  The construction of the Cherenkov Telescope Array is expected to start soon.
We will present the baseline methods and their extensions currently foreseen to
calibrate the observatory. These are bound to achieve the strong requirements
on allowed systematic uncertainties for the reconstructed gamma-ray energy and
flux scales, as well as on the pointing resolution, and on the overall duty
cycle of the observatory. Onsite calibration activities are designed to include
a robust and efficient calibration of the telescope cameras, and various
methods and instruments to achieve calibration of the overall optical
throughput of each telescope, leading to both inter-telescope calibration and
an absolute calibration of the entire observatory. One important aspect of the
onsite calibration is a correct understanding of the atmosphere above the
telescopes, which constitutes the calorimeter of this detection technique. It
is planned to be constantly monitored with state-of-the-art instruments to
obtain a full molecular and aerosol profile up to the stratosphere. In order to
guarantee the best use of the observation time, in terms of usable data, an
intelligent scheduling system is required, which gives preference to those
sources and observation programs that can cope with the given atmospheric
conditions, especially if the sky is partially covered by clouds, or slightly
contaminated by dust. Ceilometers in combination with all-sky-cameras are
plannned to provide the observatory with a fast, online and full-sky knowledge
of the expected conditions for each pointing direction. For a precise
characterization of the adopted observing direction, wide-field optical
telescopes and Raman Lidars are planned to provide information about the
height-resolved and wavelength-dependent atmospheric extinction, throughout the
field-of-view of the cameras.
