The S4 index plot indicates the recent history of the scintillation index at São Luis, MA.
The S4 index is a metric for indicating the amount of variation in the amplitude of a signal. In this situation the S4 index is an indicator of the standard deviation of the GPS satellites received signal power as seen on the ground.
The Position Error plot indicates the norm of the position error in meters of a stationary antenna on the roof of a building at São Luis, MA in Brasil. The plot is updated every minute.
The values of Position Error are limited in the vertical scale, the points that exceed this limit are cut and are not shown in this plot.
The bottom plot is an elevation/azimuth plot of the GPS satellites currently in view. The size of the circle, denoting the location of each satellite, is dependent on the S4 index. The larger the circle the larger the S4 index and vice versa.
The main causes of these variations are multipath and ionospheric electron density fluctuations. Filtering techniques can generally eliminate themultipath component leaving only the ionospheric component. This data is generated by the Cascade GPS receiver.