ELECTRONIC NAVIGATION

inertial navigation system that measures the ship’s motion in several planes and integrates the results with a high degree of accuracy. Although the methods of dead reckoning may vary, they all share the following characteristics: (1) the accuracy of the estimated position never exceeds the navigation method used to obtain the last fix, and (2) the accuracy of the estimated position deteriorates over time. ELECTRONIC NAVIGATION Simply put, electronic navigation is a form of piloting. Piloting is the branch of navigation in which a ship’s position is determined by referring to landmarks with known positions on the earth. These reference points may be bearing and distance to a single object, cross bearings on two or more objects, or two bearings on the same object with a time interval in between. Position in electronic navigation is determined in practically the same way as piloting, though there is one important difference—the landmarks from which the ship’s position is determined do not have to be visible from the ship. Instead, their bearings and ranges are obtained by electronic means. The advantages of electronic navigation are obvious. A ship’s position maybe fixed electronically in fog or heavy weather that makes it impossible to take visual fixes. Also, an electronic fix can be based on stations far beyond the range of any local bad weather. Since electronic navigation is the primary form of navigation in today’s Navy, the rest of this chapter will deal with electronic navigation and the roles played by the following systems: 1. Long Range Aid to Navigation (LORAN) 2. VLF Radio Navigation (OMEGA) 3. Ship’s Inertial Navigation System (SINS) 4. Navy Navigation Satellite System (NNSS) 5. NAVSTAR Global Positioning System (GPS) We will also briefly discuss navigation radar, surface search radar, and fathometers. We will cover TACAN in chapter 2. LORAN/OMEGA—TRANSITION AND BASIC OPERATION LORAN and OMEGA have been the “workhorse” systems for many years. However, they are being phased out. Based on the DOD policy statement reprinted below and because you may see a civilian version aboard your ship from time to time, we will simply give you an overview of the two systems. In accordance with the 1992 Federal Radio navigation Plan (FRP), NAVSTAR will become the primary reference navigation system for surface ships, submarines, and aircraft. The DOD requirement for LORAN-C and OMEGA will end 31 December 1994 and TRANSIT will be terminated in DECEMBER 1996. Land-based TACAN and VOR/DME are to be phased out by the year 2000. LORAN BASICS LORAN is a long-distance radio navigation system used by ships at sea to obtain a position fix, The system is based on the difference in the transit time required for pulsed radio signals to arrive at the LORAN receiver from multiple, synchronized, omnidirectional ashore transmitters. LORAN also takes advantage of the constant velocity of radio signals to use the time lapse between the arrival of two signals to measure the differences in distance from the transmitting stations to the point of reception. The receiving set provides a direct reading, in microseconds, of the time difference in the arrival of the signals. (Some sets automatically convert the readings into latitude and longitude.) When the time difference is measured between signals received from any two LORAN transmitter stations, a ship’s line-of- position (LOP) can be determined. 1-2