1. A method of operating a compressor of a compressor system using three models, each model of the three models describing a surge line of the compressor as a function of any two of three operating parameters of the compressor, the three operating parameters including a head (H), a flow (Q), and a speed (N), comprising: measuring operating characteristics of the compressor system using sensors; determining a current value of the three operating parameters based on at least some of the measured operating characteristics; locating operating points of the compressor on each of the three models based on the current value of the operating parameters; monitoring a variation of the located operating points over time on each of the three models to identify a sensor fault that affects the determination of one of the operating parameters; and avoiding a surge of the compressor using one model of the three models, the one model being the one model of the three models that is a function of two operating parameters unaffected by the sensor fault, to initiate a surge avoidance action when the one model of the three models indicates an impending surge.