I am a bit confused about reading technical specifications on RapidEye satellites. I assumed that Ground sampling distance (GSD) and pixel size express the same meaning, while GSD is 6.5 m, and pixel size is 5 m. Do you know the difference between the two?
Ground sampling distance is the distance between each measurement at nadir.
Pixel size refers to what is delivered when data is purchased.
Some (commercial) data providers use a smaller pixel size than the ground sampling distance. This means that the data is interpolated from the measured image grid into the delivered image grid. In the case of RapidEye, it goes from (at best) 6.5m to 5m. As such, over a distance of 20 meters, the satellite have measured 3 times, but they deliver 4 values, which are the result of interpolation.
In general, this is probably done in order to deliver data with a nice round number as the specified pixel size, or to match the competition.
The ground sampling distance given is the recording pixel size at nadir. Since the image frames captured are rather large, the actual ground sampling distance will be different off-nadir, due to earth curvature, capture angle, terrain elevation etc. The 5 m pixel size is, as I understand it, for an orthorectified image.