Some people have recently asked me what is so special about Satellite-AIS and TimeCaster™ if they only track ships to an accuracy measured in miles. GPS, they say, is installed on all ships and that tracks ships to within 10 meters! The answer lies in the question, “Who wants to know?” Despite what you see on TV, A GPS on a ship (or anywhere for that matter) only tells the ship where it is – accurate to 10 meters or so. But for someone far from the ship, that GPS position has to be communicated to that person from the ship – GPS by itself does not do that. If the ship is far from land, then only satellite phones and satellite data modem can make the connection – those are communication systems. However, the GPS position of the ship can also be broadcast out over a radio channel to anyone who might be listening. This is what AIS does – it broadcasts the ship’s ID and the GPS position, course and speed (along with other data about the ship) over a VHF radio channel for anyone within range to hear. AIS was designed to send those signals reliably out to about 20 or 30 miles (although signals can occasionally be received as far is 70-100 miles away). Anyone further away than that cannot receive the signal and therefore cannot track the ship that way. Satellite-AIS uses sophisticated space-based equipment to receive the AIS signals from very great distances (the curvature of the Earth does not shield the signals from the satellite as it would between two ships). They call it “rocket science” for a reason – Satellite AIS is hard and expensive. But when it works, it allows all ships to be detected anywhere in the world – without the cooperation of the ship’s communication system. Table 1 summarizes some of the key differences between communication systems and surveillance systems in ship tracking. Next we will look at how each system is used to track ships.
|Communications Systems||Surveillance Systems|
|Examples||Satellite phonesSatellite internet modemsVHF radios (voice)||AISSatellite AISRadarSatellite Cameras TimeCaster™ (uses AIS)|
|Key function||Connects two or more COOPERATING devices (like a phone call connects two people)||Monitors many vessels without their knowing that they are being monitored|
|Accuracy||GPS- based.Accurate to about 10 meters and can report this position whenever, and as often as desired, to any specific destination (typically the ship operator or the local port)||AIS: GPS-based.Accurate to about 10 meters at the time of transmission over the AIS.Radar and optical imagery use completely different (and expensive) technologies to achieve great accuracy (1-5 meter) but for small fields of view.|
|Tracking Challenge||Only works with cooperating shipsCommunication charges can be high.||Satellite AIS long latency, low update rates and limited probability of detection of the signal by the satellite.|
|TimeCaster™ role||Does NOT use the communication system. Provides independent validation that the ship is where it says it is.||Fixes the problem of position displays being inaccurate due to reports being spread out in time.Provides complete picture of all tracked vessels|
Table 1 How each system is used to track ships globally Communications Systems:
1) Install satellite phone/modem on ship
2) Connect to the ship’s GPS system (all ships have them)
3) Program system to send a position report to a specific computer (IP) address every X minutes. Each message will incur a fee or use a portion of pre-paid bandwidth.
4) Repeat Step 3 for everyone in the world who needs to see where the ship is.
5) Alternatively, program the system to let anyone on a list (or anyone) poll the ship at any time to get the position. Each such poll will cost the ship operator money – so they never do this.
6) Every X minutes each programmed recipient will know within about 10 meters where the ship is. No one not programmed as a recipient will have any information about the ship from this system.
To track a ship, you must:
a) Have access and ability to program the system on the ship to send you position reports; and
b) Pay all the communication bills for every position report (at 10 minute updates that’s 52,560 messages per year.
Repeat the above for each and every ship you want to track. Surveillance systems:
1) Sign up for one or more satellite AIS services (exactEarth, Orbcomm or SpaceQuest) directly or through one of their resellers.
2) Buy (or integrate software into your existing system) software that can display satellite AIS.
To track one or ALL ships:
a) Watch the screens and you will receive messages from various ships constantly –dots are placed on your map each time a message is received from a ship and left on the screen until either the next message is received from that ship (when it is replaced with the new position). If no message is received for (typically) 24 hours, the dot is deleted from your display.
b) Because of the inherent limitations of satellite systems (and at long ranges from coastal base stations), ships are detected infrequently and with wide gaps and wide delays. So the dots on your map are old and vary in age from seconds to 24 hours. About 10 to 15% of all ships are not detected for more than 24 hours and don’t appear on your display at all!
c) At any given time the dots on a global display average more than 140 miles from where this ship really is because of the age of the most recent report. That’s a search area of more than 67,000 square miles! Next:
a) Sign up for TimeCaster™ and we process those old messages into the most complete and accurate current position of all ships worldwide.
b) Our Timecasted position reports are nearly always within 13 miles of the true position of the vessel – a search area of only 530 square miles 10X better in radius and 100X better in search area
A few of the groups that need surveillance systems instead of or in addition to communication systems include: Coast guards
- Environment ministries,
- Search and rescue agencies
- Ship owners who lease their ships to third parties
- Shipping companies that need to track competitor’s ships
- Many others…
In summary, you can see that BOTH communication systems and surveillance systems have their place. “To track MY ships, I have a communication system. To track YOUR ships, I need a surveillance system.” Please don’t hesitate to contact me directly for more information, to argue with me or just to chat about this exciting industry that combines the adventures of both Sea and Space. I can be reached at firstname.lastname@example.org .