Galileo Local Component for the detection of atmospheric threats 
 
R. Warnant, M. Bavier, H. Brenot, S. Lejeune, J. Spits, S. Stankov, G. Wautelet 
 
The presence of small-scale structures in the atmosphere (ionosphere and 
troposphere) can strongly affect the reliability of GNSS high precision real 
time applications. The concepts of reliability and integrity play a crucial role 
in the development of Galileo. In particular, small-scale structures in the 
ionosphere TEC due to Travelling Ionospheric Disturbances or to geomagnetic 
storms can be the origin of strong disturbances in high precision positioning. 
 
The GALOCAD project has been submitted in response to Galileo Joint Undertaking 
call for proposals 2423. GALOCAD stands for GALileo LOCal Component for the 
Detection of Atmospheric Disturbances which affect high accuracy Galileo 
applications. The objective of GALOCAD is to develop a methodology for the 
implementation of a Galileo Local Component for the nowcasting and the 
forecasting of atmospheric threats (ionosphere and troposphere) which can 
degrade the integrity of high precision Galileo applications. The paper 
presents the results obtained in the frame of that project about the influence 
of ionospheric threats on GNSS. 
 
The philosophy of GALOCAD is the following: in a first step, small-scale 
atmospheric disturbances are detected using GNSS data. Since the end of 2003, 
Belgium is equipped with a network of 67 permanent GPS stations. The typical 
distance between the stations ranges from 4 to 30 km. Based on this dense 
network, we have characterized the behaviour of small-scale disturbances over 
Belgium. In addition, a long-term climatological study of ionospheric 
disturbances has been obtained using the measurements collected in the permanent 
GPS station Brussels (Belgium) from April 1993 up to now. 
In a second step, the effects of such atmospheric disturbances on high precision 
real time positioning is assessed based on software which simulates GNSS user 
positioning conditions on the field.