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 . m/scSpeed of light sPperiod m/cycleλwavelength cycle?Phase Cycle/sfCircular frequency DimensionSymbolQuantity 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=3x10 5 km/s, D=20,000km, Question: cycles=? (10 8 ), Observable=? λ π c P f == 1 2 dt d f ? = ∫ = t t fdt 0 ? ∫ ?=?== t t c D tfttffdt 0 )()( 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. 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= ? 4.5·10 3 Hz Phase change = 4.5 cycle 1.2 Components of Signal General Remarks 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. Two carrier signals in the L-band (22 cm), denoted L 1 and L 2 , are generated by integer multiplications of f 0 . The carrier L 3 is generated for military users only. For Block IIF satellites, the option of a carrier L 5 for civilian use will be implemented. The carriers L 1 and L 2 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. Encrypted by W-code Y-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) every millisecondf 0 /10coarse/acquisition clear/access (C/A) Repeated timeFrequencyCode Type f 0 /204600 =50x10 -6 Navigation message f 0 /20 =0.5115W-Code f 0 /10 =1.023C/A-Code f 0 =10.23P-Code 120f 0 = 1227.60 (24.4cm)Carrier L2 154f 0 = 1575.42 (19.0cm)Carrier L1 f 0 =10.23Fundamental Frequency Frequency (MHZ)Component The unmodulated carriers by Li (t) = aicos(fit) The C/A-code is placed on the L 1 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) 1 1 0 1 1Successive state 1 0 1 1 0Initial state 1 2 3 4 5Number of Cell 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 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. More accurateEasy to acquireProperties 37 one-week segments37 unique codes Code type One weekMillisecondRepetition rate ~ 30 m ~ 300 mChip length 10.23 10 6 bits per second 1.023 10 6 Bits/sChipping rate P-codeC/ A-codeParameter 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 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 MessageCode SATELLITE L1, C/A, D L2, Y. DL1, Y. D RECEIVER Carrier MessageCode Goal 1. The signal emitted from the satellite contains three components in the symbolic form (L 1 , C/A, D), (L 1 , Y, D), and (L 2 , Y, D). 2. The received C/A-code signal on L 1 is twice as powerful as the Y-code signal on L1. 3. The same ratio exists between the Y-code signals on L1 and L 2 . 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: 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 Sat. motionReceived Doppler shifted 6 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 After removal of the PRN code, the received signal still contains the navigation message which can be decoded and eliminated by high-pass filtering. The final result is the Doppler shifted carrier on which a phase measurement can be performed. 1. Squaring Technique (p. 82) Independent of PRN codes Satellite clock and the satellite orbit information are lost in the process. SNR is substantially reduced in the squaring process, In general, it is more difficult to resolve the ambiguities of the squared signals with halved wavelength. Advantage and Disadvantage 2. Cross Correlation Technique (p. 82) 3. Code Correlation plus Squaring Technique (p. 83) 4. Z-tracking technique (p. 83) The most recent quasi-codeless technique is: Z-tracking TM (The best performance in the presence of A-S). Characteristics of The Four Techniques SNR Degradation Compared to Code Correlation Technique -14 dBZ-tracking -17 dBCode correlation plus squaring -27 dBCross correlation -30 dBSquaring 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. Summary What have we learnt? Which parts are important? Assignment 5 1. How to generate GPS signal in orbit? 2. Describe the characters of C/A code, P-code and Y-code? 3. What is W-code, What is Y-code, What is relationship W-code and Y-code? 4. What is PRN? 5. How to generate C/A-code? How to generation P-code? 6. What is navigation message? Please explain in detail the characters of each subframe? 7. What is the signal processing principle? 8. How many parts does a receiver unit consists of ? What is the function of each part? 9. What is radio frequency (RF) section? Why does the multi channel has higher accuracy than single channel receiver? 10. Please describe in detail the GPS signal processing technique? 11. What is Squaring technique, Cross correlation technique, code correlation plus squaring technique and Z-tracking technique?