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Chapter 4 Major Error Sources of Satellite Observations Shorter wavelength radio signals pass through the ionosphere but are affected by it. These shorter wavelengths are used by satellites for communication and navigation purpose, and the ionosphere affects the signals rather like the way the atmosphere causes "twinkling" of the stars. Considerable efforts have, therefore, been concentrated on modeling this ionospheric parameter. Several models are available including the Chiu model (Chiu, 1975), the Bent model (Bent et al. 1972) that has been used extensively for satellite tracking, the semi-empirical SLIM model (Anderson et al, 1987) based on theoretically obtained grid values, and the FAIM model (Anderson et al, 1989) that uses the Chiu formalism together with the SLIM results. The International Reference Ionosphere (IRI) is probably the most mature of these models, having undergone more than two decades of scrutiny and improvement. At present, almost all empirical models of ionospheric parameters are limited to non-auroral, magnetically quiet conditions. Major efforts are underway to extend ionospheric predictability beyond these limitations. A promising venue seems to be the inclusion of real-time data from the newly developed automatical recording and scaling ionosondes. Following Hofmann-Wellenhof (1992), Klobuchar (1996), Seeber (1993) and Wild (1994), the phase and range refractive indices can be written as a n p = 1+ (4-10) 2 f n r a = 1− (4-11) 2 f Then first orders of ionospheric group and phase delays for range and phase observations are proportional to the integrated number of free electrons along the propagation path and inversely proportional to the square of transmission frequency and can be written as ∆S = � Ndl f ion 40. 3 p 2 34 (4-12) ∆S = − � Ndl f ion 40. 3 r (4-13) 2 where f is the carrier frequency and � Ndl is Total Electron Content (TEC), integrated along the path from ground tracking station to satellite. From Eq.(4-12) and Eq.(4-13) it can be seen that changes of range and phase caused by the ionospheric refraction may be restricted to the determination of the total electron content (TEC). TEC itself is dependent on sunspot activities, seasonal and diurnal variations, the line of sight which includes elevation and azimuth of the satellite and the position of the observation site. Usually TEC can be measured using dual frequency observations forming wide-lane linear combination (L4). In the case of code observations the total electron content is proportional to the difference of the ionospheric refraction on the two frequencies. Ionospheric error can also be removed by ionosphere-free linear combination (L3). For single frequency users, some mathematical models, for examples, Klobuchar model (Klobuchar, 1996) and single-layer model (Wild 1994) may be used. Following Wild (1994), a simple and widely used mathematic model, single-layer model (SLM) will be discussed here. SLM is based on the assumption that all free electrons are concentrated in a spherical layer of infinitesimal thickness (single layer) at a height H above the earth’s surface. From Eq.(4-12) and Eq.(4-13), ionospheric correction can be written as TVEC S f z ion 40. 3 ∆ = ± 2 cos ′ where z′ zenith distance at the intersection P of the actual signal with the single-layer (4-14)
Chapter 4 Major Error Sources of Satellite Observations TVEC surface density of the electrons in the single-layer at point P; TVEC is also called the total vertical electron content Figure 4-1 Single-Layer Model of the Ionosphere SLM reduces the complicated layer structure of the ionosphere to a single layer. The height of this idealized layer is usually set to about 300-400 km. In Eq.(4-14), TVEC can be developed as a function of Taylor series with latitude ϕ and the hour angle s of the Sun as independent variables, i.e. n m i k TVEC( , s) = ��Eik ( ϕ −ϕ 0 ) ( s − s0 ) i= 0 k= 0 where ϕ 0, s0 origin point of Taylor series development E ik ϕ (4-15) i+ k 1 ∂ TVEC = i! k! i k ∂ ϕ∂ s ϕ 0s0 , i = 0, 1, 2Λ ; k = 0, 1, Λ 35 (4-16) The hour angle of the Sun is defined as the angle between the Sun and the local meridian (positive to the west). The local solar time t is related to the hour angle of the Sun by h t = s+12 Usually, s 0 is selected to be the hour angle corresponding to the middle of the observation interval, ϕ 0 is defined as the mean value of the latitudes of all stations used to compute the model. Eq.(4-14) and the coefficients in Eq.(4-15) are introduced as unknown parameters in observation equations and estimated together with the other unknowns during data processing. Table 4-4 shows the maximum range errors of GPS dual frequencies due to ionosphere errors. Table 4-4 Maximum vertical ionospheric range error (Seeber, 1993) Frequency 1st order effect (1/f 2 ) 2nd order effect (1/f 3 ) 3rd order effect (1/f 4 ) L1 L2 L1/L2 4.2 Multipath Effect H 32.5 (m) 53.5 (m) 0.0 (m) Z 0.036 (m) 0.076 (m) 0.026 (m) 0.002 (m) 0.007 (m) 0.006 (m) Multipath error is one of the dominant error sources for satellite-based navigation systems. Although this phenomenon has been known for many years, but since 1990’ multipath effect is very interesting and hot topic in P Z´ S Single Layer
Page 1: PRECISE ORBIT DETERMINATION OF GLOB
Page 4 and 5: Summary GNSS-2 (Global Navigation S
Page 6 and 7: 3.2.2.1 Principle..................
Page 8 and 9: 9.3.3 The Effect of Distribution of
Page 10 and 11: Chapter 1 Introduction 1.2 Developm
Page 12 and 13: Chapter 2 Observations of Orbit Det
Page 26 and 27: Chapter 3 Orbit Tracking System and
Page 40 and 41: Chapter 4 Major Error Sources of Sa
Page 52 and 53: Chapter 5 Perturbation Models of IG
Page 74 and 75: Chapter 6 Algorithms of Orbit Deter
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Chapter 6 Algorithms of Orbit Deter
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Chapter 7 Orbit Determination Using
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Chapter 8 Geostationary Orbit Deter
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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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