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Old 04-11-2015, 13:19   #1
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AIS + GPS sequence cache

For all those who used not to run opencpn all the time or have a multiplexer like kplex installed on a openwrt/linux router. When opencpn opens it misses all the AIS information and it takes quite a while to scan all AIS transceivers around.

The lua program nmeaoffline solves that problem by backlogging the AIS and GPS entries for a time (default 30min) and replaying them at time of first connection.
At this moment it works only with an NMEA capable TCP-server like kplex. If needed I could adapt it to gpsd.
I tested it only with OpenCPN it could work with other plotters as well (please give feedback)
Configuration is in the source. lua and lua-sockets needs to be installed.
Just add a additional NMEA source to OpenCPN pointing to the IP and port of offlinenmea
default configuration is
nmea source: localhost port 10110
listen to port 10111

This is a proof of concept beta version
Don't use it for Navigation without proper testing.
Source is GPL

offlinenmea
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Old 05-11-2015, 01:53   #2
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Re: AIS + GPS sequence cache

But outdated data are useless for navigation...

Gerhard
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Old 05-11-2015, 03:23   #3
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Re: AIS + GPS sequence cache

Quote:
Originally Posted by CarCode View Post
But outdated data are useless for navigation...

Gerhard
Not really.
I presume the stored data is all time tagged and Ocpn will develop history trails for each target right up to the most recent and current position. It's often quite useful to see where AIS targets have been and what they have been doing.
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Old 05-11-2015, 04:02   #4
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Re: AIS + GPS sequence cache

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Originally Posted by hoolie View Post
Not really.
I presume the stored data is all time tagged and Ocpn will develop history trails for each target right up to the most recent and current position. It's often quite useful to see where AIS targets have been and what they have been doing.
Are you navigating or writing a cruising book?

Gerhard
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Old 05-11-2015, 09:05   #5
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Re: AIS + GPS sequence cache

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Originally Posted by CarCode View Post
Are you navigating or writing a cruising book?
Neither at the moment, but let me explain.

Suppose you have an AIS target nearby and you have been tracking it for 20 or 30 minutes. If it is going backwards and forwards you might assume it is a fishing vessel (it might even tell you via AIS if you're lucky) so you can make sure you choose a course that avoids fouling his gear.

If it is, say, a large tanker or container ship it might have changed course slightly 5 degrees? Then you can conclude that he probably won't collide with your sailing vessel and perhaps more importantly has seen you and as the give way vessel has changed course accordingly.

AIS tracking is important and that's why it's included as a feature of Ocpn.

As for the OP's software I think maybe it's a special requirement as certainly I would not turn off Ocpn if there was any collision risk. I suppose if your computer consumes a lot of power you could turn it off and let the router store the last 30 minutes of AIS data for an instant update when you turn it back on.
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Old 05-11-2015, 09:28   #6
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Re: AIS + GPS sequence cache

Not sure I see the point of caching GPS but a few minutes of AIS would give a more complete navigational picture on start up of a nav application (like OpenCPN) assuming the caching program was running when the nav app was not. My thought though is that more than 6 mins (frequency of static data transmission?) of caching might be overkill.
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Old 05-11-2015, 09:33   #7
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gpsd + AIS cache

To use it with gpsd instead of klpex install "socat" and run the following command before starting offlinenmea:

socat EXEC:"gpspipe -r" TCP-LISTEN:10110


As I wrote before it is most usefull to shorten the time all AIS targets show up after starting OpenCPN. If you are on the router it keeps the track and the static AIS info (like the and ship-dimensions...), moored vessels only transmit in large intervalls. some vessels AIS signals may be shaded - signal quality varies.
And offlinenmea can start long before OpenCPN even without a dedicated router it speeds up AIS target recognition.
ead of klpex install "socat" and run the following command before starting offlinenmea:

socat EXEC:"gpspipe -r" TCP-LISTEN:10110


As I wrote before it is most usefull to shorten the time all AIS targets show up after starting OpenCPN. If you are on the router it keeps the track and the static AIS info (like the and ship-dimensions...), moored vessels only transmit in large intervalls. some vessels AIS signals may be shaded - signal quality varies.
And offlinenmea can start long before OpenCPN even without a dedicated router it speeds up AIS target recognition.
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Old 05-11-2015, 15:51   #8
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Re: AIS + GPS sequence cache

gps is replayed only to give continuous tracks. I have not tested if that actually works with OpenCPN because I didn't move - please anybody testing it give a feedback
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