New price point for AIS class B+

[Brian.D]

Originally Posted by Marathon1150

... detecting other boats/ships and letting them know where we are before they are visible ...

I am sure I am stating the obvious, but AIS is only good if the other vessel is also using AIS. I think radar is a better choice because you can see a vessel (if big enough or uses a radar reflector) even in adverse conditions. JMHO

[Kelkara]

Originally Posted by Pete7

I suppose the question is if the output watts don't make any real difference, then only possible shortfall is the limit of 2000 AIS units wanting to transmit in small area. So how often does that happen?

It's a lot less than 2000 (there are actually two channels so over 4000 time slots per minute total) ... but remember that most class A units will be transmitting every 2-10s.

The way CSTDMA works is that the AIS unit decides when it wants it's next transmission (30s if moving >2kts or 3 min if slower) then selects ten random slots within 10s of that time and broadcasts in the first available one of those. If they are all taken it will abandon that transmission and wait another 30s or 3 mins (depending on boat speed) and try again.

The element of randomness has to be there to reduce clashes with other class B units. There is nothing else to prevent two class B units transmitting at the same time, risking both signals interfering.

I'm not in a crowded area, so there is absolutely no problem with the old CSTDMA class B signal ... I don't know what the threshhold is where transmissions regularly get dropped, but I presume it has become a problem in some locations or the new B+ standard would not have been developed.

[Simi 60]

Originally Posted by rslifkin

Definitely not triple here. A class B+ is just under $600 at the moment. A similarly featured class B unit is just over $400. So basically an extra $200. Once you factor in an antenna or splitter, any additional NMEA networking needed, etc. it's not a huge difference.

Cheapest I found in Australia was $1150 vs the $400 my class B cost

[Tillsbury]

I still don't quite get what significant advantage the B+ units have in the real world. It seems that all they are going to do is make it less likely that your boat will be found in the wrong place by others, when you are in a very crowded area. They don't make seeing others any better (as if others are class B they may also be delayed in a crowded area, and if class A they'll be visible whatever happens).

Under what situation are you relying on someone else finding you accurately on AIS, given that you're in a very crowded area and therefore at close quarters in a busy waterway?

I use AIS to see where and how fast the big ships are going when they're coming into and going out of harbour (and where they'll be in the future compared with my track). And I use AIS to see anyone within any distance when I'm out at sea (and I hope that I am more visible to them because of AIS). I don't care less how accurate the AIS position of another sailboat is (who is likely to be class B if they have AIS at all), because at close quarters I will look for him by eye.

I'm a bit of a tech nerd so I'm sure I would end up buying a B+ just for the sake of it if I didn't have AIS already, but I really can't see what its advantage is for a sailor.

[Brian.D]

Tillsbury, if one knows the time slot someone is using, AND, with RDF equipment, one can be triangulated and found. Simple as that.

[rslifkin]

In bad weather with radar and AIS, good AIS can be useful for knowing the name of that radar blob you may want to contact.

[Pete7]

Originally Posted by Kelkara

It's a lot less than 2000 (there are actually two channels so over 4000 time slots per minute total) ... but remember that most class A units will be transmitting every 2-10s.

Spent a couple of hours looking into this and came across this which I thought interesting and suggests that high traffic areas may not be such a big problem, unless anyone knows differently.

They use SOTDMA (Self-Organized TDMA) technology so that each automatically adjusts its transmissions to avoid interfering with others within range. In areas with high density shipping the system also shrinks the area of coverage when necessary to ensure that the system isn’t overloaded.


https://www.marineinsight.com/wp-con...Whitepaper.pdf

[Simi 60]

Originally Posted by rslifkin

In bad weather with radar and AIS, good AIS can be useful for knowing the name of that radar blob you may want to contact.

Only if that blob has AIS.
Most blobs don't.

[rslifkin]

Originally Posted by Simi 60

Only if that blob has AIS.
Most blobs don't.

Of course. That's why the ideal is to have both to gain the maximum possible information about what's going on around you.

[Cpt Pat]

Originally Posted by Kelkara

It's a lot less than 2000 (there are actually two channels so over 4000 time slots per minute total) ... but remember that most class A units will be transmitting every 2-10s.

The way CSTDMA works is that the AIS unit decides when it wants it's next transmission (30s if moving >2kts or 3 min if slower) then selects ten random slots within 10s of that time and broadcasts in the first available one of those. If they are all taken it will abandon that transmission and wait another 30s or 3 mins (depending on boat speed) and try again.

The element of randomness has to be there to reduce clashes with other class B units. There is nothing else to prevent two class B units transmitting at the same time, risking both signals interfering.

I'm not in a crowded area, so there is absolutely no problem with the old CSTDMA class B signal ... I don't know what the threshhold is where transmissions regularly get dropped, but I presume it has become a problem in some locations or the new B+ standard would not have been developed.

...and if you have some device aboard that produces high levels of radio noise, like a bad tri-color LED next to your AIS antenna at the masthead, your CSTDMA transponder will sense a "carrier" is always present and stop transmitting entirely for however long that LED is turned on. I experienced that.

[Dockhead]

Originally Posted by Pete7

. . . I suppose the question is if the output watts don't make any real difference, then only possible shortfall is the limit of 2000 AIS units wanting to transmit in small area. So how often does that happen?. .

SOTDMA AIS ("B+") has profound advantages over the older type ("B"). The messages get priority in the same way as Class A and are far less likely to be dropped. The difference in cost is really worth it. The obsolete Class B units are cheap only because they are -- obsolete.

[Dockhead]

Originally Posted by Kelkara

. . . but I presume it has become a problem in some locations or the new B+ standard would not have been developed.

As I understand the story, the whole system was intended to be SOTDMA from the beginning, but there was a patent/IP problem which prevented implementation of SOTDMA on economical B class transponders, hence the make-do solution of CSTDMA.

[Cpt Pat]

Originally Posted by Pete7

Spent a couple of hours looking into this and came across this which I thought interesting and suggests that high traffic areas may not be such a big problem, unless anyone knows differently.

They use SOTDMA (Self-Organized TDMA) technology so that each automatically adjusts its transmissions to avoid interfering with others within range. In areas with high density shipping the system also shrinks the area of coverage when necessary to ensure that the system isn’t overloaded.


https://www.marineinsight.com/wp-con...Whitepaper.pdf

Because of that SOTDMA adaptive radius behavior, the number of vessels that can be supported in the RF transport protocol is practically limitless. There are 4,500 time-slots per minute. Time-slot contentioning (data packet collisions) is controlled by the FM capture effect: receivers reject weaker and presumably more distant transmissions in the same time-slot. (From the perspective of a network designer, this collision behavior is rater inelegant because it is RF power dependent.)

The real limit is how many targets the receiving AIS equipment can transmit via serial NMEA protocol to the display (where the receiver and display are separate platforms), store in the display equipment state table, and then process. That number will vary considerably by the quality and capabilities of the display technology used. Cheaper AIS displays can saturate at around 100 targets, and some don't behave predictably once reaching target saturation (I know of one display that alternately locks up or reboots). All that data has to be stored in limited memory and processed with limited CPUs.

AIS receivers aren't RF channel constrained, they are serial transport, memory, and CPU constrained. You're unlikely to find those limits listed in cheaper display specifications.

[boatpoker]

Originally Posted by Cpt Pat

I just noticed the OP is in Canada. Canada can be even more persnickety than the US about uncertified radio devices.

Yeah but Bobby is only part time and he has 3.855 million square miles to cover

[Cpt Pat]

(Session timed out)

One of the purposes of the USCG AIS base stations is to manage the AIS transmissions near busy ports. Those base stations can control the AIS message traffic in a region, which will hopefully reduce the number of packet collisions. They can also direct the AIS vessel transmitters to use an alternate channel when there is interference on the two primary channels (cheap AIS transponder and receive-only units may not recognize those instructions - causing all redirected targets to vanish). The commands are sent on VHF channel 70 (the DSC channel). That's why all compliant AIS transponders (and any good AIS receive-only unit) will incorporate a channel 70 receiver.

The USCG botched up once, causing AIS vessel transponders to switch to alternate channels -- and then leaving them on those channels. Whereupon they sailed away transmitting indefinitely on the "wrong" channels. Oops.
https://www.navcen.uscg.gov/pdf/AIS/...0_Excerpts.pdf

The solution: The Microsoft cure-all -- reboot your AIS receiver/transponder.


BTW, as a cybersecurity practitioner, I see an AIS network vulnerability here (among many others): a Bad Guy could transmit the same command, and disrupt AIS service. There is no authentication between the transmitter and AIS vessel units built into the base station management commands, and the USCG has here provided the Bad Guys with a proof-of-concept test. We live in a dangerous world...