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Impact pressure probes may be used to measure the stagnation pressure of a spacecraft vehicle during reentry into the atmosphere. We model the flow in an impact pressure probe in the collision-free and transition regime using a kinetic simulation scheme of the Boltzmann equation. It is shown that, depending on the gasdynamic properties density, velocity, temperature of the inflow into the impact pressure probe, the measured pressure may deviate by almost a factor 2 from the stagnation pressure. Furthermore, we study the dependence of the flow in the impact pressure probe on the gas–wall interaction. The models by Maxwell, Cercignani and Lampis, and by Lord are implemented. The influence of the gas–wall interaction is sensible for small accommodation coefficients; it is more pronounced, however, for temperature and density than for the pressure.