Mapping delays AIS, GPS

[mm5aho]

I'm currently using Seaclear charting on a notebook. It has a GPS and and AIS receiver connected, so displays an image (and data) of other boats that use AIS transmitters.

But I often note that the AIS position is slow. The chart shows the other boat in a position is used to be in, not somewhere near where it is at the time. The lag can be perhaps 10 mins at times.

The AIS receiver is working and receiving data.
Looking at the incoming data screen in Seaclear, its possible to see the time stamps on the data. They are usually OK to within say 3 seconds. But the position displayed is delayed somehow.

Anyone seen anything like that ? Or have any ideas how to fix it?

[Mehitabel]

I have noticed this with my setup (NASA AIS3 engine and Garmin GPSMap 620). Just added the AIS this season so I assumed this was simply the delay imposed by the rather complex timeslot system AIS uses, but you've got me wondering now. I also have targets dropping out/lost only to appear again a few minutes later - even ones quite close. I suppose it could be due to a less than perfect antenna setup.

[mm5aho]

I added the AIS this year only for fun / interest, not really for safety, collision prevention etc that some say it's good for??
I've noted other anomalies. For example I've had boats show on the screen as being almost alongside, yet they're not there. Boats appear only to disappear, and interestingly, some whose data seems way off beam! For example yesterday, coming north up the Clyde, a huge oil tanker showed up as length 25m or something. Clearly this was more like 200, (subsequent spots showed 280m I think it was).
Its interesting to see which commercial boats show their details and which don't. A salmon farm supply boat (Fame) seems to have quite intermittent AIS. In Loch Fyne Saturday, it showed for hours, then disappeared for hours (while still visible, and while other boats spots were still coming in.)

I bought a damaged 2 channel receiver on fleabay for next to nothing, and repaired it. Looks and works fine now; must have had a big mechanical impact to bust the extruded housing.

Anomolies intermittently could I guess be expected, but the delay business is consistent. I suspect the software (Seaclear) is having trouble processing all this stuff being thrown at it - there's GPS sending a regular stream, AIS sending spurts if it receives any, and got to sorth that all out and run the mapper. Might be the notebook, but it doesn't seem to be running flatout on its CPU.

[pagoda]

I think a fair bit of the delay is down to the GPS receiving device operating internally at 4800 or 9600 Baud and the AIS input on 38400 baud. I added AIS this season for 2 north sea crossings. It works well most of the time, but not all ships switch it on, and more annoyingly, lots leave it running while tied up and unoccupied. They are a PITA as they generate alarms as you go past stationary boats...
It's useful to be able to find a far away spot on the horizon is doing 22Kts on a specific heading, which then may or may not be interesting to you. But you can find out some useful detail, including MMSI numbers.

Graeme

[marisca]

I run Belfield and OpenCPN (not together) and OpenCPN seems to deal much better with AIS targets without the lags and intermittent disappearances apparent on Belfield. I much prefer the raster mapping of Belfield but then the MkI eyeball and a paper chart is my preferred option ('cos I'm a Luddite!).

[Fingal]

I run Belfield and OpenCPN (not together) and OpenCPN seems to deal much better with AIS targets without the lags and intermittent disappearances apparent on Belfield. I much prefer the raster mapping of Belfield but then the MkI eyeball and a paper chart is my preferred option ('cos I'm a Luddite!).

It's 'cos yer old, really

[marisca]

I run Belfield and OpenCPN (not together) and OpenCPN seems to deal much better with AIS targets without the lags and intermittent disappearances apparent on Belfield. I much prefer the raster mapping of Belfield but then the MkI eyeball and a paper chart is my preferred option ('cos I'm a Luddite!).

It's 'cos yer old, really

It took you almost a month to think of that? And you think I'm old?