The erroneous GPS signal
Two researchers from the University of Liège have developed a real-time monitoring system allowing GPS users to assess the level of precision of their measurements. High precision positioning is indeed affected by large and small scale ionospheric variability. These ionospheric irregularities have been characterised and it is now possible to predict their occurrence in mid-latitude regions. The researchers in Liège have even developed an e-mail alert system: you simply have to register on their site and as soon as irregularities reach a certain level, an e-mail is automatically sent to all users (surveyors, for instance) registered on the site. The latter are therefore warned of the possible unreliability of their measurements.
Small mistakes… big consequences
Within the framework of the above-mentioned applications, the movements to be detected are in the order of centimetres, corresponding to the level of precision that sophisticated GPS’ can currently achieve, thanks to observation networks and properly adapted mathematical processing. However, in some cases the precision of the measurements is out of tolerance, with values that can exceed a metre. The problem is that professional users are only rarely informed of the inaccuracies associated with their devices. The research carried out by Gilles Wautelet and René Warnant, assistant and professor respectively at the University of Liège’s Unit of Geomatics-Geodesy and GNSS, should provide them with a better understanding. And should prevent certain inconveniences, as the main author of this research, Gilles Wautelet, underlines: “If bridges or civil structures are being monitored and the measurements are out by several decimetres when the work is being done in centimetres, there will indeed be a big problem. Also, in the case of surveyors, if they are supposed to guarantee centrimetric precision and the final measurement is 15-20 cm off, then they are no longer meeting the specifications”. In addition to the study’s theoretical framework, the two researchers therefore wished to take into account the practical importance of the problem by seeing things from the users' point of view.