Shutting down AIS

[Karanga]

Ok, I did a search on AIS base stations.

I saw many references to base stations receiving and perhaps forwarding traffic information to other base stations or traffic management centres.
I did not see any references to active interrogation of ship-borne units, and no references to modifying the behaviour of such ship-borne units. It may be that the time-division slots are manipulated (I don't know).

So, I remain dim (un-enlightened...)!

[Auspicious]

Originally Posted by StuM

AIS does not use a "transponder", it uses a transmitter and receiver.
(A transponder is a device that transmits a different signal when it receives a signal. It is portmanteau word made up from transmitter + responder). Although many manufacturers do incorrectly call their devices transponders.

The terminology is actually correct. AIS units do respond to external commands and can exchange automatic text messages and even data intended to be forwarded to other devices on the network. See AIS messages 6, 7, 10, 11, 12, 13, 15, 16, 20, 22, 23, and 25 in the IMO/IEC AIS spec or the summary on Wikipedia. AIS devices are transponders.

Aren't y'all glad you asked? *grin*

That said, I'm not sure what the specifiers had in mind and I haven't seen the capabilities used for much more than time slot management.

[Paul L]

Originally Posted by Karanga

Ok, I did a search on AIS base stations.

I saw many references to base stations receiving and perhaps forwarding traffic information to other base stations or traffic management centres.
I did not see any references to active interrogation of ship-borne units, and no references to modifying the behaviour of such ship-borne units. It may be that the time-division slots are manipulated (I don't know).

So, I remain dim (un-enlightened...)!

Ok, here's an example of base station Ais management gone awry.

USCG: Caution to AIS Users

MARINE SAFETY ALERT 7-10 --CAUTION TO AIS USERS NAVIGATING THE JAMES RIVER, YORK RIVER, UPPER CHESAPEAKE BAY, DELAWARE BAY, NEW JERSEY SHORE, AND, NEW YORK HARBOR AND APPROACHES

YOU MAY BE INADVERTENTLY OPERATING ON DIFFERENT AIS CHANNELS

Between July 27 and August 19, 2010, while conducting development testing of its Nationwide Automatic Identification System (NAIS), the Coast Guard inadvertently tele-commanded most AIS users transiting the Eastern United States between lower Connecticut and North Carolina to switch to AIS frequencies other than the AIS default frequencies (161.975 MHz - Channel 87B - 2087 and 162.025 MHz - Channel 88B - 2088). As a result, those users within uniquely defined channel management regions (as shown in the picture) will neither see nor be seen by vessels operating on the default AIS channels when within these regions. Similarly, vessels operating on default frequencies will not see or be seen by those vessels that were inadvertently switched to other frequencies. No other AIS users or areas are impacted.

One of the lesser known and potent features of AIS is its ability to operate on multiple channels within the VHF-FM marine band. This frequency agility ensures AIS can be used even when the default channels are otherwise unavailable or compromised. In such conditions, competent authorities, such as the Coast Guard, can use an AIS base station to tele-command shipborne AIS devices to switch to other more appropriate channels when within defined regions of 200 to 2000 square nautical miles. This can be done automatically (and without user intervention) through receipt of the AIS channel management message (AIS message 22) or manually entered via the AIS Minimal Keyboard Display (MKD) or similar input device. Once commanded or manually entered, the channel management information will stay in memory for 5 weeks or until an affected vessel moves more than 500 nautical miles from the defined region. AIS channel management commands can only be manually overridden or erased by the user via the unit's channel (regional frequencies) management function or automatically overridden via another channel management message for the same defined region. Reinitializing or resetting your AIS or transmission channels will not necessarily reprogram your unit back to the default channels.

Commencing September 1st and continuing for the subsequent 5 weeks, the Coast Guard will broadcast new channel management messages that will tele-command all AIS users back to default channels. This broadcast will occur each hour between hh.05:30 and hh.05:59, but may change as needed. To ensure that these messages are received, they will be broadcast on Channel 70--Digital Selective Calling (DSC), which is also monitored by all type-certified shipboard AIS. While this will ensure all AIS users will get the message regardless of what AIS channel the unit is operating on, it could however cause a minor inconvenience to owners of older DSC radios who may receive an alert (tone) upon receipt of this message. It will have no other effect on DSC radios.

AIS users are encouraged to inform others whom they believe may be affected and are therefore not being seen by others. All AIS users are reminded to maintain their AIS in effective operating condition and to validate their AIS data prior to each voyage and as needed.

This safety alert is provided for informational purposes only and does not relieve any domestic or international safety, operational or material requirement. The Coast Guard has developed policy and procedures to ensure such inadvertent broadcast do not happen again and we apologize for any inconvenience this may have caused. For further information on AIS Channel Management or reprogramming your AIS read FAQ#19 at www.navcen.uscg.gov/?pageName=AISFAQ#19 or contact cgnav@uscg.mil. Developed by the Office of Waterways Management, U.S. Coast Guard Headquarters, Washington, DC.

[Karanga]

Wow!
I investigated further, and indeed found there are more sentences available within the AIS stream, one of which could allow a base station to interrogate up to 2 units regarding status.
I didn't see a reference to changing frequencies. But that didn't surprise me. Who or what agency in its right mind would change the frequency of a unit? Therefore making it invisible to other boats/ships around it and potentially conflicting with? Plus making it impossible to "see" others?
So I disregarded that notion as fanciful and not reasonable.
But now I read it is indeed possible. And has been done. And they screwed it up!
So I am enlightened, thank you.
I am also a bit shocked that it is possible.

Tim

[Orion Jim]

A couple years ago, on a trip from New England to Bermuda, I observed the lights of a large vessel in the predawn darkness....no AIS signal. Her course and speed kept changing so I sat in the cockpit with my binoculars waiting for dawn. Dawn reveals a naval vessel a couple miles east of me which came to a full halt. All of a sudden she transmitted an AIS signal....and revealed her name....”Canadian Naval Vessel“.

[Pelagic]

Originally Posted by Orion Jim

A couple years ago, on a trip from New England to Bermuda, I observed the lights of a large vessel in the predawn darkness....no AIS signal. Her course and speed kept changing so I sat in the cockpit with my binoculars waiting for dawn. Dawn reveals a naval vessel a couple miles east of me which came to a full halt. All of a sudden she transmitted an AIS signal....and revealed her name....”Canadian Naval Vessel“.

Probably participating in some kind of excersise with US defenses

[jeffrobbins]

Sorry, just getting back to this thread. But it looks like you've found the relevant references.

Anyway, yes your transponder can be sent various messages and commands from other AIS units including base stations, search and rescue, and even other vessels. These include binary and text messages which your AIS automatically acknowledges, without your intervention, and it can also be interrogated to return your position or vessel details (name, type, etc).

Base stations can assign your AIS to transmit its position reports at different intervals, change channels, change the transmit/receive mode (eg. listen on both channels but transmit on just one) or put it into a quiet mode for a period of time. It can do this geographically by vessel type, group vessels together, or to an individual vessel.

This is mostly intended to provide a mechanism for controlling channel overloading. It also uses DSC as a backup.

[skipperpete]

After reading the preliminary report regarding the "Helge Ingstad" collision with "Sola-TS I was surprised to learn that warships transmit encrypted W/AIS that can be read by VTS and other stations that have the decription facility but by no one else.

[gatorman49]

Western Rivers seems to see excellent utilization of AIS. Towing vessels are able to receive AIS signals around bends, especially on LMR where radar signals are blocked.

Agree that it’s another tool.

Same controversy in the air with ADS-B. Signals are not encrypted. Hobbyist at home with simple receiver can receive identifying from any operating mode B transponder including military aircraft. A problem as military aircraft do operate under ATC throughout the world unless in a shooting zone.

B52s leaving Guam in VN war heading to NVN actually filed flight plans with ATC.

[David Mathis]

The opposite problem is bringing on a tend to disregard AIS or turn off the receiver. I've noticed an increase in boats docked at marinas leaving the AIS on continuously and others operating in crowded harbors (20-30 boats within a mile leaving and returning to marinas on a busy sunny day) transmitting AIS. As we approach a marina, small crowded bay, or harbor, we often get an AIS alarm. Unfortunately, we have a Lowrance VHF which is not user friendly and making changes, other than channel or volume, requires taking my eyes away from the water for longer than I should in congested areas. This has happened so frequently that the AIS receiver has been shut off. AIS is a great tool for use in open water, at night, or times of poor visibility but, unfortunately, it has become the marine version of "the boy that cried wolf".

[CapnCrunch]

Originally Posted by gatorman49

LMR where radar signals are blocked.

I was not aware of this. Can you elaborate? You are correct. AIS is very useful on the western rivers when communicating with tows. However, I would only use radar on the rivers in foggy conditions which, thankfully, I have yet to encounter.

[SaltyMetals]

I have found that the CG on S coast of Spain and CG near Sicily are also in stealth mode. Not sure if this is standard practice or just in those areas where they are trying to catch people-smugglers. One of the CG's who approached me only turned on its lights at the last minute. Both could clearly see my AIS signal and identity.
Andrew

[Pelagic]

My take on this discussion is that while AIS is a great Aid to be used in collision avoidance, it must not be relied upon for early detection (8-10nm) at Sea......For that, there is no substitution for Radar lookout.

Reasons:
1. Patrol vls turn off for Stealth purposes

2. Fishing vls the same to protect their fishing strategy

3. In areas of known Piracy, many commercial and most yachts power down AIS because of security concerns.

#3 will become the contentious issue in Asia.

[Mike Crothers]

As time goes on I expect law enforcement will be more likely to board you if you fail to transmit AIS. Not that transmitting AIS may be required but I believe transmitting certainly indicates you are not trying to hide anything. That said we have been approached twice by C&PB twice while broadcasting so go figure.

[Mike OReilly]

Originally Posted by Pelagic

My take on this discussion is that while AIS is a great Aid to be used in collision avoidance, it must not be relied upon for early detection (8-10nm) at Sea......For that, there is no substitution for Radar lookout.

This has always been, and remains, my view. AIS is useful, but it should never be the sole or even primary collision avoidance tool. Much like chartplotters, they are an approximation of reality. They may, or may not, be showing the whole picture. I'd never put all my navigational eggs in either basket.

Originally Posted by Mike Crothers

As time goes on I expect law enforcement will be more likely to board you if you fail to transmit AIS. Not that transmitting AIS may be required but I believe transmitting certainly indicates you are not trying to hide anything. That said we have been approached twice by C&PB twice while broadcasting so go figure.

I know of no evidence to support this, but you may be correct in some areas. Regardless, I would still not willingly choose to transmit AIS signals. If the cost of this is more boardings, then so be it.