  Least-squares frequency switching (LSFS) is a new method to reconstruct
signal and gain function (known as bandpass or baseline) from spectral line
observations using the frequency switching method. LSFS utilizes not only two
but a set of three or more local oscillator (LO) frequencies. The
reconstruction is based on a least squares fitting scheme. Here we present a
detailed investigation on the stability of the LSFS method in a statistical
sense and test the robustness against radio frequency interference (RFI),
receiver gain instabilities and continuum sources. It turns out, that the LSFS
method is indeed a very powerful method and is robust against most of these
problems. Nevertheless, LSFS fails in presence of RFI signals or strong line
emission. We present solutions to overcome these limitations using a flagging
mechanism or remapping of measured signals, respectively.
