FCK211 - Calibration Flight

by chance, I came across this flight:

FCK211 from THU sep.25 23:12 CEST to FRI sep.26 00:53 CEST
http://www.flightradar24.com/FCK211/46bab13

the flight path looks pretty weird - two large, perfect circles and some erratic patterns in between ...

I wonder what is the purpose of such flights, what is being calibrated?