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Old 01-10-2009, 17:35   #76
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Total confusion?

All...

Here is my understanding of embedded NMEA RMC messages:

Some AIS transponders have integrated GPS receivers. This is nice, since in this case the transponder needs no other connection to that ships existing GPS in order to calculate its position and so generate and transmit AIS messages by radio.

The radio-transmitted message stream of the transponder contains ONLY AIS messages. There is no provision for a ship to ever transmit by radio its internal RMC messages. What carnage would result! Every AIS receiver in the area would receive lots of RMC messages, and have no way to associate them with any ship. Thankfully, this never happens.

The hardwired data stream out of that transponder may contain AIS messages generated by itself for its ownship, received AIS messages from other ships, as well as RMC messages from its integrated GPS receiver.

Now, if we connect this combined AIS/RMC wired data stream to opencpn, we get the desired result: opencpn sees RMC messages for ownship (that's good) and received AIS messages from nearby ships (also good), as well as an AIS message representing ownship.

This latter ownship AIS message may be confusing in real life. There would be an AIS target right on top of ownship all the time. Collision with myself would be always imminent.

I have never seen such a setup, and don't know anyone with such a transponder to enroll as a test case.....

On the other hand, if we filter the RMC messages in opencpn, then this mariner will have no positioning data for her own ship, unless of course she installs another GPS receiver of some sort.

Further, some users have combined AIS/GPS receivers. No transponder. If this is the only GPS aboard that ship, that is the one that opencpn must use. The RMC message must not be filtered out.

Now the TCP AIS data stream complication: I suppose a mariner with such a combined AIS/GPS transponder/receiver may want to feed its received AIS data stream to an aggregator such as SxxTech, in order to gain access to the worldwide data stream. If they upload the hardwired output of their own transponder, there will be embedded RMC messages.
Thankfully, most of the AIS receivers feeding data to the upload site are not transponders, but simple receivers like most of us use. Thats why the number of RMC messages is low.

A similar situation obtains when a simple AIS receiver and GPS receiver are multiplexed in software before being sent upstream to the aggregator. There will be embedded RMC messages in the resulting TCP data stream seen by the rest of the world.

What to do:
If I ran the world, I would insist that TCP retransmission of AIS data streams be purged of RMC messages before being sent out as TCP traffic. RMC messages in this context are gibberish, and mean nothing useful. I consider the current situation an easily avoidable error on the part of the re-transmitters.

Realistically, though, this is what opencpn does now...
a. Real cruisers using TCP AIS data streams for actual navigation will see their ownship jump all around the world randomly. Opencpn will not be usable. Just as well, since TCP AIS data streams are in no way suitable for navigation due to their unmanageable TX/RX delays. If you insist, get another computer and run two systems side-by-side, or run two instances of opencpn as virtual machines. One has a real GPS, for navigation, one has TCP AIS data for entertainment.
b. Armchair cruisers may set Toolbox->NMEA Data Source to "None". No embedded RMC messages will be processed.

If all that hasn't increase the confusion quotient, I don't know what will....

Dave
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Old 01-10-2009, 18:41   #77
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Total confusion.. #76..

Here is a sample of the combined AIS/GPS messages received from the S.....h network.
The GPS message is received approximately every second...

!AIVDM,1,1,,B,13u<tmg0000nR?FQ0k:Ra45P08Ob,0*22
!AIVDM,1,1,,A,13u=JE0P010mGi8Q1KlE=gwb0HOc,0*50
!AIVDM,1,1,,B,13:laf00000nA3tQ0Vu4E`K`0`Oc,0*2A
$GPRMC,190954.00,A,5741.48320,N,01141.09197,E,0.00 4,,280909,,,A*77
!AIVDM,1,1,,A,13uC6lP0000k7sLQfJ:P03Cb0`Oj,0*3B
!AIVDM,1,1,,A,33ALbL0P@tPqiavP3v<:tpQb011P,0*4A
!AIVDM,1,1,,B,13kvHD5000PqJddP:41P0FIV0D0V,0*1C

As a point of interest,the S.....h AIS browser does not display this GPS message.

Would it not be possible to upload the NMEA GPS message on it's own to the S.....h network via 3G ? (For those without an AIS receiver/transponder?)
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Old 01-10-2009, 21:50   #78
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how to capture AIS data without an AIS receiver

Is anybody sucessfully porting AIS data to openCPN without the use of a dedicated AIS receiver? It seems to me that the whole idea of transmitting AIS data over channels 87 & 88 was that anyone with a VHF would be able to get the data & use it, but after spending much time trying to do so, using many different methods, I am unable to do so.

Am I missing something?
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Old 02-10-2009, 05:23   #79
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It can evidently be done using what is called a discriminator and using the radio's sound card. Otherwise there are i believe also VHF sets sold with AIS receiver included.
Some people also are able to connect to one of the FREE AIS networks over the internet by wi-fi or using 3G networks where available but you will not see your own boat unless you have at least an in-expensive AIS receiver like the SR160 or SR162.

There have also been people connecting their GPS data in a similar manner but with only the most essential info.
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Old 02-10-2009, 08:18   #80
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It can evidently be done using what is called a discriminator and using the radio's sound card.
But have you done it? My post specifically says "Is anybody sucessfully porting AIS data to openCPN without the use of a dedicated AIS receiver?".

Regarding AIS data via internet, from what I understand, in order to receive their feed, you've got to send them your local AIS feed...catch 22..if I can't capture the data, I can't get the data.
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Old 02-10-2009, 08:25   #81
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I have not personally done it but you can check it out from the experts on shipplotter.com Not sure if they use a dedicated VHF or an amateur radio with VHF band.

You are right about the sharing idea,it's a two way street...
One other way to SEE the AIS targets is to have a look at the websites of one of the FREE providers:
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Old 04-10-2009, 01:57   #82
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Cheers for 1.3.4.
Couldn't resist it, just because it is possible.
Thomas
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Old 04-10-2009, 04:46   #83
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He he , great Thomas...you're getting there!

I wonder how a raster map qualifies for conversion?
What else is needed?

Regards
Tore
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Old 04-10-2009, 05:42   #84
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This is more fun than AIS...he he!!
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Old 06-10-2009, 00:19   #85
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Hi Jerrymc i am currently using an old Uniden Polaris radio that has been "discriminated" by myself after reading through www.discriminator.nl
One then has to download AISMon (free) which "translates" the sound signal, and then a virtual comm port emulator. Sounds a bit complicated but it is actually very easy.
Running the system very well with OpenCPN
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Old 06-10-2009, 07:14   #86
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i am currently using an old Uniden Polaris radio that has been "discriminated" by myself ..
Running the system very well with OpenCPN
Thanks for the reply..questions:
How did you do the tap? Did you order an interface from discriminator.nl or make it up yourself? Then, is data input to serial port? I know about AISMON.

Did you put a splitter on your VHF antenna or install an AIS antenna?

How much coverage? Prox how many miles can you see AIS signals?

I sail out of Los Angeles Harbor and the "big ship" traffic is considerable. As Catalina is most common destination, one has to negotiate incoming/outbound traffic lanes making what should be an enjoyable couple of hours into a sometimes tense "white-knuckle" event. AIS is the answer, I think, but before spending considerable money & effort, I really want to try it out..have an older Standard Horizon VHF that I'm willing to experiment with, as it's no great loss if I screw it up, but wanted to know if anyone had actually sucessfully done so..

thanks again,

Jerry
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Old 13-10-2009, 11:33   #87
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In search of low cost map solutions I found this site: A small company in the Ukraine producing maps from scanned raster images in various formats. Perhaps a cooperation would be possible? Anyway,the list of charts they offer are very resonable at Eur 3.80.
They appear to be scanning existing Hydrographic charts, the data of which is almost certainly copyrighted.

But, if you want to go down this road, then there is a simpler and cheaper, albeit more labour intensive method:
there is a site on the Internet (PM me if you don't know the url) where MapMedia charts can be freely downloaded. Their collection runs to approx 32Gb (you may wonder how I know this ...), and it's straightforward to display these charts using MaxSea.

By taking a series of screen captures - I use Gadwin Systems 'PrintScreen' - and importing these into PaintShopPro - it's then possible to re-create the original Hydrographic raster chart. I've already done this for a few covering the UK's North Sea, and found that it's a far easier technique than scanning the equivalent paper charts.

It's a tad time-consuming to patch the screen captures together (but no more work than working with paper charts), and obviously this only has to be done once for each chart - so that if a large number of people each re-created half a dozen charts, the workload wouldn't be too onerous.

What results from this technique is a high-quality raster chart, identical to the original paper chart, in any common graphics format, albeit 10 years out-of-date. It's then easy to download a file containing cumulative corrections for each chart from the UKHO.
Corrections can then be made with any graphics editor, bringing the chart fully up-to-date, just as if it were a paper chart.
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Old 13-10-2009, 13:52   #88
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Can't see the point of downloading charts which can only be read by MaxSea? Not everybody have this program. In any case,you propose to take several screen captures for later re-assembly,in which format? The chart also has to be calibrated somehow? And finally what format compatible with OpenCPN? Think I'd prefer to spend my Eur 8.50
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Old 13-10-2009, 17:47   #89
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Quote:
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Can't see the point of downloading charts which can only be read by MaxSea? Not everybody have this program. In any case,you propose to take several screen captures for later re-assembly,in which format? The chart also has to be calibrated somehow? And finally what format compatible with OpenCPN? Think I'd prefer to spend my Eur 8.50
At EUR 3.80 Per chart, the 221 Brazil raster charts would be EUR 839.80. Does that still sound like a good deal?

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Old 14-10-2009, 03:00   #90
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Can't see the point of downloading charts which can only be read by MaxSea?
With the exception of the USA and NZ (and maybe one or two more), show me another source of free kosher Hydrographic raster charts ...
Quote:
Not everybody have this program.
Easily remedied.
Quote:
In any case,you propose to take several screen captures for later re-assembly,in which format? The chart also has to be calibrated somehow? And finally what format compatible with OpenCPN?
Any common graphics format (I thought I said this ?) - I use .png , but use whatever your graphics editor handles best.
Then use Irfanview (free) to convert between graphic formats.
Calibration ? The charts are WGS84 and have precise Lat/Longs marked at each corner ready for MapCal (if you're a SeaClear fan) - and as for an OpenCPN format ? Hey, I can't do everything !
Indeed, there's no way I can even test these charts on OpenCPN, as 1.3.4. falls over on my machine (Win98Lite).
Quote:
Think I'd prefer to spend my Eur 8.50
For one chart, for one person - sure, that make sense - they look like pretty good charts too.

But I was thinking more along the lines of a community effort to produce several thousand charts covering the whole globe. Now that would be a helluva lot of work, even for a team of people, but luckily not particularly hi-tech so anybody with a computer could contribute.

I would be the first to agree that what I'm suggesting is far from being a perfect solution, so if anyone can come up with a better idea ....
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