  The SCUBA HAlf Degree Extragalactic Survey (SHADES) is a major blank-field
extragalactic sub-mm survey underway at the James Clerk Maxwell
telescope.SHADES aims to cover half a square degree at 450+850microns to a
4sigma depth of ~8mJy at 850microns. Two fields are being observed, the
Subaru/XMM--Newton Deep Field (SXDF) and the Lockman Hole East. The survey has
3 main aims to investigate: i) the cosmic history of massive dust enshrouded
star-formation activity, ii) the clustering properties of sub-mm-selected
galaxies in order to determine whether these objects could be progenitors of
present-day massive ellipticals, and iii) the fraction of sub-mm-selected
sources that harbour AGN. To achieve these aims requires the sub-mm data be
combined with co-spatial information spanning the radio-to-X-ray frequency
range, the resulting extensive multi-wavelength dataset providing complete
photometric redshift information accurate to delta z <~0.5 and detailed SEDs
for the vast majority of the sub-mm-selected sources. In this paper, the first
of a series on SHADES, we present an overview of the motivation for the survey,
describe the survey strategy and provide a detailed description of the primary
data analysis pipeline. As of Feb. 2004, 720arcmin^2 had been mapped with SCUBA
(about 40% of the anticipated final total) to a median 1sigma depth of 2.2mJy
per beam at 850microns (25mJy per beam at 450microns), and a source density of
650+/-50 sources/deg^2 >3sigma at 850microns, uncorrected for Eddington bias. A
refined re-analysis of the original 8-mJy survey Lockman hole data was carried
out in order to evaluate the new data reduction pipeline. Of the 17 most secure
sources in the original sample, 12 have been re-confirmed, including 10 of the
11 for which radio IDs were previously secured.(Abridged)

Astrophysics