Towards a European Instrument Control Unit for the Laser Interferometer aboard NGGM

The ESA-led Next Generation Gravity Mission, as the successor to the GRACE-FO mission, will use a Laser Tracking Instrument (LTI) as its primary and only inter-satellite ranging instrument, which will be developed entirely in Europe. The Instrument Control Unit (ICU), as one development to replace prior US contributions, is expected to acquire range information through phase readout of the laser interference, control the Optical Bench Electronics, stabilize the laser frequency, and communicate with the Onboard Computer. As a joint venture development with the Laser Interferometer Space Antenna (LISA) at AEI, an improved algorithm for Differential Wave Front (DWS) sensing is implemented, featuring higher robustness due to independent gain settings for ranging and DWS information. In addition, LTI pointing angles of the optical bench are used as a sensor to control satellite attitude, and the Scale-Factor Measurement System is included in the ICU as a major difference to the GRACE-C mission.

Here we present the ICU Engineering Model (ICU EM) design, its most important performance requirements, and first results from the test campaign, which aims to achieve a two times more ambitious ranging noise requirement by means of phase-tracking with a noise below 40nm/rtHz x NSF(f) and DWS of 5nrad/rtHz x NSF. These measurements require a stabilized 80MHz system clock referenced with less than 50psec/rtHz x NSF(f) to a 10MHz ultra stable oscillator of the spacecraft. The laser frequency is stabilized either to a cavity with an absolute laser frequency accuracy below 20 ppb or locking to the incoming laser frequency with 13MHz offset, depending on the satellite's role.

We specifically address the development and features of the FPGA firmware, which comprises, next to the digital signal processing pipelines for phase tracking, steering mirror, and laser control, also house-keeping and diagnostic functionalities. The first results from electrical and optical testing suggest that the ICU will meet the performance requirements and enable uninterrupted range measurements with sub-nanometer resolution, hence, continue the laser ranging dataset that has been initiated with the GRACE-FO mission.