Remote sensing radar instruments rely typically on active transmitters that have a power demand, which is difficult to cope with on CubeSats due to their size limitations. A passive reflectometer overcomes this problem by receiving the reflected beam from a known precise signal source in space. Due to the used frequency band, the signal modulation and the availability of their transmitter's ephemerides, GNSS signals are perfectly usable for this purpose. The PRETTY (Passive REflecTometry and dosimeTrY) satellite is currently developed by TU Graz, RUAG Space and Seibersdorf Laboratories under a contract with ESA. PRETTY hosts two payloads: The first payload is a passive GNSS based reflectometer. The main scientific goal is the precise altimetric determination of ocean and sea-ice surfaces using the interferometric phase-delay altimetry approach. The interferometric approach avoids the local generation of the original signal by additionally receiving of the direct (not reflected) signal from the signal source, which subsequently is correlated with the reflected beam. This methodology will be applied for the first time in space. The second payload is a dosimeter for analysis of the space radiation environment in the PRETTY orbit by collecting data from different sensors. The dosimeter is capable of providing information on the total ionizing dose (TID) as well as on the linear energy transfer (LET) of the space radiation environment. The satellite mission has entered the detailed design phase and a launch is scheduled for early 2021, with a nominal operational lifetime of one year. In the present publication, we introduce the mission and observation concept, as well as the current status of the project.