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Old 07-07-2006, 10:30   #16
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Old 07-07-2006, 20:24   #17
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Quote:
Originally Posted by AdamY
I dont get why we cant make one of these. If you could take a VHF, tune it to the frequency and then feed that undisciminated signal into a laptop using a standard stereo mini cable you would be in business, this should be doable for much less than the prices these AIS systems are charging.
It could be done if somebody felt like putting the effort in to it. There are two frequencies to monitor; it is unclear to me whether you can get by monitoring only one. The data rate is too high (9600 bps) to use the speaker output from the radio, so you need to get the FM discriminator output directly. The sound card input should have a high enough sampling rate to distinguish the bits.

After that, it's "only" a matter of writing a program to extract the HDLC frame from the audio input, extracting the data from the transmitted packet, and finding a way to do something useful with the data. Probably you would end up writing a Windows device driver that looks like a serial port sending NMEA data, so it would work with existing applications.

The hard part of all this is the detail of design and programming. If you did this as a commercial endeavor, you would have expensive software instead of expensive hardware. (ok, you still need the radio...) If you do it as a hobby, your time is free.
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Old 08-07-2006, 02:08   #18
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The original NASA engine only used one channel, and this provided most of the required information. However, they have found it necessary to now use both A and B channels to get the full information.

Why re-invent the wheel, shipplotter is already designed to take either sound through the soundcard, or nmea output to provide the data.
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Old 08-07-2006, 09:18   #19
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"If you could take a VHF, tune it to the frequency and then feed that undisciminated signal into a laptop using a standard stereo mini cable you would be in business, this should be doable for much less than the prices these AIS systems are charging."

Not long ago on another board someone had commented they did that just by using their ICOM PC1000 and their laptop.
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Old 08-07-2006, 09:45   #20
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I use an SR161 AIS with Nobeltec VNS and it is a terrific device. Get ship traffic out to ~25 mi off of a stern rail mounted VHF antenna, depending on height of ship antenna. Nice to find something that works just like it is supposed to. You also get free navigation software that supports it with the package from Milltech Marine, but I haven't tried it. Besides all the ship related stuff, get time and distance of closest approach, warnings if within specified limits.
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Old 09-07-2006, 07:56   #21
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Thumbs up Splitter for AIS

Hi folks,
if you use an splitter for AIS and VHF you need one which is active.
Asap VHF is on air, the splitter must activley disconnect the AIS from the splitter. This must happen in a time less than 30ms otherwise the VHF power goes into the AIS receiver. This is not easy and you need a good stuff of equipment.
Of course youŽll lose some db, which is normal for a splitter.

Have a look to:
http://www.easyais.com

WBR
Kojakal
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Old 09-07-2006, 08:55   #22
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The trouble with the splitter is that it knocks 3 db off the reception (cause it is splitting it) and on the edge of VHF coverage, that will make a significant difference between being able to hear what is said in reply to a mayday and a watery grave.
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