And on a practical level?


Currently, the model we can derive from this study isn’t implemented either in the receivers or in the GNSS processing software. This will only be possible when the causes of the irregularities can be definitely established.

In the meantime, ULg's geomatics unit offers free real-time monitoring (available on the website: www.gnss-ulg.be) of the state of the ionosphere, measuring its impact on high precision positioning within the entire network of GPS stations in Belgium, i.e. some sixty reference stations. This monitoring is accompanied by a proactive approach: “We know that surveyors receive training to make sure they are always up to date. The idea is therefore to come to these gatherings and to offer them this aid that can be used for planning or, in any case, the verification of their GPS measurements. They can therefore check all their measurements by observing the ionospheric conditions when readings are taken in the field", explains Gilles Wautelet. And in order not to miss any disturbance, an e-mail alert system has been set up: “you simply have to register on our website. As soon as the irregularities reach a certain level, an e-mail is automatically sent to all the users registered on our website. Obviously, there is a slight latency as we have to process the data first”, the geodesy specialist points out.


If there is a degradation in the ionospheric conditions visible on the website, the user can thus take the appropriate decision: return to the field to take another reading if the ionospheric error is too great or keep the measurements, knowing approximately the level of inaccuracy attributed to ionospheric variability. “However, it wouldn’t be wise to “correct” the measurements on the basis of our diagnostic since this only reflects the ionospheric conditions found by the stations that we have processed, and not that of the user in the field", Gilles Wautelet specifies. Indeed, the position errors available on the website can’t simply be applied such as they are to all users in the field; every measurement environment is unique, with its own configuration of satellites in the user’s sky and its own errors due to the close environment. The ULg researcher compares this problem with a more explicit case: “It’s a bit like wanting to know the precise wind speed at the bottom of a valley when you only know the wind speed in a place outside the valley. While we can indeed obtain a good estimation of this speed, we won’t ever be able to obtain the exact value in our valley: we know that local influences such as topography or plant cover have a significant effect on wind speed. What we observe at point A is therefore not applicable to point B, whether we’re talking about wind speed or the ionosphere.”