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Old 19-10-2016, 06:08   #1
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AIS..how accurate is yours???

Currently there are six boats in the marina transmitting AIS.

This is what I am seeing on Marinetraffic when I look at past tracks....none of these boats have moved in the last month or so.....

Three including mine have nice steady positions.... three don't.

I have no idea what AIS these boats have or where they are getting their GPS info from.... mine is an AMEC with the GPS puck affixed to the inside of a cabin window.

The first boat is on the hard but while I was having breakfast appeared two pens away ( on the AIS), third boat is mine and this is what the past track of two of the others looks like.

To get an idea of scale 'Constantino Kochifas' is a coaster of about 80 metres LOA.

Food for thought for them as thinks AIS has rendered radar and the Mk1 eyeball obsolete.
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Old 19-10-2016, 07:04   #2
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Re: AIS..how accurate is yours???

It's as accurate as the GPS position it's fed with.

Note that your screen will show a moving boat at its last reported position (I believe it's against the standard to show a projected position). With Class B broadcasting only every 30 seconds, that can be a seriously out of date position.

Another reason to use a Class A set.


It was discussed to death in another thread, but AIS and radar are not substitutes for each other. Different things altogether. Where I sail, both are more or less essential. I would really hesitate to go to sea, other than a short coastal hop in good wx, without radar.
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Old 19-10-2016, 07:21   #3
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Re: AIS..how accurate is yours???

Quote:
Originally Posted by Dockhead View Post
It's as accurate as the GPS position it's fed with.
......
I thought that went without saying.
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Old 19-10-2016, 07:52   #4
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Re: AIS..how accurate is yours???

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Originally Posted by El Pinguino View Post
I thought that went without saying.
So was your question just rhetorical? Or did I misunderstand what you were asking about?
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Old 19-10-2016, 08:01   #5
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Re: AIS..how accurate is yours???

Plain gps kicks.

Very common on disconnect / reconnect cycles but also otherwise.

Also gps antena placement may influence. Below deck can get lost at times.

b.
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Old 19-10-2016, 08:19   #6
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Re: AIS..how accurate is yours???

I just turned mine on to check, and it's pretty close. My boat and all the ones around me are in the water, so I guess that's good.

Sent from a boat somewhere
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Old 19-10-2016, 08:19   #7
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Re: AIS..how accurate is yours???

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So was your question just rhetorical? Or did I misunderstand what you were asking about?
When I said 'AIS' I was considering the entire setup...and thinking that maybe people would consider it worthwhile to externaly monitor their own system.

I would wager that quite a few have never considered to quality of the gps input.
How many people have a GPS puck in a bad location .... 50% of my small sample.
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Old 19-10-2016, 09:00   #8
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Re: AIS..how accurate is yours???

El Pinquino, I checked some boat tracks, of boats in marinas, in the Vancouver area and didn't see anything like what you have posted. Although some boats danced around a bit they were all close to or on the dock . Many were very stationary.

One thing I noticed was that most boats have turned their AIS transponder off while at the dock.

That one boat on the hard (at your position) must have a bad connection to the GPS.

One more item to add to the check list.
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Old 19-10-2016, 11:24   #9
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Re: AIS..how accurate is yours???

It's likely not a bad connection to GPS. it may be an out of date satellite picked up, or that signal is NOT using a WAAS GPS capable received.

GPS satellites send 3 signals-
Identification
Time
Date

That's it. Depending on the accuracy of the deciphered signal, or time code, is how you get location. Also, GPS uses 3 signals of time to get accuracy, if only 2 are picked up the deciphered time can place your location off by 100s of ft or meters.
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Old 19-10-2016, 20:07   #10
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Re: AIS..how accurate is yours???

Today's track from an Android tablet...

Looks not dissimilar.......

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Old 19-10-2016, 20:18   #11
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Re: AIS..how accurate is yours???

Couldn't edit my post.

GPS satellites also send Almanac data, and the general location of the satellite. The almanac is the data for all GPS satellites, while the other is the location of the satellite you are receiving the signal from. The data is transmitted every 30 seconds.

As for accuracy - it depends on the number of satellites your GPS receiver is picking up, and how quickly the system can decipher and calculate the atomic time and location. I believe the atomic time is sent down to the nano seconds, but may actually be transmitted in pico seconds, but requires an additional cypher to calculate the additional data. 1 nano second is roughly 12 inches of accuracy, and most GPS's are accurate to about 3 meters (95% confidence).

Looking at the image above - I have to ask - is there something that might have been done to change the processor. Were you fiddling with anything? That track isn't smooth, which leads me to believe that something changed that might have slowed down the GPS signal being calculated, or maybe it was switching between satellites.
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Old 20-10-2016, 10:55   #12
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Re: AIS..how accurate is yours???

While on the subject ( if I shed go to another thread, my apologies) what is the range you would expect with A IS? Mine is on a Raymarine splitter from the vhf antenna the top of the mast which is about 17.5 metres ( 57 ft). Think the best i have got is 3-5Nm. I expected more.

As part of this thread I thought all AIS installations use their own dedicated gps antenna, which is therefore likely to be recent technology and very accurate. I am also puzzled by the unexplained movements of vessel positions. Have those at anchor been eliminated?
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Old 20-10-2016, 11:05   #13
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Re: AIS..how accurate is yours???

My AIS ant is a dedicated one mounted on the taffrail about 2 metres above the sea.... I receive at 15 miles on a regular basis.
To check at what distance you are being received at go to MarineTraffic and find your yacht. Then check the location of the station that is monitoring you and you will get an idea of how well you are getting out. I am typically received at 15 miles.

I think the dodgy GPS position issue is down to badly positioned (within the boat) AIS GPS pucks.
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Old 20-10-2016, 12:16   #14
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Re: AIS..how accurate is yours???

Yes, but surely the AIS TX/RX is using TWO antennae. 1) a vhf antenna probably on a splitter with the existing antenna at the masthead and 2) a gps antenna which can be mounted lower down on the rail or a transom arch.

You say you can receive at 15 Nm. Fantastic. Must be something wrong with mine. In line with one of the others on this thread have you actually checked at what distance other ships can see you? I was told by a marine electronics expert that marine traffic.Com is relying on land based signals to place your position on a map on an Internet site. I just sailed from UK across Biscay down to Gib and across to the Adriatic but about the only time my family could track me on marinetraffic.com was when I entered or left a port and I only stopped every 400-600 Nm so they had long periods when they clad see nothing.
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Old 20-10-2016, 12:23   #15
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Re: AIS..how accurate is yours???

Quote:
Originally Posted by SaltyMetals View Post
While on the subject ( if I shed go to another thread, my apologies) what is the range you would expect with A IS? Mine is on a Raymarine splitter from the vhf antenna the top of the mast which is about 17.5 metres ( 57 ft). Think the best i have got is 3-5Nm. I expected more.

As part of this thread I thought all AIS installations use their own dedicated gps antenna, which is therefore likely to be recent technology and very accurate. I am also puzzled by the unexplained movements of vessel positions. Have those at anchor been eliminated?
How long is a piece of string?

Range varies from a couple of miles to 100 miles or more depending a long list of factors, including even atmospheric conditions.

A ship with a Class A set and good antenna mounted high, I would expect to see at a minimum 30 miles off.

My antenna is much lower -- at the first spreader, 10 meters up. But it is a dedicated antenna not run through a splitter, and connected with RG213 cable with no breaks all the way from the antenna to the transceiver.


As to checking how well other vessels see YOU -- I have always thought, that it is as important to know this, as it is to do a radio check. I regularly all up vessels and ask them to tell me if they see me. Ships see me at 20 miles or so, on average.

Other pleasure vessels are very much a mixed bag. Many can't see me at all. I think the quality of installations varies greatly.
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