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AIS broadcast data message – what is in it?

The information that is broadcasted by the AIS unit is sorted into three independent data reports and transmitted at set schedules according to the agreed standards.

Marine traffic is interested in the vessels name, its status according the Rules of the Road, its course and speed, rate of turn (RoT) and what the CPA and TCPA are. This information is packed in the so-called Static and Dynamic data packages, all other data is for the primary use of the maritime authorities.

Static data (update every 6 minutes)
(pre-programmed and does not change)

o Ship’s name and call sign
o IMO number
o Length & beam
o Location of antenna
o Ship’s type

Voyage related data (update every 6 minutes)
(to be inputted every new voyage):

o Draft
o Cargo information
o Destination and ETA
o Other relevant information

Dynamic data (update varies)
(automatically derived from ship’s interfaces):

o Time
o Ship’s position
o Course over ground
o Speed over ground
o Gyro heading and Rate of Turn
o Navigational status (according to Rules of the Road)

The Dynamic reports are updated depending on ship’s speed and status, as follows:

o At anchor 3 minutes
o 0 – 14 knots 10 seconds
o 0 – 14 knots & changing course 3.3 seconds
o 14 – 23 knots 6 seconds
o 14 – 23 knots & changing course 2 seconds
o 23+ knots 2 seconds
o 23+ knots & changing course 2 seconds

The trouble is that that AIS on some commercial vessels I have met at sea are showing “At Anchor” when they are clearly underway and going at a fair rate of knots… this means the so called dynamic data is being refreshed every 3 minutes – more than enough time for a collision event!


Laptop on board

I wanted to use the Raymarine C120 as the primary means of electronic chartplotting while on board – it is interfaced to all the instruments as well as the the DSC radio, radar, and AIS.


But while onshore, at home, at anchor or in a marina it is often more convenient to use a laptop. It is now so easy to download grib files, look up weather forecasts, email, browse the web, and forums and so on.

I thought I would use the laptop for two main additional purposes.

a) Chartplotting using the software I already had before installing the Raymarine kit on board ie. Garmin’s Map source software which uses the Bluechart charts – all proprietory to Garmin.

b) Interface to the Target SSB radio so that I could get Navtex, RTTY and Weatherfax transmissions and store them on the laptop for reference while on passage.

c) Transfer data to / from the laptop / C120 and the RC400 using a 1Gb Sandisk Compact Flash Card


DSC00008I have been able to achieve all of this but I must admit that using the SSB is a lot more tricky than I anticipated. The picture here shows the audio cable from the SSB radio into the MIC input of the laptop….


The laptop is an ACER Aspire 5020WLMi and is – in hindsight – probably way over specified now that I have experience of using it in action..it has a built in card reader (very useful) 1Gb of RAM and a 100Gb HDD – but at least it will cope with advances in nav software which like all PC software is destined to bloat and bloat..


SSB On board

Well it seemed like a good idea to get an SSB instead of the dedicated Navtex displays that are on the market…there isn’t much price difference and with an SSB you can receive Weather fax, RTTY and Navtex transmissions anywhere in the world and all for free – no subscriptions required!

DSC00010sThis is the SSB that I installed it is the NASA Target HF3M SSB with dedicated antenna and the audio PC cable to interface with the laptop’s soundcard.

This does cover the spectrum from 30Khz to 30Mhz – so you can listen to AM radio (yes, you would have to be a long way offshore to want to do that !) and also receive the teletext transmissions called RTTY, and also the free Weatherfax




I have found the whole process very fragile however – maybe I just need some more patience to wait for the timed broadcasts and then for the slow acoustic modem speed transmissions – remember 300bd modems?…well this is slowwww….

It is best to do this at anchor or in a marina, so that the signal has a reasonable chance of getting through in a stable fashion.

I keep a folder of frequencies and times of broadcasts handy so that you can set up at the appropriate moment instead of having the laptop on for hours on end. This may be an argument for having the fixed receivers from people like NASA instead of the SSB radio coupled to a a laptop….?


Fitting the RC400

I decided that I would use an RC400 handheld in the cockpit since it DSC00012s would:

a) take less room than any of the smallest C series Raymarine displays
b) Cost a quarter of the price
c) Use Navionics GOLD charts – the same as the C120 chartplotter at the nav table
d) be able to receive and transmit waypoints and tracks using NMEA (or so I hoped)
e) Not be so large or bright that it would interfere with night vision
f) could run off the ships power circuit – saving batteries and the ‘crash’ events that happen when the battery power runs low…(another post later on that one!)

DSC00014sAs it turned out all the above criteria were satisfied except (d). The unit can xmit/rcv NMEA but I have not been able to use that facility since the C120 has only one NMEA port and that is set to 38,400bd so it can receive the AIS data – which only comes at that speed.


I havent been able to work round this yet but I am hopeful that somehow I can use the Brookhouse Mux to recv NMEA waypoint/track data from the C120 at 38,400 and xmit it at 4,800bd – I havent got round to this yet – I am just enjoying the fact that the RC400 works – works off ships power – and fits in really well at the helm without getting in my way.


Electronic Navigation – New System

This is my new system..
– GPS Raystar 125 (2nd GPS)
– Chart plotter – C120
– AIS (outputting at 38400 to Brookhouse and then to the C120)
– Raymarine 18″ 2Kw Radar

Connecting to a laptop
– ACER Aspire 5022WLMi

– E85001 – NMEA0183/Seatalk I/f box to output NMEA0183 to serial and then via a serial to USB adaptor into the laptop

– Brookhouse Multiplexer to I/f AIS to C120 at 38400 and to output on its built in USB cable straight into the laptop

This means that two independent programs can operate on the laptop both getting data in from the ships instruments

All of this now works well after many hours of head scratching and torn knuckles 😉