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 ? Examine specifics of GPS signals ? Multiple modulations on same carrier signal ? Structure of signals ? GPS receiver operation and satellite acquisition. 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 ? 0 ωt+? 0 t 0 t 1 y=A e sin(ωt+ ? 0 ) A e 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 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= ? 4.5·10 3 Hz Phase change = 4.5 cycle 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 f 0 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 f 0 . 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. 6 Principles of the Global Positioning System 2005-3-18 1111 GPS Signal Fundamental frequency 10.23MHz L 1 carrier phase 1575.42MHz L 2 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. 7 Principles of the Global Positioning System 2005-3-18 13 Principles of the Global Positioning System 2005-3-18 1414 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". 8 Principles of the Global Positioning System 2005-3-18 1515 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 signal. ? 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. Principles of the Global Positioning System 2005-3-18 1616 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. 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 10 Principles of the Global Positioning System 2005-3-18 19 0 V o -V o 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 √2V o -√2V o (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 11 Principles of the Global Positioning System 2005-3-18 21 Composite: Sum of C/A and P code 0 C/A code bits (h) 0 Composite C/A+P(Y) signals ~3 μseconds (g) 0 P(Y) code bits ~20 μseconds (i) Principles of the Global Positioning System 2005-3-18 22 Composite GPS signal ? Last few slides show construction of composite signal ? There are sets of phase reversals on the L1 signal: C/A code at rate of 1.023 MHz and the P(Y) code add 90 o out of phase at a rate of 10.23 MHz ? How do you the GPS signal if you don’t know both codes (since each reverses the sign and should average to zero)? 12 Principles of the Global Positioning System 2005-3-18 23 Characteristics of C/A and P Code z In order to protect the P-code against spoofing, the P- code is encrypted to the Y-cod by A-S. z 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. Principles of the Global Positioning System 2005-3-18 24 123 45 1 2 3 4 5 6 7 8 9 10 30s 6s 0.02s 0.6s 25 pages 10 bits 30 bits GPS Navigation Message 13 Principles of the Global Positioning System 2005-3-18 25 GPS Navigation Message TLM = Telemetry Word HOW = Handover Word (contains Z-count) 1500 BITS 30 SEC. SUBFRAME NUMBER TLM HOW CLOCK CORRECTION TLM HOW EPHEMERIS TLM HOW EPHEMERIS TLM HOW IONOSPHERE, ETC. TLM HOW ALMANAC EACH FRAME: -10 30-BIT WORDS, 6 SEC . 1 2 3 4 5 Principles of the Global Positioning System 2005-3-18 26 Each subframe 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. The TOW count (sometimes called Z-count) is a multiple number of 1.5-second intervals since the beginning of the current GPS week. ? 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. 14 Principles of the Global Positioning System 2005-3-18 27 The First Subframe contains z The GPS week number, z A prediction of the user range accuracy, z Indicators of the satellite health and z The age of the data, z An estimation of the signal group delay, and z 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 z Ionosphere z UTC data z Various flags z Almanac data (i.e., low-accuracy orbital data) for satellites beyond the nominal 24 constellation. Principles of the Global Positioning System 2005-3-18 28 The Fifth Subframe has ? Almanac data ? Health status for the first 24 satellites in orbit. The Fourth + Fifth Subframe commonly ? 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. 15 Principles of the Global Positioning System 2005-3-18 29 Component Frequency [MHz] Ratio of fundamental frequency f o Wavelength [cm] Fundamental frequency f o 10.23 1 2932.6 L1 Carrier 1,575.42 154?f o 19.04 L2 Carrier 1,227.60 120?f o 24.45 P-code 10.23 1 2932.6 C/A code 1.023 f o /10 29326 W-code 0.5115 f o /20 58651 Navigation message 50?10 -6 f o /204,600 N/A GPS Signal Summary Table Principles of the Global Positioning System 2005-3-18 30 Signal Processing 16 Principles of the Global Positioning System 2005-3-18 31 General Characters 1. The signal emitted from the satellite contains three components in the symbolic form z (L1 , C/A, D), z (L1 , Y, D), and z (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. Principles of the Global Positioning System 2005-3-18 32 Antenna and Preamplifier Control and Interface unit Micro- processor Power Supply Unit Data Storage Code tracking loop Carrier tracking loop RF Multiple channels GPS Receiver 17 Principles of the Global Positioning System 2005-3-18 33 1. Antenna Design: 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. 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. Principles of the Global Positioning System 2005-3-18 34 2. Microprocessor: 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. 18 Principles of the Global Positioning System 2005-3-18 35 3. Storage: 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. Additionally, the receiver can be interfaced to an external computer. 4. Power: z An (optional) internal power (rechargeable batteries) z External batteries z Other power supplies. Principles of the Global Positioning System 2005-3-18 36 19 Principles of the Global Positioning System 2005-3-18 37 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? Principles of the Global Positioning System 2005-3-18 38 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. 20 Principles of the Global Positioning System 2005-3-18 39 GPS Signal Processing Techniques Principles of the Global Positioning System 2005-3-18 40 Correlation technique is performed by 1. A reference carrier is generated in receiver which is then biphase modulated with a replica of the known PRN code. 2. The resulting reference signal is correlated with the received satellite signal. Code correlation technique provides all components of the satellite signal 1. Satellite clock reading 2. Navigation message 3. Un-modulated carrier z After removal of the PRN code, the received signal still contains the navigation message which can be decoded and eliminated by high-pass filtering. z The final result is the Doppler shifted carrier on which a phase measurement can be performed. 21 Principles of the Global Positioning System 2005-3-18 41 1. Squaring Technique In general, it is more difficult to resolve the ambiguities of the squared signals with halved wavelength. Advantage and Disadvantage z Independent of PRN codes z Satellite clock and the satellite orbit information are lost in the process. z SNR is substantially reduced in the squaring process Principles of the Global Positioning System 2005-3-18 42 2. Cross Correlation Technique 3. Code Correlation plus Squaring Technique 4. Z-tracking technique The most recent quasi-codeless technique is: Z-tracking TM (The best performance in the presence of A-S). 22 Principles of the Global Positioning System 2005-3-18 43 Characteristics of The Four Techniques 1. All four approaches to recover the L2 carrier in the presence of A-S suffer from a substantial degradation in the SNR. 2. Without exception, no codeless or quasi-codeless technique recovers GPS signal information as well as the code correlation technique. 3. Weaker signals are more sensitive to high ionospheric activities and interfering (jamming) signals which may even cause a loss of lock. Principles of the Global Positioning System 2005-3-18 44 Assignment 1. Describe the characters of C/A code, P-code and Y-code? 2. What is PRN? 3. How to generate C/A-code? How to generation P-code? 4. What is navigation message? Please explain in detail the characters of each subframe? 5. What is the signal processing principle? 6. How many parts does a receiver unit consists of ? What is the function of each part? 7. Why does the multi channel has higher accuracy than single channel receiver? 8. Please describe in detail the GPS signal processing technique? 9. What is Squaring technique, Cross correlation technique, code correlation plus squaring technique and Z-tracking technique?