AIS ZIG-ZAG

[nohal]

Originally Posted by cagney

It took some time, but now I have seen, and caught, the zig-zag again.....

Here is the track of the ship Las Palmas in Las Palmas Harbour Gran Canaria.

Attachment 111627

and the track-file.

Attachment 111628

Thomas

Thomas...
Ready to cross? What strikes my attention right away is the precision of the info in the GPX (just 4 decimal places), it would be really interesting to see what they actually transmit.

Pavel

[svlamorocha]

Originally Posted by cagney

It took some time, but now I have seen, and caught, the zig-zag again.....

Here is the track of the ship Las Palmas in Las Palmas Harbour Gran Canaria.

Attachment 111627

and the track-file.

Attachment 111628

Thomas

Thanks. I looked at the numbers in the gpx file and the picture seems very clear to me.

The lat and lon are rounded to 1/100 of a minute, which gives a 18-meter "jump" in Northing/lat and a slightly smaller jump in Easting/lon.

For comparison, both a) the AIS standard that defines the AIS signal transmitted by radio from the other ship to your boat, and b) the NMEA 0183 AIS sentence transmitted between your AIS hardware and your computer, call for accuracy of 1/10000 of a minute.

Therefore there are two significant digits that have been lost somewhere.

I bet that the two missing significant figures got chopped off somewhere in the computer, but it is easy to check that. Do you have the means to read the NMEA data in the wire between AIS received and the computer?

[cagney]

The gpx-track was created by the "Record Track" button in the target query box. I saw the target to late to to use the VDR plugin (that records the raw incoming NMEA data). The track on the screen-dump is using the raw NMEA data though.
@Pavel I have not checked the code but I suppose that "Record Track" drops a few decimals?
Since I saw this zig-zag last time I have certainly encountered several hundreds of AIS targets, so I have a hard time seeing this as a general OpenCPN problem.

Let's try to catch a VDR record of the vessel "Las Palmas", and see were that leads to. Here are the last 5 ports:
Date / Time Port / Country
Oct 27, 2015, 12:04 UTC LAS PALMAS DE GRAN CANARIA , SPAIN
Oct 24, 2015, 12:02 UTC SEVILLA, SPAIN
Oct 20, 2015, 10:00 UTC LEIXOES, PORTUGAL
Oct 17, 2015, 13:04 UTC ROTTERDAM, NETHERLANDS
Oct 10, 2015, 09:03 UTC CADIZ, SPAIN
Now heading for Agadir, Morocco.

Along this route there are many O users. What about some help from our French, Dutch or Spanish friends?

@Pavel
We're getting there. Probably leaving early next week for BVI.

Thomas

[cagney]

Another interesting target just arrived.

Empress MMSI 249056000



and the vdr file. Just delete the .pdf extension.

vdr-Empress-249056000.txt.pdf

Thomas