Southwest Jiaotong University Principles of the Global Positioning System Lecture 04 YUAN LInguo EmailIgyuan@163.com Dept of Surveying Engineering, Southwest Jiaotong University Today's Lecture Examine specifics of GPS signals Multiple modulations on same carrier signal Structure of signals GPS receiver operation and satellite acquisition Principles of the Global Positioning System 2005-3-18(2
1 Principles of the Global Positioning System Lecture 04 YUAN Linguo Email: lgyuan@163.com Dept. of Surveying Engineering, Southwest Jiaotong University Principles of the Global Positioning System 2005-3-18 2 Today’s Lecture ¾ Examine specifics of GPS signals ¾ Multiple modulations on same carrier signal ¾ Structure of signals ¾ GPS receiver operation and satellite acquisition
Physical Fundamentals GPs signal propagation from the satellite to the user by means of electromagnetic waves. Such waves are generated by an oscillating (alternating) electric force The field strength decreases with increasing distance from the transmitting source The propagation conforms Maxwell's laws &e Principles of the Global Positioning System 20053-18(3 Physical Fundamentals ottp Aesin(at+(po) Principles of the Global Positioning System 2005-3-18(4
2 Principles of the Global Positioning System 2005-3-18 3 Physical Fundamentals Principles of the Global Positioning System 2005-3-18 4 Physical Fundamentals ϕ0 ωt+ϕ0 t0 t1 y=Aesin(ωt+ ϕ0) Ae
Physical Fundamentals This attenuation is usually expressed in decibels(dB). By definition, an attenuation of n db means that the original field strength is decreased by a factor of 10-0.In Physical quantities Quantity Symbol Dimension Circular frequency wavelength λP m/cvcle Speed of light m Principles of the Global Positioning System 20053-18(5 Integer cycles are equivalent to multiples of 2T radians Another unit for cycles per second(cps )is Hertz(Hz) ∫=2nn= By Phase 0=m=0(=6)=0=2 Example Known: F=1.5GHz, c=3x10 km/s, D=20,000km Question: cycles=?(10), Observable=? Principles of the Global Positioning System 20053-18(6
3 Principles of the Global Positioning System 2005-3-18 5 Physical Fundamentals This attenuation is usually expressed in decibels (dB). By definition, an attenuation of n dB means that the original field strength is decreased by a factor of 10-0.1n. Principles of the Global Positioning System 2005-3-18 6
In the case of a moving emitter or a moving receiver, the received frequency is Doppler shifted fr(received)+ fe(emitted) by an amount Af which, apart from relativistic effects, is proportional to the radial velocity Vp dp/dt= p of the emitter with respect to the receiver 電 Principles of the Global Positioning System 2005-3-18(7 Af=? Example GPS mean velocity=3.9km/s Radial velocity=0; thus, No Doppler effect Max radial velocity =0.9km/s, GPS transmitted f=1. 5GHz Doppler frequency shift Af-? 4.5103Hz Phase change=4.5 cycle Principles of the Global Positioning System 20053-18(8
4 Principles of the Global Positioning System 2005-3-18 7 Principles of the Global Positioning System 2005-3-18 8 ∆f = ? Example: GPS mean velocity = 3.9km/s Radial velocity = 0; thus, No Doppler effect Max radial velocity = 0.9km/s, GPS transmitted f = 1.5GHz Doppler frequency shift ∆f= ? 4.5·103Hz Phase change = 4.5 cycle
Components of signal General Remarks The oscillators on board the satellites generate a fundamental frequency fo with a stability in the range of 10-over one day for the block Il satellites Two carrier signals in the L-band(22 cm), denoted LI and L2, are generated by integer multiplications offo The carrier L3 is generated for military users onl For Block IIF satellites, the option of a carrier L5 for civilian use will be implemented Principles of the Global Positioning System 2005-3-18(9 The carriers LI and L2 are modulated by codes to provide satellite clock readings to the receiver and to transmit information, e.g., orbital parameters cle carrier modulated Principles of the Global Positioning System 20053-18(0 5
5 Principles of the Global Positioning System 2005-3-18 9 Components of Signal General Remarks z The oscillators on board the satellites generate a fundamental frequency f0 with a stability in the range of 10-13 over one day for the Block II satellites. z Two carrier signals in the L-band (22 cm), denoted L1 and L2, are generated by integer multiplications of f0. z The carrier L3 is generated for military users only. z For Block IIF satellites, the option of a carrier L5 for civilian use will be implemented. Principles of the Global Positioning System 2005-3-18 10 The carriers L1 and L2 are modulated by codes to provide satellite clock readings to the receiver and to transmit information, e.g., orbital parameters