GRAS's zenith-pointing antenna tracks a number of GPS satellites in range. This is indicated by the blue lines. The track-line changes to yellow once the signal goes into occultation as it is in range of the GAVA antenna – this is an example of a set event. From these signals, profiles can be derived for the lowermost 80 kilometres of the Earth's atmosphere. The red line indicates where a rise event begins and the signal is received by the GVA antenna.