Investigations on the Scale Factor Readout of the Laser Interferometer aboard NGGM

The GRACE-FO mission was launched in 2018 and has been providing valuable data since then. However, developments for future missions have been initiated: the US-German GRACE-C Mission with a planned launch in 2028 and the ESA-led Next Generation Gravity Mission (NGGM) with a launch window around 2031. Both use laser interferometers as their primary and only inter-satellite ranging instruments. Accurate knowledge of the laser wavelength, or equivalently the laser frequency, is required to convert the measured phase to a range with a unit of meters. In GRACE-FO, this conversion (or scale) factor can be determined with the main microwave ranging instrument. However, future missions will require a dedicated wavelength or scale factor readout. Additional phase modulations to be applied to the laser light, in parallel with the conventional cavity locking scheme, have been proposed by the Australian National University (ANU) to achieve such a readout. This technique is now being implemented as a novel Scale Factor Unit (SFU) for the Laser Ranging Interferometer (LRI) onboard the GRACE-C mission, and is also being investigated for the European Laser Tracking Instrument (LTI) of NGGM. In this talk we will explain the measurement principle, show first results from a breadboard setup of our Scale Factor Measurement System (SFMS) and give an outlook on the next steps towards an implementation for the NGGM LTI.