2.1 Phase and Group velocity(why we use tw frequencies A single electromagnetic wave propagating: The velocity of phase. λ f GPS Li,2 2. Atmospheric Effect (n. 97-118) A group of waves with different ncies, the group ropagation velocity is ith refractive 2.2 ionospheric Refraction Refractive Index for single and group wave are The ionosphere, extending In various lavers from about 50 km to 1000 km above earth, is a dispersive medium n=1+ with respect to the gPs radio signal With different velocities, a group delay and a phase dvance pproximates the phase Why the phase observation can reach higher efractive index accuracy than the pseudoranges observation c4 1. GPS code measurements are delayed and the carrier phases are advanced. 2. The code pseudoranges are measured too long and the carrier phase pseudoranges are measured too short compared to the geometric range between the satellite and the receiver I Ionospheric Refraction (IR): A Measuring the TEC (p. 102 The difference between measured and geometric range is called IR 2. Estimating the TEC (p. 102) 2. Relationship: IR(Alon)& TEC (p. 100) 3. Computing the Effect of tEC by Model D103 TEC 4. Eliminating the Effect of TEc(p. 104) 3. TEC: Total Electron Content Taking all these effects into account, a gPs pseudorange TEC=N ionospheric the TEC computed by models, or eliminated.2 2. Atmospheric Effect (p. 97-118) 2.1 Phase and Group Velocity (Why we use two frequencies?) A single electromagnetic wave propagating: The velocity of phase. v f ph = λ A group of waves with different frequencies, the group velocity 2 λ d λ df v gr = Propagation velocity is related with refractive index. GPS L1, L2 The ionosphere, extending in various layers from about 50 km to 1000 km above earth, is a dispersive medium with respect to the GPS radio signal. 2.2 Ionospheric Refraction = + 2 + 3 + 4 + Λ 2 3 4 1 f c f c f c n ph The following series of approximates the phase refractive index. With different velocities, a group delay and a phase advance. 2 2 1 f c nph = + 2 2 1 f c ngr = − Refractive Index for single and group wave are 1. GPS code measurements are delayed and the carrier phases are advanced. 2. The code pseudoranges are measured too long and the carrier phase pseudoranges are measured too short compared to the geometric range between the satellite and the receiver. Why the phase observation can reach higher accuracy than the pseudoranges observation? 1. Ionospheric Refraction (IR): ∆Iono The difference between measured and geometric range is called IR. 2. Relationship: IR(∆Iono) & TEC (p.100) TEC f Iono ph 2 40.3 ∆ = − TEC f Iono gr 2 40.3 ∆ = 3. TEC: Total Electron Content ∫ TEC = Neds0 Error caused by the ionospheric & the determination of the TEC. The TEC may be measured, estimated, its effect computed by models, or eliminated. Taking all these effects into account, a GPS pseudorange may be wrong from about 0.15 m to 50 m. 1. Measuring the TEC (p.102) 2. Estimating the TEC (p.102) 3. Computing the Effect of TEC by Model (p.103) 4. Eliminating the Effect of TEC (p.104)