Where are we ?

When I eventually made contact with the blogosphere again yesterday, I was impressed to see how the progress of the Ital Contessa while I was on board had been tracked by some of the keen members of Bray Sailing Club. Some of you have asked me where this information comes from so here's a brief explanation (as far as I understand it !) :

The first source of data that some people were looking at is AIS data. The photo on the right shows a busy AIS screen as the Ital Contessa left the Straits of Gibraltar and entered the Mediterranean. AIS stands for Automatic Identification System and is a means whereby ships transmit data by VHF. The data transmitted is a combination of dynamic and static information. The dynamic data includes information like navigational status (e.g. at anchor, underway by engine, underway by sail, not under command, etc.), speed, latitude and longitude. The static data contains information such as the vessel's callsign, destination, estimated time of arrival, draught (which will obviously vary depending on cargo carried) and ship dimensions.

The second source of information was from the website www.sailwx.info. The Ital Contessa (along with many other oceangoing vessels) carries weather reporting equipment and, in return for the Officers on watch carrying out regular weather observations and reporting them back to a central reporting station, the ship's position is then plotted and made available to view on a public website.

The principal differences between the two data sources are : as AIS is transmitted by VHF, it is normally only receivable within line of sight. On board, this meant that other vessels normally appeared on the AIS display when they were about 30 miles away (depending on the heights of VHF antennae and weather conditions). This may seem like a long way off, but two container ships each doing 25 knots towards the other will meet after 36 minutes so it's not all that long when you consider how long it takes to turn or stop one of these ships at speed. When a vessel appeared on the AIS display, the Officer of the Watch always looked at the detailed data being transmitted. This helped him to make decisions about likely changes of course that the other vessel might be about to make, what sort of vessel he should be looking out for, etc. In addition, the radar display provided much more detailed information (based on the "target" vessel's most recent speed and course) about likely crossing times, Closest Point of Approach (CPA) and Time to Closest Point of Approach (TCPA). This information was used at an early stage to decide whether a course alteration was required. Standing instructions on the bridge were that if the Closest Point of Approach was one nautical mile or less, then a course alteration should be made at an early stage. If for any reason this was not possible or there was any doubt about the other vessel's intentions, then the Master was to be called.

AIS data (as it is a VHF transmission) is receivable by anyone with suitable equpment (either a dedicated AIS receiver or a modified VHF radio) and within receiving range. There is a large community of ship watchers who receive AIS data on their own domestic equipment at home and share it with various websites who then make this information available to all and sundry. One of the most popular such sites in the British Isles is www.shipais.com (most members are in Liverpool or on the Clyde but data from Dublin is also shared by a user in Ireland). However, the possibility of tracking the progress of an oceangoing vessel across the whole world is very limited as this depends on the availability of enough shore based receivers who "share" their data. Naturally, the vessel is out of range of any shore receivers at all for long periods of time.

The other source of information is from the weather reports. On the Ital Contessa at any rate, these are taken at intervals of more or less six hours. The observations (including the observing vessel's position) are then encoded and compressed into a very small data file which is transmitted to a central station by a satellite phone link. When the receiving station receives the data and includes it in their weather charts, they also (as a quid pro quo) publish the position of the vessel at the time the observations were made. However, as some of you will have noticed, this system is not without its shortcomings. Firstly, it relies on someone on board carrying out the observations and sending the file. (This is not a compulsory watch duty and so is not always carried out.) Then the receiving station needs to publish the data received. Normally this happens quite quickly but some of you will have noticed that the last reported position of the Ital Contessa dates from 4th December, although I know for a fact that many updates have been sent since then so it is not a particularly reliable source of infomation.

So, for the landlubbers among you, I hope this clears up some of the mysteries about how you can find the position of a seemingly invisible ship in the middle of the Indian Ocean...