APPENDIX
C
Navigation Information Systems
This appendix describes navigation information systems mentioned in the report, excluding vessel traffic services (VTS), which were discussed at length. The listings and descriptions are not intended to be comprehensive. Rather, the information is intended to provide the lay reader with sufficient background to understand the purpose and mechanism of each system.
Aids to navigation are devices for assisting a navigator in determining a vessel's position in a waterway and/or a safe course to steer. They also warn of dangers or obstructions. Traditional aids include buoys, lights, beacons, and sound signals. Satellite navigation systems and other electronic aids to navigation are becoming increasingly important.
Buoys are used as reference points for vessels to prevent groundings and to help with alignment in channels. Buoys may be lighted, unlighted, or equipped to provide electronically enhanced radar signals.
Conventional and low-light closed circuit television (CCTV) can provide visual surveillance of small areas of waterways where other surveillance systems are inadequate. CCTV is usually used to identify vessels or determine visibility and weather conditions in remote locations. Capabilities range from fixed-focus, daylight-only, stationary cameras to remote-controlled, 24-hour cameras that can pan, tilt, and zoom and are mounted in environmentally controlled enclosures. Although some automated processing of CCTV imagery is feasible, most data from CCTV displays must be processed by human operators.
An electronic chart is a digitized version of a nautical chart, with graphic representations of water depths, shore-lines, topographical features, aids to navigation, and hazards. At present, electronic charts have no legal status and are used only to supplement paper charts, which must be carried on all ships. The development of accurate electronic charts for all U.S. waters awaits the updating of hydrographic surveys and digitizing of the resulting data.
An electronic chart display and information system (ECDIS) uses a high-resolution color display system integrated with radar, radionavigation systems, depth sounder, gyro compass and electronic chart data to aid pilots and other vessel operators. The principal application will be on board, but using ECDIS in shore-based control centers will ensure that identical traffic images are available to both vessels and VTS. Widespread adoption of ECDIS awaits the development of international standards and further digitizing of nautical charts.
The global positioning system (GPS) is a three-dimensional, spaced-based military radio navigation system implemented and managed by the U.S. Department of Defense. It consists of 24 satellites orbiting in six circular planes at an altitude of 20,200 kilometers (10,900 nautical miles). GPS capabilities have been extended into the civil sector, with one service provided for the military and another for the civil users. The military service provides predictable positioning accuracy to within 22 meters (m) horizontally, 27.7 m vertically, and time transfer accuracy within 200 nanoseconds. The civil service provides predictable positioning accuracy of 100 m horizontally, 156 m vertically, and time transfer within 340 nanoseconds. Availability is advertised at 99.8 percent where there is defined coverage.
The differential global positioning system (DGPS) provides accurate position fixes for civilian users by eliminating most effects of selective availability (a procedure in which GPS satellite signals are “dithered” randomly to preserve military integrity). Marine radio beacons are being modified to transmit GPS corrections to civilian users. With a properly modified receiver, users within range of the radio beacon station can apply corrections to improve accuracy to better than ±10 m. Special coding is used to overcome adverse effects of noise and lightning bursts on the transmitted corrections. The availability of the radio beacons used for DGPS is advertised at a minimum of 99.7 percent. Shipboard DGPS receivers usually provide real-time readouts of latitude and longitude. Small displays often are included, showing direction of movement, direction to waypoints, course to steer left or right of a designated azimuth, and speed over ground.
Nautical charts provide graphic representations of water depths, shorelines, topographical features, aids to navigation, and hazards. By international agreement, paper charts must be carded on all ships.
Notices to Mariners, published by each Coast Guard district, contain chart corrections and other information of interest. Other publications for mariners include U.S. Coast Pilots.
Radar can measure range and bearing accurately. Shipboard radar is usually used to survey an area, avoid collisions, and aid navigation. In VTS, shore-based radars provide all-weather surveillance and tracking of vessels. Radar provides independent surveillance of vessels not required to participate in VTS and monitors other waterway features, such as the position of buoys. Various levels of radar system performance can be selected, ranging from very large antenna arrays for highly accurate tracking to low-cost systems with shipboard type antennas for general surveillance.
Radio beacons are nondirectional radio transmitting stations. A radio direction finder (RDF) is a short-range navigation aid using radio beacon bearings to fix navigation positions.
The physical oceanographic real-time system (PORTS) is a real-time meteorological and hydrological measurement and information distribution system developed by the National Oceanic and Atmospheric Administration (NOAA). PORTS provides networked instruments to measure current, wind velocity and wind direction. REALDATA, another NOAA system, reports current tides and provides users with computer software to compare actual and predicted tide levels. PORTS provides (through software) a consolidated graphic display of real-time outputs from all accessible environmental sensors in the area. Detailed PORTS information is generally transmitted by telephone (digital data and audio advisories), and users may dial up to hear weather, current and tidal conditions, or to have detailed PORTS information displayed.