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Old 08-02-2020, 15:36   #1
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AIS question

Earlier today a cruiser came by in her dinghy asking if we had AIS. After affirming we did she asked if we could look to see if her boat showed up. Yes, the boat showed up in the list, but did not show up on radar or chart plotter. She also asked another cruiser to look and they were able to see her both ways.

She was transmitting on a Raymarine (donít know the model). My equipment is a Shakespeare AIS anntena, Digital Yacht AIS, Garmin GPS Map 741xs, Garmin radar.

So my question - why am I able to see her on my list of boats transmitting but not on my chart or radar? Iíve never know this to happen before, but maybe Iím missing many targets.

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Old 08-02-2020, 15:39   #2
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Re: AIS question

I m going to guess you have some sort of filter activated on your plotter, like maybe don’t display non moving targets?
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Old 08-02-2020, 15:39   #3
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AIS question

Were you stationary?

Do you have a heading sensor attached to your plotter?

Ours wonít show ais targets in the radar display without a heading sensor.
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Old 08-02-2020, 15:40   #4
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Re: AIS question

do you see any AIS targets on the radar /plotter ?

could it be that your friend had only just turned her AIS on and your system was taking a few minutes to register the data ? did the target appear later ?

did you have some targets (eg non-dangerous or stationary) filtered out ?

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Old 08-02-2020, 16:11   #5
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Re: AIS question

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Originally Posted by chrisr View Post
could it be that your friend had only just turned her AIS on and your system was taking a few minutes to register the data ? did the target appear later ?
This would be my first guess. Where was the "list" that it showed up on?

My Vesper AIS transceiver will remember targets for a while, but obviously the chartplotter or radar can only display those that have been received since they were powered on.

AIS transmits less frequently when at anchor than when underway. With Class B it could be several minutes. Something makes me want to say six minutes, but I might stand corrected on that.

Whatever the number, if the AIS receiver was on when the friend's boat last transmitted, but the chartplotter or radar hadn't fully booted up yet, it would be very possible that you'd see it on the AIS but not the other devices for several minutes. Also, the static information (boat name, size etc.) is transmitted even less frequently. The other devices might wait for that before displaying the target.
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Old 08-02-2020, 16:22   #6
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AIS question

We were both stationary. I do not have a heading sensor attached to my plotter. But other boats in the vicinity were showing. Distance was about .4 miles. All other boats were between .2 and much farther - probably at least 50 boats on the list and showing on both plotter and radar. I also tried different ranges on both the chart plotter and radar with nothing different happening.

There were MANY targets showing at a nearby marina. The list is one of the choices in the menus from both chart plotter and radar.

I thought the same about waiting to see if it showed up. I waited about 1 1/2 hours and nothing changed with regard to her boat.

I donít have any filters on that Iím aware of. Most of the boats showing were non-dangerous boats - in slips and not moving.

When first powered it took about 5 minutes to display the boat name instead of the MMSI. That is how it usually is.

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Old 08-02-2020, 16:41   #7
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Re: AIS question

Did you check if her boat was shown on any mobile application like MarineTraffic?
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Old 08-02-2020, 16:44   #8
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Re: AIS question

Her boat was not. But many times when Iíve looked for boats on that they donít show up.
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Old 10-02-2020, 08:50   #9
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Re: AIS question

I sent Digital Yacht my question. They said that if a boat is not moving the transmission is less frequent AND it may appear in the target list but may not show on the plotter.

So, Iím hoping that is the reason her boat didnít show up, although the other boats that were not moving did show up.
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