AIS Alarm Issues

We have an AIS system which shows us on the chart plotter the position, speed, and course of other vessels, on Wings and it is generally a great navigational and safety aid. Probably the second most important improvement to cruising since GPS. However, we have some problems. Read this if you are interested in AIS and our experience.

Here is a recap of our AIS adventures:

Installation

We installed a West Marine AIS1000 in 2010. It was inexpensive and easily available. West Marine programmed it as required by law in the US. The unit is a bit clunky but is weather sealed and rugged (not essential for our installation) but it installed easily and worked right out of the box with little zero further set-up. We really did not want to install a second VHF antenna or the cabling of one so we tried a West Marine VHF signal splitter, which has worked perfectly. Our AIS and VHF share a single cable and mast head antenna. Our cabling to the mast head is RG9U and our losses seem acceptable. We've done limited operational testing but when we have, both class A and Class B equipped vessels have seen us on their displays at reasonable distances. So the installation works.

We've routed the outputs to both the nav station PC for use on OpenCPN and to a Standard Horizon Matrix GX2000 VHF radio which has a AIS display. This permits us to monitor AIS without continuously running the Dell PC we use in the Nav station. (This is a power saving decision). The GX2000 also requires a separate GPS signal and we route our Lowrance GPS output to that (and to the Dell, for redundancy).

The biggest benefit we see from the AIS system is a watch-standing function. Using CPA and TCPA alarms we get over the horizon warnings about commercial traffic, very, very helpful and comforting during passages. In bad weather or when the watch stander is drowsy, the AIS lets us know somebody is coming. Of course during transits of the Straits of Malacca, offshore of East Africa, and near the coasts of Brazil and Venezuela, all very high traffic areas, the visual display on the chart plotter has made a world of difference. Commercial traffic is no longer a significant worry although we will be happier when all fishing vessels and small craft also carry it.

CPA and TCPA

The OpenCPN handling of targets and alarm setting is much better than the GX2000, which has been a bit of a disappointment. In fact if there is one issue we have with our installation it is in the alarm handling of the GX2000. In the first place it cannot logically combine CPA settings with TCPA. So with a CPA setting to give an CPA alarm for any vessel which will come within 2 miles, and a TCPA setting to give an alarm for any vessel whose closest point will be in the next 30 minutes, for example, the GX2000 gives in alarms for all vessels which will either get within 2 miles, no matter how much time will pass before that happens, and all vessels whose closest point will occur in the next 30 minutes, even if their closest point is 15 miles! It is a logical "OR" instead of a logical "AND". There is no prioritization as there is with the Vesper Marine AIS Watchmate. Open CPN works better in this regard.

Re-occurring nuisance alarms

The next issue, which is really aggravating, happens on both units, but it is much more serious on the GX2000. This relates to the instantaneous CPA and TCPA calculations which are dramatically affected by our vessel's yawing and surging in sea states common on an ocean passage. Even when our course is such that we will pass well clear of an oncoming (or overtaking) vessel, when a wave makes us yaw towards the target both our OpenCPN and the GX2000 wake up and give alarms. However, in the next second when we yaw back onto our course the calculation changes and the alarm clears. Why this is such an issue is that the alarms are loud enough to wake the dead (set so intentionally) and having a loud alarm go off every few seconds soon becomes a safety issue itself: how do you operate the vessel (let alone talk on the radio) when you have a loud alarm gonging away in the nav station. You only choice is to turn off the CPA and TCPA alarms, which then exposes you to other possible situations which are not then being monitored. Here is an example: An overtaking vessel is 30 minutes away from a CPA of three miles, but the wave actions cause you to swerve towards that vessel's path every few seconds. The CPA or TCPA alarms sounds, the clears itself, then sounds, then clears itself. You can't stop it with either system other than by turning off the alarms entirely. So that is what you do to get some peace and you manually monitor the overtaking vessel. Meanwhile a second vessel comes over the horizon and sneaks up on you, no alarms! With the GX2000 it is even more serious. With this unit it takes a few seconds to turn off the alarms, due to the number of keystrokes required, however, whenever a new alarm sounds (which happens on every wave, with every yaw or surge of our own vessel) the radio's display and controls switch back to AIS alarm mode even if you are in the middle of disabling the alarm, and you have to start over with the keystroke sequence. Often it is impossible to complete the keystroke sequence to turn off the alarms once they commence. Right when you might need to be on deck watching a commercial vessel, or trying to talk to him, you are instead engaged trying to kill the alarm. We've found that when the GX2000 starts sounding the alarms we have to turn off the GPS so the radio stops calculating CPA and TCPA. Not a good solution. I've spoken at length with Standard Horizon technical people about this but they don't offer any solution, and actually seem skeptical that the situation even occurs. I guess in a laboratory setting, or even on inshore waters where waves don't cause a vessel to swerve or surf, the problem does not occur. On OpenCPN I have suggested a solution, where "Acknowledge" commands, which turn off an alarm for a specific vessel, be retained even after the calculations remove the target from the CPA or TCPA limits. Currently the "Acknowledge" command goes away when the CPA or TCPA calculation says the target is no longer a threat. However, if it comes back, it is a new threat and the alarms sounds again. I'd like the target list screen to show which targets have recently caused an alarm to sound, and allow me to "ignore" these by ticking a box, for a specified period or time, or until an further risk threshold is reached.

Perhaps this all sounds a bit esoteric, but when it happens, believe me, it's aggravating to the point of being dangerous, and imagine trying to explain this to a newbie watch stander who might be non-technical and possibly a bit seasick. In the end it is the captain who gets up and deals with it, which is OK, I don't mind, but it is confidence shattering to the person on watch. I will be interested in hearing about the experience of others regarding this problem, and particularly if Watch Mate handles it better.

SSB interference

One other issue we've run against is the tendency, in some atmospheric conditions, for the SSB radio to kill the GPS computation of the West Marine AIS unit. Not every time, but sometimes, when we transmit on the SSB, like when we connect to Sailmail, the AIS loses its GPS almanac and therefore its position solution, which kills the transponder. We can still receive AIS information but we don't send ours without positional information. The unit recovers itself in about 45 minutes. It took us a while to figure out what was happening, in the meantime West Marine sent us three GPS antennas thinking that the antenna itself was the problem. Now we have some spares, and we understand the recovery cycle.

It seems like these are software issues which were not easily anticipated, and are quite complex to solve, and so we understand why they have not been, so far. But until they do, we're dissatisfied.

Fred Roswold, SV Wings, Cartagena