1. Monitoring the ionospheric activity using GNSS
Monitoring the ionospheric activity using GNSS. From dual frequency GPS to multi-constellation multi-frequency GNSS
R. Warnant, B. Bidaine, M. Lonchay, J. Spits, G. Wautelet
Unit of Geomatics – Geodesy and GNSS

2. Outline
Overview of past, curent and future research activities related to ionospheric effects on GNSS
TEC reconstruction with GPS and opportunities with multi-constellation multi-frequency GNSS
Monitoring the integrity of GNSS high precision real-time applications wrt « ionospheric threats »

3. TEC reconstruction

4. TEC with GPS L1/L2 (1)
The principle of TEC reconstruction using dual frequency (L1/L2) GPS code and phase measurements first proposed by Lanyi and Roth (1988)
Developed during the nineties (very controversial topic !)
Most ionosphere physicist’s did not believe in it
Most Geodesist’s were convinced that the « ionosphere free » combination was THE SOLUTION and that TEC reconstruction was just useless for Geodesy

5. TEC with GPS L1/L2 (2)
The goal was to try to have a better understanding of ionospheric effects on GPS
Correlation between TEC and position time series ??
TEC can be reconstructed from the geometric-free (GF) combination of L1/L2 phase measurements
The main problem is to compute the GF ambiguity which is not an integer

6. TEC with GPS L1/L2 (3)
TEC reconstruction with GPS is now a well recognized tool in Geodesy and Ionosphere Physics
Use of TEC maps in ambiguity resolution procedures (BERNESE)
Correction of higher order iono effects in IF
Use of TEC to study ionosphere reaction to geomagnetic storms
In terms of accuracy, not much progress done during the last 10 years
Accuracy ranges between 2 and 5 TECU (1 TECU = 1016 e- m-2 )

7. TEC with GPS L1/L2 (4)
TEC at Brussels from 2000 to 2012

8. TEC local variability

9. Ionosphere and real time positioning
Real-time Positioning techniques like RTK are based on the assumption that ionospheric effects are similar at the reference station and at user position (both in differential and relative modes)

10. Local Variability in TEC
Therefore, local irregular structures (few km) in the ionosphere (TEC) can strongly degrade real time positioning accuracy
Users are not necessarily aware about the problem
This is a limitation to the reliability of future Galileo services which are supposed to provide certifed accuracy levels

11. Monitoring GNSS « integrity » wrt ionosphere
Research in order to develop a prototype Galileo Local Component for the monitoring of Galileo «integrity» with respect to ionospheric threats :
nowcasting : to inform users (in real time) about the ionosphere influence on their applications (can Galileo certified accuracy be reached ?)
forecasting : to forecast a few hours in advance the occurrence of ionospheric disturbances which could degrade significantly Galileo accuracy

12. Nowcasting Ionospheric effects
Detection of irregular structures in the ionosphere which can degrade GNSS accuracy based on a dense network of GNSS stations
Rate of TEC (level 1)
Double differences (level 2)
Assessment of the effect of these ionospheric structures on GNSS high accuracy applications
Software which simulates user « positioning conditions » on the field (level 3)

13. Small-scale structures in ionosphere (1)
Detection of small-scale structures using a «single-station method»
Ionospheric small-scale disturbances are moving
 Detection possible by monitoring Rate of TEC at single station
Rate of TEC (RoTEC) is monitored using the geometric free combination of GPS dual frequency measurements (no ambiguity resolution)

14. Small-scale structures in ionosphere (2)
Method validated on Brussels GPS data (1993-now)
Two types of structures detected :
Travelling Ionospheric Disturbances (TID’s)
Noise-like structures
Detailled climatology of these structures has been performed

15. Travelling Ionospheric Disturbances

16. Noise-like structures
20 November 2003 severe geomagnetic storm

18. Level 1: Rate of TEC (1)
RoTEC (TEC change with time) is an easy to compute parameter allowing to detect the occurrence of local ionospheric activity which is a possible threat for GNSS
BUT differential applications depend on differential ionospheric effects between user and reference station (TEC difference in space)
Therefore RoTEC only give a « qualitative » assessment of ionospheric effects

19. Level 1: Rate of TEC (2)
Based on the number and amplitude of detected ionospheric irregular structures, assessment of ionospheric effects on differential GNSS using a colour scale (green, orange, red, black)

20. Double differences (1)
Double Differences (DD) are differences of observations made by 2 receivers (A: ref station, B: user) on 2 satellites (i,j) in view in the 2 stations
In DD, all the error sources which are common to measurements performed by receivers A and B cancel i j A B

21. Double differences (2)
DD of L1 or L2 contain residual differential atmospheric (iono+tropo) effects between A and B (depends on distance)
DD of geometric free combination of L1 and L2 allows to isolate the differential ionospheric error
BUT requires ambiguity resolution !

22. Residual iono effects from DD (1)
Quiet activity, 11 km baseline

23. Residual iono effects from DD (2)
Medium amplitude TID, 11 km baseline

24. Residual iono effects from DD (3)
20 November 2003 geomagnetic storm, 11 km baseline

25. Level 2: Double differences
DD allow to assess differential iono effects on individual measurements : this « refines » the information given by RoTEC
BUT users are NOT interested in TEC maps, TID’s, DD, … BUT in POSITIONING ERRORS.

26. Level 3: Positioning error
Development of software which reproduces user positioning conditions on the field
It computes positions in the same way GNSS users do
Based on permanent station data (known positions) which play the role of « user » and « reference station »
« extracts » the part of the error budget due to the ionosphere (for users who have already solved their phase ambiguities)

27. Effects on positions (quiet ionosphere)

28. Effects on positions (TID)

29. Effects on positions (severe storm)
Errors up to a few meters if the disturbances appear
at the time users are solving their phase amibiguities

30. Effect of a TID on the Belgian reference Network

31. Forecasting TEC variability
Severe geomagnetic storms are the origin of increased local variability in TEC
Development of the MAK model (collaboration with GI-BAS) to forecast K geomagnetic index in Belgium
Based on Solar wind parameters
Possibility to issue warnings
Based on data from 2002 to 2011, development of a statistical model allowing to forecast the occurrence of iono irregularities
PCA to model daily variability (shape of irregularities)
Low-order polynomial and harmonic function to model influence of season and Solar activity
Autoregressive formulation to adapt the model to current conditions

32. Added value of new GNSS

33. New GNSS Triple frequency GNSS become available
Third frequency with GPS (L5)
Four frequencies for Galileo (E1,E5a,E5a+b,E5b)
QZSS (Japan)
Beidou
GLONASS K (L3)

34. Multi-frequency TEC (1)
The availability of multi-frequency GNSS opens new opportunities for ionosphere monitoring
Third frequency  possible to solve integer ambiguities on each carrier separetely using undifferenced single station data.
The GF ambiguities could be computed based on the integer ambiguities on each carrier
Other methods (not using GF) will be available
In particular could be obtained as a by-product of multi-frequency multi GNSS

35. Multi-frequency TEC (2)
Justine Spits (2012) proposed a method based on Galileo E1, E5a and E5b data
Wide-Lane combination E5a/E5b (λ = 9,76 m)
 N5b – N5a
Triple frequency combination (Differenced wide-lane)
 N1 – N5b

36. Multi-frequency TEC (3)
Triple frequency combination (TF multipath combination) using the information from the previous 2 steps
 N5b
 N1 and N5a
GF ambiguities can be solved
 TEC
Validated on GIOVE A and GIOVE B data from 2008 (low activity)

37. Multi-frequency TEC accuracy
If the ambiguities have been solved to the right integer
If one neglects the influence of phase biases
Under « reasonable » multipath conditions
If different effects are corrected :
Phase center offset and phase center variations
Phase wind-up effect
Then TEC could be reconstructed at 0,20 TECU (99 %)

38. The weaknesses : influence of phase biases
Phase bias variability could be non negligible on short periods of time (case of GPS PRN#25)
They could be large enough to prevent ambiguity resolution
Their influence could be large enough in the geometric free combination to affect TEC accuracy at a level of a few 0,1 TECU
Therefore, they have to be estimated
Could be done on a network basis as it is done for code biases

39. Conclusions
GPS dual frequency data have been used for more than 20 years to monitor the ionospheric activity
In particular, local irregularities in TEC which can pose a threat to real time positioning can be detected
This « tool » is now well recognized both by Geodesist ’s and Ionosphere Physicist’s
The availability of TF GNSS offers the opportunity to reconstruct TEC at a few tenths of TECU (improvement of one order of magnitude)
But the problem of phase biases must be solved