Studies on the Accuracies of Position Fixes Obtained by Marine GPS and their Applications

摘要

1) Development of a ship's position measurement systems and their accuracies From the view point of safety and economy, it is very important for a ship on the ocean to find its position. For a long time, terrestrial navigation by taking of cross bearings and celestial navigation makeing use of celestial bodies were available. In the 1940s, radio navigation developed for military use progressed rapidly with the advance of electronic techniques after that. Especially, Omega and Navy Navigation Satellite System (NNSS) are epochal systems, because their coverage are worldwide. But, the accuracy of Omega is not good enough, and NNSS has a disadvantage of uncontinuity. Global Positioning System (GPS) was initiated in the 1970s, and was completed in 1994. This system makes up for several disadvantages of usual navigation systems of all sorts; not only it enabled us to find the ship's position but also make the applications of the ship's movement and current measurement possible. The purpose of this study is to make clear the positioning accuracy of the GPS and usefulness of using the GPS by elucidating the accuracy of Loran C, the measurement of surface current and the analysis of a ship's movement with GPS. 2) Process of the development and positioning accuracy of GPS According to the development of GPS, the author measured the GPS position fix at the standing fixed station and several ports of call of the training ship in our country and overseas, and obtained the positioning accuracy of each of them and made clear the yearly variation. There were six or seven satellites available in the early stage of development till October 1989, so the usable time for position fixes was limited to six or seven hours. But the accuracy of three dimensional high level positioning was better than at the present completed stage. Namely, the standard deviation (SD) was only ten meters odd, and maximum position error was about fifty meters. Since November 1989, full-scale working-type satellites has been launched, so the number of usable satellites has increased. But, the Department of Defense (DOD) turned on Selective Availability (SA) to the working-type satellites since March 1990, and, moreover, positioning accuracy was not stable till the first half of 1990 because of some system test. Since August 1990, when Iraq Army invaded Kuwait,positioning accuracy has improved and good accuracy has been continuously obtained after the end of the Gulf War, that is to say, fourteen or fifteen satellites were available in September 1990, and DOD turned off SA or decreased its error. Constant error was less than 10 meters of difference of latitude (D.lat), and about 40 meters of departure (Dep), and SD was about 10 meters. SA was reactivated on GPS satellites in November 1991, and it is continuing now in 1994. We see the effect of SA, having an increase of SD (about 30 meters), but, constant error and SD are stable now with constant value. 3) Application examples ① Measurement of Loran C error However superior GPS may be, it is necessary for us to use another system at the same time as a backup system. So, the author combined, for the simultaneous measurement, GPS and Loran C, which was the most suitable as a backup system, and made clear the Loran C errors. The Loran C errors are variable according to various factors. Therefore, we must obtain the positioning accuracy in each area of the sea. So the author showed the Loran C errors in and around the Goto-nada, the eastern area of the East China Sea and the western area of the North Pacific Ocean, respectively. The ship's position obtained by Loran C in and around the Goto-nada was 100～400 meters southward and 10 meters westward ～30 meters eastward from the datum position. The constant error in the eastern part of the East China Sea was large for the long propagation distance over the land, that is, 600～900 meters southward, and 200～300 meters eastward. In the western area of the North Pacific Ocean, the author evaluated the accuracy in three different areas. Positioning error was less than 0.5 nautical miles (n.m.) in the two areas where theoretical semidiameter error is within 1 n.m., and the positioning error in the other area which was more than 1000 n.m. distant from each transmitting station was less than 1 n.m., if the reception of signals was good. ② Measurement of the surface current It was confirmed that the direction and velocity the surface current obtained by pursuit of a Radar Buoy in relation to the ship's position with GPS are almost the same as those of the current measured with electric current meter so we measured surface current in the ocean too. In this chapter we discuss a comparative study of two surfac currents; one obtained by pursuit of Reflector, the other calculated by the direction and velocity of ultrasonic current meter. As a result, it was found that surface current was obtained by position of GPS which was information over the ground and information through the water of ultrasonic current meter. ③ Measurement of turning circle The turning test of a training ship with 1000 gross tonnage and the maneuverability of a small ship for research test were done with GPS. Generally, a turning test is conducted at full speed and the maximum rudder angle only when the ship is build up. No turning test is done at slow speed and a sharp rudder angle in consideration of the ship's arrival or, leave and the ship handling for avoidance. So, the author set up three stages of the speed and rudder angle, that is to say, Full, Half and Slow, and 30, 15 and 5 degrees, and measured the left and right turning circles. Maximum advance and transfer were obtained by turning tracks, and therefore,the measurement of turning circle with GPS was found useful and practical enough. As mentioned above, this study clarified the usefulness of GPS by means of applications of the specific quality of the high accuracy position fixes of GPS.