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
GPS Signal Fundamental 10.23MHZ L, carrier phase C/A code P code Navigation 157542MHz103MH|10.23MH S0BPS Ly carrier phase P code 1201260Nz 10.23MHZ OBPS 電 Principles of the Global Positioning System 20053-181(1 Two codes are used for the satellite clock readings both characterized by a pseudorandom noise (PRN) sequence Code Type frequency Repeated time coarse/acquisition f10 everv millisecond lear/access(C/A) precision(or protected) once every 266.4 code (p-code) W-code encrypt the P-code to transmitted in 30 the Y-code, f 20 seconds Y-code Encrypted by W-code Principles of the Global Positioning System 2005-3-18
6 Principles of the Global Positioning System 2005-3-18 11 11 GPS Signal Fundamental frequency 10.23MHz L1 carrier phase 1575.42MHz L2 carrier phase 1227.60MHz C/A code 1.023MHz P code 10.23MHz P code 10.23MHz Navigation message 50BPS Navigation message 50BPS ×154 ×120 ÷10 ÷204600 Principles of the Global Positioning System 2005-3-18 12 Two codes are used for the satellite clock readings, both characterized by a pseudorandom noise (PRN) sequence
Component Frequency(MH) Fundamental Frequency fo=10.23 Carrier li 1546=157542(190cm) Carrier l2 120=122760(244cm) P-Cod =10.23 C/A-Code J10=1023 W-Code f20=0.5115 Navigation message f204600=50X106 The unmodulated carriers by Li(t)=acos(it) The C/A-code is placed on the Li carrier in phase quadrature (i.e, 90 offset)with the P-code 電 Principles of the Global Positioning System 2005-3-18(13 A Random code? The Pseudo random Code(PrC)or Pseudo random Noise code, PRN, is a fundamental part of GPS Physically it's just a very complicated digital code, or in other words, a complicated sequence of "on"and"off pulses. The signal is so complicated that it almost looks like random electrical noise. Hence the name "Pseudo- Random Principles of the Global Positioning System 2005-3-181414
7 Principles of the Global Positioning System 2005-3-18 13 Principles of the Global Positioning System 2005-3-18 14 14 A Random Code? The Pseudo Random Code (PRC) or Pseudo Random Noise code, PRN, is a fundamental part of GPS. Physically it's just a very complicated digital code, or in other words, a complicated sequence of "on" and "off" pulses. The signal is so complicated that it almost looks like random electrical noise. Hence the name "PseudoRandom
A Random code? Since each satellite has its own unique Pseudo- Random Code, this complexity also guarantees that the receiver won,'t accidentally pick up another satellite's So all the satellites can use the same frequency without jamming each other. And it makes it more difficult for a hostile force to jam the system In fact the Pseudo Random Code gives the dod a way to control access to the system 電 Principles of the Global Positioning System 20053181515 a Random code? Another reason for the complexity of the Pseudo Random Code, is crucial to making GPS economical The codes make it possible to use information theory to "amplify"the GPS signal And that's why GPS receivers don' t need big satellite dishes to receive the gPs signals Principles of the Global Positioning System 2005-3-181616
8 Principles of the Global Positioning System 2005-3-18 15 15 A Random Code? ¾ Since each satellite has its own unique PseudoRandom Code, this complexity also guarantees that the receiver won't accidentally pick up another satellite's signal. ¾ So all the satellites can use the same frequency without jamming each other. And it makes it more difficult for a hostile force to jam the system. ¾ In fact the Pseudo Random Code gives the DoD a way to control access to the system. Principles of the Global Positioning System 2005-3-18 16 16 A Random Code? ¾ Another reason for the complexity of the Pseudo Random Code, is crucial to making GPS economical. ¾ The codes make it possible to use information theory to “amplify” the GPS signal. And that's why GPS receivers don't need big satellite dishes to receive the GPS signals
Pseudorandom Noise Codes(PRN) What is PRN? Operating Principle: The generation of the PRN sequences in the codes is based on the use of hardware devices called tapped feedback shift registers Storage cells Number of Cell 3 4 5 ( bit) Clock pulseInitial state Clock puls Successive state output The new value of the leftmost cell is determined by the binary sum of two defined cell The chose of the defining cells is arbitrary and determines the property of the resulting code 電 Principles of the Global Positioning System 2005-3-18(17 Signal modulation L1 CARRIER 1575.42 MHZ ∞⑧ CA CODE 1.023MHZ Mixer NAVISYSTEM DATA 50 Hz 「L「「「 P-CODE 10.23 MHZ mutt wuuI L2 CARRIER 1227.6 MHZ MWwMWmWW990WWM XX>L2SIGNAL GPS SATELLITE SIGNALS Principles of the Global Positioning System 2005-3-18
9 Principles of the Global Positioning System 2005-3-18 17 Pseudorandom Noise Codes (PRN) Principles of the Global Positioning System 2005-3-18 18 Signal modulation
CA Code modulation CIA bi-phase modulated signal A0M0owW 電 Princip es of the Global Positioning System 20053-18(19 P-Code generation Po carrier 9 uadrature modulated s 046400 P-code rate should 10 times higher than CIA code Principles of the Global Positioning System 2005-3- 10
10 Principles of the Global Positioning System 2005-3-18 19 0 Vo -Vo C/A carrier (a) 0 C/A code bits (b) 0 C/A bi-phase modulated signal (c) CA Code Modulation Principles of the Global Positioning System 2005-3-18 20 P-Code generation 0 P(Y) carrier √2Vo -√2Vo (d) 0 P(Y) code bits (e) 0 P(Y) bi-phase quadrature modulated signal (f) P-code rate should 10 times higher than C/A code
