cET 318 faith Lecture 1. Signal Structure 5. Satellite Signal Book: p 71-85 Dr Guoqing Zhou 1.1 Physical Fundamentals This attenuation is usually expressed in decibels (dB). By definition, an attenuation of n dB means signal propagation from the that the original field strength is decreased by ite to the user by means of factor of 10-0In electromagnetic waves. Such waves Physical quantities (alternating) electric force. Dimension Circular frequency The field strength decreases ith inereasing distance period from the transmitting source. conforms Speed of light are equivalent to multiples of 2r radians. Another unit for cycles per second (cps )is Hertz(Hz) of a moving emitter or a me eceived frequency is Doppler sh do g=「fd f(received)≠e( emitted by an amount Af which, apart from relativistic effects, is proportional to the radial velocity ve dp/dt p of the emitter with respect to Known: F=1.5GHz, c=3x10 km/s, D-20, 000km Question: cycles=?(10), Observable=?
1 Dr. Guoqing Zhou 5. Satellite Signal CET 318 Book: p. 71-85 1. Signal Structure 1.1 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 propagation conforms to Maxwell's laws. The field strength decreases with increasing distance from the transmitting source. 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. Speed of light c m/s period P s wavelength λ m/cycle Phase ϕ cycle Circular frequency f Cycle/s Quantity Symbol Dimension Physical quantities Integer cycles are equivalent to multiples of 2π radians. Another unit for cycles per second (cps ) is Hertz (Hz) Example: Known: F=1.5GHz, c=3x105 km/s, D=20,000km, Question: cycles=? (108), Observable=? λ π c P f = = 1 2 dt d f ϕ = ∫ = t t fdt 0 ϕ ∫ = = − = − t t c D fdt f t t f t 0 ( ) ( ) ϕ 0 By Phase In the case of a moving emitter or a moving receiver, the received frequency is Doppler shifted. fr (received) ≠ fe (emitted) by an amount ∆f which, apart from relativistic effects, is proportional to the radial velocity vρ = dρ/dt = ρ of the emitter with respect to the receiver
Af= " 1.2 Components of SigI E scillators on board the satellites GPS m elocity=3.9km a Radial velocity =0; thus, No Doppler effect of 10- over one day for the Block ll satellite Max radial velocity =0.9km Two carrier signals in the L-band (22 cm), denoted L GPS transmitted f= 1,5GHz and Lz, are generated by integer multiplications of fo The carrier Ly is generated for military users only Doppler frequency shift Af=? 4.5103Hz For Block IIF satellites, the option of a carrier Ls for Phase change= 4.5 evele The carriers Li and L2 are modulated by codes to provide Two codes are used for the satellite clock readings, both satellite clock readings to the receiver and to transmit characterized by a pseudorandom noise(PRN)sequence Code Type Frequency Repeated time 10 every millisecond clear/access(C/A) 后 nce every 266.4 w-code encrypt the P-code ansmitted in 30 the Y-code seconds nodulated Y-code ncrypted by w-code fRequency(MHZ) Pseudorandom Noise Codes (PRN) Fundamental Frequency What is PRN?(p. 73) Carrier li 1546=157542(90cm) Operating Principle: The generation of the PRN 4cm) sequences in the codes is based on the use of hardware devices called tapped feed back shift registers =1023 C/A-Code 6/10=1023 - Number of Cell 1 2 3 4 5(1bit) 后20=0.5115 Clock pulse,Initial st 10110 k pulse. Successive state11011 The unmodulated carriers by Li(t)=acos(ft) The new value of the leftmost cell is determined by the binary sum of two defined cell The C/A-code is placed on the Li carrier in phase The chose of the defining cells is arbitrary and determines the quadrature(i.e, 90 offset)with the P-code property of the resulting code
2 ∆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 1.2 Components of Signal General Remarks • 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. • Two carrier signals in the L-band (22 cm), denoted L1 and L2, are generated by integer multiplications of f0. • The carrier L3 is generated for military users only. • For Block IIF satellites, the option of a carrier L5 for civilian use will be implemented. 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. Two codes are used for the satellite clock readings, both characterized by a pseudorandom noise (PRN) sequence. Y-code Encrypted by W-code transmitted in 30 seconds encrypt the P-code to the Y-code, f 0/20 W-code once every 266.4 days f 0 precision (or protected) code (P-code) f every millisecond 0 coarse/acquisition /10 clear/access (C/A) Code Type Frequency Repeated time f0/204600 =50x10 Navigation message -6 f0 W-Code /20 =0.5115 f0 C/A-Code /10 =1.023 f P-Code 0 =10.23 120f Carrier L2 0 = 1227.60 (24.4cm) 154f Carrier L1 0 = 1575.42 (19.0cm) f Fundamental Frequency 0 =10.23 Component Frequency (MHZ) The unmodulated carriers by Li(t) = aicos(fit) The C/A-code is placed on the L1 carrier in phase quadrature (i.e., 90° offset) with the P-code. Operating Principle: The generation of the PRN sequences in the codes is based on the use of hardware devices called tapped feedback shift registers Pseudorandom Noise Codes (PRN) Storage cells What is PRN? (p. 73) Successive state 1 1 0 1 1 Initial state 1 0 1 1 0 Number of Cell 1 2 3 4 5 Clock pulse Clock pulse (1 bit) 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. Chips
Characteristics of c/A and P Code I C/A-code Generation(p. 75) Parameter/A-code P-code 2. P-code Generation(p. 75) Chipping rate 1.023.10 Bits/s 10 23.10 bits per second 30 ode type 37 unique codes 37 one-week segment More accurate ince the y-code is the modulo two sum of the p-code and the encrypting W-code, access to P-code is only possible when the secret conversion algorithm is known Navigation Message Each subframe The navigation message contains Starts with the telemetry word (TLM)containing a message containing bits with five 2. The second word is hand-over word (HOW) Various correction data subframe identification Time-of-week (Tow), which count for the epoch at the start (leading edge)of the next subframe subframe 2 The Tow count (sometimes called Z-count) is a multiple number of 1. 5-second intervals since the beginning of the One subframe is transmitted in 6 seconds and contains 10 words with 30 bits (a word/0. 6 seconds) A receiver requires at least 30 seconds to lock on a satellite in order to receive the complete navigation message F iThe gps a ame contains week number. The Fifth Subframe has A prediction of the user rang Almanac data Indicators of the satellite health and The age of the dat Health status for the first 24 satellites in orbit Three coefficients for a qu polynomial to model The Fourth Fifth Subframe commonly the satellite clock correction rd Subframe transmit the broadcast Are broadcast by each satellite. Therefore, by tracking phemerides of the satellite only one satellite, the almanac data of all the other satellites in orbit are obtained The Fourth Subframe are reserved for military use and contain The contents are changed in every message and have a Various flags The total information is packed into 25 pages and requires Almanac data(i.e 12.5 minutes for transmission satellites beyond the nominal 24 constellation
3 1. C/A-code Generation (p. 75) 2. P-code Generation (p.75) Characteristics of C/A and P Code • In order to protect the P-code against spoofing, the P-code is encrypted to the Y-cod by A-S. • Since the Y-code is the modulo two sum of the P-code and the encrypting W-code, access to P-code is only possible when the secret conversion algorithm is known. Properties Easy to acquire More accurate Code type 37 unique codes 37 one-week segments Repetition rate Millisecond One week Chip length ~ 300 m ~ 30 m 10.23•106 1.023•10 bits per second Chipping rate 6 Bits/s Parameter C/ A-code P-code Navigation Message The navigation message contains • Satellite health status, • Satellite clock, • Satellite orbit, and • Various correction data. Total message containing 1500 bits with five subframes. 1. Starts with the telemetry word (TLM) containing a synchronization pattern and some diagnostic messages. 2. The second word is hand-over word (HOW). • A subframe identification • Some flags, • Time-of-week (TOW), which count for the epoch at the start (leading edge) of the next subframe. Each subframe • One subframe is transmitted in 6 seconds and contains 10 words with 30 bits (a word/0.6 seconds). • A receiver requires at least 30 seconds to lock on a satellite in order to receive the complete navigation message. The TOW count (sometimes called Z-count) is a multiple number of 1.5-second intervals since the beginning of the current GPS week. The First Subframe contains • The GPS week number, • A prediction of the user range accuracy, • Indicators of the satellite health and • The age of the data, • An estimation of the signal group delay, and • Three coefficients for a quadratic polynomial to model the satellite clock correction. The Second & Third Subframe transmit the broadcast ephemerides of the satellite The Fourth Subframe are reserved for military use and contain • Ionosphere • UTC data • Various flags • Almanac data (i.e., low-accuracy orbital data) for satellites beyond the nominal 24 constellation. The Fifth Subframe has – Almanac data – Health status for the first 24 satellites in orbit. • Are broadcast by each satellite. Therefore, by tracking only one satellite, the almanac data of all the other satellites in orbit are obtained. • The contents are changed in every message and have a repetition rate of 25. • The total information is packed into 25 pages and requires 12.5 minutes for transmission. The Fourth + Fifth Subframe commonly
ATELLITE 2. Signal Processing L,CA可[.D叵2. RECEIVER Carri Message is the recovery of the signal components Goal extraction of the codes for the satellite clock readings and the navigation message General Characters 5.2.1 Receiver Desig 1. The signal emitted from the satellite contains three components in the symbolic form variety of receivers on the market used for different purposes (I C/A, D). (navigation, surveying, time transfer)and with different features. Lp, Y, D), and (L2,Y,D) contains Signal reception Basic concept of 2. The received C/A-code signal on L, is twice as powerful as the Y-code sign 3. The same ratio exists between the Y-code signals on Ll and L Control device Storage device 4. The B T IR satellites may transmit signals with less Microprocess he present constellation, since the nominal power lower than the present signal strength. Power supply 1. Antenna Design: 2. Microprocessor: 1. The microprocessor controls the entire system and 2. The electronic center should be close to its geometric center and should be insensitiy enables real-time navigation by means of code tation and inclination This becomes particularly important in kinematic 2. The control device provides interactive communication with the receiver elevation or multipath signals. At present, best achieved by choke ring antennas. 3. Commands can be keyed in and diagnostic or other satellites above the horizon and, after pre transmits signals to the radio frequency(RF 4. The control device is, therefore, usually designed as ntenna may be designed for only the primary k both LI and L2 carriers. Most of the antennas sold today are microstrip antennas
4 2. Signal Processing is the recovery of the signal components • reconstruction of the carrier wave • extraction of the codes for the satellite clock readings and the navigation message. Carrier Code Message SATELLITE L1, C/A, D L1, Y. D L2, Y. D RECEIVER Carrier Code Message Goal 1. The signal emitted from the satellite contains three components in the symbolic form • (L1, C/A, D), • (L1, Y, D), and • (L2, Y, D). 2. The received C/A-code signal on L1 is twice as powerful as the Y-code signal on L1. 3. The same ratio exists between the Y-code signals on L1 and L2. 4. The Block IIR satellites may transmit signals with less power than the present constellation, since the nominal power is 6 dB lower than the present signal strength. General Characters 5.2.1 Receiver Design A variety of receivers on the market used for different purposes (navigation, surveying, time transfer) and with different features. The receiver unit contains –Signal reception –Signal processing Basic concept of a receiver unit 1. One important design criterion is the sensitivity of the phase center. 2. The electronic center should be close to its geometric center and should be insensitive to rotation and inclination. This becomes particularly important in kinematic applications. 3. Antenna should have a gain pattern which filters low elevation or multipath signals. At present, this is best achieved by choke ring antennas. 1. Antenna Design: An omnidirectional antenna receives the signals of all satellites above the horizon and, after preamplification, transmits signals to the radio frequency (RF) section. The antenna may be designed for only the primary carrier L1 or for both L1 and L2 carriers. Most of the antennas sold today are microstrip antennas. 1. The microprocessor controls the entire system and enables real-time navigation by means of code pseudoranges. 2. The control device provides interactive communication with the receiver. 3. Commands can be keyed in and diagnostic or other messages can be displayed. 4. The control device is, therefore, usually designed as keyboard display unit. 2. Microprocessor:
3. Storage: Radio Frequency(RF) Section Heart Various media are presently used Signal input Discrimination cassette drives, C/A-codes(unique for each satellite) magnetic bubbles or other nonvolatile storage Sat.+ Monitoring the Doppler shift Additionally, the receiver can be interfaced to an external computer The RF section processes the incoming signals employing 4. Power: Single frequency units process only the LI signal An(optional)internal power(rechargeable batteries) Dual frequency instruments process both the LI and L2 External batteries Other power supplie The data collected by a dual frequency receiver enable a mbination where the ionospheric refraction can be eliminated. An important feature of the rF is the number of Inels (the number of satellites trac Multichannel receivers are more accurate and less nsitive to loss of signal lock but can have inter annel biases older instruments used a limited number of physical channels and alternated satellite tracking I. Inter channel biases can be virtually eliminated by rapidly sequencing(20 milliseconds) satellites in by calibration and out of the same channel 2. Receivers with sequencing channels are Current receivers assign one satellite each to a less expensive, but slower. physical channel where the satellites are are seldom used for surveying. continuously tracked 3. Hybrid receivers use a combination of techniques How Many maximum Channels does a GPS receiver has? Unmodulated Doppler shifted 2.2 GPS Signal Processing Techniques Code range Phase lock loop(PLL) loop(DLL)」 Code correl. ase offset of Remove PRn two phase
5 Storing the observables and the navigation message so that they are available for later processing. Various media are presently used: – microchips, – cassette drives, – magnetic bubbles or – other nonvolatile storage. 3. Storage: • An (optional) internal power (rechargeable batteries) • External batteries • Other power supplies. 4. Power: Additionally, the receiver can be interfaced to an external computer. The RF section processes the incoming signals employing separate channels. – Single frequency units process only the L1 signal. – Dual frequency instruments process both the L1 and L2 signals. The data collected by a dual frequency receiver enable a combination where the ionospheric refraction can be eliminated. Radio Frequency (RF) Section Heart Signal input Discrimination RF C/A-codes (unique for each satellite) Monitoring the Doppler shift Antenna Sat. Sat. An important feature of the RF is the number of channels (the number of satellites tracked simultaneously). – Older instruments used a limited number of physical channels and alternated satellite tracking by rapidly sequencing (20 milliseconds) satellites in and out of the same channel. – Current receivers assign one satellite each to a physical channel where the satellites are continuously tracked. How Many Maximum Channels does a GPS receiver has? Multichannel receivers are more accurate and less sensitive to loss of signal lock but can have inter channel biases. 1. Inter channel biases can be virtually eliminated by calibration. 2. Receivers with sequencing channels are • less expensive, but slower. • are seldom used for surveying. 3. Hybrid receivers use a combination of techniques. 2.2 GPS Signal Processing Techniques Code range determination Delay lock loop (DLL) Code correl. tech. Remove PRN High-pass filter Doppler shifted Unmodulated Phase lock loop (PLL) Compared with Carrier replica Fractional phase offset of two phase Result Received Sat. motion Doppler shifted