Description: As part of the EU-funded joint research project "Metrology for low-frequency sound and vibration - 19ENV03 Infra-AUV" laboratory calibration methods for seismometers in the low frequency range down to 0.01 Hz have been developed. These procedures provide the possibility of traceable on-site calibration during operation for field sensors without disturbing the regular measurements and allow an accurate determination of amplitude and phase along with measurement uncertainties. This is of special interest for stations of the Comprehensive Nuclear-Test-Ban Treaty Organization’s (CTBTO) International Monitoring System (IMS), which are required to be operational 100% of the time.The on-site calibration procedure requires a reference sensor co-located to the operational station sensor and the record of sufficient coherent excitation signals within the relevant frequency range. The gain ratio between the station sensor under test and the reference sensor is calculated and subsequently multiplied with the precise frequency response of the reference provided by the newly developed laboratory calibration methods, yielding the frequency response function for both amplitude and phase of the station sensors including site-specific factors.We present results of on-site calibration tests performed at the seismic IMS station PS19 in Germany using reference seismometers calibrated by laser interferometry in the laboratory. The results deviate less than 5% from the nominal response function for frequencies between 0.01 Hz and 20 Hz for all components. The traceable determination of the response for the individual components in detail improves the sensor and data quality; subsequently waveform amplitudes can be estimated correctly.