The document discusses signal selection criteria and remedies for satellite navigation systems like GPS. It covers 5 criteria: 1) acceptable received power levels with reasonable antenna patterns and ionospheric delay, remedied by choosing L-band frequency. 2) rejection of multipath signals using circular polarization. 3) meeting power spectral density constraints using spread spectrum signaling. 4) providing multiple access using code division multiple access (CDMA). 5) providing ionospheric correction using dual frequencies like L1 and L2. It provides details on GPS signal frequencies, modulation, and PRN codes.

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

20. BPSK Modulation Scheme

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

24. C/A Code generator

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

27. Signal transmission

28. THANK YOU