The following is a description of the fitting of an Automatic Identification System (AIS) Transponder to SV Crystal Blues by Neil and Ley Langford. It originally appeared on their web site.
AIS Part 1 – A Primer
My father taught me to sail when I was about 8 years old – he’s the worried one in the old photo at right (probably because my brother Peter is on mainsheet). With great and delightful understatement, he always said that “a collision at sea can ruin your whole day”. He’s absolutely right of course, though nowadays we can use AIS technology to help avoid those “ruined days”. This is the first of three posts regarding the system, and basically describes the technology. Future posts will cover our installation experiences and the system in operation.
Some years ago the International Maritime Organisation (IMO) ratified a standard requiring all ships over 300 tons to carry an Automatic Identification System (AIS) transponder. This was a major step forward in collision avoidance for ships at sea. The system really works – AIS equipped ships constantly transmit information including name, MMSI number, position, speed, course, rate of turn, cargo carried etc etc. Commercial vessels within range receive that data, which is then displayed on dedicated screens or (in most cases) overlaid onto radar or chart plotting screens. The result is that AIS equipped vessels are readily identified, tracked and avoided.
This is a significant primary safety system, and many in the yachting community have taken advantage by purchasing low cost AIS receivers – these display ship locations on navigation chart plotters, or on suitably equiped navigation computers. Whilst an AIS receiver system is a good thing to have, I always believed that the best safety system required the big ships to see me as well – I wanted a transponder that would transmit and receive.
Nowadays we can all see and be seen, with low cost
AIS transponders available to the cruising and pleasure craft
community. The “big ship” Class A systems are expensive, so the IMO has
also ratified a simpler version called AIS Class B, for pleasure
There are important differences between the
two, however they are designed to work together. Class A systems use
dedicated GPS receivers for position information and system timing.
They then transmit a wide range of vessel data, and do so quite
frequently, using coded data bursts on VHF channels 87 & 88. The
system uses a protocol called SOTDMA to keep everything organised, with
GPS derived time managing the broadcast slots. With 2250 time slots on
channel every second, the dual channel system provides up to 4500 time
slots. Class A systems transmit at up to 12.5 watts. A ship travelling
at more than 14 knots will transmit dynamic data every 6 seconds. A
course change increases the burst rate to every 2 seconds.
us in the chart plotter image at right, the black circle and arrows on
the lower right, moving west. At left of the image are AIS equipped
ships entering and exiting Singapore Straits, into the Sth. China Sea.
Click the image to enlarge.
Class B systems also use a
dedicated GPS receiver, but transmit a more restricted data set (no
rate of turn, destination, ETA or cargo information) and do so less
frequently, using less power (maximum 2 watts). The control protocol is
CSTDMA (Carrier Sense Time Division Multiple Access). Basically the
Class B systems listen for a gap in the Class A traffic, then transmit.
There is no guarantee that any individual data burst will be
successfull, however the system transmits a burst every 30 seconds when
underway. Even in Singapore, with literally hundreds of ships
transmitting close by, I’ve watched very solid returns from Class B
equipped vessels in the Singapore Straits. The system can certainly
process lots of traffic – our transponder identified over 1000 targets
(!) in 48 hours on our recent passage from Singapore to Langkawi.
Most of the approved Class B systems use a common internal circuit board, made by Software Radio Technology
(SRT) in the UK. SRT was part of the IMO advisory panel that set the
standard, so its no surprise they have complying product on the market.
Our Comar CSB200 AIS transponder (user manual here) uses the SRT circuit board.
is already compulsory on pleasure craft in some parts of the world (eg
South Korea), and I believe it will become mandatory in many countries.
In Singapore, pleasure craft must carry either an AIS-B transponder or
one of the local HARTS transponders that use cellphone technology as
the data link to shore based monitoring stations (more info on HARTS is
here – thanks to Terry Sargent on SV Valhalla for the document).
more background information on the politics, technology and products
behind AIS, I suggest you spend awhile reading the AIS links on the
excellent PANBO blogsite here. A very good background story, published by Yachting World, is also available here. Our own installation experiences will be posted next.
AIS Part 2 – The Installation Experience
stands for Automatic Identification System, an active primary safety
system for vessels at sea. The black arrow in the photo at right,
captured from our PC screen, is Crystal Blues. The red arrows (targets)
are ships we want to avoid. Clicking on a target reveals the data box
seen at bottom left – lots of information on the ship in question
(click the image at right to enlarge).
I met a cruising sailor last night who said that my first AIS story (here)
was very technical – oops, I guess it is a technical subject. Not sure
that I can eliminate the technicalities, but I will try to explain
them. First though, an essential technology primer …. I strongly
recommend you spend time roaming the excellent PANBO website, specially the AIS pages. Also, here are links for two very informative and useful documents, covering co-axial connectors and co-axial cabling. OK, on with the story ….
We purchased our Class B AIS transponder from Oceantalk in Singapore – in fact we bought four of them, as several other boats wanted to install the system. The unit is a Comar CSB.200, manufactured in the United Kingdom. It was supplied by Oceantalk with a Shakespeare VHF whip antenna and a Sanav GPS antenna.
Class B AIS transponders require a dedicated GPS receiver and a
dedicated VHF antenna. No arguments please – if you want this powerful
safety system, you have to install the extra antennae. No, you cannot
share signals from existing systems, however the AIS derived GPS
position information is available to you as a separate NMEA signal, for
chart plotting purposes. You’d better plan carefully for antenna
locations, bracket positions and cable runs.
antenna is a simple patch antenna with a low noise amplifier that sends
the received signals (as RF) direct to the AIS system for timing
analysis – in other words, the external receiver is just an amplified
antenna, and all the complex decoding and mathematical computations to
derive your vessel position occur inside the AIS transponder. This is a
specific requirement of the IMO AIS
regulatory framework – it basically ensures that no one can feed fake
vessel positions into the system. That is good to know …
the VHF side, AIS Class B uses just 2 watts of radiated power to send
its reports outwards (the big ships get 12 watts). It is easy to
receive signals from big-ship class A systems, however you better pay
attention when installing your VHF cables, connectors and antenna. You
want to ensure that all of your 2 watts is actually radiated into the
ether. Robin Kidd from Oceantalk stressed this point – make sure your
VHF cabling and connection work is good. Installation requires the
– Physically mount the AIS box. A U-bracket
is supplied, but we used industrial strength adhesive Velcro to mount
it on a vertical bulkhead (see photo).
– Run the cables for VHF and GPS antennae
– Install and connect both antennae
– Run cable for DC power with a fuse in line
– Run the data cable to the chart plotter/display system
– Terminate everything and then commission the system
course your chart plotter must be compatible with AIS messages in NMEA
format to display the targets and information. If it isn’t, you can use
this very neat AIS display from Vesper Marine, the AIS Watchmate, or a more serious display (with charts) made by Comar, the CSD.200.
to the installation. If you power the box from a shared DC circuit
breaker (ours is on our navigation instruments circuit) you should
include a 5amp fuse in the power feed. Be careful with the VHF antenna
cabling – OK, its just RG.58, but you’ve only got 2 watts to radiate,
so make sure you use high quality connectors and fittings.
GPS receiver supplied by Oceantalk is the RV-76, made by San Hose
Technology in Taiwan. It includes a nice 10 metre pre-terminated cable.
Its very thin, and easy to run through the boat, but it turns out to be
RG.174, which has very high losses (attenuation) at these frequencies (1.5ghz).
If you use the supplied cable as-is, with its existing terminations, it
will work just fine. However if you need to cut, join, extend or splice
(as we did), then you’ll have to use a more suitable cable (RG.223).
After attempting to extend the supplied cable, and getting no satellite
signal, we changed to a Bedea RG.223 with Telegartner crimp connectors – voila, tons of signal. We purchased the cable and connectors from Coastal Electronics in Singapore, though similar cable is made by Belden and others. Make sure you use the correct crimping tool (see photo).
the VHF side, the Comar AIS box will actually measure and report the
SWR (reflected energy ratio) on your VHF transmission line during
commissioning – you’ll soon know if your VHF cabling and connectors are
good or not. Click on the photo below at right to see typical values.
all that hard work done the rest was easy – plug the box into our PC
using the supplied cable and configure the COM port on the PC (baud
rate etc) . Then load the Comar software (supplied) and configure the
unit. At this point you’ll be asked to input your vessel identifier,
which is the unique “MMSI number” issued by your National Marine
Authority. If you don’t have an MMSI number you’d better apply for one
now, because you cannot transmit using AIS without one. A Comar Class B
transponder will stay in “receive only” mode until you give it your
MMSI number. The real trick is that this number can only be entered
ONCE by the user – mess it up and you have to send the box back to the
dealer for resetting.
I’ve just learned that Class B systems are FINALLY approved for use in the USA (story here),
but that they are not allowing users to configure the MMSI identifiers.
I bet that will be fun to administer…. seems the installers or
retailers will have to configure the box before handover. The Comar
configuration software is simple to use, neat and logical.
use only good cables and connectors, get yourself an MMSI number and
enjoy the results. Our next story on AIS will conclude the series with
our user experiences
AIS Part 3 – Safety At Sea, with AIS Onboard
helps us avoid the cruising sailors greatest danger – a collision at
sea. Of course the system isn’t perfect, however it is a powerful ally
in collision avoidance and I wouldn’t want to be without it. Our
previous two stories (read them here and here) introduced the system and discussed a typical cruising boat installation – in this story we share our operational experiences.
Singapore Straits is the busiest shipping lane in the world, and the
nearby Malacca Straits are not far behind in the traffic stakes. These
two waterways provided a strenuous testing ground for our new Comar
AIS installation. Our very first test was conducted on a three day
passage from Sarawak (northern Borneo) to Singapore. Departing Kuching,
capital of Sarawak, we tracked several ships on screen that were well
over our visual (and radar) horizon. We also noted a fixed AIS base
station on a mountain top near Kuching, reporting itself as being
highly accurate in its position. As a newcomer to AIS this puzzled me,
however we could see that base station for over 100 nautical miles into
the Sth China Sea, so it was a welcome reference. You can see it in
this image, the purple dot near the bottom, with its own MMSI number of
Travelling from Borneo to Singapore there are
not a lot of ships – hence not a lot of AIS traffic and therefore a
great signal to noise ratio. In that low noise / low traffic
environment our system was receiving and plotting ship locations more
than 120 nautical miles away. It was great to know well in advance the
traffic that was likely to cross our path. We also noted that military
support vessels don’t have to run their transponder all the time – we
passed within 1/4 mile of a small fleet oiler that didn’t exist on AIS
– though I’m pretty sure they knew where we were, and we had her on
radar for hours. Whilst military vessels have a nominated identity in
the AIS world, they don’t always broadcast their location, for obvious
we approached Singapore the traffic density (and target numbers)
increased, and the maximum receiving range fell off. This is a logical
consequence of the increased traffic levels, and illustrates how good
the system really is – when traffic is dense the closer vessels
dominate – which is exactly what we want for effective collision
avoidance. In this image you can see vessels arriving and departing the
Straits (red & green triangles) plus anchored vessels (purple
Departing Singapore for Langkawi, we cleared
immigration at the Western Quarantine Anchorage. Already we had
hundreds of vessels showing on our display, but the software handled
things well and it was never confusing – at all times the closest and
most threatening targets were clearly visible. In a dense traffic
situation (harbour / river / channel) it pays to zoom in close on the
chart plotter display screen, so that only local targets are visible –
the ones that matter. One hour out we plotted a Class B transponder,
the first we’d seen on screen, and watched this small motor vessel
cross two shipping lanes and then run past our port side. Despite
several hundred Class A transponders broadcasting close by, the Class B
vessel was consistently visible. The 30 second reporting frequency of
Class B systems was just evident – this was a quick motor boat, and the
Class B position updates were just a little lumpy.
into Malaysian waters we headed north west up the Malacca Straits,
estimating three days for our passage to Langkawi. This was our sixth
transit of the Malacca Straits, but our first with AIS, and what a
great difference it makes. Every large commercial vessel showed up on
our chart plotter, giving us very early warning of their speed and
course. We habitually stick to the eastern edge of the main shipping
channel in the straits, hoping to keep out of the ships way and
also to avoid the fish nets and fish traps that pepper the inshore
waters. Passing Port Klang has always been a challenge – it’s a major
shipping port and many ships turn into and out of the Straits channel,
but this time it was easy. This image, zoomed out on the chart display,
shows the traffic density – we’re the black circle and arrow in the
middle. Seriously, the traffic just parted for us – ships running up
the straits turned early or late and always gave us plenty of sea room
– it was very clear that our AIS transmissions were being watched. What
a joy. We usually keep a dual lookout in that region, one of us on port
and the other on starboard, however this time it simply wasn’t
necessary – I slept soundly for that part of the passage. Amazingly, in
that three day transit the AIS system logged over 2000 targets. That’s
a lot of ships.
always found that most commercial mariners will try to be cooperative
if they have the right information – AIS certainly gives them that.
More importantly, on most large vessels the voyage data recorder (info here),
like the one at left, will securely record your yacht’s transmissions –
no one can deny your existence. This has to encourage larger vessels to
comply with SOLAS rules.
The recording of AIS targets is very useful – should your tiny sailboat
go missing one day, every big ship that you’ve passed will have a time
and date stamped record of your past position to assist the search
authorities. Furthermore, many shore based AIS stations (and there are
thousands already in place) will see you as you pass within range and
report your position – check it out here.
my last story I mentioned that the AIS transponder provided a GPS
position output in standard NMEA format. It is tempting to think of
this as a useful backup to our primary GPS receivers, but think again.
It seems that the AIS GPS positions are often fine tuned with
differential information broadcast by local AIS base stations (hence
that “high accuracy” base station we observed in Borneo). Also, AIS GPS
hardware is built to far more stringent standards (IEC61108)
than conventional receivers. This means that your AIS GPS system is
most likely more accurate than your existing primary GPS in many
coastal situations – so you should use the AIS position data as a first
On Crystal Blues our AIS data is received and displayed by our PC based chart plotting software – Transas Navigator Pro.
The software displays our vessel, plus all the AIS & MARPA radar
targets, as live moving indicators overlaid onto the electronic chart.
This is an incredibly powerful tool, derived from commercial shipping
AIS is in its infancy, but it clearly has a
future on your boat. Thousands of aids to navigation already carry AIS
transponders, so that you can see them even when the weather is thick.
Some are suggesting that “virtual aids” will soon appear on your AIS
display – electronic marker buoys that can be placed rapidly if a new
wreck or other danger appears. These can be activated almost instantly
by maritime authorities, well before a real buoy can be positioned. And
in a distress situation, any commercial vessel can accurately locate
Crystal Blues with ease, once within range.
This week I received an email from Jeff Robbins, developer of the highly rated Vesper Marine Watchmate
AIS display. The Watchmate is a very simple (low power) LCD screen that
interprets data from any AIS unit and displays it essentially the way
you need it. To quote Jeff “Watchmate prioritises targets … the most
important collision risks appear first. It also filters the data to
eliminate false alarms when operating in harbour or crowded situations.
And it has a user selectable “profile”, that allows the user to select
with a single button their sailing situation (eg. anchored,
Phew – Jeff has done his
homework, and his display screen seems probably the best way to handle
AIS information if you don’t already have a compatible chart plotting
system, and a very smart way to go even if you do. Of course it works
with transponders or simple receivers.
Whilst we don’t
have Jeff’s screen we’re pleased with the extra safety and navigational
information we receive from our AIS sytem, and even more pleased that
the big boats out there know exactly where we are, day and night –
rain, hail or shine. AIS transponders are a great advance in safety for
both coastal and ocean sailors. Cruising author and boat designer Steve Dashew
says of AIS “I would put an AIS B way ahead of most other “necessities”
for a cruising yacht, especially when cruising in areas with lots of