Why do VHF radios only come with a AIS receiver not a transceiver?

[roland stockham]

This is apples and pears. The AIS component in a VHF radio is designed to use mis number to communicate with other radios and to relay a is broadcasts such as distress messages. The AIS transponder broadcasts and receives the target location component of AIS. If you were to combine it in another istrument I would suggest it would be the chartplottter since the goal it to display target positions on a chart. Most radios do not include a screen that can do that

[svmobert]

Originally Posted by roland stockham

This is apples and pears. The AIS component in a VHF radio is designed to use mis number to communicate with other radios and to relay a is broadcasts such as distress messages. The AIS transponder broadcasts and receives the target location component of AIS. If you were to combine it in another istrument I would suggest it would be the chartplottter since the goal it to display target positions on a chart. Most radios do not include a screen that can do that

Actually, in general, the VHF radios that have an AIS receiver built-in also have a radar like screen to show the targets in relation to your position, and allow you to select them for calling So they do have that.

That said, the screen is relatively small and there are no charts or other relevant information, so yes, Chartplotter is better.

I would said if they can get the cost of the AIS add-on to $100 or less on top of the cost of the VHF and/or Chartplotter that they embed it into, then combining would be a no brainer. But if adding an AIS transponder also adds $500 to the cost (or the price), then I think the market gets minimized and hence the manufacturers have less of a reason to do it. Embedding in the chartplotter means I need to be in the market for a new chartplotter AND a new AIS transponder at the same time, a slice of the market, not the whole AIS market. A separate AIS transponder fills the entire market need.

[tanglewood]

One issue with a combined VHF+AIS transponder is what happens when you have more than one of them on your boat. As boats get larger, it's very common to have multiple VHFs. Now you have two devices transmitting boat info? And does each receiver correctly handle reception of it's own boat info coming from another transmitter. Based on what I've seen so far, I would expect the vendors to totally f&*(ck this up. Heck Icom still doesn't have the M506 working correctly on N2K with two stations, and that's receive only. And that's been out for how long? 5 years?

[transmitterdan]

It would seem that having two active AIS transmitters on the same vessel is at least a violation of the spirit if not the letter of the regulations. All of the protocols and rules surrounding AIS presuppose one and only one active transmitter per vessel. Receivers are obligated to keep historical data indexed by MMSI in order to give an accurate picture of a vessel's situation. It's impossible, under the present ITU AIS recommendations, to have two transmitters active at once without causing confusion at the receiver. So I believe it would most likely be a rule violation to have two active transmitters with same MMSI. If two AIS transmitters are on board one must be kept in no-transmit mode.

Two active receivers isn't a rule violation since receivers aren't regulated like transmitters. But certainly, the way NMEA messages are dispatched from all receivers in real time precludes any sane method of dealing with two receivers on the same MFD.

[tanglewood]

Originally Posted by transmitterdan

Two active receivers isn't a rule violation since receivers aren't regulated like transmitters. But certainly, the way NMEA messages are dispatched from all receivers in real time precludes any sane method of dealing with two receivers on the same MFD.

Yes, that's even a problem with dual receivers, and I encountered exactly that same issue some time back. I ran into vagueries over whether your own MMSI messages should be filtered by the AIS receiver, or by the display device. I found plotters, for example, that assume your own MMSI traffic would be filtered by the AIS device, and an AIS device that assumed it would be filtered by the plotter. The result was a continuous collision alarm with yourself. At the time I thought a backup AIS receiver in my VHF was a good complement to my stand alone transceiver, but discovered that the vendors had apparently never considered such a scenario. The solution was to swap the VHFs for models without AIS receivers.


Sometimes single purpose devices work a lot better, and there is a lot less to replace when they go bad or get outdated.

[transmitterdan]

Tangle,

The vendors can do nothing about the problem. The root cause is the AIS standard itself. Receivers do not get all the vessel data in one go. It takes time for the complete vessel data set to populate. That's because the standard is written that way due to the nature of the system (Time Division Multiplex). Why go around blaming vendors or software for what is an inherent feature of the system specification?

The system specification assumes one and only transmit device per unique MMSI. The US has finally allowed more than one MMSI per vessel to handle DSC radios and PLBs. Not sure about the rest of the world.

[tanglewood]

Originally Posted by transmitterdan

Tangle,

The vendors can do nothing about the problem. The root cause is the AIS standard itself. Receivers do not get all the vessel data in one go. It takes time for the complete vessel data set to populate. That's because the standard is written that way due to the nature of the system (Time Division Multiplex). Why go around blaming vendors or software for what is an inherent feature of the system specification?

The system specification assumes one and only transmit device per unique MMSI. The US has finally allowed more than one MMSI per vessel to handle DSC radios and PLBs. Not sure about the rest of the world.

I'm not understanding why fragmented receive messages cause any problem. All are self-describing with the MMSI (required for reassembly anyway). So even when they come in scattered over time, you still know who they pertain to.



I think the problem emerged because nobody ever thought about it, so it was never specified how to handle. To be specific, I'm talking about filtering out your own ship's messages rather than showing them as a colliding vessel, and in particular how that should be done when there are multiple AIS receivers. Heck, AIS receivers aren't even a specified standard. Colloquially people refer to them as Class C, but there is actually no such thing as Class C. It's just a marketing make-believe term.


With a transceiver, I'm pretty sure own-ship messages are tagged, so that's specified and can be handled. But not necessarily to case for receive only. At a minimum, the receiver needs to have you MMSI number if it's going to tag or filter. Yet the norm with AIS is that in the absence of an MMSI, you continue to receive and only suspend transmission. And that in turn creates the problem.

[transmitterdan]

A transceiver unit should never hear its own MMSI because the receive section is blanked during transmission. So hearing one's own MMSI can only happen if two receivers are on board. We already know that's a bad idea. But there are other reasons not to have multiple receivers.

Fragmented messages are just how it works. The ship information like name and other parameters are sent much less frequently than the navigation data (speed, location, heading, etc.). So a display must hold a data base of information to match up with incoming messages before it can show the full vessel status. Multiple receivers may output similar but not exactly the same information thus confusing a display. Also, the duplicate messages generate unnecessary waste of memory and processing time. Every position message will not be an exact duplicate because the relative location of the targets will never match. That's because the receive antenna locations on your vessel are approximated by you. So the relative position of a target when reported by two receivers will likely jump around and this can cause displays to get confused if they try to compute relative closing speed and bearing. It's impossible to avoid the problems. Don't use multiple AIS systems at the same time is all I can recommend.

[svmobert]

I am at the Pacific Marine Expo and ran into the USCG person responsible for regulating AIS. Since I had the chance I asked about VHF/AIS combo units. First, he claims that no manufacturers have actually tried to submit a VHF with AIS transponder for type-certification as of yet. While certification would be an uphill battle anyway, he also gave me some insight into why that might be an issue.

First, with two transmitters in the same case, you'd have a whole bunch of shielding issues to deal with in a small space. Not insurmountable but definitely a design consideration.

Second, any unit with a splitter built-in would automatically break the AIS certification, because the certification requires that AIS transmissions occur at minimum intervals and anything that can prevent transmission for longer than that interval would break it. Antenna splitters themselves are not regulated, they are not allowed nor disallowed. All the FCC/USCG would have to do is hold down the mic button for long enough to break the minimum interval requirement and the unit would fail certification.

Regarding AIS/VHF Splitters I asked for an opinion and he said that in general he was against them, but depends on the use case. Two antennas (VHF and AIS) are supposed to be at least 30ft apart which is not possible on a lot of boats, and he prefers maximum height where possible. Translating that, for a single mast sloop, he would prefer a shared masthead antenna with splitter vs two antennas with the AIS being on the stern rail or somewhere low.

Where a vessel has two or more masts, or larger vessels, then two high mounted antennas is preferred.

Lastly, he shared that all of this will likely be solved when the future digital VHF standards start implementation. The radios will become more software defined, VHF, DSC, and AIS would all be implemented in software, with more data being transmitted/received over the air, and at this point any problems with coordinating VHF, DSC, and AIS transmissions and reception would be mitigated in software, rendering the current issues moot.

Just thought I would share what I learned.

[aeronautic1]

Can you imagine 10’s of thousands of unregistered AIS transmitters floating around the US. You think a keyed mic on CH 16 is annoying, imaging all the false alerts with no record of contacts that the USCG would have to respond to. In case you forgot, recreation boats do not have to register their VHF

[aeronautic1]

Good schnitt mon

[aeronautic1]

Good schnitt mon!

Originally Posted by Sea-TechSystems

I am at the Pacific Marine Expo and ran into the USCG person responsible for regulating AIS. Since I had the chance I asked about VHF/AIS combo units. First, he claims that no manufacturers have actually tried to submit a VHF with AIS transponder for type-certification as of yet. While certification would be an uphill battle anyway, he also gave me some insight into why that might be an issue.

First, with two transmitters in the same case, you'd have a whole bunch of shielding issues to deal with in a small space. Not insurmountable but definitely a design consideration.

Second, any unit with a splitter built-in would automatically break the AIS certification, because the certification requires that AIS transmissions occur at minimum intervals and anything that can prevent transmission for longer than that interval would break it. Antenna splitters themselves are not regulated, they are not allowed nor disallowed. All the FCC/USCG would have to do is hold down the mic button for long enough to break the minimum interval requirement and the unit would fail certification.

Regarding AIS/VHF Splitters I asked for an opinion and he said that in general he was against them, but depends on the use case. Two antennas (VHF and AIS) are supposed to be at least 30ft apart which is not possible on a lot of boats, and he prefers maximum height where possible. Translating that, for a single mast sloop, he would prefer a shared masthead antenna with splitter vs two antennas with the AIS being on the stern rail or somewhere low.

Where a vessel has two or more masts, or larger vessels, then two high mounted antennas is preferred.

Lastly, he shared that all of this will likely be solved when the future digital VHF standards start implementation. The radios will become more software defined, VHF, DSC, and AIS would all be implemented in software, with more data being transmitted/received over the air, and at this point any problems with coordinating VHF, DSC, and AIS transmissions and reception would be mitigated in software, rendering the current issues moot.

Just thought I would share what I learned.

[tanglewood]

Originally Posted by aeronautic1

Can you imagine 10’s of thousands of unregistered AIS transmitters floating around the US. You think a keyed mic on CH 16 is annoying, imaging all the false alerts with no record of contacts that the USCG would have to respond to. In case you forgot, recreation boats do not have to register their VHF

But an AIS device does need to be registered, so not really a problem.

[transmitterdan]

Originally Posted by tanglewood

But an AIS device does need to be registered, so not really a problem.

Actually all AIS transceivers need to be registered. In the US you must have an MMSI registered before purchasing an AIS transceiver so the MMSI can be loaded before delivery. I think all countries require MMSI registry. Otherwise, what good is AIS?

[continuouswave]

In the USA the FCC has already considered the problem of AIS transmitters that are not registered properly: the selling vendor MUST pre-configure the device with the customer's MMSI and GNSS sensor location data.