system 36 is zero the instantaneous frequency of the input to this demodulation system is equal to f c frequency demodulated signals are smoothed using a low pass filter 38 to reduce the influence of noise and the resulting frequency signature is processed by system 40 to extract signal characteristics such as for example amplitude rate of change of the signal spectral content etc the extracted signal characteristics identified as stall precursor measure which may be stored in system 40 the band pass filter 34 frequency demodulation system 36 low pass filter 38 and stall precursor measure system 40 may all be implemented in an integrated unit 31 sensor data may also be processed using a plurality of frequency demodulation algorithms operating in parallel thus increasing the confidence of stall precursor detection a number of stall precursor magnitudes obtained from respective sensors may be combined in a system 42 and the combined magnitude is compared in a comparator 43 with a combined baseline stall magnitude inferred from a look up table 44 to define an upper limit of compressor degradation the look up table 44 may be populated with several sets of baseline compressor values as a function of underlying compressor operating parameters the level and detailed nature of frequency variation for a baseline compressor is known a priori as a function of the underlying compressor operating parameters which provides a basis for inferring the health of compressor 14 the difference between measured precursor magnitude s and the baseline stall measure via existing transfer functions is used to estimate a degraded compressor operating map and a corresponding compressor operability measure is obtained i e operating stall margin is computed to compare to a design target the operability of compressor 14 is then deemed sufficient or not if the compressor.