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

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 »

TEC reconstruction

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

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

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 )

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

TEC local variability

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)

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

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

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)

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)

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

Travelling Ionospheric Disturbances

Noise-like structures 20 November 2003 severe geomagnetic storm

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

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)

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

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 !

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

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

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

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.

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)

Effects on positions (quiet ionosphere)

Effects on positions (TID)

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

Effect of a TID on the Belgian reference Network

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

Added value of new GNSS

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)

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

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

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)

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 %)

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

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