Gps signal structure
- 2. Signal selection criteria - 1 The signal must provide Acceptable received signal power with reasonable power levels and Earth coverage satellite antenna patterns Antenna design considerations Minimum ionosperic delay and fluctuations
- 3. Remedy- 1 Choose L band frequency 1GHz – 2 GHz
- 4. Why L band ? Ionospheric delays are enormous for frequency ranges below 500 MHz and above 10 GHz Frequencies above 2 GHz requires beam antenna for signal reception Dual frequency requirement – so high band width signal preferred
- 5. GPS Signal frequencies Navigation signal modulation L1- Link 1, Carrier frequency = 1575.420 MHz L2- Link 2, Carrier frequency = 1227.600 MHz Nuclear detection system (NDS) and NDS analysis package (NAP) L3- Link 3, Carrier frequency = 1381.050 MHz L4- Link 4, Carrier frequency = 1379.913 MHz Additional signal link L5- Link 5, Carrier frequency = 1176.450 MHz L2C for improving accuracy of navigation and easy tracking for civil use.
- 6. Signal selection criteria - 2 Rejection of multipath signals Signals reaching the receiver after reflection Noise and flutter
- 7. Remedy- 2 Use Circularly polarized electromagnetic waves Right Hand Circularly Polarized wave is used for GPS Reflection changes the direction of polarization Reflected waves get attenuated by the choke ring antenna.
- 8. Signal selection criteria - 3 Power spectral density constrains The GPS signals received on the earth sufficiently low so as to avoid interference with the terrestrial microwave line-of- sight communication ITU regulations for each microwave band
- 9. Remedy- 3 Spread spectrum signaling
- 10. Why Spread Spectrum Signaling ? It can be proved by Shannon Hartley channel-capacity theorem C = B × log2 (1 + S/N) It states that one can maintain or even increase communication performance (high C) by allowing or injecting more bandwidth (high B), even when signal power is below the noise floor.
- 11. Spread Spectrum Signaling This can be achieved when a data modulated carrier signal is multiplied by a wide band spreading wave form Direct Signal spread spectrum- DS-SS used in GPS
- 12. Spread Spectrum Signaling De-spreading operation
- 13. Signal selection criteria - 4 Multiple access Capability of accessing multiple signal simultaneously
- 14. Remedy- 4 Code Division Multiple Access (CDMA)
- 15. CDMA Spread spectrum signalling aids CDMA Low bit rate data is encoded with a high bit rate PRN The receiver knows the PRN of each satellite to reconstruct data
- 16. CDMA Encoding Two distinct CDMA encodings are used Coarse/ Acquisition or C/A code at 10.23 Mcps Precision code or P code at 1.023 Mcps
- 17. Signal selection criteria - 5 Provide ionospheric correction Ionosphere causes delays the microwave signals
- 18. Remedy- 5 Use dual frequency L1 and L2
- 19. Signal modulation C/A code is modulated onto the L1 signal P code is modulated on to L1 and L2 carrier The navigation data is modulo-2 added to the C/A and P code before modulation Binary phase shift keying (BPSK) is used as digital modulation technique
- 21. PRN Generators
- 22. P codes The P code can continue without repeating for 38 weeks The P code is subdivided into 38 segments of 1 week code and assigned to each satellites The P code is very difficult to acquire without acquisition aids
- 23. P codes acquisition High speed clocking of the receiver Different high speed algorithms are available With the help of HOW in the navigation data
- 25. C/A code Designed for rapid acquisition 1023 bits in 1ms Used for satellite multiple access technique
- 26. Navigation data 50 bps signal Modulo-2 added with P and C/A code GPS week number (5th Jan 1980) and satellite health information, an ephemeris and an almanac
- 28. THANK YOU