AIS Part 1 - A System 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 vessels.

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 each 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.

Thats 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.

AIS 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).

For 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.

Coastal Internet Update - Google At Sea

We continue to receive questions about internet access afloat, so an update seems appropriate. To my delight, our Sierra Wireless 3G/GPRS modem continues to provide amazing service. With a DiGi sim card installed we had perfectly reliable internet access for more than 70% of the voyage from Singapore to Langkawi. Service dropped out at One Fathom Bank, north of Port Klang, but came back in again as we approached Pangkor Island and held the coast to Penang. It dropped again for a few hours between Penang and Langkawi, but we have seen the service running quite well more than 16km offshore. In Malaysian waters we certainly don't bother with wi-fi anymore. Whilst true 3G and/or Edge service is hard to find, the DiGi GPRS is fast, stable and incredibly widespread. At about 2 ringit (70c) per day, we love it. The Sierra Wireless modem will work anywhere in the world, though costs will vary in different countries. For more detailed information, click here & here.

1000 Miles To Phuket

Latitude 06 degrees 32.31 North, Longitude 099 degrees 25.16 East. The Southern Andaman Sea

Its 1000 nautical miles (just under 2000km) from our long-house anchorage in Borneo to Phuket in Thailand. Right now we're just 90nm from Phuket and should arrive there tomorrow morning. Then we start work in earnest, as Crystal Blues will come out of the water onto the hard stand for a six month intensive refit and paint job.

We departed Sarawak, Borneo, on July 23rd, and paused in Singapore for a 4 day break that (typically) turned into a 2 week layover. Why does this always happen to us ? Catching up with friends is part of it, and being thoroughly entertained by the Stonham family (we call it being "Stonhamed") also took its toll. However we did spend a lot of time checking out equipment for an upcoming video shoot, completed our AIS transponder installation, re-installed our wifi antenna system to the davits aft and purchased a lot of the materials needed for the refit.

Our good friend Robert Goh, who lives in Singapore, is a newly initiated cruising sailor. He's also a 1st rate alpinist and mountain climber. Unfortunately he was caught up in the sad disaster that engulfed the climbing teams on K2 in Pakistan this season - Robert wasn't hurt, he was below the ice fall and he returned to base camp safely. Check his website here. His summit attempt has now been cancelled and he's naturally very disappointed. Robert's partner Elaine spent time with us as the disaster progressed, and is naturally quite relieved that he's on his way home.

We escaped from Singapore on Wednesday August 13, heading north up the Malacca Straits and arrived in Langkawi (Malaysia) exactly 2 days and 11 hours later. Every hour of that passage was on the engine - the "new" Cummins engine has now clocked over 2000 hours. There was no useful wind for the entire trip. The even newer Comar Automatic Identification System also proved its worth - more on that in a future story. 36 hours anchored at Kuah town in Langkawi let us abuse our credit cards whilst stocking up on essential duty free "beverages" and adding further to the collection of refit materials now clogging the boat.

Stashed below are 2 sheets of Dow RTM Styrofoam insulation for our new freezer box (each 2.6m long), plus a roll of vinyl marine hooding for Ley to make new covers. On deck we have a new aluminium extension ladder for access to the boat on the hard. A deck mount air-con unit loaned by SV Tweed fills the salon, thanks Jon & Pam (thats the Tweed crew in the image at left, breakfasting on Roti in Langkawi). Our aft cabin is full of grog, hardware, stainless steel angle, paint and spare parts. Its now a big floating hardware store, and it all has to come off the boat and go into storage when we arrive, along with everything else on the vessel - food, clothing, utensils, parts, tools, mast, rigging etc. This will take a while, though we hope to complete the transfer before we haul out on September 4th.

We departed Langkawi this morning and have a fine weather forecast for the next 24 hours - a little wind this time, so we're now motor sailing with 7 knots of breeze just aft the beam. Flat seas and sunny weather, little rain, its unusually mild for the South West monsoon. Ley and I expect to be in Thailand for 6 months, and will be renting a house for the duration, somewhere close to Phuket Boat Lagoon. Friends and family are always welcome, so please email or call us if you'd like to visit.

A Cruising Life For Sale ...

We met Jon & Sandra Stonham in Borneo two years ago. They've became fine friends, and we've travelled extensively together (see our Andaman travels here). They also have a terrific website - see it here. However they've decided its time to stop roving and put the girls (Nicola and Alex) into "real" schools. So they're settling here in Singapore, where Jon is starting a new business and Sandra has accepted a position in the finance industry. All of which means that their beloved Tui Tai, a centre cockpit Tayana 47 designed by Robert Perry, is for sale.

The family moved off the boat three days ago, into a lovely big apartment, and the girls are learning to sleep in a "real" house again. Jon and Sandra haven't been getting much sleep at all, as their beds arrived from Hong Kong without mattresses. Oops.

Tui Tai was launched in 1993, is registered in Hong Kong and now waits patiently for a new owner at One Degree 15 Marina here in Singapore. She has a fabulous inventory, and has never wanted for maintenance. Download the inventory document here. She sails very well, is quicker than Crystal Blues in light airs, and sleeps six in three cabins. Tui Tai has air conditioning, a 6kva generator, an R.O. water maker, substantial systems and lots of comfort. Many of the electronic systems have been upgraded in the past two years. A solid cruising boat with impecable credentials. Interested ? An email to jon"at"thestonhams.com would kick things off nicely.