Passive remote sensing from space with signals of opportunity has been studied for a long time beginning in the early 90s . The technique uses already existing signals, transmitted from satellites, which are reflected from ground in order to determine properties of earth surface. GNSS signals are the main interest here, due to their well-known signal properties. The Passive reflectometer acts as a bistatic radar, hence no expensive and power-consuming radar transmitter are needed. The received signal is correlated with a local code replica, where the resulting waveform shows properties of the reflected surface area. Instead of using a local code replica, the PRETTY mission will correlate the received reflected signal with the received direct signal. This technique is known as the interferometric approach. The main advantage for the interferometric approach is, that one is not bound to use known signals but can also exploit signals with unknown data modulation, opening up the possibility to use more generic signals for earth observation.The PRETTY satellite architecture will be based on the OPS-SAT architecture with modifications to accommodate the two payloads. RUAG Space GmbH as prime contractor will design the passive reflectometer payload, and Seibersdorf Laboratories is responsible for a novel dosimeter payload for measuring the space radiation environment during the PRETTY space mission. The dosimeter system will assess Total Ionizing Dose (TID) by the use of radiation integrating sensors, as well as LET spectral information, which is related to Single-Event Effects (SEE) in electronic components. Technical University Graz contributes the satellite platform based on the OPS-SAT architecture . Furthermore TU Graz conducts the Manufacturing, Assembly, Integration and Test (MAIT) activities and will be in charge for the operation of the satellite. The scientific advisory group to be established will support the design of the payload and the evaluation of the raw data from the operation of the satellite. Within the present paper we will describe the architecture of the passive reflectometer payload within this 3U CubeSat mission and discuss operational routines and constraints.