  The Karlsruhe Tritium Neutrino (KATRIN) experiment will measure the absolute
mass scale of neutrinos with a sensitivity of $\m_{\nu}$ = 200 meV/c$^2$ by
high-precision spectroscopy close to the tritium beta-decay endpoint at 18.6
keV. Its Windowless Gaseous Tritium Source (WGTS) is a beta-decay source of
high intensity ($10^{11}$/s) and stability, where high-purity molecular tritium
at 30 K is circulated in a closed loop with a yearly throughput of 10 kg. To
limit systematic effects the column density of the source has to be stabilised
at the 0.1% level. This requires extensive sensor instrumentation and dedicated
control and monitoring systems for parameters such as the beam tube
temperature, injection pressure, gas composition and others. Here we give an
overview of these systems including a dedicated Laser-Raman system as well as
several beta-decay activity monitors. We also report on results of the WGTS
demonstrator and other large-scale test experiments giving proof-of-principle
that all parameters relevant to the systematics can be controlled and monitored
on the 0.1% level or better. As a result of these works, the WGTS systematics
can be controlled within stringent margins, enabling the KATRIN experiment to
explore the neutrino mass scale with the design sensitivity.

Instrumentation and Detectors