Cross-Spectrum Measurement Statistics: Uncertainties and Detection Limit

The cross-spectrum method consists in measuring a signal c( t) simultaneously with two independent instruments. Each of these instruments contributes to the global noise by its intrinsic (white) noise, whereas the signal c( t) that we want to characterize could be a (red) noise. We first define the real part of the cross spectrum as a relevant estimator. Then, we characterize the probability density function (pdf) of this estimator knowing the noise level (direct problem) as a Variance-gamma (VG) distribution. Next, we solve the ``inverse problem'' due to Bayes' theorem to obtain an upper limit of the noise level knowing the estimate. Checked bymassiveMonte Carlo simulations, VG proves to be perfectly reliable for any number of degrees of freedom (DOFs). Finally, we compare this method with anothermethod using the Karhunen-Loevetransform(KLT). We find an upper limit of the signal level slightly different as the one of VG since KLT better considers the available information.