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We present the rationale for and the observational description of ASPECS: TheALMA SPECtroscopic Survey in the Hubble Ultra-Deep Field (UDF), thecosmological deep field that has the deepest multi-wavelength data available.Our overarching goal is to obtain an unbiased census of molecular gas and dustcontinuum emission in high-redshift (z$>$0.5) galaxies. The $\sim$1$'$ regioncovered within the UDF was chosen to overlap with the deepest available imagingfrom HST. Our ALMA observations consist of full frequency scans in band 3(84-115 GHz) and band 6 (212-272 GHz) at approximately uniform line sensitivity($L'_{\rm CO}\sim$2$\times$10$^{9}$ K km/s pc$^2$), and continuum noise levelsof 3.8 $\mu$Jy beam$^{-1}$ and 12.7 $\mu$Jy beam$^{-1}$, respectively. Themolecular surveys cover the different rotational transitions of the COmolecule, leading to essentially full redshift coverage. The [CII] emissionline is also covered at redshifts $6.0<z<8.0$. We present a customizedalgorithm to identify line candidates in the molecular line scans, and quantifyour ability to recover artificial sources from our data. Based on whethermultiple CO lines are detected, and whether optical spectroscopic redshifts aswell as optical counterparts exist, we constrain the most likely lineidentification. We report 10 (11) CO line candidates in the 3mm (1mm) band, andour statistical analysis shows that $<$4 of these (in each band) are likelyspurious. Less than 1/3 of the total CO flux in the low-J CO line candidatesare from sources that are not associated with an optical/NIR counterpart. Wealso present continuum maps of both the band 3 and band 6 observations. Thedata presented here form the basis of a number of dedicated studies that arepresented in subsequent papers.

ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Survey Description