  Most cosmic microwave background experiments observe the sky along circular
or near-circular scans on the celestial sphere. For such experiments, we show
that simple linear systems connect the Fourier spectra of temperature and
polarization time-ordered data to the harmonic spectra of T, E and B on the
sphere. We show how this can be used to estimate those spectra directly from
data streams. In addition, the inversion of the linear system that connects
Fourier spectra to angular power spectra offers a natural way to down-weight
those modes of observation most contaminated by low-frequency noise, ground
pickup, or fluctuations of atmospheric emission on large angular scale. This
can be of interest for the analysis of future CMB data sets, as an alternative
or in complement to other approaches that involve map-making as a first
analysis step.
