  A method is presented for investigating the periodic signal content of time
series in which a number of signals is present, such as arising from the
observation of multiperiodic oscillating stars in observational
asteroseismology. Standard Fourier analysis tends only to be effective in cases
when the data are perfectly regularly sampled. During normal telescope
operation it is often the case that there are large, diurnal, gaps in the data,
that data are missing, or that the data are not regularly sampled at all. For
this reason it is advantageous to perform the analysis as much as possible in
the time domain. Furthermore, for quantitative analyses of the frequency
content and power of all real signals, it is of importance to have good
estimates of the errors on these parameters. This is easiest to perform if one
can use linear combinations of the measurements. Here such a linear method is
described. The method is based in part on well-known techniques in radio
technology used in every FM radio receiver, and in part on the SOLA inverse
method
