Development of a GAlileo LOcal Component for the nowcasting and forecasting of Atmospheric Disturbances affecting the integrity of high precision Galileo applications
Background & Objectives
Atmospheric effects on GNSS signal propagation remain an important error source for GNSS applications. In particular, high-accuracy real-time positioning is affected by small-scale irregular structures in the ionospheric plasma or in the neutral atmosphere water vapour. For example, the occurrence of local irregular structures in the atmosphere due to disturbed space weather conditions or to strong thunderstorms can be the origin of ‘out of tolerance’ errors. GNSS users are not necessarily aware when such events occur. This is a major concern for Galileo services which will offer certified accuracy levels to their customers.
Atmospheric effects on GNSS signal propagation remain an important limitation to the reliability of GNSS applications. The goal of GALOCAD is to create a prototype Galileo local component which will assess the effects of the atmosphere on Galileo high-accuracy positioning through ‘nowcasting’ and forecasting.
Small-scale atmospheric structures can be studied using GPS measurements. Nevertheless, until now the ‘density’ of available GNSS stations in Europe was not sufficient to make a detailed study of these atmospheric (ionospheric and tropospheric) structures which can have a ‘very local’ behaviour. Since the end of 2003, Belgium has been equipped with a network of 67 permanent GPS stations. The typical distance between the stations ranges from 4 to 30 km. The idea of GALOCAD is to use data from this network to perform a detailed study of atmospheric small-scale structures, to build a model of representative small-scale activity, and to assess the influence of these structures on the reliability of Galileo precise positioning applications.
In addition, the project will study the correlation which exists between small-scale ionospheric activity and geophysical parameters (like the local geomagnetic K index): the existence of such a correlation between small-scale structures and local K would enable forecasting the occurrence of degraded positioning conditions a few hours in advance. Then, the results of these investigations will be used to develop and to validate a prototype Galileo local component for ‘nowcasting’ and forecasting the effect of atmospheric disturbances on the integrity of high-precision Galileo applications.
Based on a dense network of GNSS stations in Belgium, the goal of GALOCAD is to create a prototype Galileo local component (over Belgium) which will assess in real time, and forecast a few hours in advance, the effects of the atmosphere on Galileo high accuracy positioning. In other words, this local component will monitor the integrity of Galileo with respect to ‘atmospheric threats’: - ‘nowcasting’: on the one hand, the service will inform users (in real time) about the atmosphere influence on their applications (can Galileo certified accuracy be reached?) - forecasting: on the other hand, the service will forecast, a few hours in advance, the occurrence of ionospheric disturbances which could degrade significantly Galileo accuracy.
Work performed & results
The expected outcomes are: Improving Galileo product reliability: The idea of GALOCAD is not to provide improved atmospheric corrections but to inform users when a Galileo certified positioning accuracy level is not reached due to atmospheric and space weather conditions. In other words, GALOCAD will provide a location dependent Galileo ‘integrity’ monitoring with respect to atmospheric disturbances. Forecasting ‘positioning conditions’ for the next few hours: GALOCAD will forecast the occurrence of geophysical phenomena or space weather conditions a few hours in advance, which can be the origin of degradations of Galileo positioning accuracy. The service will warn Galileo users when degraded positioning conditions are expected.