Precise Orbit Determination of Global Navigation Satellite System of ...

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Chapter 7 Orbit Determination Using Carrier Phase Observation can be added as extra parameters in satellite state vector, but the satellite dynamic and observation equations will become complicated. 7.2. Problems of Carrier Phase Observation 7.2.1 Ambiguity Solution Convergence Normally ambiguities as parameters are solved during orbit determination at the same time as the other parameters such as satellite positions, velocities and satellite dynamic parameters. Other ambiguity solution approaches will be discussed in a later section. The probability of ambiguity solution is dependent on the satellite types and the empirical formular can be expressed as, s& P( Ambiguity) = f (&, s PDOP) =κ (7-14) PDOP where κ coefficient related to frequency, atmosphere and so on P probability &s change rate of line-of-sight between tracking station and satellite The probability of ambiguity solution related to types of satellite orbits using GPS L1 frequency as an example is approximately shown in Figure 7-1. Possibility 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 ICO 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 Altitude of Satellite (100xkilometer) Figure 7-1 Ambiguity Solution Related to the Altitude of Satellite In the figure above, ICO (Intermediate Circular Orbits) is a circular orbit at an altitude of around 10,000 km. Based on mathematic point-of-view and simulation tests for various satellite constellations, the relations of ambiguity solution with ICO, MEO/GPS, IGSO and GEO satellites are listed in Table 7-1. Table 7-1 Ambiguity Convergence NI 100% ICO/(10000km) 7 m 1 h MEO/GPS 20 m 4 h IGSO 2 h 25 h GEO ∞ ∞ 88 MEO/GPS IGSO
Chapter 7 Orbit Determination Using Carrier Phase Observation In Table 7-1, NI means that almost all of ambiguities are rounded to nearest integers. 100% means the exact integer ambiguities solved (possibly under simulation test). From Eq.(7-14), Figure 7-1 and Table 7-1, it is obvious that the ambiguity solution is strongly dependent on the rate of change of line-of-sight between tracking station and satellite as well as PDOP. Hence, the lower the satellite orbit, the better for the ambiguity solution. For GEO satellite, if no special techniques such as wide band and multi-frequency are used, the initial ambiguity cannot be directly solved during orbit determination. This problem will be shown in the next section. 7.2.2 Geostationary Satellite Orbit Determination using Carrier Phase Observation Due to the slow movement of geostationary satellite relative to the surface of the Earth, the initial ambiguities of carrier phase observations are very difficult to be solved (see Figure 7-2). For detailed information about the distribution of tracking stations, sample rate of observation, true initial ambiguities and orbital arc length are referred to Chapter 9, Software and Simulation Results. Difference (meter) Ambiguities (cyc) 500 400 300 200 100 0 -100 -200 -300 -400 -500 180 160 140 120 100 80 60 40 20 0 -20 0 24 48 72 96 120 144 168 192 216 240 Tim e (hour) Figure 7-2 Results of Orbit Determination using Phase observation of GEO λ=-10° 0 24 48 72 96 120 144 168 192 216 240 Tim e (hour) Figure 7-3 Ambiguities of GEO Orbit Determination Figure 7-2 shows the results of GEO satellite orbit determination using carrier phase observations. Figure 7-3 shows the initial ambiguity solutions. The solution of the initial ambiguities is strongly dependent on the changes of geometry between satellites and tracking stations. GEO satellites move very slowly relative to the earth, which makes the initial ambiguities difficult to be distinguished from other parameters. The initial ambiguities for GEO satellite can be solved only before orbit determination, i.e. during observation preprocessing phase. In 89 dx dy dz 1 2 3 4 5 6 7 8
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PRECISE ORBIT DETERMINATION OF GLOB
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Summary GNSS-2 (Global Navigation S
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3.2.2.1 Principle..................
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9.3.3 The Effect of Distribution of
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Chapter 1 Introduction 1.2 Developm
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Chapter 2 Observations of Orbit Det
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Chapter 3 Orbit Tracking System and
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Chapter 4 Major Error Sources of Sa
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Page 46 and 47: Chapter 4 Major Error Sources of Sa
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Chapter 9 Software and Simulation R
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Conclusion 144
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References Carpino M. (1995): SAEG
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References Klobuchar, J. A.(1996):
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References Traquilla, J. M. and J.
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Acknowledgments 152
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