Gravity field modelling for the Hannover 10 m atom interferometer

Stable gravimetric measurements over timescales from several days to decades are required to provide relevant insight into geophysical processes or to realise a gravimetric reference frame. Users of absolute gravimeters participate in CIPM or RMO key comparisons with a metrological reference in order to monitor the temporal stability of the instruments and determine the bias to that reference. These comparisons provide the reference values of highest accuracy, around 10 nm/s², compared to the calibration against a single gravimeter operated at a metrological institute. The construction of stationary, large scale atom interferometers paves the way towards a new measurement standard in absolute gravimetry used as a reference with a potential stability better than 1 nm/s² at 1 second integration time. At the Leibniz University Hannover, we are currently building such a very long baseline atom interferometer with a 10 m high vertical free fall zone. Additionally, a 3D model of the instrument and its environment is adapted to derive the change of gravity due to the setup of the instrument. The model is then compared to episodic gravimetric measurements. The knowledge of local gravity and its gradient along and around the baseline is required to establish the instrument's uncertainty budget and enable transfers of gravimetric measurements to nearby devices for comparison and calibration purposes. We report on the progress of the gravimetric measurements and modelling of g inside and near the instrument in parallel to the construction of the atom interferometer