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It can "see" the piers of the Huey P. Long Bridge and nearby vessel traffic long before radar picks them up. The AIS is a shipboard broadcast system that acts like a transponder operating in the VHF maritime band, capable of han- dling 4,500 reports per minute and updating as often as every two seconds. It uses Self-Organizing Time Division Multiple Access (SOTDMA) technology to meet this high broadcast rate and ensure reliable ship-to-ship operation. Each station determines its own transmission schedule (slot), based on data link traffic history and knowledge of future actions by other stations. A position report from one AIS station fits into one of 2,250 time slots established every 60 seconds (60 seconds = 1 frame). AIS stations continuously synchronize themselves to each other to avoid overlap of slot transmissions. Slot selection by an AIS station is randomized within a defined interval, and tagged with a random timeout of between zero and eight frames. When a station changes its slot assignment, it pre-announces both the new loca- tion and the timeout for that location. In this way, new stations (e.g., towboats), including those stations that suddenly come with- in radio range close of other vessels, will always be received by those vessels. The required ship reporting capacity according to the IMO per- formance standard amounts to a minimum of 2,000 time slots per minute, but the AIS system provides 4,500 time slots per minute. The SOTDMA broadcast mode allows the system to be overloaded by 400 to 500% and still provides nearly 100% throughput for ships closer than 8 to 10 NM to each other in a ship-to-ship mode. In the event of system overload, only targets further away will be subject to drop-out in order to give preference to nearer targets that are a primary concern to ship operators. In practice, the capacity of the system is nearly unlimited in the ship-to-ship mode, allowing for a great number of ships to be accommodated at the same time. The system coverage range is similar to other VHF applications, essentially depending on the height of the antenna and the power of the transmitter. Its propagation is slightly better than that of radar, due to the longer wavelength, so it's possible to "see" around bends and behind islands if the land masses are not too high. A typ- ical value to be expected at sea is nominally 20 nautical miles, but such ranges are not usually required on the rivers. A Moment's Negligence It was a bright warm Sunday morning on May 26, 2002. Downstream from the I-40 bridge fishermen on the Arkansas River heard a deafening crash in the vicinity of the bridge. They looked up to see that an enormous slab of the overhead interstate highway had fallen onto a barge. They watched helplessly as a west-bound semi flew through the yawning gap in the bridge deck and plunged 85 ft to the river, followed quickly by several other vehicles. Horrified, but realizing highway traffic was not seeing the gap, one of the fisherman desperately shot a signal flare that miraculously hit the windshield of a semi, causing it to come to a screeching stop at the edge of the yawning gap. Other vehicles then realized some- thing was wrong and crammed on their brakes, but not before many vehicles had fallen. A few seconds earlier, a two-barge Magnolia Marine tow had gone off course and slammed into a bridge pier. The towboat Robert Y. Love was piloted that day by a man who suffered from an ailment later described by the NTSB as syncope. The NTSB report stated that Captain Joe Dedmon remembers seeing the bridge ahead, recalls that the tow was in proper position with respect to the channel, but that he blacked out before making the necessary turn to starboard. Tragically, the only man sharing the watch with the captain had just gone below to awaken the mate for his shift. For a few critical seconds, there was no effective lookout in the wheelhouse, no one to make a course correction. What was the probability that the cap- tain would black out for 10 seconds during the same 10 seconds that his lookout went below? It was only 1 in 2.76 billion. It was tru- ly "a moment's negligence." The NTSB investigation showed there was no problem with wind or adverse weather. A blood test on the captain showed nothing irregular. The captain did not lose his license, but his guilt about the accident caused him to leave the river forever. Now what do we think of the Supreme Court's 1865 decision? Do we still think it was too extreme? At a bridge too far west for many coal people to take notice, a barge collision with a bridge pier had resulted in the loss of 14 lives and tens of millions of dollars in losses and damages. Could it have happened on the Ohio, the Tennessee or the Mississippi Rivers with longer tows of coal barges and longer bridges? Dave Gambrel is a private consultant in coal transportation and is the former director of transportation for Peabody Energy. Prior to entering the coal industry, he was an electronics engineer serving as a project manager for Honeywell Systems in Seattle. Brian Tetreault is a navigation systems specialist for the U. S. Army Corps of Engineers Research and Development Center Coastal and Hydraulics Laboratory. His projects include developing naviga- tion systems to improve safety, efficiency, and reliability of inland and coastal waterways. Acknowledgement The authors acknowledge the personal assistance given by Captain Jerry Tinkey and by the contribution of professor Craig Allen of the University of Washington, author of Farwell's Rules of the Nautical Road, Eighth Edition. t r a n s p o r t t i p s c o n t i n u e d 28 www.coalage.com February 2015 (L) An AIS-equipped vessel can see a target around a bend such as Mulatto Bend, and (R) multiple vessels can rapidly synchronize their positions with one another. "A moment's negligence on his part may involve the loss of his vessel with all the property and the lives of all on board."