The Research On NAVTEX Assistant Navigation System

The NAVTEX is an important part of GMDSS. Its simplex function and lower intellectualizes are not adapt to the development of modern ship. The paper is based on the old-typed NAVTEX system. Add the area radio navigation function on it so that it can meet the desire of the modern ship.The radio broadcast character is one of important factors which can influence the precision of the navigation system in different environment, calculate the ground-wave field strength off seashore 400km on the frequency of 4.2MHz. The thesis modeled and simulated the distribution of the electric-field strength on the non-uniform areas using Matlab.Based on the theory of DSSS Communication and range positioning system, spread spectrum code emitted by navigation station has correlative operation with the signal produced by receiver, through measuring the place of maximal correlative function, we can measure the spreading time of the navigation station's signal, so we can measure the distance between navigation station and receiver. Therefore the capability of spread spectrum code is the key to the capability of area navigation system. In this thesis parallel phase-shifting sequence as spread spectrum code is given, which has better capability.In the NAVTEX assistant navigation system, suppose that all of navigation stations and the users are on the same horizontal level. According to the definition of the GDOP (geometric dilution of precision) of GPS, the GDOP expression of NAVEX navigation system is deduced in this paper and using the matrix theory the lowest GDOP and that GDOP is smaller with more navigation stations are proved in theory and simulated the fig of the equal precision. We can use GDOP as the rule of locate the navigation stations and analyze the performance of the stations.