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The derivation of radial velocities from large numbers of spectra thattypically result from survey work, requires automation. However, except for theclassical cases of slowly rotating late-type spectra, existing methods ofmeasuring Doppler shifts require fine-tuning to avoid a loss of accuracy due tothe idiosyncrasies of individual spectra. The radial velocity spectrometer(RVS) on the Gaia mission, which will start operating very soon, prompted a newattempt at creating a measurement pipeline to handle a wide variety of spectraltypes.  The present paper describes the theoretical background on which this softwareis based. However, apart from the assumption that only synthetic templates areused, we do not rely on any of the characteristics of this instrument, so ourresults should be relevant for most telescope-detector combinations.  We propose an approach based on the simultaneous use of several alternativemeasurement methods, each having its own merits and drawbacks, and conveyingthe spectral information in a different way, leading to different values forthe measurement. A comparison or a combination of the various results eitherleads to a "best estimate" or indicates to the user that the observed spectrumis problematic and should be analysed manually.  We selected three methods and analysed the relationships and differencesbetween them from a unified point of view; with each method an appropriateestimator for the individual random error is chosen. We also develop aprocedure for tackling the problem of template mismatch in a systematic way.Furthermore, we propose several tests for studying and comparing theperformance of the various methods as a function of the atmospheric parametersof the observed objects. Finally, we describe a procedure for obtaining aknowledge-based combination of the various Doppler-shift measurements.

A multi-method approach to radial-velocity measurement for single-object spectra