Figure shows the auto-spectral density for a signal from an accelerometer which was attached to the front body of a car directly above its front suspension while it was driven at 60 mph round a test track used for ride assessment and optimisation.   The signal was calibrated in milli-g (mg) units, i.e. 1 unit is 0.00981 m/s².

a)   Discuss the prominent features of the response.                                      

b) Estimate the rms value of the measured acceleration and the maximum acceleration you would expect in a five-minute recording of the signal.   State any assumptions that you make and comment on the validity of any assumptions.

c)  How would you check if the process that produced the signal is stationary?

d) How would you check if the process that produced the signal is ergodic?