Antihydrogen, the lightest atom consisting purely of antimatter, is an ideal laboratory to study the CPT symmetry by comparison to hydrogen. With respect to absolute precision, transitions within the ground-state hyperfine structure (GS-HFS) are most appealing by virtue of their small energy separation. ASACUSA proposed employing a beam of cold antihydrogen atoms in a Rabi-type experiment to determine the GS-HFS in a field-free region. Here we present a measurement of the zero-field hydrogen GS-HFS using the spectroscopy apparatus of ASACUSAs antihydrogen experiment. The measured value of $ u_mathrm{HF}$=$1~420~405~748.4(3.4)(1.6)~textrm{Hz}$ with a relative precision of $Delta$$ u_mathrm{HF}$/$ u_mathrm{HF}$=$2.7times10^{-9}$ constitutes the most precise determination of this quantity in a beam and verifies the developed spectroscopy methods for the antihydrogen HFS experiment to the ppb level. Together with the recently presented observation of antihydrogen atoms $2.7~textrm{m}$ downstream of the production region, the prerequisites for a measurement with antihydrogen are now available within the ASACUSA collaboration.
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