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Radio frequency interference (RFI) on L-band affecting GNSS receivers might lead to unacceptable performance degradations when operating with GNSS only. Flights using GNSS based procedures and operated under Instrument Meteorological Conditions (IMC), are particularly concerned about such a threat. This paper focuses on the impact of RFI on GPS/SBAS receivers. Based on a recorded RFI event, the behavior of the receivers' estimated protection level is analyzed. In order to understand the behavior of the HPL, the corresponding algorithm is analyzed in depth. It is shown, that RFI impacts only few HPL input parameters. The size of HPL is only dictated by the increased pseudorange noise as well as the number of tracked satellite signals. The latter is directly related to the receivers' antenna pattern describing the expected Carrier to Noise Ratio (C/No). Hence, the vehicles' attitude plays a relevant role, too. Different simulations are presented which describe the HPL behavior in a more general way.