AIS: Standalone or Overlay

[duncan_ellison]

+1 Overlay
+1 Transponder

Raymarine here and the overlay of radar, AIS and chart is awsome for coastal work.

Put your AIS antenna as high up as possible. Don't be persuaded to put it on the pushpit rail unless there's absolutely nowhere else to put it. You will get MUCH greater distance if you can get the antenna atop the mast even toi the extent of using a splitter.

FWIW, despite the negativity on this forum about AIS/VHF antenna splitters, I would go the same way again. There are some very good ACTIVE ones out there designed for this exact purpose and they neither interfere with your VHF or the AIS in any meaningful way. Just be sure that if you get an AIS transponder you also get a splitter which is designed for AIS Tx - some are designed for recieve only.

Duncan
S/V TALISA

[beetle]

Originally Posted by goboatingnow

Cant see why youd never not use an overlay, it is the only way, The only reason to non-overlay , is that you dont have the technology to use overlays

If the only mechanism you have to view AIS data (overlay or otherwise) is on a laptop, then the laptop is a single point of failure that is both fragile and expensive to operate (from an electron-consumption viewpoint).

That's why it can be desireable to operate a stand-alone AIS display unit that is small, hardened, and has very low power consumption - you can operate the AIS continously without expending the energy on a laptop.

There's also little value in a laptop's chartplotter capability mid-ocean as there's usually not much relevant information on a chart of, say, mid-Pacific 800 miles off the coast, therefore little additional information is obtained by overlaying an AIS display on a chart plotter.

The same is true of a radar - overlaying radar is nice, but why operate two devices (the radar display and the laptop display) when you get all the information you need from just the radar display?

When you're near-shore and the chart contains additional information, then there is value in turning on the laptop and bring in AIS and overaly the AIS data. It's also handy to overlay the GRIB weather data, if that is available as well.

At least the above was my thinking when putting together the setup I have on the boat. The driver for my decision process was to eliminate as many single-point-of-failures as possible, and have redundancy where possible (and stay within budget).

- rob/beetle

[donradcliffe]

We have the West Marine transponder, a standalone Vesper Watchmate with an excellent alarm system, and overlay on OpenCPN. I also agree that the standalone is better for offshore, and the overlay is better for inshore.

For example, the night before last, we were heading down the coast about 20 miles off Jacksonville, FL. We saw the ships on the standalone, but the overlay showed them following the defined approach channels to the Jacksonville entrance. One ship which looked fine on the standalone was following a shipping lane which made a turn, and sure enough, when it made its turn the CPA was going to be close. Eventually the CPA alarm on the Vesper went off, but the overlay allowed us to predict that event half an hour before that happened.

Offshore, I want to be able to turn the charting system and overlay off to conserve power, and that's where the standalone system is best.

[Donner74]

Awesome, awesome! Thanks to all of the great feedback. Will definitely be using the overlay feature on the NSE, it's a no brainer. The unit is capable of accepting, displaying and filter AIS data, and while I'm still not sure which one, I'm gonna spring for the dedicated radar-like AIS display as well for future offshore excursions where I won't necessarily need my NSE on at all times. This has been super helpful and greatly appreciated. Now on to my next decision! - Greg

[Paul Elliott]

Originally Posted by goboatingnow

Cant see why youd never not use an overlay, it is the only way, The only reason to non-overlay , is that you dont have the technology to use overlays

Dave

I see that Beetle has already replied to this, so let me agree with him. I have the option to display AIS as an overlay on my chartplotters, and on the charting program on my laptop. When at sea far from land I prefer to display AIS without the overlay on a quasi-radar style display (also on the laptop). I use my NavMonPc for this, but there are other good AIS display programs out there.

If I am running the radar, I do like to overlay AIS on top of that. When sailing coastal or inshore, the chartplotter/overlay is my preferred display. Actually, inshore my eyeballs are what I mainly use.

The small low-power stand-alone units such as the NASA/SiiTech and Vesper can be very useful (these don't have chart overlay), and the power drain is much less than most chartplotters. I know I keep bringing up power budgets, but if you are making multi-day passages this can be very important.

One more thing: A valuable feature with AIS is a proximity alarm. Chartplotters and stand-alone units have these alarms (which can be very flexible), but while many laptop charting programs can display AIS, few of them provide an alarm.

[Donner74]

Originally Posted by Paul Elliott

A valuable feature with AIS is a proximity alarm. Chartplotters and stand-alone units have these alarms (which can be very flexible), but while many laptop charting programs can display AIS, few of them provide an alarm.

Paul thanks for your input ... I am sold on overlay. I do have one question though about the alarm you mention. I do want to choose a unit that offers alarms, as I do believe they are key to getting the most from AIS, but I am concerned about the level of intelligence behind the alarm. Proximity for example seems like it would be useful offshore, but in my more crowded cruising grounds it would annoy the heck out of me. I'm hoping these units are more intuitive than that. I really don't need to know about ships that aren't moving, or those that are headed away from me. I am more interested in the ships that pose the greatest collision risk. It seems to me that in a busy area, that's not always going to be the closest ship.

Greg

[Jens1]

Overlay.
It is the best way to
- interpret the RADAR screen correctly,
- interpret the intention of a target in combination with the chart situation better
- identify incorrect AIS data.
Finally I think overlaying it is the most convenient way. All commercial vessels will use overlay if their system is able to provide such technology.

[witzgall]

If you are choosing one OR the other, I would go with the standalone, specifically, the Vespa marine. We spent a considerable amount of time looking at the AIS offerings of different vendors at the Annapolis Sailboat show this fall. The low power, ease of use, EXTERNAL alarm, and intelligent alarming features of the Vespa made for a very convincing solution. We are waiting for their transponder to come onto the market.

They have also indicated that this will have a built-in anchor alarm. Another power-saving bonus, so you don't have to run a chartplotter while at anchor.

Chris

Chris

[Paul Elliott]

[AIS Alarms]

Greg, you are absolutely right about the usability of alarms. I'm not familiar with options that all the different programs/units provide, but what I provide in NavMonPc is probably fairly typical so let me describe that:

There is a basic proximity alarm. Mine looks to see if the target is approaching or receeding: has the TCPA (Time of Closest Point of Approach) already passed? If the target is approaching and comes inside the guard zone, then the alarm is triggered. Receeding targets do not trigger an alarm. Some AIS monitors trigger on any target inside the guard zone. This guard zone is circular and the radius can be programmed.

There is also a more intelligent alarm, that looks at the CPA (Closest Point of Approach) and TCPA. This will trigger an alarm when a target is on a course that will approach inside a certain distance within a certain time. This alarm is more flexible and probably more useful.

(Note that when I say "approach" or "course", I am speaking of the combined vector: Your course/speed and the other ship's course/speed.)

Some displays will let you choose to not show stopped vessels. I don't know if their alarms will ignore these -- mine doesn't.

My alarms are filtered, so a ship that is momentarily heading your way while turning won't trigger the alarm. The condition has to exist for several seconds. I assume that most systems provide some sort of "anti-nuisance" alarm filtering.

Some systems provide a "snooze button" that lets you acknowledge and silence a particular alarm, but still be sensitive to new ships. This is very useful.

There are systems such as the Vesper (I believe), that provide useful on-screen information showing "danger zones". My program lets you extend the displayed course/speed vectors to the TCPA, so you can see where you and the ship will be at the CPA.

In practice you will probably turn off the alarms when you are in heavy ship traffic. I never run the alarm when sailing in San Francisco bay, or the approaches. There are just too many ships, they are always changing direction, and I'm already paying attention.

When I'm in coastal waters I will set the alarms for a fairly close guard zone -- perhaps a mile. On the high seas I usually set the alarm to max range, because several days can go by between ships, and I like to know about them -- it's interesting. In the middle of the ocean I never see most of the ships that the AIS detects. If there are more ships where you sail and you get tired of the alarm, just set it to a couple of miles.

Some systems let you store several alarm settings. I don't, but it's a good feature and I will eventually implement this.

As you suspected, there is no "one size fits all" solution. Any of the available systems provide value, and you need to spend some quality time with whatever you choose. Note that your own boat's course and speed can be quite erratic, don't assume that what's shown as a close-call won't turn into a direct hit.

You might enjoy playing with NavMonPc. On the "Downloads" page I have a NMEA file that was captured during the start of the 2008 Pacific Cup. This file includes plenty of AIS traffic, and my program lets you play back the file so you can experiment with the AIS display and settings.

[Paul Elliott]

Originally Posted by Paul Elliott

You might enjoy playing with NavMonPc. On the "Downloads" page I have a NMEA file that was captured during the start of the 2008 Pacific Cup. This file includes plenty of AIS traffic, and my program lets you play back the file so you can experiment with the AIS display and settings.

By the way, you can also test other programs using the playback feature of NavMonPc. It provides virtual serial ports and a TCP/IP server so you can send the NMEA data to any other program. It's not a bad way to evaluate a program from your home -- you don't need to be out sailing the boat. (I'd rather be sailing the boat, but sometimes it's just not practical!)

[jim_thomsen]

We have been using an overlay for about 5 years and I can't image not having it now. When crossing both the Atlantic and Pacific I don't think we ever had more than one AIS target, so it's not that cluttered.
When ours was really cluttered (the English Channel, Gibraltar, approaching and leaving Panama) it's nice to have all the targets showing in relationship to your position. It's confusing enough when there are multiple targets all coming toward you.
Something that ours does not do, maybe other models/software do, but I wish it did is when it says the Closest Point of Approach is .2 miles it does not tell you whether this is in front or behind you. When we have a CPA of less than 1/2 mile it's hard to know which way to turn. Usually I just try one way and see if the CPA increases. Is there a better way?
We also have the radar on overlay and will often pick up a target before their AIS signal appears.

[captainKJ]

Why spend the money on AIS to begin with,,, just another piece of electronic equipment to go bad. I use AIS every day in the commercial world and find that it is not used much to begin with. I wok in the Gulf Of Mexico, very busy port, Radar and a good lookout are all that is needed,,,,for the most part we do not even use the Arpa feature on the radar when we get close to shore,,,,to many targets,

[Paul Elliott]

Originally Posted by jim_thomsen

Something that ours does not do, maybe other models/software do, but I wish it did is when it says the Closest Point of Approach is .2 miles it does not tell you whether this is in front or behind you. When we have a CPA of less than 1/2 mile it's hard to know which way to turn. Usually I just try one way and see if the CPA increases. Is there a better way?
We also have the radar on overlay and will often pick up a target before their AIS signal appears.

In NavMonPc you can click on a checkbox to extend the displayed vectors to the CPA position. I believe that the Vesper unit shows a CPA region on the screen. I've used systems such as you describe where you have to figure this stuff out yourself, and it really should be simpler. Of course you need to sanity-check everything. I keep a hand-compass in easy reach, so I can double-check the bearings when closing with a ship.

May I ask where your AIS antenna is? I often pick up AIS targets well over the visual and radar horizon. Occasionally I can see them by eye long before I get their AIS signal. My antenna is on the upper spreader, perhaps 40ft.

[s/v Jedi]

for all the ppl here that swear by overlay (and I think never used something else). Here's an example of a screen from one of our trips where you get the red "no-go" area. The photo is taken too late for a real good example because the ship is already passing us, but you get a good idea of what it does.

I have never seen features like this on overlay systems.

(I found a better example shown in 2nd picture)




ciao!
Nick.

[Jon Hacking]

Originally Posted by beetle

For AIS when offshore redundancy and minimal power consumption is a good thinig ... If you have the budget, do both.

The dedicated display units remove the dependancy on external hardware, which is nice. But they do cost more, & their displays are not usually as good as a chart-plotter, & certainly not as good as a computer. AIS is showing LOTS of information. A bigger, hi-res screen is MUCH better at getting all that info into your brain.

For offshore work, we don't keep our computer running all the time (we use OpenCPN for both navigation & AIS). But it's quick to turn on if we see a light coming at us, which is usually good enough. For coastal hopping, especially here in the (extremely crowded) Malacca Straits, we have plenty of electrons so we just leave the computer on.

I talk more about this (& other AIS issues) on our Cruising Info AIS page.

FWIW, we just upgraded from an AIS receiver to a full transceiver ($495 at Milltech Marine). The Camino-101 has no display but uses almost no power. I just couldn't justify the extra cost of a display unit.