm going to guess that the separate GPS on an AIS is for simplicity and safety. You can't rely on networking another GPS and wouldn't know the quality or age of the gps. For the few dollars that a GPS board costs, it is easier to just include one in your product. Hell, even my camera contains a GPS. As for the external GPS antenna, you can't count on a signal inside of a boat, so external is better.

[C420sailor]

Noise jamming a GPS is easy peasy.

Deceptive jamming a GPS is a whole other ball game.

GPS signal under a fiberglass deck is actually pretty good. If it is balsa cored and waterlogged? Not so much. Water is a huge signal attenuator for GPS—it’s why a thick jungle canopy is such a problem.

I have a GPS at my nav station on its own internal antenna and it gets exquisite signal.

[goboatingnow]

Originally Posted by StoneCrab

You might find these links interesting.



The Iranians spoofed the gps signal going to a USA Drone and captured it a number of years ago. GPS spoofing is possible and real.



A separate GPS antenna wouldn't make a difference if someone had a spoofing device. Alternately, truckers use a GPS jamming device to go off-book.



https://nakedsecurity.sophos.com/2012/07/02/drone-hackedwith-1000-spoofer/



https://www.schneier.com/blog/archiv...n_the_cap.html



I'm going to guess that the separate GPS on an AIS is for simplicity and safety. You can't rely on networking another GPS and wouldn't know the quality or age of the gps. For the few dollars that a GPS board costs, it is easier to just include one in your product. Hell, even my camera contains a GPS. As for the external GPS antenna, you can't count on a signal inside of a boat, so external is better.

No the Iranians jammed the drones GPS and then it went into auto pilot mode they then took control

Gps spoofing is very difficult to do. Truckers just jam the signal.

Local gps jamming is relatively easy

Hence the need to avoid a nmea gps position feed to the ais. That’s easily manipulated by anyone with a pc.

[CarinaPDX]

I really admire your paranoia, but I still believe it is wrong. If you think it was done to block manipulation then cite a source. Clearly the government was not going to count on boat owners' skills to connect GPS to an AIS, and rightly so. We have threads here from time to time where DSC VHF owners want to attach a GPS and the fun begins... Often there are just wrong answers given. Selecting the correct GPS receiver does require a bit of knowledge, and most GPS receivers on the open market do not fit the bill. As a practical matter AIS had to be sold as a single package for a plug-and-play solution. As I have said before, the GPS modules talk in NMEA 0183 - with exceptions for really old units like Garmin - although the signal levels may be 5V/0V, not RS-422 levels. It should be no problem to take an old AIS like mine and connect a different GPS module to it. But that is pointless - one friend of mine used an old VHF handheld he had around and hooked it up to his computer and used some software to decode AIS. To reverse that and create false signals is pretty simple for a reasonably able programmer - no need to hack an AIS box. But I am talking about the intent of the original designers/specifiers of AIS, and since then security is now a larger concern for all programs. I see no logical, or even security, reason to not output GPS locations - in fact it is downright dumb. All of the other ships' positions only make sense when compared to one's own ship position so what would be accomplished by hiding that? The explanation for that change would make interesting reading...

Greg