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ŠUMARSKI LIST 7-8/2016 str. 43     <-- 43 -->        PDF

and carrier phase observations. After this study that presents the mathematical model of PPP technique, Kouba and Héroux (2001) gives the acceleration on PPP. In PPP method, the satellite ephemerides data and the clock estimations have been provided by permanent GPS/GNSS reference stations network. In this point, the most important source of accurate orbital and satellite clock data is IGS. Table 2 represents the comprehensive information about the accurate GPS satellite ephemerides and the accurate clock data provided by IGS. According to the developments on satellite geodesy, precise orbit and clock products are obtained by organizations such as IGS, JPL etc., and these are presented to the users.
Due to these developments, PPP technique becomes the most effective and novel method on GPS positioning. PPP is an absolute positioning technique, which provides cm or dm level point accuracy in static or kinematic mode depending on observation duration with a dual- frequency receiver. PPP uses undifferenced ionospheric-free both carrier-phase (Ф) and code pseudorange (P) observations collected by dual-frequency receiver for data processing. This technique provides precise positioning by using precise ephemeris and clock products provided by IGS and other organizations and by considering other corrections such as satellite effects (satellite antenna offsets and phase wind-up), site displacement effect (solid earth tides, polar tides, ocean loading, earth rotation parameters) and compatibility considerations (products formats, reference frames, receiver antenna phase center offsets, modeling/observation conventions) (Gao and Shen, 2002; Rizos et al., 2012; Kouba and Héroux, 2001; Gao and Shen, 2001; Zumberge, 1997).
As stated in both Zumbarge et al. (1997) and Kouba and Heroux (2001), the ionospheric-free combinations of dual-frequency GPS pseudorange (P) and carrier-phase observations (Ф) are related to the user position, clock, troposphere and ambiguity parameters according to the following simplified observation equations:
                P = ρ + C(dT- dt) + Tr + εP      (1)
                Ф = ρ + C(dT-dt) + Tr + Nλ + εФ           (2)
P is the ionosphere-free combination of P1 and P2 pseudoranges (P3)=(2.546P1-1.546P2)
Ф is the ionosphere-free combination of L1 and L2 carrier-phases (L3)=(2.546 λ1 Ф1-1.546 λ2 Ф2)
ρ is the geometrical range computed as a function of satellite and station coordinates
C is the vacuum speed of light
dT is the station receiver clock offset from the GPS time
dt is the satellite clock offset from the GPS time
Tr is the signal path delay due to the neutral-atmosphere (primarily the troposphere)
N is the non-integer ambiguity of the carrier-phase ionosphere-free combination
λ1, λ2, λ are the of the carrier-phases L1, L2 and L3-combined (10.7 cm) wavelengths, respectively
εP, εФ are the relevant measurement noise components, including multipath.
Despite the advantages of PPP technique, the most problematic issue is requirement of long convergence time for solving the carrier phase integer ambiguity. It is the long convergence times (of the order of 20 minutes or more) necessary for the ambiguity float solution to converge so as to ensure centimeter-level positioning accuracy (Rizos et al., 2012). Thus, increasing the observation duration is the significant factor for improving the accuracy of point positioning