Status of Laser Ranging Interferometry for GRACE-C and NGGM

The Laser Ranging Interferometer (LRI) onboard GRACE-FO has successfully demonstrated the technology that enables biased range observations with noise levels in the nanometer/sqrt(Hz) range or even below. The telemetry of this instrument has been analyzed in great detail and many lessons have been learned during instrument development and data analysis. The two upcoming gravimetric satellite missions, GRACE-C and NGGM, will both use laser-based systems as their primary and only ranging instruments.

We present the current status of the laser ranging interferometry of NGGM and GRACE-C, highlighting the redundancy implementation concept and some of the recent design changes and optimizations. We argue that with the new firmware already tested on GRACE-FO and the fact that a different type of thruster will be used on GRACE-C, the phase jump perturbations (seen on GRACE-FO) should not occur when using the attitude control thruster. The slightly modified timing architecture and the use of a different oscillator are also acceptable changes that will affect the data below other dominant noise levels, such as laser frequency noise. In addition, both future missions include a novel readout scheme for the absolute laser frequency, i.e. the scale factor of the LRI, since it can no longer be estimated by cross-correlation of KBR and LRI.

We will discuss some of the differences between the instruments of the US-German GRACE-C and the all-European NGGM baseline, with special emphasis on the instrument control unit of the NGGM laser tracking instrument currently under development.