# Towards NGGM: Laser Tracking Instrument for the Next Generation of Gravity Missions
## 5 Conclusions  
Gravity missions, such as GOCE, GRACE and GRACE-FO, provide essential data to monitor Earth's system, specifically valuable in our aim to better understand the mechanisms of climate change. The precise tracking of the distance between pairs of satellites is key to provide required data, and the technology to enable this by means of laser interferometry is currently under development. The technology demonstrator of the laser ranging interferometer on GRACE-FO already works extremely well in orbit, since more than 4 years have passed with very few interruptions and far superior ranging performance compared to the microwave ranging instrument. Future missions, such as the NGGM, aim to significantly improve performance by combining the improved ranging performance of the laser interferometry with improved accelerometers and platform performance, and ideally the implementation of at least two pairs of satellites. The transfer of the laser ranging interferometer from a technology demonstrator to a main instrument with higher performance and reliability is pursued by the ESA with the pre-development activity of the so-called laser tracking instrument--LTI. In addition, independent from the current programmatic baseline, which foresees work share between Europe and the US with hardware contributions from both sides to both satellite pairs, the aim of this activity is also to gain the capability to implement the LTI with only European suppliers for all units in case it should be needed. After a detailed trade between different instrument concepts over recent years, the off-axis transponder concept LTI was selected for the NGGM. With the existing implementation of this concept as a demonstrator on GRACE-FO, a lot of knowledge and technology are already available. The requirements and required technologies for the mission and the individual units are well understood, enabling a clear development plan towards implementation in the NGGM. On GRACE-FO, the tilt-to-length coupling, the thruster shocks and the absolute laser frequency knowledge (relevant to derive the scale factor between the phase and range) limited the performance of the LRI. For the NGGM, it is, therefore, foreseen that the tilt-to-length coupling needs to be reduced with a reduced satellite pointing noise and tilt-to-length calibration of the LTI. Linear thrusters would eliminate the thruster shocks and the absolute laser frequency knowledge is foreseen to be improved by an order of magnitude either by using a dedicated scale factor measurement system or dedicatedpost-processing. With these modifications the LTI is expected to meet all NGGM mission requirements. The unit level design as well as the LTI system engineering activities are progressing in accordance with the timeline of potential mission prime contractors. The focus of the development of the units, which we already provided for GRACE-FO, lies in the necessary modifications for increased reliability and mechanical loads. Increased redundancies, be it full redundancy on a unit level, as foreseen in the fully redundant LTI configuration or partial redundancy, e.g., with respect to photoreceivers, beam collimators and steering mirror electronics on the optical bench, as well as increased parts quality for some electronics, were considered and implemented in the design. The intent was to keep what worked well on GRACE-FO and limit the modifications to what was needed for reliability and changed the mechanical loads. The ICU design was based on the LISA developments and was a newly developed unit for the NGGM. The laser remained the same as that used for GRACE-FO, with minor internal modifications. The foreseen laser stabilization unit was from the OSRC development, which ran in parallel.