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  AIS-SART: Response of Chart Plotter

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Author Topic:   AIS-SART: Response of Chart Plotter
jimh posted 12-10-2014 12:11 PM ET (US)   Profile for jimh   Send Email to jimh  
In a recent thread the topic of a new type of Search and Rescue Transmitter (SART) using the Automatic Identification System (AIS), or AIS-SART, was discussed:

http://continuouswave.com/ubb/Forum6/HTML/003680.html

In that discussion there was speculation about the behavior of a typical recreational grade marine chart plotter in responding to input from an AIS receiver than included NMEA-0183 sentences in the !AIVDM format from an AIS-SART device. I was curious about this behavior and wished to test my own chart plotter, a Lowrance HDS-8.

Since I lacked an AIS-SART device to generate a suitable AIS sentence for testing, I contacted a manufacturer of AIS-SART devices, Weatherdock AG, in Germany. They were very kind and quickly responded to my inquiry. I asked them if they could send me a typical NMEA-0183 sentences that would be sent from an AIS receive upon reception of a signal from one of their AIS-SART devices. This was a lot simpler than sending me an AIS-SART to test with, and they kindly complied. Many thanks to Maximilian Schuster at Weatherdock AG for the data.

The AIS receiver output sentence I used was:

!AIVDM,1,1,,B,1>MA>E>P000NoHHMEB=P0?wt1t00,0*4C

Using an on-line AIS decoder at http://rl.se/aivdm , I tested this sentence. The response of the decoder is shown below:


On-line AIS decoder output from test sentence.

Next, I configured my laptop computer to connect to the NMEA-0183 input of my Lowrance HDS-8 chart plotter. (I give some details about this below.) I sent the !AIVDM sentence from Weatherdock AG to the HDS-8.

The HDS-8 responded by plotting an AIS target on the chart plotter. Since this target was 4,000-miles from the position of the HDS-8, I had to a long scroll of the map over to Germany to see the target. Here is the appearance of the chart plotter screen:

Screen capture from HDS-8 showing AIS target from AIS-SART device.
Response of HDS-8 chart plotter to input from simulated AIS receiver with AIS-SART target in range.

The HDS-8 did make any sort of alarm sound or put on-screen any sort of alert. It just silently added the target to the chart display. The chart plotter also offers more details about the target in another screen, seen here:

Screen capture: Details of AIS target
Details of AIS target from HDS-8 interpretation of data.

Conclusions

The HDS plotted the AIS message from the AIS-SART in its proper location, but it did not provide any sort of alerting or alarm. The details of the AIS message are not quite clear. The HDS reports the "NavStatus = unknown", but the Navigational Status should be the contents of the Type 14 message contained in the data. Notice the on-line decoder said "Navigation status = 14". Neither really showed the text of that message.

Details of NMEA-0183 connection
On my MacBook Pro laptop, I connect a USB-to-Serial adaptor, a KeySpan USA-19HS. I use the MacOS terminal program and run the SCREEN utility. I cut and paste the text of the !AIVDM sentence into the terminal window, sending it to the USB device, which is connected to the HDS. The interconnection is done with my Universal NMEA-0183 interface, making it just a matter of plugging the two device's NMEA connections into my backplane connector. (See an earlier article for more details.) This permits me to send data to the HDS-8 from my laptop terminal shell.

jimh posted 12-10-2014 12:59 PM ET (US)     Profile for jimh  Send Email to jimh     
I also tried to decode the AIS-SART message on another on-line decoder at:

http://www.aggsoft.com/ais-decoder.htm

This decoder seemed to be very robust and able to decode in great detail most AIS messages, but the AIS-SART stumped it. It returned the result "Unable to decode message" when given the test sentence to analyze.

Having tested the AIS-SART message with three decoders (the HDS and the two on-line ones), the results seem quite mixed. Only two out of three could decode the message. Of the two that decoded the message, neither showed the text payload information contained in the Type#14 data. The chart plotter did not raise any alert or alarm, and its plot of the AIS-SART target was the same as for a normal Class-A transponder message.

I don't expect to encounter any real-world AIS-SART messages when out boating, unless for some reason I happened to be in radio range of another boater or vessel who happens to have one and has a man-overboard or other emergency, so I can't say I am overly concerned about my HDS-8 not giving me an alarm or alert. In the context of the reply I received from the U.S. Coast Guard that AIS-SART signals are for homing, not for alerting, perhaps the response of the chart plotter is not especially inappropriate.

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