雨自文大電园地 Principles of the Global Positioning System Lecture 06 YUAN Linguo Email:Igyuan@home.switu.edu.cn Dept of Surveying Engineering, Southwest Jiaotong University Error Categories 1. Errors related to GPs Satellites 2. Errors related to GPS signal propagation in atmosphere 3. Error related to gPs receivers Gv Principles of the Global Positioning System 2005-4-1
1 Principles of the Global Positioning System Lecture 06 YUAN Linguo Email: lgyuan@home.swjtu.edu.cn Dept. of Surveying Engineering, Southwest Jiaotong University Principles of the Global Positioning System 2005-4-1 2 Error Categories 1. Errors related to GPS Satellites 2. Errors related to GPS signal propagation in atmosphere 3. Error related to GPS receivers
Error sources 1. Errors Related to gPs satellites Ephemerides error Clock error · Relativistic effect 2. Errors Related to GPS Signal Propagation ionospheric refraction Tropospheric refraction Multi-path effect 3. Error Related to GPS Receivers · Receiver clock error Receiver position Antenna geometric center wv Principles of the Global Positioning System 2005-4-1(3) Basic Measurement of Eliminating these Errors 1. Correction by Models 2. Observation Approaches(relative survey DGPS), Observation Time(time, date) 3. GPS Receiver Selection(Hardware: good antenna for multipath, avoid building GE Principles of the Global Positioning System 20054-1(4
2 Principles of the Global Positioning System 2005-4-1 3 Error sources 1. Errors Related to GPS Satellites • Ephemerides error • Clock error • Relativistic effect 2. Errors Related to GPS Signal Propagation • Ionospheric refraction • Tropospheric refraction • Multi-path effect 3. Error Related to GPS Receivers • Receiver clock error • Receiver position • Antenna geometric center Principles of the Global Positioning System 2005-4-1 4 Basic Measurement of Eliminating these Errors 1. Correction by Models 2. Observation Approaches (relative surveying, DGPS), Observation Time (time, date) 3. GPS Receiver Selection (Hardware: good antenna for multipath, avoid building)
GPS Major Error Sources a Timing errors: receiver and satellite including SA satellite clock (as a difference between the precise and broadcast clocks ): 0.1-0.2 microseconds which corresponds to 30-60 m error in range first-order clock errors are removed by differencing technique wv Principles of the Global Positioning System 2005-4-1(5) Clock errors PRN 03 June 14) Clock NosA(ns)2000 Time(hrs) Principles of the Global Positioning System 2005-4-1(6
3 Principles of the Global Positioning System 2005-4-1 5 GPS Major Error Sources Timing errors: receiver and satellite, including SA satellite clock (as a difference between the precise and broadcast clocks ): 0.1-0.2 microseconds which corresponds to 30-60 m error in range first-order clock errors are removed by differencing technique Principles of the Global Positioning System 2005-4-1 6 Clock errors -200 0 200 400 600 800 0 4 8 12 16 20 24 PRN 03 (June 14) Clock SA (ns) 1999 Clock NoSA (ns) 2000 Clock error (ns) Time (hrs)
GPS Major Error Sources Orbital errors and Selective Availability(SA) o nominal error for the broadcast ephemeris: 1-5 m on average a precise(post-mission orbits are good up to 5-10 cm and better; available with 24-hour delay a Selective Availability: not observed on the orbit a first-order orbital errors are removed by differencing technique wv Principles of the Global Positioning System Basic atmospheric structure 30° Troposphere is where the temperature stops decreasing in the atmosphere. (10 km altitude) stratosphere Gv Principles of the Global Positioning System 2005-4-1 8
4 Principles of the Global Positioning System 2005-4-1 7 GPS Major Error Sources Orbital errors and Selective Availability (SA) nominal error for the broadcast ephemeris: 1-5 m on average precise (post-mission) orbits are good up to 5-10 cm and better; available with 24-hour delay Selective Availability: not observed on the orbit first-order orbital errors are removed by differencing technique Principles of the Global Positioning System 2005-4-1 8 Basic atmospheric structure Troposphere is where the temperature stops decreasing in the atmosphere. (~10 km altitude)
Atmospheric Errors on GPS Range Boundary between iono GPS stellite orbit Geometrie distanee troposphere Center of mass of the earth av Principles of the Global Positioning System 005-4-1(9) GPS Major Error Sources Propagation media ionosphere(50-1000km) the presence of free electrons in the geomagnetic field causes a nonlinear dispersion of electromagnetic waves traveling through the ionized medium group delay (code range is measured too long)and phase advance(phase range is measured too short), frequency dependent; can reach -150 m near n=1-3+. group refractive index(group of waves, such as code GPS signal) nm=1+-+.. phase refractive index constant C,=-403N [H=]thus n >n since electron density N is always positive PRIncples of the Global Positioning System 20054-1(10
5 Principles of the Global Positioning System 2005-4-1 9 troposphere ionosphere Geometric distance Actual signal path Boundary between iono and troposphere Atmospheric Errors on GPS Range Principles of the Global Positioning System 2005-4-1 10 • Propagation media • ionosphere (50-1000km) • the presence of free electrons in the geomagnetic field causes a nonlinear dispersion of electromagnetic waves traveling through the ionized medium • group delay (code range is measured too long) and phase advance (phase range is measured too short) , frequency dependent; can reach ~150 m near the horizon; constant 40.3 [ ] thus since electron density is always positive 1 phase refractive index 1 group refractive index (group of waves,such as code GPS signal) 2 2 2 2 2 2 e gr ph e ph gr c N Hz n n N f c n f c n = − > = + + = − + … … GPS Major Error Sources