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 head H, flow Q, and 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 characteristics; locating operating points of the compressor on each of the three models based on the current value of the operating parameters; identifying a sensor fault that affects the determination of at least one of the operating parameters; and avoiding surge of the compressor using one model of the three models, the one model being a model that is a function of two operating parameters unaffected by the sensor fault.