  A long-term multi-purpose observational programme has started with
HARPS-N@TNG aimed to characterise the global architectural properties of
exoplanetary systems. In this first paper we fully characterise the transiting
system Qatar-1. We exploit HARPS-N high-precision radial velocity measurements
obtained during a transit to measure the Rossiter-McLaughlin effect in the
Qatar-1 system, and out-of-transit measurements to redetermine the
spectroscopic orbit. New photometric transit light-curves are analysed and a
spectroscopic characterisation of the host star atmospheric parameters is
performed based on various methods (line equivalent width ratios, spectral
synthesis, spectral energy distribution). We achieved a significant improvement
in the accuracy of the orbital parameters and derived the spin-orbit alignment
of the system; this information, combined with the spectroscopic determination
of the host star properties, allows us to derive the fundamental physical
parameters for star and planet (masses and radii). The orbital solution for the
Qatar-1 system is consistent with a circular orbit and the system presents a
sky-projected obliquity of lambda = -8.4+-7.1 deg. The planet, with a mass of
1.33+-0.05 M_J, is found to be significantly more massive than previously
reported. The host star is confirmed to be metal-rich ([Fe/H]= 0.20+-0.10) and
slowly rotating (vsinI = 1.7+-0.3 km/s), though moderately active, as indicated
by strong chromospheric emission in the Ca II H&K line cores (logR'_HK about
-4.60). The system is well aligned and fits well within the general lambda vs
Teff trend. We definitely rule out any significant orbital eccentricity. The
evolutionary status of the system is inferred based on gyrochronology, and the
present orbital configuration and timescale for orbital decay are discussed in
terms of star-planet tidal interactions.
