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CruisingWiki

ARPA v AIS

Most commercial vessels will use radar and very soon every one over 300 gross GRT will have AIS too (July 2007).

Why would AIS be a good adjunct to the use of ARPA?

Typically the ARPA will track the most reflective part of the vessel, on a VLCC this would be the superstructure. Before a change of heading of such a ship is detected on the ARPA, the VLCC will probably have turned 30-40 degrees, equivalent to 4 or 5 minutes. Another 1 to 2 minutes are needed to obtain a steady vector of the target, for smaller vessels this is somewhat less, however an average of five to six minutes between wheel-over and recognition of this fact on ARPA is common.

In contrast, the data of an AIS-ship, sailing at a speed greater than 14 knots and turning, will be updated every 2 seconds, showing the course alteration virtually from the moment that the wheel was put over.

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Who should have AIS?

AIS introduction schedule:

Mandatory carriage requirements are set for all ships of 300 GRT and over engaged on international voyages, and those of 500 GRT and over not on international voyages as well as all passenger vessels irrespective of size.

All ships constructed on or after July 1st, 2002 must be fitted with AIS. Existing ships engaged in international voyages must be fitted with AIS as follows:

  • all passenger ships not later than July 1st, 2003
  • all tankers not later than the first safety equipment survey after July 1st, 2003
  • other vessels of 50,000 GRT and over not later than July 1st, 2004
  • other vessels of 10,000 GRT to 50,000 GRT not later than July 1st, 2005
  • other vessels of 3,000 GRT to 10,000 GRT not later than July 1st, 2006
  • other vessels of 300 GRT to 3000 GRT not later than July 1st, 2007

Existing ships not engaged on international voyages, constructed before July 1st, 2002, must be fitted not later than July 1st, 2008.

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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!

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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.

DSC00011s

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..

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Connecting AIS – NASA AIS receiver

I used the NASA AIS receiver. It has worked fine and if you look at other posts here you can see some very congested screen shots of AIS data in the English Channel.

I also find that I can ‘see’ AIS transmissions of boats up to 12nm out at sea from inside the marina and inside the port of Newhaven, England.

NASA AISThe NASA device does look a bit cheap and one big complaint is the useless manual and the lack of any form of lights on the device to tell you it has power or is receiving or transmitting anything on the NMEA bus. Very frustrating when you are trying to troubleshoot connections. If you are particularly adept you may want to follow this advice and install your own lights.

But it does work and I have plugged it into the Brookhouse Multiplexer and from there I get two outputs. One is the NMEA 2000 (at 38,400bd) feeding into the C120 chartplotter and the other is the signal on the built in USB bus which feeds straight into a laptop without any tempremental Serial/USB converter in the way. This also means that you can reliably feed two signals into the laptop in parallel running two programs for instance without the data getting corrupted.

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