AIS, this and that #1
Whereas so many of you are interested in AIS, and so many news bits keep coming in, I’m starting a “this and that” series…
* Nobeltec has put up the best looking AIS 100 manual to date. Above is a wiring illustration, including how to pass GPS data through the AIS 100, though Nobeltec does not recommend doing so. (For them it’s largely an issue of technical support, but I also understand that the AIS 100 only passes through the GPRMC data string, which may mean that you won’t be able to see, say, how many satellites a GPS is receiving. Anyone know for sure? {The answer is yes, confirmed by GPSNavX in comments})
* Meanwhile, NASA has apparently changed the firmware in the AIS 100 so that it now automatically switches channels every 36 seconds, which seems like a good idea. Supposedly it is possible to update older units, but I don’t know how, or where you can get the update (yet).
* Navicon, a Danish company doing a lot of AIS software development, has introduced a Java-based charting program called AIS Navigator.
* True Heading, a Swedish AIS company, has developed a splitter for sharing a single antenna with an AIS receiver and a regular VHF radio. Look in Products/Accessories. They also have a new product called Blue-Pilot which uses Bluetooth to make a Class A transponder’s “pilot port” wireless. Better yet, it is supposedly able to find and correct problems in the plug installation (which are said to be common). Look in Products/AIS, where you’ll also see the Smart Radio 161 single channel receiver recommended for shore use only.
From the Nasa Marine Engine Specification..
“Data format (GPS RMC sentences from Engine to PC)
NMEA RMC sentences received on a separate input channel at 4,800 Baud (on pin 9 of the 9-pin DB-9 socket) will be copied through on the 38,400 Baud output channel whenever they are available. RMC sentences are re-transmitted only if the CRC check on the incoming data is passed, and are sent whenever they are available.”
So this means if you connect your GPS into the Nasa AIS marine receiver you will only get the $GPRMC message passed through and only if it has a check-sum (most do). All other messages will be lost, such as $GPWPL or $GPGSV which are the active waypoint or satellite information. One should avoid running the GPS through the Nasa receiver if possible. Ideally a seperate port should be used for GPS (4,800) and AIS (38,400).
I would like to Thank you for keeping me informed about developments in marine electronics. AIS in particular you are doing an outstanding job of educating the recreational boater of this new technology. Based on information you have posted I purchased two NASA AIS engines. I will install one on my boat this weekend, the other is a gift for a friend.
Pat
Well, thank you kindly, Pat. I must say I really enjoy working in this medium. But I’d have to add that it’s certainly not paying the bills, just yet. One day soon you may see more sponsorship, plus a way to show appreciation with Paypal or a credit card 😉
ShipModul has a device for combining AIS with NMEA-183 from multiple sources.
http://www.shipmodul.com/en
I bought a NASA AIS with integral display ($350 in the door) Hook up was straight forward and I’m watching traffic in the West Bay with some contacts out to 30 miles. I see Sitex has an identical model, are they in bed together now?
Hello admin, nice site ! Good content, eautiful design, thank !
Very nice design 😉 Good work,webmaster!
Although the Nasa AIS Engine is being discussed on this blog as a ‘single frequency’ unit it has the capability of receiving on both frequencies. The recently introduced software by Nasa, PC Display for AIS Engine, now at ver 1.03, will automatically change between the two frequencies at 36 second intervala or it can be locked on either frequency. This capability, however, is the ONLY thing I like about the new Nasa software. All other features are amateurish compared to such software as Yacht-AIS, Sea Clear II, and Software on Board. Perhaps the other software designers will figure out how Nasa does this automatic switching ???
Y-Tronic is fast on the draw! Just downloaded the upgraded 1.8 version of Yacht-AIS dated 2006-Mar-29, and am pleased to find it can now do automatic switching between frequencies for the Nasa AIS Engine. BTW, their new URL is http://www.y-tronic.com Many thanks to Rainer at Y-Tronic.
I think your site is very good and complete, but the information you have here
Regarding switching AIS frequencies in client software. I think this may not be ideal.
If the active AIS transmitter on the ship alternates between two frequencies and if the AIS receiver can only receive on one frequency but switches between the two, there is a non-trivial chance that you won’t hear some ships at all, or at least for a long time. You will end up always being on the wrong frequency when they transmit. It a little counter intuitive, but happens in the real world.
The technical term for this is called “self synchronization”. For example a networking paper on this is http://portal.acm.org/citation.cfm?coll=GUIDE&dl=GUIDE&id=166241
I also wonder if there is anything built into AIS to keep two ships from transmitting at the same time? For example, adding a random time interval after the fixed interval.
In theory, this is indeed possible, but unlikely as the actual reporting rate varies with speed and rate of turn etc. s any self synchronisation would soon be lost.
As to Tx at the same time, there is a complicated protocol (SOTDMA for Class A, or Carrier Sense for Class B) that handles this so it is unlikely to occur.
To be safe I am going to get one of the two channel models. Probably the EasyAIS and splitter. Seems like a very nice combination.
Portable AIS Receiver:
Anybody has seen this? It is a South African Company and sells a portable AIS, powered by 6 AA cells. I do not know if it is sell in USA. Anybody knows a dealer in US for this brand?
http://maritec.co.za/index.php?pr=ArkEars_Portable
That is actually the Sine Qua Non AIS test unit.
http://www.sinequanonth.co.za/
Battery powered, but not exactly what you would call affordable at about $2k USD
And with a receiver sensitivity of -47dBm I wouldn’t count on hearing anything other than your own AIS.
bob
Reguarding the 1 channel/2 channel, there is no ‘real’ reason to switch channels at all other than getting message 5 (ship data like name and destination) which is transmitted every 6 minutes.
The problem with the Class A spec is that if you stay on 1 channel there is a possibility that you may never hear a Message 5, depending on the reporting rate of the ship it has the potential to always fall on the other channel.
Realistically I don’t think that’s a big problem. So with a 1 channel receiver that doesn’t switch it may take you up to 12 minutes to get the Message 5 with the ship’s name, destination and callsign.
bob
Well, here’s another problem for single frequency receivers:
https://panbo.com/yae/archives/001024.html
and remember that Class B transponders will be broadcasting at much slower rates:
https://panbo.com/yae/archives/001145.html
I think dual frequency will get less expensive and will become the norm.
Thank you for the information on the Portable AIS: really for this price, we can buy a two channel AIS receiver AND a C80 Chart plotter…
Regarding the need for a 2 channel AIS receiver, the price of SR162 is more than double of SR161;I think that, instead is better to buy an SR260/61 AIS Modem in China for about US$500.
I have nearly finished all high-level s/w for an AIS tranmitter. but now i need an OEM GMSK TDMA modem for realising my project. Pls give me some address or forward me to buy a modem power around 2W if possible.