The sweeping Langmuir probe (SLP) instrument is a payload of the European Space Agency’s (ESA) in-orbit demonstrator PicoSatellite for Atmospheric and Space Science Observations (PICASSO) cubesat, measuring the plasma environment in a high-inclination low earth orbit (LEO) orbit. Because of the small size of the spacecraft (0.3 m ×0.1 m ×0.1 m), the spacecraft floating potential can significantly drift when sweeping the probe bias voltage, affecting the current–voltage characteristic from which plasma parameters are retrieved. In LEO, the spacecraft generally encounters a high-density, cold (less than 1 eV) plasma environment, but due to the high orbital inclination, PICASSO passes the auroral regions exposing the spacecraft to precipitating fluxes of hot (several kiloelectron volts) electrons with potentially harmful, high-level spacecraft charging as a result. To investigate the spacecraft charging and the in-orbit operation of the Langmuir probes in both of these regimes, simulations of PICASSO were carried out in Spacecraft Plasma Interaction System (SPIS). The conditions of the LEO plasma and the constraints imposed by the particle-in-cell (PIC) models on the mesh size make this type of plasma simulations a very challenging task.