The erroneous GPS signal
2/10/14

Gilles Wautelet based his research on this premise: “the objective of my work is to model the irregularities of what is known as the Total Electron Content (TEC), which is the total number of electrons between the satellite and the station, or more precisely, to identify the recurring behaviours as well as the amplitude of these irregularities according to the season, local time or solar activity". It is also necessary to precisely define what we mean by “irregularity”. Various types of irregularities exist depending where we are on Earth, as the young researcher reveals: “we’re quite lucky in Europe, in our mid-latitude sector. In fact, there are very few extreme irregularities compared with the magnetic equator or the poles. And the situation is different depending on whether you’re in the northern or southern hemisphere, or even in Japan or the United States even though these are also mid-latitude sectors. In Europe, there is an average variability which mainly manifests itself as Travelling Ionospheric Disturbances (TID's), itinerant waves that spread throughout the ionosphere". The geomagnetic storms (solar storms) are another major type of irregularity. In this case, coronal mass ejections, directed towards the Earth, interact with the geomagnetic field, thus creating a variability in the ionosphere. Although greater than the variability due to TID’s, this nevertheless occurs far less frequently.

Magnetic fieldWhile it is possible to predict the effects of a solar storm several hours in advance thanks to imaging satellites capable of detecting ionised clouds, it is far more complicated with regard to TID’s: "these are observations made over several years. We know that TID’s are generally observed in autumn and winter, and around midday. We can therefore only predict TID’s on a climatological basis, thanks to a certain recurring characteristic. Of course, this isn‘t checked every day. For the time being, it’s possible to predict their occurrence fairly accurately, but not their source", Gilles Wautelet explains.

As for René Warnant, he is very satisfied with the study carried out at the GPS stations installed in Belgium and whose raw data was processed by software developed at ULg: “this is the first complete statistical study of the different types of ionospheric irregularities affecting GPS in mid-latitudes in Europe. The study covers a period of 10 years (editor’s note: from 2002 to 2011), representing practically the duration of a solar cycle (approximately 11 years). This point is important because the “behaviour” of ionospheric irregularities depends on solar activity”.

Study results

This research has revealed a number of points (2). First of all, irregularities are observed maximum 9% of the time, which means that they aren’t frequently observed, even during periods of high solar activity. Secondly, ionospheric irregularities can be divided into two major categories: those associated with spatial events (Space Weather – SW) and those occurring during “quiet-time”. Even if SW irregularities are responsible for the largest TEC fluctuations, their contribution oscillates between 0 (solar minimum) and 25% (solar maximum) of the total annual amount. Consequently, the study of occurrence rates and the amplitude analysis were focused on “quiet-time” irregularities, which form the majority of irregularities detected in mid-latitudes. The latter are classified into two groups, i.e. Winter Daytime (WD) and Summer Night time (SN). While WD is responsible for almost 75% of annual irregularities in “quiet-time”, this is quite rare in SN (less than 10%). The analysis also revealed the amplitude of WD irregularities is proportional to the TEC. On the other hand, the amplitude of SN irregularities seems to be negatively correlated with the TEC, with the largest values generally observed during periods of low solar activity. Considering these occurrence rates and amplitude characteristics, it would seem that the WD irregularities correspond to daytime TID’s, sometimes referred to as “classical” TID’s. However, the physical origins of SN irregularities are more difficult to establish.

(2) « Climatological study of ionospheric irregularities over the European mid-latitude sector with GPS ». Journal of Geodesy. DOI : 10.1007/s00190-013-0678-4

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