[en] The electron stretcher facility ELSA provides an electron beam of a few hundred pA used for the generation of bremsstrahlung photons probing the nucleon structure in a detector setup. For the correct interpretation of the events registered, the persistence of the beam position over time is crucial. Its continuous monitoring has been enabled by setting up a measurement system based on resonant cavities. Position signals at a frequency of 1.5 GHz and below one aW of power can be abstracted from the beam without degrading its quality. After frequency down-conversion to a few kHz, a narrow bandwidth detection performed by lock-in amplifiers separates them from noise. A maximum sample rate of 9 Hz and a resolution of one tenth of a millimeter could be achieved. The position signals have to be normalized to the beam current which is monitored by another dedicated resonator. The measurement precision down to a few pA allows for the accelerator extraction mechanism to be controlled by a feedback loop in order to obtain the respective requested current. (orig.)