# Towards NGGM: Laser Tracking Instrument for the Next Generation of Gravity Missions
## 3 LTI Units
### Retroreflector Unit (RRU)  
The retroreflector unit consisted of an off-axis three-mirror retroreflector with its vertex located in the satellite center of mass. It routed the transmitted beam from the optical bench to the other spacecraft. Placing the vertex of each RRU at the satellite CoM enabled the LTI to measure the distance variation of the CoM of the two satellites to a nanometer accuracy without the need to actually have a physical mirror in the CoM, which is usually occupied by the accelerometer test mass.  
Key requirements for the RRU were a beam co-alignment error of less than half the beam divergence (the requirement placed on the RRU was 40 \(\upmu\)rad), a wavefront distortion of less than \(\lambda/15\) per surface over two times the beam diameter and an optical path length change/vertex movement of less than some hundred nm/K.  
In GRACE-FO, the lateral beam separation required of the retroreflector was 600 mm, driven by the diameter of the cold gas tank. For the NGGM, the beam separation would likely be in the order of 500 mm. Figure 10a shows, for reference, the RRU of the GRACE-FO LRI, consisting of a highly stable lightweight hybrid structure comprising a carbon-fiber (CFRP) and titanium frame and two bonded zerodur mirror assemblies. While this design met all optical performance requirements of the LTI, it would be subjected to higher environmental loads on the NGGM than one GRACE-FO. An adaptation of the design was, thus, necessary to mitigate the higher loads, in addition to implementing a different beam separation and mounting interface. The concept of the modified RRU is shown in Figure 10b.  
Figure 9: Optical bench concept for NGGM. (**a**) Cross-sectional view of the optical bench; (**b**) CAD picture of optical bench body with attached collimators, steering mirror and quadrant photoreceivers.  
The RRU at first glance is a relatively simple structure, but it is not. To achieve the demanding co-alignment and ranging performance over the full mission duration, stringent requirements were placed on the CFRP tube with respect to the CTE homogeneity, the twist and tilt of the tube endfaces with a moisture release and temperature variation, the CFRP-to-zerodur interface stability, as well as on the zerodur-to-zerodur interfaces.  
To achieve the CTE homogeneity, twist and tilt of the CFRP, the basic CFRP layer design both needed to be specifically optimized and special care had to be taken to ensure that the fiber-matrix ratio was kept homogeneous over the diameter. The CFRP-to-zerodur interfaces needed to be CTE-compensated to avoid thermal stress, leading to tilting, surface deformation and potentially cracking. The zerodur-to-zerodur interfaces (seven in total) were all optically contacted and, then, after the required alignment accuracy was achieved through the iteration of the optical contacting and potentially re-polishing, they were reinforced by injecting glue in dedicated glue pockets in the optically contacted surfaces. This approach enabled to meet the challenging co-alignment and ranging noise requirements of the overall structure and survive the launch loads.  
With the GRACE-FO heritage, where this technology was successfully applied and is still working perfectly in orbit, a TRL of six was assessed. The design modifications focused on the different beam spacing, the higher launch loads as well as the fine-tuning of the various interfaces.