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The time-variable velocity fields of solar-type stars limit the precision ofradial-velocity determinations of their planets' masses, obstructing detectionof Earth twins. Since 2015 July we have been monitoring disc-integratedsunlight in daytime using a purpose-built solar telescope and fibre feed to theHARPS-N stellar radial-velocity spectrometer. We present and analyse the solarradial-velocity measurements and cross-correlation function (CCF) parametersobtained in the first 3 years of observation, interpreting them in the contextof spatially-resolved solar observations. We describe a Bayesian mixture-modelapproach to automated data-quality monitoring. We provide dynamical and dailydifferential-extinction corrections to place the radial velocities in theheliocentric reference frame, and the CCF shape parameters in the siderealframe. We achieve a photon-noise limited radial-velocity precision better than0.43 m s$^{-1}$ per 5-minute observation. The day-to-day precision is limitedby zero-point calibration uncertainty with an RMS scatter of about 0.4 ms$^{-1}$. We find significant signals from granulation and solar activity.Within a day, granulation noise dominates, with an amplitude of about 0.4 ms$^{-1}$ and an autocorrelation half-life of 15 minutes. On longer timescales,activity dominates. Sunspot groups broaden the CCF as they cross the solardisc. Facular regions temporarily reduce the intrinsic asymmetry of the CCF.The radial-velocity increase that accompanies an active-region passage has atypical amplitude of 5 m s$^{-1}$ and is correlated with the line asymmetry,but leads it by 3 days. Spectral line-shape variability thus shows promise as aproxy for recovering the true radial velocity.

Three years of Sun-as-a-star radial-velocity observations on the approach to solar minimum