Satellite Navigation: Lecture 3 -_gnss_receivers

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Satellite Navigation: Lecture 3 -_gnss_receivers

  1. 1. Satellite NavigationPrinciple and performance of GPS receiversAE4E08 GPS Block IIF satellite – Boeing North AmericaChristian TiberiusCourse 2010 – 2011, lecture 3
  2. 2. Today’s topics• Introduction – basic idea• Link budget• Signal de-modulation• Receiver architecture• Measurement precision topics in part III and IV of Misra&Enge book, instead …: GPS Receiver Architectures and Measurements c on exp cise o of s sition by Michael S. Braasch and A.J. van Dierendonck ubje ct Proceedings of the IEEE, Vol. 87, No. 1, January 1999 pp. 48-64 2Satellite Navigation (AE4E08) – Lecture 3
  3. 3. Receiver architecture overview of a GNSS receiver – main building blocks its purpose? output: - pseudorange code measurement - carrier phase measurement - Doppler measurement - C/N0 measurement (signal strength)Satellite Navigation (AE4E08) – Lecture 3
  4. 4. GPS receiver architecture - functionality basic idea Red-crowned amazon from: Misra and EngeSatellite Navigation (AE4E08) – Lecture 3
  5. 5. The GPS Signal - recapFREQUENCY DOMAIN TIME DOMAIN Binary Phase Shift Keying (BPSK) modulation CARRIER (spread spectrum (SS) modulation) CARRIER with Pseudo Random Noise (PRN) f0 = 1.5 GHz f code sequences: PRN-CODE( sin x ) 2 - SPECTRUM 1 1 0 0 0 0 1 1 1 • C/A code on L1 - 1.023 Mbits/sec x SPREAD • P(Y) code on L1 and L2 - 10.23 Mbits/sec SPECTRUM SIGNAL f0 = 1.5 GHz 2.046 MHz 2.046 MHz sin x 2 ( ) - SPECTRUM CA CODEsee also Figure 2.5 in Misra&Enge x P CODE 1227.6 MHz 1575.42 MHz f 20.46 MHz 20.46 MHz L2 SIGNAL L1 SIGNAL Satellite Navigation (AE4E08) – Lecture 3
  6. 6. The GPS Signal at the Receiver Antenna• Signal delayed due to the travel time • speed of light in vacuum “ “ • atmospheric delays• Signal has undergone a Doppler shift (freq)• Signal is very weak (amplitude) • ordinary spherical weakening (~158 dB) • atmospheric absorption (small, 1-2 dB) typical signal-to-noise ratio (SNR) for the C/A code signal is ~1/80 (-19dB) well below noise level dB? see § 2.3.3 Misra&Enge Satellite Navigation (AE4E08) – Lecture 3
  7. 7. Link budget - 1 Table 1 from IEEE-article C/A-code at 1575.42 MHz satellite antenna: directs signal in beam (not omni-directional) EIRP: 478.63 W (or 26.8 dBW) 26.8 dBW = 10 log10 478.63 W ⎛ λ ⎞ 2 spherical spreading free − space loss factor = ⎜ ⎟ factor = 5.73 * 10-19 ⎝ 4πR ⎠ -182.4 dB = 10 log10 5.73e-19Satellite Navigation (AE4E08) – Lecture 3 includes effective area of (omni-directional) receiver antenna - see § 10.2 Misra&Enge AE = λ2/4π
  8. 8. Power density of received GNSS signal from: Misra and EngeSatellite Navigation (AE4E08) – Lecture 3
  9. 9. Link budget - 2 atmospheric loss: -2 dB -2 dB = 10 log10 0.63 (hence factor of 0.63) ⇒ received signal power: 26.8 dBW 478.63 W -182.4 dB x 5.73 * 10-19 -2.0 dB x 0.63 + -157.6 dBW 1.738 * 10-16 Wnoise power at receiver: 1.413*10-14 W (or –138.5 dBW) in 2 MHz bandwidth -138.5 dBW = 10 log10 1.413e-14 W Satellite Navigation (AE4E08) – Lecture 3
  10. 10. Link budget - 3 signal power [W] Signal-to-Noise ratio (SNR) SNR = noise power [W] signal power: -157.6 dBW 1.738 * 10-16 W noise power: -138.5 dBW / 1.413 *10-14 W -at receiver on Earth SNR: -19.1 dB 0.0123 Fig. 6 (a) from IEEE-articleraw GPS signal?‘nothing’ to see !⇒ signal indeed far below noise-floor 1 millisecond of received signal - sampled at 5 MHz (amplified & filtered) Satellite Navigation (AE4E08) – Lecture 3
  11. 11. Signal de-modulation• tracking• work-out on the blackboard …Satellite Navigation (AE4E08) – Lecture 3
  12. 12. Correlation Demo is on blackboard.Satellite Navigation (AE4E08) – Lecture 3
  13. 13. Correlation • shift s1(t) τ =0 • multiply s2(t-τ) • integrate τ =0 s1(t).s2(t-τ)∫ s1(t).s2(t- τ) ⎯⎯ τ →Satellite Navigation (AE4E08) – Lecture 3
  14. 14. Correlation • shift s1(t) τ = 0.5 • multiply s2(t-τ) • integrate τ = 0.5 s1(t).s2(t-τ)∫ s1(t).s2(t- τ) ⎯⎯ τ →Satellite Navigation (AE4E08) – Lecture 3
  15. 15. Correlation • shift s1(t) τ =1 • multiply s2(t-τ) • integrate τ =1 s1(t).s2(t-τ)∫ s1(t).s2(t- τ) ⎯⎯ τ →Satellite Navigation (AE4E08) – Lecture 3
  16. 16. Correlation • shift s1(t) τ = 1.5 • multiply s2(t-τ) • integrate τ = 1.5 s1(t).s2(t-τ)∫ s1(t).s2(t- τ) ⎯⎯ τ →Satellite Navigation (AE4E08) – Lecture 3
  17. 17. Correlation • shift s1(t) τ =2 • multiply s2(t-τ) • integrate τ =2 s1(t).s2(t-τ)∫ s1(t).s2(t- τ) ⎯⎯ τ →Satellite Navigation (AE4E08) – Lecture 3
  18. 18. Correlation • shift s1(t) τ = 2.5 • multiply s2(t-τ) • integrate τ = 2.5 s1(t).s2(t-τ)∫ s1(t).s2(t- τ) ⎯⎯ τ →Satellite Navigation (AE4E08) – Lecture 3
  19. 19. Correlation • shift s1(t) τ =3 • multiply s2(t-τ) • integrate τ =3 s1(t).s2(t-τ)∫ s1(t).s2(t- τ) ⎯⎯ τ →Satellite Navigation (AE4E08) – Lecture 3
  20. 20. Correlation s1(t) • shift • multiply s2(t-τ) • integrate s1(t).s2(t-τ)∫ s1(t).s2(t- τ) ⎯⎯ τ →Satellite Navigation (AE4E08) – Lecture 3
  21. 21. Two stages of receiver operation• acquisition (searching)• tracking (following) – providing measurementsSatellite Navigation (AE4E08) – Lecture 3
  22. 22. Receiver block diagram Fig. 5 from IEEE-article next slide …Satellite Navigation (AE4E08) – Lecture 3
  23. 23. Fig. 7 from IEEE-article baseband processing In-phase cos sin I and Q channel, to avoid Quadrature-phase lack of signal when ϕ=0, π/2, π, 3π/2 baseband signal processing block diagram = block 3 in Fig. 5.Satellite Navigation (AE4E08) – Lecture 3
  24. 24. code trackingDelay Lock Loop (DLL) code tracking controls delay pseudorangeSatellite Navigation (AE4E08) – Lecture 3
  25. 25. carrier phase trackingPhase Lock Loop (PLL) *)actually Costas-loop carrier tracking controls frequency carrier phase (Doppler shift)*) sometimes also Carrier Tracking Loop (CTL) Satellite Navigation (AE4E08) – Lecture 3
  26. 26. data nav. msg. when PLL is locked in data (nav. msg.) can be extractedSatellite Navigation (AE4E08) – Lecture 3
  27. 27. GNSS receiverDLL + PLL per signal, per satellitetoday’s high-end GPS receiver has typicallybetween 24 and 48 channels: multi-constellation GNSS receiver: - CA-code signal on L1 much more! - P(Y)-code signal on L1 - P(Y)-code signal on L2to track up to 12-16 GPS satellites simultaneouslygenerally each satellite is tracked independently(each GPS satellite has its own unique PRN ranging code (pulse sequence)) Satellite Navigation (AE4E08) – Lecture 3
  28. 28. Link budget – de-spreading signal power at receiver: 1.738*10-16 W (or –157.6 dBW) What if we could confine ourselves to a much smaller bandwidth …? noise power at receiver: 3.54*10-19 W (or –184.5 dBW) in 50 Hz bandwidth -184.5 dBW = 10 log10 3.54e-19 W signal power: -157.6 dBW 1.738 * 10-16 W noise power: -184.5 dBW / 3.54 *10-19 W -at receiver on Earth SNR: +26.9 dB 490 then, GPS signal has been raised well above noise floor !! Satellite Navigation (AE4E08) – Lecture 3 ⇒ read data when tracking
  29. 29. Signal to Noise Ratio (SNR) signal power [W]Signal-to-Noise ratio (SNR) SNR = noise power [W] in the same band (of course total noise power depends on bandwidth considered)normalize SNR to 1 Hz bandwidth:carrier to noise density ratio c/no = SNR * Blogarithmic scale [dB-Hz] C/No = 10 log10 c/no see example in eq. (16) IEEE-articleto present signal strength independent of spreading / de-spreading stage Satellite Navigation (AE4E08) – Lecture 3
  30. 30. Performancesignal key-parameters• power: C/No (code and phase)• (PRN-code) chip-rate (code)• (carrier) wavelength (phase)• signal bandwidth (code) but transmitted bandwidth not infinitenext to receiver parameters as: e.g. antenna gain, and (code and carrier) tracking loop bandwidthsSatellite Navigation (AE4E08) – Lecture 3
  31. 31. Measurement precision: code BL σc ≈ λc standard deviation in [m] 2 c / nowith BL code tracking loop bandwidth (0.1 - 5 Hz) c / no carrier-to-noise density ratio λc PRN code ‘wavelength’ [m] (1 chip = 293 m for CA-code)measurement noise due to thermal noise, coherent DLL,for standard 1-chip Early-Late spacing (and assuming infinite signal bandwidth)Satellite Navigation (AE4E08) – Lecture 3
  32. 32. Measurement precision: phase BP λ σP ≈ standard deviation in [m] c / no 2π with BP carrier tracking loop bandwidth (5 - 15 Hz) to accommodate c / no carrier-to-noise density ratio vehicle / platform dynamics (and local oscillator noise) λ wavelength [m] (~0.20 m) measurement noise due to thermal noise (and neglecting squaring loss)Satellite Navigation (AE4E08) – Lecture 3
  33. 33. Summary and outlookStudy:• IEEE-paper by Braasch&VanDierendonck (Blackboard)Next: GPS measurements and error sourcesAssignment 1 Future GNSS (deadline 2 December)Satellite Navigation (AE4E08) – Lecture 3

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