April - 1 May, Evolution to Modernized GNSS Ionospheric Scintillation and TEC Monitoring

Transcription

1 Workshop on Science Applications of GNSS in Developing Countries (11-27 April), followed by the: Seminar on Development and Use of the Ionospheric NeQuick Model (30 April-1 May) 11 April - 1 May, 2012 Evolution to Modernized GNSS Ionospheric Scintillation and TEC Monitoring A. J. Van Dierendonck AJ Systems/GPS Silicon Valley USA

2 Evolution to Modernized GNSS Ionospheric Scintillation and TEC Monitoring Dr. A.J. Van Dierendonck, AJ Systems 4/18/2012 African Workshop

3 Tutorial Outline Short Review of GPS Receivers Emphasizing what functions are affected by scintillation Emphasizing modifications implemented for measuring scintillation effects Amplitude and Phase Scintillation Measurements Measurement Limitations How well does the receiver perform in a scintillation environment? How can a GNSS receiver be designed to better operate in a scintillation environment? TEC Measurements Measuring TEC or satellite and/or receiver inter-frequency biases? Example Measurements GPS Satellites SBAS Geostationary Satellites 4/18/2012 African Workshop

4 Multiple Frequency GNSS Receiver Functional Block Diagram Antenna Repeat for Each Frequency Automatic Gain C ontrol RF Pre-f ilter RF Low Noise Amplif ier RF Down Conv erter Analog IF A/D Conv erter Digital IF Digital Receiv er Channel 1 2 N Oscillator (Normally TCXO) Frequency Sy nthesizer AGC Control Receiv er Processor, Nav igation Unit Assumption: Receiver measures carrier phase and C/N 0 Measurement Output 4/18/2012 African Workshop

5 GNSS Receiver Modifications for Scintillation Monitoring 4/18/2012 African Workshop

6 Receiver Modifications to Measure TEC and Scintillation Antenna Choose Appropriate Antenna Automatic Gain C ontrol RF Pre-f ilter RF Low Noise Amplif ier RF Down Conv erter Analog IF A/D Conv erter Digital IF Digital Receiv er Channel 1 2 N Oscillator (Normally TCXO) Slave to Low Noise OCXO Frequency Sy nthesizer AGC Control Add Scintillation and TEC Measurement Software Receiv er Processor, Nav igation Unit 4/18/2012 African Workshop

7 Measuring Amplitude Scintillation 4/18/2012 African Workshop

8 Typical Receiver Channel for Amplitude (Power) Measurements Integrate & Dump I Ek Digital IF Cos I Carrier NCO Sin Q Early Prompt Late Applicable Code Generator Integrate & Dump Integrate & Dump Integrate & Dump Integrate & Dump Integrate & Dump I I Q Q Q Pk Lk Ek Pk Lk WBP I Q Only use Prompt I and Q Samples for Power Measurements Pk Pk k Pk Pk k 1 k 1 NBP I Q 4/18/2012 African Workshop

9 Signal Intensity Samples Signal Intensity samples are based upon Narrowband (NBP) and Wideband (WBP) Power Measurements (50 samples/second) SIk NBPk WBPk Difference between NBP and WBP is proportional to received signal power Theoretically cancels noise power in the mean Practically, it doesn t completely correction made later Samples collected and stored over 60 seconds Thus, 3000 samples every minute These 50 sps samples are available as an output 4/18/2012 African Workshop

10 Computing S4 (1) Total S4 is standard deviation of normalized Signal Intensity S4 Total SI 2 k SI k SI 2 k 2 Scale factor of Signal Intensity is ambiguous, but this normalization with average value over 60 seconds takes care of that Desirable to remove the effects of receiver noise, theoretically computed as S4N 1 0 Sˆ N 19 Sˆ N 0 0 This is square root of expected value of S4 2, given noise only ŜN 0 is average measured signal-to-noise density over 60 second period also an output, as well as the above noise contribution 4/18/2012 African Workshop

11 Computing S4 (2) Noise contribution is removed as follows: 2 SIk SIk S4Corrected 1 2 Sˆ SI N ˆ 0 19 S N k 0 2 If square-root argument is negative, set to 0 (means noise dominates any amplitude scintillation) This corrected value is computed off-line Option also exists to compute average value of SI k as low-pass filtered value This presents potentially unstable normalization because of filter delay results in inflated S4 values 4/18/2012 African Workshop

12 Low-Pass Filtering Introduces Delay in Normalization In low-passed version (denominator) does not line up with raw version, increasing the variance Possible to correct for the delay, but requires raw data buffering that is not desirable 4/18/2012 African Workshop

13 Measuring Amplitude Scintillation Summary Amplitude Scintillation Measure GNSS signal-plus-noise power Remove, as well as one can, noise power Relatively straight-forward Some detrending issues separating scintillation fades from multipath fading a detrending bandwidth issue Detrending using averaging proves to be more stable than filtering, but results in higher S4 due to multipath fading 4/18/2012 African Workshop

14 Measuring Phase Scintillation 4/18/2012 African Workshop

15 Some History Relative to Measuring Phase Scintillation Effects GPS Silicon Valley inherited commercialized scintillation monitoring technology from a US Air Force Small Business Innovation Research (SBIR) program Toughest challenge on that program was measuring phase scintillation with standard GPS receivers using Temperature Compensated Crystal Oscillators (TCXOs) TCXO phase noise masked phase scintillation effects Problem solved using good Oven Controlled Oscillators (OCXOs) These upgraded receivers provide good phase scintillation measurements Even then, there are limitations to operation in a scintillation environment 4/18/2012 African Workshop

16 Measuring Phase Scintillation Effects To measure phase scintillation, GPS receiver must track signal phase using a phase lock loop (PLL) Normally, weakest link in a GPS receiver Measurements include perturbations of receiver and satellite oscillators Mostly, these perturbations cannot be removed with detrending Longer-term phase includes signal Doppler, multipath and ionosphere TEC (and oscillator frequency offset), mostly removed with detrending Typically, measurement bandwidth is the PLL loop bandwidth Wide bandwidth makes loop more sensitive to amplitude fading, and thus, loss of lock Narrow bandwidth makes loop more robust, but filters out higherfrequency phase scintillation effects Loop can be configured to have narrow loop bandwidth for robustness, but still provide wide bandwidth phase data 4/18/2012 African Workshop

17 PLL Model with Wideband Phase Estimator Phase Discriminator measures current PLL 50-Hz phase error added back onto phase estimate 4/18/2012 African Workshop

18 Legacy Measurements of TEC Measure difference of GPS PN code phase on L1 and L2, smoothed against negative L1/L2 difference in carrier phase Legacy monitors use semi-codeless technique to measure on L2 Does not enhance ability to measure scintillation Semi-codeless L2 has 15 to 35 db less signal power recovery than L1 However, can use very low bandwidth PLL, aided with L1 Doppler phase, regaining 14 to 17 db, depending upon C/N 0 Limitations Typically not available if L1 C/N 0 drops below 38 db-hz Must contend with L1/L2 biases Satellite biases (Tau_GD and C/A-to-P) and receiver and antenna L1/L2 biases Real-time accuracies on the order of 1 2 TECU, after calibration Also, very much affected by multipath 4/18/2012 African Workshop

19 Evolution to Modernized GNSS 4/18/2012 African Workshop

20 Legacy GSV 4004B & Antenna GSV4004B GPS IONOSPHERIC SCINTILLATION AND TEC MONITOR AND OPTIONAL GPS702GG ANTENNA 4/18/2012 African Workshop

21 Features of GPStation-6 GISTM Features Channel Configuration Signal Tracking Ionospheric Measurments Scintillation Indices TEC (Code and Carrier) Communication Interface GISTM Receiver (Bold Red Indicates New Features) 120 independent channels GPS (L1, semi-codeless L2P, L2C, L5) GLONASS (L1, L2-C/A, L2P) Galileo (E1, E5A/B, E5 Altboc) SBAS (L1, L5), Compass (Upgradable) 50 Hz phase and amplitude data (raw or detrended-raw) GPS (L1 C/A, L2C, L5), GLONASS (L1, L2) Galileo (E1, E5), SBAS (L1, L5) GPS (L1/L2P, L1/L2C, L1/L5), GLONASS (L1/L2) Galileo (E1/E5A), SBAS (L1/L5) (1 Hz raw and 4/minute smoothed) USB/RS-232/RS-422, I/O (PPS, Event, Position Valid) 4/18/2012 African Workshop

22 Mean C/No (db-hz) Number of Satellites Improvements by Adding L2C and L GSV4004B (GPS Only) GPStation-6 (GPS + GLONASS) L1 C/A L2 P(Y) L2C L Elevation Angle (deg) Observation Period (Hrs) Measured at Calgary, AB, Canada GPS Modernization improves Signal Quality C/N 0 Adding Constellations increases Number of Ionospheric Pierce Points 4/18/2012 African Workshop

23 Comparison of L1C/A - L2C and L1C/A - L2P(Y) for Measuring TEC Negative TEC because receiver is not delay calibrated 4/18/2012 African Workshop

24 L2P(Y)/L2C TEC Performance Differences Not much difference in displayed performance 3 db loss in L2C I/Q multiplexing Wider tracking loop bandwidth on L2C Multipath errors dominate lower chipping rate on L2C However, L2C tracking much more robust and less dependent on L1 aiding Larger TEC bias using L2P(Y) More filter delay of wideband signal 4/18/2012 African Workshop

25 Amplitude Scintillation Index (S 4 ) Phase Scintillation Index ( ) (60-seconds) (rad) GPS Scintillation Measurement Comparisons GSV4004B GPStation GSV4004B GPStation Calama, Chile 0 21:00 22:00 23:00 00:00 01:00 02:00 Local Time (Hrs) S4 Legacy vs Modernized 0 21:00 22:00 23:00 00:00 01:00 02:00 Local Time (Hrs) Legacy vs Modernized Comparison shows excellent backward compatibility 4/18/2012 African Workshop

26 Amplitude Scintillation Index (S 4 ) Phase Scintillation Index ( ) (60-seconds) (rad) Modernized Monitor Includes GLONASS PRN 1 PRN 2 PRN PRN 1 PRN 2 PRN :00 22:00 22:00 23:00 00:00 Local Time (Hrs) 0 21:00 22:00 22:00 23:00 00:00 Local Time (Hrs) S4 4/18/2012 African Workshop

27 SBAS GEO Measurements 4/18/2012 African Workshop

28 Sigma Code/Carrier Divergence - meters Legacy SBAS S4 Measurements in Non-Scintillating Environment Standing wave multipath detrends out very well Code/carrier divergence due to crossing Doppler of 2 GEOs GPS TOW - Hours 4/18/2012 African Workshop

29 Sigma_Code_Carrier Divergence - meters Easy to Distinguish between Multipath and Amplitude Scintillation from GEOs No scintillation Slow varying standing wave multipath Everything above the line is likely multipath fading plus noise 0.5 Sigma_PR < X Corrected_S Corrected S4 4/18/2012 African Workshop

30 Amplitude Scintillation Index (S 4 ) Phase Scintillation Index ( ) (60-seconds) (rad) Modernized SBAS Measurements Same Performance as Legacy Receiver PRN 133 PRN PRN 133 PRN :00 22:00 23:00 00:00 01:00 02:00 03:00 Local Time (Hrs) S4 Time difference is due to pierce point location difference :00 22:00 23:00 00:00 01:00 02:00 03:00 Local Time (Hrs) Noise is due to GEO payload transponder phase noise Some phase scintillation observable between 9 and 10 pm 4/18/2012 African Workshop

31 Scintillation Monitoring Limitations That Apply to Both Legacy and Modernized Monitors 4/18/2012 African Workshop

32 General GNSS Receiver Limitations in Scintillation Environment Phase Scintillation Generally, not a problem at L1 or L5, or on L2C Unless a very narrow tracking bandwidth is used No worse than low-grade TCXO typically found in GPS Receivers Requires relative wide bandwidth PLL for phase tracking Larger problem for semi-codeless P(Y) on L2 Very narrow bandwidth PLL coupled with erroneous (required) aiding with L1 phase (doesn t agree with Doppler aiding) Amplitude Scintillation Primary culprit for loss of phase lock Deep and long fades steal signal from PLL Narrower bandwidth is better, but could require a better oscillator, and may lose lock due to strong phase scintillation False alarms from lock detectors during fades (apparent loss of lock) Loss of data (symbols) from SBAS signals 4/18/2012 African Workshop

33 Phase Scintillation Limitations 4/18/2012 African Workshop

34 GNSS Scintillation Monitor Limitations in Phase Scintillation Environment Can t measure scintillation at semi-codeless L2 P(Y) Loop bandwidths too narrow Measurement limitations on coded signals (L1, L2C and L5) dominated by receiver oscillator Typical receiver oscillator phase noise masks phase scintillation (See PSDs and plots in next charts) Thermal Noise limitation is about db- Hz OCXO phase noise typically better than 0.05 radians Limitation can be overcome by differencing phase between satellites Creates a requirement for high-rate data collection and substantial post processing 4/18/2012 African Workshop

35 Power Spectral Density - db-rad 2 /Hz Phase Noise PSD Comparisons TCXO Spectral Density OCXO Spectral Density Weak VHF Scintillation Scaled to L1 Stronger Antofagosta Scintillation TCXO Differenced Spectral Density SBAS GEO Signal Frequency Offset - Hz 4/18/2012 African Workshop

36 Antofagosto Phase Scintillation vs. TCXO Phase Noise = radians = 0.46 radians 4/18/2012 African Workshop

37 Tradeoffs Regarding Using Low-Noise Oscillators (OCXOs) Cost of low-noise OCXOs has diminished somewhat over recent years The cost driver is their packaging with the receiver (low-volume quantities) This packaging must also meeting international radiation and conductive emission (CE) requirements As stated, TCXO noise can be eliminated by differencing phase across satellites Creates a data storage and post-processing burden Receiver tracking bandwidth must be kept high, preventing tracking in noisy conditions and during deep fades 4/18/2012 African Workshop

38 Amplitude Scintillation Limitations 4/18/2012 African Workshop

39 Scintillation Monitor Limitations in Amplitude Scintillation Environment Amplitude Scintillation High S4 can cause loss of phase lock S4 is still usually valid it is based upon non-coherent power measurements, at least for short to medium length fades See state diagram Multipath fading limits minimum S4 capability Longer duration, but shallow fades Can be detected and eliminated because multipath also causes code/carrier phase divergence scintillation does not 4/18/2012 African Workshop

40 c/no (db Hz) Fade Depths and Widths Using 50 Hz Amplitude Samples 50 50Hz c/no for 8 November UT Tromso GSV time (minutes since start of hour) 4/18/2012 African Workshop

41 Sigma_Code_Carrier Divergence - meters Distinguishing Between Amplitude Scintillation and Multipath Fading 1 No Scintillation Varying Multipath All GPS Satellites Sigma_PR < X Corrected_S Corrected S4 Everything above the line is likely multipath fading plus noise 4/18/2012 African Workshop

42 Sigma CCDiv - meters Distinguishing Between Amplitude Scintillation and Multipath Fading 1.2 PRN_30 PRN_22 PRN_14 Moderate Scintillation Varying Multipath All GPS Satellites Multipath Noise/Multipath PRN_29 PRN_6 PRN_24 PRN_5 PRN_9 PRN_28 PRN_10 PRN_ Amplitude Scintillation Uncorrected S4 4/18/2012 African Workshop

43 Sigma_Code_Carrier Divergence - meters Multipath Fading Tracking SBAS Signals 3 No Scintillation, Slow Varying Multipath 2 SBAS Geostationary Satellites Everything above the line is likely multipath fading plus noise Sigma_PR < X Corrected_S Corrected S4 4/18/2012 African Workshop

44 Signal Tracking State Diagram Not necessarily implemented in all receivers, but is in Scintillation Monitors described here 4/18/2012 African Workshop

45 Example Phase Measurements Collected in San Francisco Area Non-Scintillation Environment 4/18/2012 African Workshop

46 Sig_Phi - Radians C/N0 - db-hz, Elev Angle - Degrees Typical Plot of 1, 3 and 10 Second Sigma-Phi from All Satellites in View Sig_Phi_1 Sig_Phi_3 Sig_Phi_10 ElevAngle C/N GPS TOW - Hours 0 4/18/2012 African Workshop

47 Sig_Phi - Radians Lock Time - Sec SBAS GEO Phase Measurements Phase Degraded by GEO Transponder Code/Carrier Control However, constant 45 degree elevation no multipath effects Sig_Phi_1 Sig_Phi_3 Sig_Phi_10 Lock Time GPS TOW - Hours 0 4/18/2012 African Workshop