To characterize the impact of a RFI signal on the effective carrier-to-noise power density ratio, C/N0, of a GNSS receiver is very important for civil aviation standardization procedures in order to define Radio Frequency Interference (RFI)/jamming robustness test for two reasons. First, the prediction of the effective C/N0 is fundamental to predict the minimum performance attainable by an airborne GNSS receiver and to adjust the tests accordingly. Second, the receiver’s estimation of the effective C/N0 can allow the detection of the jamming presence and thus can help the receiver to adapt to the RFI situation. This problematic is analyzed with the introduction of three C/N0 definitions, statistical C/N0 which is the statistical prediction of the effective C/N0, real C/N0 which is the true effective C/N0 faced by the receiver and estimated C/N0 which is the value provided by a C/N0 estimator. Three conditions of validity are given to establish a relationship of equality between the three terms: ergodicity, centered RFI contribution at the correlator output and C/N0 estimator characteristics. To evaluate this last condition, Narrow-Wideband Power Ratio (NWPR), Signal-to-Noise Variance (SNV), Beaulieu and Moments Methods (MM) estimators are inspected and their expected mean value in presence of a generic RFI is theoretically derived. Finally, a full analysis is conducted for a RFI chirp signal showing that not all chirp signals are able to allow a statistical prediction of its effective C/N0 (condition of validity 1) and that NWPR and SNV are the more robust estimators (condition of validity 3) against this type of RFI.