Multirate Sampling and Generalized Alias as a Source of Errors in Magnetometer Experiments

It is customary to time share or commutate the return data link from spacecraft. This means that the data transmission is periodically interrupted. Sampling of time series data under these conditions results in violation of the fast rate Nyquist limit with the consequent folding of the spectra density distribution function of the data unless the presample filter bandwidth is determined by the commutation frequency. In the time domain, the consequence can be to replicate components of the spectrum about various frequencies including zero. These conditions are applied to a hypothetical magnetometer experiment and it is shown that, depending upon the frequency content, the data can be intrinsically aliased at the commutation switching rate of the telemetry system and, further, second order aliased in the data reduction sampling procedure. The presence of the doubly aliased time series for each of the magnetic field components can lead to potentially serious errors depending upon the form of the spectral amplitude functions which, in turn, are unspecifiable if folded.