Comar AIS Receiver and Splitter

[roger.waite]

FWIW, one of the UK sailing magazines (Practical Boat Owner, I think) reviewed AIS units in the last few months. Main focus was on receive only units. Compares features, and detection ranges relative to a model chosen as a reference.

[Tom Spohn]

The SR161 is a very good receiver, but your display has to be able to receive RS232 signals. Works great with my Furuno, but some units may not be supported.

[fairbank56]

The Comar splitters have a fail-safe feature that keeps your VHF connected in the event of failure or power loss to the splitter. If you want the best of both worlds, install that extra antenna on the rail via a good quality coax switch. Then you can use the masthead antenna for both AIS and VHF and in the event of unlikely catastrophic failure of the splitter, you can switch to the back-up antenna for your VHF. If your worried about the expense and possible failure of a coax switch , don't use one. Just have the coax plug secured next to the VHF's antenna jack so you can manually switch it in an emergency.

Eric

[goboatingnow]

Lets face it - do you really want to compromise a not inexpensive device designed to improve your safety at sea?

Sorry, that statement could be applied to all electronics on your boat. so what. The fundemental point is that an active splitter is very useful for class B transponders as it allows you to use your masthead mounted antenna. Most of the time that antenna is transmitting nothing, might as well use it for AIS.

Since all you good people are carrying a VHF emergency antenna, anyway, you have a backup if the spliiter fails ( which it wouldnt anyway).

why do people constantly run down a piece of technology simply becuase it isnt what they would use.!

[Watermann]

Originally Posted by bewitched

Much better to avoid them - I'm definately with you on that one - most of the time you can avoid a 'risk of collision' situation before it even develops.

But the reference to big ships not keeping a proper watch - what are the views on this? I would have thought that now that AIS is required on everything over 300 tonnes (I think) that it actually improves the situation. Wouldn't they just set the proximity alarm on the AIS, set the proximity alarm on the Radar and then anything that has a decent radar signal and AIS transponder is pretty much always going to set off an alarm.

Can anyone with experience of being on the bridge of these types of vessels comment? What is standard operating procedure?

Hi bewitched,

I am a new poster here and a deck officer on American-flagged merchant vessels. I can tell you generally (very generally) what our operating standards are (or at least, my personal ones, as this can vary ship to, and captain to captain). I grew up sailing and racing and did so through college too (currently have a wooden trawler, though), so I have perspective on both sides of the integration of pleasure and commercial traffic.

First, unlike some European and a lot of Asian countries, where one-man watches are perfectly acceptable, we keep at least two people on watch when at sea - a watch officer (mate) and a lookout/helmsman, usually an AB. Steering of the ship is generally left to the autopilot and its degree of integration can vary.

Second, in regards to AIS, it is a very powerful tool, but, like any other piece of electronics does not replace a good set of eyes, knowledge and experience.

Equipment can and does vary ship to ship and how the AIS is integrated into the other systems (if it is at all) influences what kind of AIS "watch" you are capable of. The legal bare-minimum display is the unit's own stand-alone LCD that usually just lists target names (or MMSI#), range and true bearing. Some ships still only have that display. Increasingly, especially as older radars and chart systems are replaced with newer units, AIS is integrated with other equipment and targets can be overlayed onto radar and the ECS/ECDIS. In this case, target limits (CPA in miles and TCPA in minutes) can be set by the watch officer.

In open sea with little or no traffic, I prefer limits of 1.0nm at 12 minutes. I set the ARPA target limits to the same, along with the vector lengths to the same time as the TCPA (ie, everything is 12 minutes). I also like to aquire AIS targets as ARPA targets as well. The computed CPA and TCPA between the two can vary quite a bit, particularly in crossing situations. I do not regularly utilize the radar's guard zones, but I know some officers who do. The radars themselves are generally set at 12 and 24 miles for the X and S-Bands respectively. I prefer north-up, relative motion with true vectors and relative trails. Of course, this is adjusted as conditions warrant.

The Master's standing orders may command a minimum CPA greater than 1.0. As traffic density increases or you're making landfall into pilotage waters, these limits are gradually decreased. The objective is to find a balance where you're getting an early enough warning of a potential risk of collision, but not answering so many alarms that you begin to ignore it or just disable it all together. These alarms are both visual and aural and can get annoying if sounding repeatedly.

I hear often a lot of claims back and forth on ships "turning off" Class B targets. In my experiences, this has never had to happen and I'm not even aware of any AIS receivers that can do that easily (if they can, it's buried in some menu). HOWEVER, the radars and ECDIS certainly can filter out Class B (or Class A, if needed) targets on their own displays, usually with one click. Given that bridge crews do 99% of their AIS monitoring from these displays, it should certainly be kept in mind by those with Class B transceivers that you are one mouse click away from being filtered out. It is not done routinely, but the equipment certainly is capable of doing it.

I've probably over-complicated this post, but that's just because there are still differences in operating procedures throughout the shipping world and they all vary with different nationalities, companies, vessels and masters. It's hard to say flat-out what the "standard operating procedures" are for large ships.

I will close out with saying that even though we have a lot of "toys", there is still much emphasis being placed on good visual look-outs. We're fortunate to have very commanding views of the water around us and big windows - I like to take advantage of them. Simply saying that "I would have thought that now that AIS is required on everything over 300 tonnes (I think) that it actually improves the situation" is a recipe for complacency and an eventual problem. Just like when driving a car, I prefer to be on the defense, operating within the confines of the law, company policy and the master's orders.

If you have any more questions regarding this, please let me know. I'm happy to offer my perspective and experience.

[goboatingnow]

IMO specified equipment cannot disable class A or B targets. What it does allow is not dangerous targets to be ignored.

[Watermann]

While that is true of the AIS unit itself, AIS target overlays on the radars and ECDIS certainly can be filtered by class type. The target still exists on the AIS, but it is simply not displayed.

Also, remember that 'dangerous targets' are as specified by the user. The appropriate CPA and TCPA must be set by the user. In this sense, it is entirely possible to have all of your targets considered 'dangerous' or none of them (min CPA and/or TCPA set to zero).

[bewitched]

Hi Waterman - Thanks for that in depth post - very much appreciated.

Can you comment on the extent to which sailing boats carrying of Class B Transponders have become more 'detectable' to you. Is it a significant improvement over a good radar signal?

[Watermann]

bewitched,

In my experiences, recreational boats of all types with AIS are still few and far between, but it is becoming more popular, particularly with offshore cruisers who may or may not be soloing. There seem to be a lot more Class B equipped vessels in Europe and parts of Asia than in U.S. waters, but much of that, I suspect, is due to regulation (or impending regulation) regarding mandatory Class B carrying vessels.

A Class B target, on the surface, looks the same as a Class A - you get the symbol, vector, nav data, and our unit computes the CPA and TCPA. Frankly, that's all I care about. While it can be nice to know vessel type, destination and all that jazz, what it really comes down to is avoiding collisions. At a BARE MINIMUM the presence of an AIS target (of any kind) on my display is enough to warrant a visual look, especially if this is the first time I have noticed you. That right there is good enough reason to install a Class B system, if you're on the fence of doing so (sidenote - small boats with white hulls are the worst at picking out of a sea that is white with foam, especially if there is glare on the water). Class A of course, is more sophisticated, but B is better than nothing. Even just having a name to use to call on the radio is very handy (one of my favorite benefits of AIS - no more "calling so-and-so at this lat/lon on course blah blah blah, speed blah blah").

Now, as for AIS detection vs. radar (and subsequently, ARPA), there is some stuff to consider. First is that AIS targets typically are detected at a greater range (often, well beyond the geographic horizon), particularly at times when sea or rain clutter is high and small vessels at longer ranges may be masked from radar. As I mentioned earlier, I am of the habit of plotting the actual radar returns of AIS targets in order to get ARPA data as well. It's interesting to compare the data side-by-side and there often are discrepencies, although, usually, not significant enough to alter the situation - that's partially why we have minimum CPAs. Notwithstanding the AIS data transmission intervals (and these vary between the Classes), AIS data is more "instant" than ARPA (for a computer fed with accurate numbers, it's all basic vector analysis), particularly when both or either vessel make course or speed changes. ARPA data (once acquired) is only updated once per antenna sweep and major alterations in the course or speed of either vessel takes time to become apparent on radar and only then can the ARPA computer make its newest calculation. At very close ranges or in the case of lots of clutter, ARPA data becomes more and more degraded before it can finally be considered unusable (you should already be looking out the window).

On the other hand, one of the major things to keep in mind with regards to AIS is that you're relying on someone else to have and provide accurate navigation data, transmit that to you and your receiver interprets that data correctly. Radar is all controlled from your own ship. It's your signal going out, being received and interpreted on your own display and equipment. All else being equal, if push comes to shove, I am much more tempted to trust radar. I have seen some wild static data from other ships' AIS units, especially in the dimensions or GPS antenna locations. Both of these are critical to accurate AIS CPA and TCPA calculations. If entered inaccurately, the units are basically calculating the CPA's of the two vessel's GPS antennas. Given the accuracy of GPS these days, in a situation with two 1000' ships crossing at right angles, this can introduce a significant error, leading to the vessels getting closer than they should have.

In short, there are advantages to both sides that come about if a small vessel installs Class B AIS (or Class A, for that matter). But there are some significant limitations and this should not be lost on the users aboard either vessel. I would also always install bigger, brighter navigation lights and a good, effective radar reflector before or along with Class B AIS. I understand the power/rigging limitations of an offshore sailor, but these things do really make a difference.

If you ever have the opportunity to ride a ship on the bridge, take it. It usually is an eye-opener into how small and insignificant even "big" boats are to a ship. You'll be able to see how the bridge team incorporates their natural senses with technology and how that leads to different decisions made based on their knowledge and experience.

Again, sorry for the length, but I hope this answered your question.

[roger.waite]

Originally Posted by bewitched

Now fairbank56 has explained the system, I can see benefits of both set ups:

Use a splitter and get better range

Use a dedicated antenna and have less bits to go wrong (and have a 2nd emergency antenna ready to go incidentally).

Sound right IMO. Unless we put in or have a second masthead antenna. Offshore, in particular, I want detection range.

[chrisjs]

I wrestled with the same issues recently and decided in favor of a Comar receive only unit ($330) with a dedicated masthead antenna - though since we are trawler folks our mast is only 26ft!
I was tempted to go for the West Marine Class B transceiver at about $500 but decided against it for 2 reasons. Firstly, it was unclear whether the WM unit is a dual channel unit (I suspect it is not) whereas the Comar is a true dual channel unit and updates twice as frequently as a single channel unit. There is a good discussion of this issue on the Milltech Marine web site. Secondly, since I already had a spare coax going up the mast, it was easy to install a dedicated AIS antenna next to 2 existing VHF antennas at the masthead. However after discussions with antenna companies I came to understand that if I installed an AIS transceiver, there was a good chance that either the AIS transmission would fry the VHF or the VHF transmission would fry the AIS. Not a good scenario. Apparently the antennas need to be at least 6ft above or below one another to avoid problems. Hence the perennial boat compromise - receive only, watch out for the big guys, and hope to see them when they are far off because of better signal reception and more frequent updating of the AIS.

[Paul Elliott]

The West Marine transponder contains a true dual-channel receiver (and transmitter). As a matter of fact, *all* transponders, Class A and Class B, are dual-channel devices. Some receive-only units are single-channel-at-a-time designs, but all transponders must be dual-channel.

I've got an ACR Class-B transponder, with its antenna mounted on the upper spreader (my regular VHF antenna is at the top of the mast). Prior to that, I had a receive-only unit with the antenna on the stern rail. Both antenna locations gave quite usable range, but the spreader antenna is slightly better.

Even with the transponder, you still should watch out for the big guys!

[richardhula]

Originally Posted by richardhula

Lets face it - do you really want to compromise a not inexpensive device designed to improve your safety at sea?

Originally Posted by goboatingnow

Sorry, that statement could be applied to all electronics on your boat. so what.

Most of the time that antenna is transmitting nothing, might as well use it for AIS.

Not being used for transmitting much but in use for receiving 100% of the time.

why do people constantly run down a piece of technology simply becuase it isnt what they would use.!

Not generally my policy but in this case the concept is so flawed. Even if the splitter could be hypothetically 100% reliable with no signal losses, what about reliability & losses of extra connectors and cable?

I may be wrong but I doubt you would see antenna splitters in use in a commercial environment.

[MarkJ]

The concept of AIS isn't flawed at all. Its probably the most wonderful advance since GPS!

We want a good Class B transmitter and I want it right on the helm in my chart plotter.

with safety stuff so basic as this I wionder why the governmenst dont subsidise it?


the other thing about AIS is with AIS you can hit anything and it wont sink you immediately, small fishing boats, logs. Only without AIS can you die 'instantly'.

[donradcliffe]

We have the West Marine Class B transponder with a dedicated pushpit antenna, plus a Watchmate display/alarm unit. It as an issue with averaging vs instantaneous SOG/COG data from the built-in GPS, but so does the Comar, and it will eventually be sorted out.

It is a big step up from my single channel recieve only unit, for not much more money. I have the AIS mounted near the VHF, and can switch the antennas if I think the AIS range is an issue, but the ships seem to pick me up about 10 miles out--compared to about 5-6 on radar or 2-4 on visual.

AIS transponders are getting more popular by the week--I just passed English Harbor in Antigua, and picked up 36 transponders--all from yachts. If this trend continues, I can see where the commerical shipping would use a filter near ports--I just hope they remember to turn it off in the open sea.

The Watchmate has 4 alarm modes--anchor/harbor/coastal/offshore. The default on the offshore was a CPA of 2 miles, but I have cut it down to 1. I put in a switchable external 105 db alarm for the offshore work, and it will wake up the entire boat if something big is bearing down on us.