1. Effects of ionospheric small- scale structures on GNSS G. WAUTELET Royal Meteorological Institute of Belgium Ionospheric Radio Systems & Techniques (IRST) Edinburgh, 28 – 30 April 2009

2. 2 OUTLINE Introduction 1.“One-station” method 2.Small-scale structures and double differences (DD) 3.Small-scale structures and relative positioning Conclusions – future work

3. 3 INTRODUCTION Ionosphere = main error source of GNSS Models exist (Klobuchar, NeQuick) BUT… occurrence of small-scale structures (local scale) which induce positioning error in the case of high accuracy applications (e.g. Real-Time Kinematics or RTK) GOAL = detect and assess the influence of iono small-scale structures on GNSS precise applications

4. 4 INTRODUCTION 3 steps  3 sections : 1.Detection of structures at 1 GPS station 2.Assess the influence of those structures on double differences (relative positioning basic observables)  in terms of L 1 (or L 2 ) cycles 3.Assess the influence of those structures on precise relative positioning like RTK  in terms of meters (user units)

5. 1. “One-station” method

6. 6 1.1. Methodology Objective: isolate the high frequency changes in the TEC (Total Electron Content) observed at a given GPS station HOW? Using the Geometric-Free (GF) combination of GPS phase measurements (GPS system uses 2 frequencies) 5 steps

7. 7 1.1. Methodology 1.Computation of the TEC at each observation epoch [TECU/min] 2.Computation of the verticalized temporal gradients of TEC (ΔVTEC) at each observation epoch

8. 8 1.1. Methodology If σ > 0,08 TECU/min, ionospheric event detected 15 min 3.Polynomial fitting of temporal series of ΔVTEC 4.Residuals computation: « ΔVTEC – polynomial » called Rate of TEC (RoTEC) 5.Every 15 min, computation of Std. Dev. σ of RoTEC

9. 9 Travelling ionospheric disturbances (TID’s) 1.2. Two main types of structures - Cause: interaction between gravity waves and ionosphere - Observation: wave-like fluctuation of the electronic density - Different classes: SSTID's, MSTID's, LSTID's - Origin: non geomagnetic for SSTID's and MSTID's geomagnetic for LSTID's

10. 10 “Noise-like” structures (NLS) 1.2. Two main types of structures - Cause : geomagnetic phenomena (CIR, storms) - Observation : RoTEC fluctuates randomly (« noise ») - Order of magnitude NLS > order of magnitude TID's

11. 11 1.3. Climatological study Time interval = 1994 – 2008 (i.e. more than a solar cycle) Reference station = BRUS (Brussels) Goal of the study = counting of the number of ionospheric events detected in function of time. Different temporal dependencies (time scales) of the occurrence of such structures will be analyzed:  Solar cycle dependence  Seasonal dependence  Local time dependence

12. 12 1.3. Climatological study Solar cycle dependence On average, number of events is higher during solar maximum (2001, 2003) than during solar minimum (1996, 2007)... BUT seems to show a strong monthly dependence

13. 13 1.3. Climatological study Seasonal dependence  More small-scale structures during autumn/winter months  Stuctures more numerous during solar max Computation of the monthly mean of the number of events: solar max (2001) and solar min (2006) are the most representative.

14. 14 1.3. Climatological study Local time dependence Computation of the total number of events for each 15 min time interval : solar max (2001) and solar min (2006) are the most representative.  Maximum around 10 h  Secondary max during nighttime

15. 15 1.3. Climatological study Type of structure detected Computation of the total number of events for each 15 min time interval for year 2001. We consider (red) or not (green) the days for which Kp max > 5 (stormy days). Offset between the 2 graphs : phenomena due to geomagnetic storms occur all the time : Noise- like structures Most of structures are not connected to geomagnetic activity : Travelling Ionospheric Disturbances (TID's)

16. 16 Conclusions 1.3. Climatological study 2 main types of small-scale ionospheric structures have been detected : Noise-like structures : very few and occur all the time TID's : numerous and are season and time-dependent (more during autumn/winter than during spring/summer). 2 main types : Daytime TID's : around 10 h, very numerous Nighttime TID's : between 22 h and 2 h, less numerous than daytime ones

17. 2. Small-scale structures and double differences (DD)

18. 18 2.1. Methodology Relative positioning = determination of a baseline between 2 receivers with an accuracy of a few cm Basic observable = double differences of phase measurements (DD) Advantages : cancellation of all error sources common to the two stations  no clocks/orbit errors  usualy, atmospheric residual errors are negligible BUT… Equation (neglecting multipath and noise): REFERENCE STATION with a known position USER with an unknown position 10-20 km A B Residual ionosphere can however be a threat for such high-accuracy applications

19. 19 2.1. Methodology Objective = compute the ionospheric residual term for a given baseline Use of the GF combination of double-differenced (DD) phase measurements: a) Neglecting multipath and noise, we compute the ambiguity term and we obtain : = ionospheric residual term in DD (every 30 s) b) We express this term into cycles of L 1 carrier

20. 20 2.2. Nominal conditions 11.3 km 8.9 km 4.1 km

21. 21 2.2. Nominal conditions Statistics of (in L 1 cycles) GILL – LEEU (11.3 km) BRUS – GILL (4.1 km) LEEU – BRUS (8.9 km) P2.5-0.135-0.116-0.105 P97.50.1380.1080.110 over 11 days during winter 2008 (DOY 300-310)  solar min For quiet days and in 95% of cases, residual ionosphere in DD represents less than 0.15 L 1 cycle

22. 22 2.3. Results on case study Baseline of 11 km (GILL – LEEU) near Brussels (typical RTK baseline) 3 different (typical) ionospheric conditions : – quiet (DOY 310/08) – occurrence of medium amplitude TID (DOY 359/04) – occurrence of geomagnetic storm (DOY 324/03) RoTEC max [ TECU/min] # events at BRUS Kp max 310/080.30920.3 359/040.837442 324/038.9332309

23. 23 2.3. Results on case study Quiet day : DOY 310/08 95% of values (nominal conditions) MAX < 0.4 L 1 cycle

24. 24 2.3. Results on case study Medium-amplitude TID : DOY 359/04 MAX ~ 1 L 1 cycle

25. 25 2.3. Results on case study Geomagn. storm : DOY 324/03 MAX ~ 2.5 L 1 cycles

26. 26 During TID's or geomagnetic storms, the residual delay due to the ionosphere is significantly larger than the nominal value (0.15 cycle), and even larger than 0.5 cycle → risk to fix the ambiguity to a wrong integer value → large positioning error 2.3. Results: summary

27. 3. Small-scale structures and relative positioning

28. 28 (b) Computation of the user position every 30 s using a least square adjustment corrected for the ionospheric effect computed in section 2 : residual error is mainly troposphere (a) Computation of the user position every 30 s using a least square adjustment : residual errors are mainly troposphere and ionosphere 3.1. Methodology Objective = assess the part due to the ionosphere in the positioning error (c) Positioning error due to ionosphere obtained by substraction (a) - (b) (ΔX 1, ΔY 1, ΔZ 1 ) (ΔX 2, ΔY 2, ΔZ 2 ) (ΔX 1 – ΔX 2 ), (ΔY 1 – ΔY 2 ), (ΔZ 1 – ΔZ 2 )

29. 29 3.2. Nominal conditions Statistics of (in meters) GILL – LEEU (11.3 km) BRUS – GILL (4.1 km) LEEU – BRUS (8.9 km) P95 0.0330.0290.030 over 11 days during winter 2008 (DOY 300 - 310)  solar min For quiet days and in 95% of cases, residual ionosphere in DD is responsible for about 3 cm  can be approximated to the ambient noise and multipath Express the error in terms of distance:

30. 30 3.3. Results on case study Quiet day : DOY 310/08 MAX < 5 cm 95% of values (nominal conditions) outlier

31. 31 3.3. Results on case study Medium-amplitude TID : DOY 359/04 MAX ~ 15 cm

32. 32 3.3. Results on case study Geomagn. storm : DOY 324/03 MAX ~ 65 cm

33. 33 3.3. Results: summary During TID's or geomagnetic storms, the positioning error due to the ionosphere is significantly larger than the nominal value (3 cm). → medium ampl. TID: ~ 15cm → geomagn. storm: ~ 65 cm

34. 34 CONCLUSIONS Three levels of observation of the ionosphere: One-station: detects iono irregularities in time (temporal gradients) and allows to perform a climatological analysis of the iono irregularities for a mid-latitude station Double differences: detect spatial gradients in ionosphere and allow to see the contribution of the ionosphere in the DD, which is especially important for ambiguity resolution Relative positioning: assess the influence of spatial gradients in the ionosphere on positioning error (quantitative assessment) These three levels allow us to determine the phenomena observed and their effects on relative positioning  integrated tool for a service which warns users when degraded positioning errors occur or are expected

35. Thank you for your attention!

36. Effects of ionospheric small- scale structures on GNSS G. WAUTELET Royal Meteorological Institute of Belgium Ionospheric Radio Systems & Techniques (IRST) Edinburgh, 28 – 30 April 2009