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What’s that
Ship?
or AIS for Landlubbers
by Stan Swan
What is that ship sailing past? Where’s it heading? When is it due in
port? What is its speed? How big is it? For all these questions – and
many more – the answer is AIS: Automatic Identification System. You
may even follow it, on line, on your own computer – from anywhere!
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R
eaders may recall a few months
ago we featured a means of
identifying aircraft and following their progress using a hardware
and web product called AirNav Radar
Box (SILICON CHIP, November 2008).
Now you can do much the same
thing with virtually all commercial
shipping and even many of the larger
pleasure craft you see off our coasts.
Somewhat akin to AirNav Radar
Box, you have the choice of tracking
in real time (using a VHF receiver or
scanner) if you live close enough to
the coast or alternatively, using the
internet where details of that ship
may well have been posted. But we
are getting slightly ahead of ourselves!
Ship watching
Even viewed from the shore, ships
often conjure up an exotic sense of far
away places and tap an eternal human
wanderlust.
Many a ship-watcher has idly gazed
at passing vessels and pondered their
destination and movements – particularly if one’s vantage point is from a
becalmed yacht in a busy sea lane!
Both at a romantic and utilitarian
level, even a casual interest in shipping movements from a coastal location near sea lanes could mean an
emerging VHF wireless technology
called AIS – Automatic Identification
System – may take your fancy.
Essentially a SOLAS (Safety of Life
At Sea) collision avoidance system
designed for professional seafarers,
AIS shows immense potential even
for landlubbers! It’s a fascinating blend
of software, radio, electronics and enavigation, yet can be experienced on
skinflint budgets.
Interested? But it sounds like digital
sea-sickness? Gain your AIS sea legs
instead (and to whet your appetite!) go
to www.marinetraffic.com/ais/!
Just select a busy region (perhaps
the Straits of Gibraltar) from the global
map and zoom in to suit. Mouse-hovering over a coloured display symbol
gives vessel details, while a right
mouse click shows ship images and
near-live vessel tracks.
The versatile real-time insights
gained from this are quite astounding and should enthuse even hesitant
landlubbers!
Keeping track . . . of a sailing ship with AIS. The Spirit of New Zealand off
the northeast coast of NZ. You can even see how the ship is tacking from the
southeast before it makes its due-westerly run into Kawau Island. There’s even
a photo of the Spirit under canvas so you can see what she looks like!
Collision avoidance
“Full speed astern!” Over past centuries of powered shipping, no doubt
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The Channel is getting pretty crowded! This is just a tiny section with AIS showing cargo vessels, passenger vessels, tankers, ships under way, ships moored . . .
August 2009 13
Commercial AIS equipment – at right
the text-only SIMRAD A170, photographed on the bridge of the Norwegian
coastal vessel MF Bastø II by Ulf
Larsen. (Courtesy Wikipedia).
Below is a Smart Radio SR161 AIS
Receiver, an entry-leve single channel
scanning receiver designed for both
recreational and professional use.
(Courtesy Milltech Marine).
this urgent command has been the last
request handled by many engine-room
telegraphs!
Radar (when fitted) has long helped
identify possible shipping hazards
but the returned radar trace requires
skilled interpretation. As well, microwave signals may be masked by
nearby terrain.
Thanks to GPS, your position may
be accurately known but the intentions
of the “tanker on my port bow!” (suddenly appearing around a headland
or looming out of the fog) may be a
mystery.
VHF voice communication may
help clarify things but evasive action
takes precious time – especially in
congested waters.
Aside from the ever-present dangers
of weather, reefs and pirates, mariners
have to increasingly contend with the
possibility of accidentally colliding
with each other.
The English Channel is so crowded
with sea traffic that old salts swear
white lines and traffic lights may soon
be needed to prevent collisions!
When compared with aircraft, vessel speeds are orders of magnitudes
slower. But aircraft have the third
dimension available to climb or dive
to avoid each other – ships naturally
work in a more restrictive 2D environment and may also need several
This ship parking area is off the port of Newcastle, NSW. At left is a Google Earth overlay showing the ships lucky enough
to have entered the Hunter River, while the queue at right shows those still waiting. Do you reckon the captain of one
vessel might have got bored waiting and decided to do a hot lap to keep the crew busy?
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On the left a “ShipPlotter” example shows single ship AIS details, plotted against a low resolution shoreline sourced
from www.sailwx.info Detailed marine charts can be costly, although the 1:250,000 global coastline extractions (freely
available as “mapgen” downloads from http://rimmer.ngdc.noaa.gov/) may be satisfactory for shoreside users.
nautical miles to come to a stop, go
astern or even to make an evasive turn.
Automatic Identification
System (AIS)
An amateur radio APRS (Automatic
Packet Reporting System) GPS-based
technique has been in use since the
1980s and it’s tempting to think such
enthusiasts paved the way for commercial services again – much as
amateurs did for “useless” shortwave
broadcasting back in the 1920s.
ACARS (Aircraft Communications
Addressing and Reporting System)
began about 1990 and by the late 1990s
an international shipboard based collision avoidance positioning system
emerged.
This marine development has grown
into perhaps the most significant navigational safety technology since the
introduction of radar.
AIS is strictly called UAIS (Universal Automatic Identification System )
but the “U” is often dropped to make
it simpler. This wireless technology
enables commercial vessels to “see”
and clearly identify each other, whatever the conditions (night, fog, storms,
congestion).
AIS does this by continuously transmitting a vessel’s identity, position,
speed and course to all other nearby
AIS-equipped vessels or shore stations
within VHF range (typically 20 nautical miles or ~ 30km).
These low-power and low-speed
data signals are openly sent on VHF
marine channels 87B (161.975MHz)
and 88B (162.025MHz) as brief
(~30ms) 9600 bps data bursts and can
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be readily received and decoded by
others for textual or graphical display.
So-called Class “A” AIS transponders have been compulsory on vessels
over 300 tonnes since 2004 but even
smaller leisure craft are now fitting the
less informative “Class B” version as
part of their e-navigation tools.
AIS acceptance and adoption has
been very rapid and current worldwide usage is already (mid 2009)
estimated at over 40,000 vessels.
AIS benefits
Although radar and GPS may be
thought adequate instruments, there
are seven major areas where AIS improves over them:
• Longer wavelength – AIS can “see”
targets around islands and capes in
coastal waters (as microwave radar
doesn’t penetrate hills etc)
• Ships identified – AIS “translates”
radar echoes into ship names and
types - useful when calling them on
VHF!
• More information – AIS improves
prediction of a target’s intentions by
taking into account its course and
rate of turn (ROT).
• More range – at sea AIS receivers
typically cover to the horizon.
• Two-way information – it alerts
others to your vessel’s nature and
intentions.
• Low power – only small transmitters are needed– typically just 2-12
watts (radar is much higher power
and has intricate moving parts).
• Inshore vessel information can be
gained, perhaps globally via the
web.
Typically the end result can be echarted displays showing details of all
AIS-enabled craft within VHF range,
along with their course and speed and
Decoding software,such as NMEA Plus (trial download) or online via
http://rl.se/aivdm, conveniently allows the cryptic AIS data messages to be
shown as plain text. Although useful when first testing ones AIS receiving
setup, graphical displays may be naturally more enlightening.
August 2009 15
Rottnest Island, off
Perth/Fremantle
(WA), with the Swan
River at right. This
image comes from
www.marinetraffic.
com Below is an
extracted chart of
the same area
sourced via NOAA’s
free 1:250,000 global
coastline resource
(http://rimmer.nadc.
noaa.gov/).
“NavMonPc” can
AIS-read and zoom
these mapgen format
charts.
even if they are at anchor or berthed.
Although essentially minimising the
risk of collision, AIS allows harbour
facilities to be productively managed
and it’s also useful for marine search
and rescue operations (SAR aircraft
now increasingly carry AIS), since it
can identify and exactly pinpoint the
position of an AIS-transmitting ship
in distress.
AIS information details
At its most basic level, an AIS transmitter just continually sends the host
vessel’s call sign and GPS position,
alerting others to its whereabouts.
An intricate method of time sharing
the radio channel ensures that even
with a large number of vessels nearby,
blocking of individual transmissions
is unlikely.
AIS ships typically automatically
transmit dynamic voyage information at intervals of between 2 and 12
seconds (depending on speed and
proximity to others), or every few
minutes if at anchor.
Details include:
• MMSI number (Maritime Mobile
Service Identity - a series of nine
digits)
• Navigation status, eg, ‘at anchor’ or
‘underway’
• Ground speed, from 0 to 102 knots
in steps of 0.1 knots
• Rate of turn, 0 to 720 degrees per
minute
• Position (GPS-sourced latitude and
longitude)
• Heading and Course over Ground
(COG)
• Time stamp.
Every six minutes vessel information is also transmitted, typically:
• Callsign
• Ship’s name and unique ID number
• Type of ship
• Beam (width), length and draught
• Onboard GPS antenna location
Further voyage related information
such as cargo, destination, ETA and
route plan, short messages etc, is generally manually entered and updated
by the bridge team.
Rusty English officers are prone to
entering such amusing typos as Sidney
instead of Sydney!
Nature of AIS data
Two-way ship-to-ship and shipto-shore AIS is usually preferred
by mariners, as it allows invaluable
information to be automatically exchanged between all AIS-enabled
craft, maritime traffic stations, coastguard authorities and even buoys and
lighthouses.
However, such equipment and its
professional installation can run to
several thousand dollars.
Dedicated AIS sets pick the 9600
You might expect to find a lot more ships in what Cook described as one of the world’s great harbours . . . but ever since
the NSW government’s rather questionable decision to close Sydney Harbour as a working port, five vessels is about it!
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bps GMSK (Gaussian Minimum Shift
Keying) FM signal off the receiver’s
discriminator and process the entire
25kHz data bandwidth before later
narrowing occurs.
GMSK (which is also most notably
used in the GSM cellular system)
enjoys the impulse noise immunity
of FSK (Frequency Shift Keying) but
the digital data stream is first filtered,
thereby yielding high spectral efficiency.
If you live near a seaport and
have a NBFM (narrow band FM) receiver covering the 161.975MHz and
162.025MHz AIS frequencies (either
will do), the brief data bursts arising
from nearby commercial ships, boats
and ferries can usually be heard as
scratchy “pops”. Signals are often
quite weak, so the receiver squelch
may need to be opened.
Unfortunately, in most scanners receiver filters and de-emphasis circuits
trim off much of the incoming signal’s
intelligence, limiting raw audio decoding. However, it’s feasible to modify
even cheap VHF receivers and scanners to make them more acceptable
for AIS service without compromising
“normal” usage.
We show details on how to do this to
a typical scanner shortly, with decoding via PC soundcard.
The conversion is straightforward
on even older VHF scanners and a
comprehensive resource site www.
Vessel tracks,
which fade
after selectable
times on plotting
programs, offer
outstanding
course insights.
Although the
AIS course data
is near real time,
it may be some
minutes before
ships transmit
their name and
voyage details.
Google Earth
pictures are all
very pretty but a
simple graphical
screen, as seen
here, is often far
better to view
ship details and
paths without
the clutter of
buildings.
siliconchip.com.au
Stats from a Sydney
AIS station feeding into
www.marinetraffic.
com. Freakish AIS reception from far offshore
usually relates to VHF
tropospheric ducting
– common in summer
(and at night) when hot
dry settled anticyclones
overlay cooler, moist
ocean air. NSW endured a heatwave early
February 2009 !
discriminator.nl/ details the technique
for hundreds of different VHF sets.
Modern, more upmarket VHF
receivers increasingly offer this
“discriminator tap” output for direct
soundcard input of the raw data.
Such direct access to raw data signals may be useful for other digital
modes as well – soundcard data decoding is very well established!
NMEA
Just as modern aircraft are predominantly “fly-by-wire”, modern
ships have long enjoyed all-electronic
“glass bridges”. The National Marine
Electronics Association (NMEA) has
developed a specification to suit.
Specialised AIS receivers (such as
the Smart Radio SR161) will probably include an NMEA port for direct
interfacing to various pieces of marine electronic equipment, allowing
information to be automatically sent
between computers, GPS, radar, echo
sounders, chart plotters and the like.
Although cryptic, for most purposes
NMEA can be considered an RS232compatible 4800 bps, 8 data bits, no
parity and one stop bit (4800,8,N,1)
protocol.
However AIS encoding assigns each
ASCII character to 6 binary bits (unlike normal ASCII which uses 8 bits).
Here are typical 64-bit plain text AIS
data strings –
!AIVDM,1,1,,B,1INS<8<at>P001cnWFEdSmh00bT0000,0*38
!AIVDO,1,1,,A,100000002lP7NcvM<at>jupq79`0000,0*3A
It’s certainly not easy to make sense
of these!
An on line NMEA sentence decoder
at http://rl.se/aivdm relates the first to
an anchored ship near Athens (Greece)
at latitude 37.91 N , longitude 23.56
E, while the second vessel travels
through the English Channel at a heady
18 knots. (AIVDM = other vessels,
while AIVDO means your own ship.)
Fortunately there’s no need to sweat
the NMEA decoding! Convenient
software is available, both for online
and local use, that allows smooth PC
decoding (and perhaps even plotting)
of the shipping data contained in the
cryptic NMEA AIS sentences.
Shore-side reception
and decoding.
Let’s imagine you’re based near the
busy seaport of Fremantle (WA) and
are now interested in setting up a costeffective AIS monitor.
As any “Freo” resident will confirm,
even with a global downturn, dozens
of ships continually ply the Swan
River mouth between the mainland
and offshore Rottnest Island.
What ARE they? Given the open and
informative nature of AIS, such interest has soared amongst enthusiasts
keen to monitor local shipping moveAugust 2009 17
The Ship on
the First Floor
When Stan Swan sent his draft and
pictures over for this feature, we were
intrigued to find a ship “at anchor” a
little north-west of Manly, on Sydney’s
Northern Beaches.
Those who know the area – as we do
(very well!) – will know that there is a
lagoon a little north-west of Manly but
anything higher than about 30cm would
not get through the pipe to the ocean.
There’s also a dam in the hills above
Manly – but we doubted any of small
pleasure craft which sometimes use the
dam would have AIS fitted.
On closer examination, we found that
the “ship” was actually “anchored” in a
street in Brookvale, only a couple of hundred metres from the SILICON CHIP offices.
Then Stan had another look at the data
and found the “ship” was actually “under
test” and appeared to be tacking back and
forth on the first floor of an office building. In fact, he managed to print out the
track of the ship, shown below. It certainly
shows the accuracy of the GPS tracking
signal which is part of AIS!
Umm . . . maybe the AIS transmitter
should have been turned off or at least
fed into a dummy load?
18 Silicon Chip
ments – both ashore and when afloat.
Before embarking on a full AIS quest
it’s first worthwhile experimenting
with available VHF receivers (and
perhaps simple antenna) to see if
161.975MHz/162.025MHz signals can
even be heard locally.
As the two AIS channels are marine
hi-band VHF ones, almost any VHF
radio or old scanner will normally
receive them.
Cheap US “Weather Band” receivers
(~162.500MHz) are so near the AIS
frequencies that retuning to suit may
even be possible, as may tweaking
older two metre (144-145MHz) ham
band gear.
However, some older VHF receivers
may be insensitive, subject to interference or prone to drifting, making full
AIS decoding performance possibly
frustrating.
Dedicated AIS commercial transponder and charting setups can cost
many thousands of dollars and best
suit the professional or well-heeled.
Assuming signals have been detected,
more cost effective approaches hence
may tempt.
1. Purchase or construction.
Aside from just buying a dedicated
AIS receiver, making one’s own
complete receiver/decoder is not
impossible. Several AIS-capable ICs
are available (most notably CML’s
CMX7032 and CMX7042) to handle
all the data massaging – with just a
162MHz RF front-end needed.
However such a constructional
challenge best suits the real enthusiast, even then one with a reasonable
level of skill. So both landlubber ship
followers and small vessels keen to
receive and decode AIS are advised to
look at simpler (and cheaper) options.
2. Discriminator tap + soundcard.
As outlined overleaf, basic local AIS
reception really only requires the following, with most of the (Windows)
software free:
• A sensitive, stable ~162MHz VHF
receiver with discriminator output
(see below)
• A suitable VHF antenna (the higher
the better - refer “Slim JIM” below)
• An mono-to-stereo audio cable between your receiver and PC soundcard input
• A Windows PC with 16-bit soundcard – even older laptops will do fine.
• Soundcard demodulating and
NMEA producing software (AISMon 2.2.0)
• A virtual serial port program (VSPD
XP5) to feed the decoding program
• Either decoding software to turn
NMEA strings into plain text
(NMEA Plus), or
• Simple graphical software to e-chart
the NMEA output (NavMonPc)
Although increasingly considered
a cartographic display norm, Google
Earth-style charting of local shipping
movements requires powerful software and perhaps local e-charts, both
of which can be costly. Naturally a web
connection may ease this but remote
coastal sites or underway vessels may
not enjoy this convenience.
The esteemed “ShipPlotter” (Portuguese; 21 day trial then around
$50) provides acceptable displays,
including linked satellite views and it
directly handles the soundcard signal
processing and NMEA feeds.
In spite of these processing needs,
almost any PC will do, as the AIS data
handling is relatively pedestrian –
there’s even a Win98 version.
Although there are numerous other
tracking programs, ShipPlotter is presently considered the best value AIS
package but it has near overwhelming features with a steep learning
curve. Even the alternative (and free)
“SeaClear” may take days to fine-tune
soundcard settings and charts to your
needs.
In contrast the simpler (but comprehensive, global and free) 1:250,000
coastline chart extractions used by
NavMonPc may better suit. Remember that ships naturally are at sea and
Google Earth terrestrial details may
hence be incidental (most Google
Earth satpics don’t go very far out to
sea).
3. Web Server and on line display
Although this looks more ambitious,
in fact setup is streamlined compared
with the 2nd option, as such dedicated
sites as www.marinetraffic.com (University of the Aegean, Greece) handle
much of the final NMEA decoding and
graphical display.
Naturally internet access is necessary. Both you and global ship watchers can monitor real-time marine
traffic while half a world away (and
miles from the ocean) in Alice Springs.
That’s right – no receiver is needed!
Web-served AIS display viewing is
highly recommended initially, as the
nature and variety of zoomable graphical ship tracking in popular locations
will be quickly experienced.
siliconchip.com.au
Getting your PC to work its soundcard decoding magic requires three
linked programs – an NMEA decoder, com port emulator and AIS displayer.
AISMon 2.2.0 has been found a robust initial NMEA decoder, with its
blue signal input bar invaluable for audio level setting. Click a 48000
sample rate, 4800 baud rate and serial output on perhaps COM1.
Decoded NMEA file saving (and even an internet
feed address) can be set from here too – this may be
handy for examining the NMEA strings. Connect the
discriminator tap to sound card input cable and click
Start Monitoring – the level should show mid range
blue bars. Sync’s and Message ID numbers should
soon appear, with Demodulator Counts (100% is best)
often relating to soundcard overload, distant ships or
a
poor local antenna. Once verified as working this
program can be minimized.
VSPD – a Virtual Serial Port Driver (14 day trial
however) next must be run to provide com port pairs.
Com1 from the AisMon feed is simply paired to give a Com3
output. Numerous emulator offerings exist and programs such
VSPE (free) may well suit.
NavMonPc, a comprehensive (but lean) e-navigation program
completes the chain. Start this program, select File and set
Serial Channel A as Com3 4800 baud. Clicking Done should
finally display decoded NMEA messages as they arrive –
recheck all earlier settings and connections if they don’t.
Options allows entering (as DD MM.mmm) your local latitude
and longitude – just approximate values will initially do –
with Use This Position. Clicking AIS then gives a circular
display, with targets shown as triangles, although true vessel
outlines can be selected at high zoom levels if the bridge
crew have entered them correctly. Local NOAA charts can
be obtained from http://rimmer.ngdc.noaa.gov/. Tweak your
receiving locality details if need be. After all the hard work,
spend some time enjoying the settings, tracks and zoom levels!
This may even be all you need,
especially if living inland beyond
marine VHF range.
You can help contribute data!
Our Fremantle example is one such
area that web-based AIS info is lacking – at the time of writing, virtually
none of the WA coast (including the
very busy Pilbara) had web-based AIS
coverage. This will no doubt change as
more and more enthusiasts contribute
their received AIS data to the world
wide web.
Of course, not having web-based
AIS doesn’t stop you receiving it direct
– and if you are at a fixed site with
VHF AIS reception, you can become
active in supplying YOUR soundcardgathered AIS data to a global audience.
By altruistically providing your
own local AIS web feed you can help
extend the global AIS footprint.
All that’s required is to enter the IP
port address supplied by the hosting
service and ensure a reliable upload
feed.
A modest few megabytes per day of
uploaded data seems typical of harbours such as Wellington and although
essentially just a public service, bensiliconchip.com.au
eficial educational and promotional
spinoffs may result.
Serious ship watchers who are keen
to provide coastal coverage in busy regions still not served may even manage
a free professional AIS receiver from
site hosts such as www.vesseltracker.
com. Their motivation behind such
largesse is no doubt commercial!
Web monitoring concerns
Although enthusiasm is globally
very high (especially in Europe) for
AIS adoption, both at the super-tanker
and recreational boating level, concerns have been raised over web-based
AIS data availability.
The International Maritime Organization (IMO) Safety Committee
especially feels that web publication
of freely-available AIS-generated
ship data could be detrimental to the
safety and security of ships and port
facilities.
Given recent piracy, it’s indeed
feasible that criminals may be monitoring inshore ship movements with
devious intent.
But ships in global trouble spots
can, of course, simply turn off their
AIS transmitter or remove sensitive
voyage entries – or even spoof their
true identity (eg, a cruise liner or oil
tanker anywhere near the Somali coast
appears on AIS to be a destroyer!).
Taken overall, public AIS information, both locally-monitored or
web-based, seems so informative that
beneficial aspects win the day.
Just being able to pop into an internet café and web-locate a moored
dockland vessel may save the bacon
of lost crew floundering around in the
fog as sailing time approaches!
Web-published information is not,
of course, intended to enhance safety
at sea. Any internet-provided AIS data
may be obsolete, in error or incomplete
and best suits general shore-side informative purposes.
On-board seamanship and navigation aids are naturally still required.
Article links and AIS resources are
hosted at www.manuka.orconhosting.
net.nz/ais.htm
OVERLEAF:
How to modify a Uniden
scanner (without affecting
its normal usage!) to receive
160MHz AIS transmissions.
August 2009 19
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