GALOCAD GAlileo LOcal Component for

nowcasting and forecasting Atmospheric Disturbances

R. Warnant*, G. Wautelet*, S. Lejeune*, H. Brenot*, J. Spits*, S. Stankov*, K. Stegen*, I. Kutiev**

*Royal Meteorological Institute, Brussels, Belgium**Geophysical Institute of BAS, Sofia, Bulgaria

Framework

Selected in the frame of call for proposal issued by GSA (GNSS Supervising Authority)

Consortium of 2 Institutes

Royal Meteorological Institute of Belgium Geophysical Institute of the Bulgarian Academy of Sciences

Project duration: November 2006 – July 2008

Background of the project (1/4)

GNSS are used in the frame of many different types of applications

Precision of positions ranges from a few mm to about 20 m depending positioning technique:

Absolute or differential Real-time or post-processingCode or phase observable

Background of the project (2/4)

Differential positioning allows to reach the best positioning precision in real time

Users receive « differential corrections » from a reference station Improved user position

Background of the project (3/4)

Real Time Kinematic (RTK) gives few cm precision in real time on distances up to 20 km

RTK requires phase ambiguity resolution

Assumption: user and reference station experience same atmospheric effects

Background of the project (4/4)

Therefore, local irregular structures (few km) in the atmosphere (TEC, IWV) can strongly degrade positioning precision

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

Objectives

Research in order to develop a prototype Galileo Local Component for the monitoring of Galileo «integrity» with respect to atmospheric threats :

nowcasting : to inform users (in real time) about the atmosphere 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 (1/2)

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 (2/2)

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 (1/2)

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 (2/2)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 (1/2)

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 AA and BB cancel

AA BB

ii jj

Double differences (2/2)

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 compute the differential ionospheric error

BUT requires ambiguity resolution !

Residual iono effects from DD (1/3)

Quiet activity, 11 km baseline

Residual iono effects from DD (2/3)

Medium amplitude TID, 11 km baseline

Residual iono effects from DD (3/3)

20 November 2003 geomagnetic storm, 11 km baseline

Level 2: Double differences

RTK positions are computed using a least square process which involves measurements made on all satellites in view

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 (know 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

Conclusions and outlook (1/3)

First attempts in order to build a Galileo Local Component for nowcasting the effects of ionospheric threats have been presented.

Ionospheric threats are detected at 3 levels (which validate each other): RoTEC, DD, error on position

Method has been validated on a few days which represent typical ionospheric conditions but further validation is necessary on more data.

Practical validation « on the field » with users is also foreseen

Conclusions and outlook (2/3)

Level 1 will be available on a new web site at the beginning of 2009.

Prototype version of Level 2 and 3 for a geographical area around Brussels will be available at the end of 2009

Same kind of work has been done for the neutral atmosphere (effects of strong thunderstorms, heavy rainfalls, …)

New model for local K index forecasting has also been developed and will also be operational at the begining of 2009

Conclusions and outlook (3/3)

More details can be found at poster session :

“Ionospheric variability which degrades the precision of real time GNSS applications”

by Gilles Wautelet, Sandrine Lejeune, René Warnant