# Towards NGGM: Laser Tracking Instrument for the Next Generation of Gravity Missions
## 2 LTI Design Principle
### Lessons Learned from GRACE-FO  
The LTI transponder design presented here was based on the GRACE-FO LRI heritage, which the authors already contributed in [9]. Though this instrument was built as a technology demonstrator with reduced requirements on reliability and redundancy, it worked extremely well with almost no unintended interruptions and experienced no degradation after 4 years in orbit. The ranging data were well understood. It was limited at high frequencies (>100 mHz) by the cavity stability to an equivalent ranging noise level of a few 100 picometers per square root of Hertz (pm/\(\sqrt{\text{Hz}}\)).  
Several performance-relevant effects were observed with the LRI, which we considered relevant to improve the NGGM, in addition to the increased redundancy to fulfil the needed reliability as a main instrument. These were mainly the low-frequency tilt-to-length coupling, microshocks of the platform and the absolute laser frequency knowledge:  
* At mHz frequencies, the tilt-to-length coupling caused a ranging noise level of approx. 500 nm/\(\sqrt{\text{Hz}}\). Though the level did not limit the gravity field quality for GRACE-FO since the post-fit residuals were much higher, it needed to be reduced to benefit from the improved foreseen accelerometer performance on the NGGM. The tilt-to-length coupling, given as the product of satellite pointing jitter and coupling factor related to unavoidable residual alignment imperfections, could, in principle, be reduced in post-processing [20]. The satellite pointing angles in yaw and pitch could be measured with star cameras or--more precisely--by the LTI steering mirror angles, which were controlled based on the interferometric DWS signals, while the corresponding coupling factors were determined with maneuvers that were also used to determine the center of-mass location. For the NGGM, we planned to implement this correction, which has not been included in the official GRACE-FO LRI processing thus far. In addition, the NGGM satellite platforms are likely to provide a significantly lower pointing noise and perform the fine pointing based on the steering mirror or DWS signal, further improving the tilt-to-length coupling.
* The GRACE-FO LRI lasers were frequently disturbed by micro-shocks produced when some particular attitude control thrusters were activated. The disturbances yielded non-physical steps in the measured micrometer range, which could be removed to a large extent with some non-trivial post-processing from the range data, a process called deglitching [9]. It is, nonetheless, a high priority that future missions ensure that such phase jumps are suppressed, primarily by reducing the shocks using less noisy thrusters, damping or through modifications to the laser-locking control loop.
* Laser interferometry relied on the readout of the optical phase of the laser light traveling between the satellites. The phase changes had to be converted to a range using the absolute laser frequency, i.e., the wavelength. Although the reference cavity (see Section 3.3 Laser Stabilization Unit) provided high stability of the frequency, its absolute value had to be determined by other means to a high accuracy of 1 to 10 MHz for the 282 THz light in order to constrain the LTI/LRI scale factor. In GRACE-FO, the LRI scale factor was determined with cross-correlating microwave and laser observations. In the NGGM, which did not have a microwave instrument anymore, a dedicated unit was under discussion to measure the absolute frequency (see Section 4).