Globalinės padėties sistemos palydovų signalų vėlavimas dėl jonosferos poveikio

Abstract

GPS signals are transmitted through the atmosphere from a satellite to a receiver. The atmosphere could be divided into different layers, according to their physical characteristics and their influence on radio waves propagation. Ionosphere is a disperse medium for radio waves and propagation velocity depends on the frequency of GPS signals. It can have many effects on GPS, but here die author pays attention to ionospheric delay only. It is critical for single frequency GPS receivers, while two frequencies GPS receivers’ user can almost completely eliminate the effect of the ionospheric delay. Free electrons in the ionosphere produce most of the effects on the GPS signal modulation. The delay is proportional to the integrated number of free electrons along the transmission path and inversely proportional to the square of the transmission frequency. Typical zenith delays range from a few meters to a maximum of 15 meters. Of course, delays could reach even 100 meters at low elevation angles. There are two ways to eliminate this error. The first is to use an ionosphere model. Such model should be accurate to a few meters of vertical errors (up to 75% of ionospheric delay). The second way uses both broadcast frequencies for measuring delay directly by double frequency GPS receivers. The ionospheric group delay can be approximately expressed as [8]: [....] (1) The differenceof ionospheric delay δI: [....] (2) From (2) we obtain: [....] (3) By using some actual GPS observation data the ionospheric delay for double differences is computed. The data collected in Lithuania through the international EUVN’97 GPS campaign were used. By using GPPS software and MATLAB codes (written by Kai Borre) ionospheric delay for double differences is computed for 6 baselines of different length: [....] (4) N1 ir N2 values are from GPPS solutions for L1 and L2 respectively. The influence of the satellite elevation angle on ionospheric delay for double differences is analysed and plotted. Relation between the ionospheric delay for double differences and GPS receivers’ separation is determined.