# Towards NGGM: Laser Tracking Instrument for the Next Generation of Gravity Missions
## 3 LTI Units
### Instrument Control Unit (ICU)/Phasemeter  
The ICU constitutes the core digital signal processing and control unit of the LTI. The ICU's primary function is to extract nanometer-stable translation and nanoradian-stable tilt measurements from the interferometric signals of the two optical beams superimposed on the optical bench. Gravity field information was encoded as tiny phase fluctuations in the translation measurements, which was filtered, decimated and formatted in science packets by using the ICU before streaming to the on-board-computer (OBC) for downlink and on-ground data post-processing afterwards. The pitch and yaw tilt measurements based on the DWS signal were used in-flight to point the laser beam to the remote spacecraft with high precision, either in a fast steering mirror control loop [21; 22] or by actuating the whole satellite platform. The ICU was also incorporated as the function to lock the laser head to a Fabry-Perot Cavity resonance by implementing a Pound-Drever-Hall (PDH) scheme [23], in which the phase measurement was implemented via self-heterodyning the back-reflected light with spectral sidebands on the light coupled into the cavity. These sidebands were imprinted via phase modulation in a dedicated electro-optical modulator. This allowed to suppress the laser frequency noise of the LTI down to the requirement described in [19]. To keep a stable optical link performance, and after conducting the laser link acquisition procedures [19; 24], the ICU on the transponder satellite fed back the beat note phase information into an internal frequency controller. The controller actuated the laser internal piezo actuator and thermal loops in a laser offset phase-locking scheme, operating the transponder laser head with a configurable loop bandwidth of, typically, 10 kHz [25; 26]. Figure 4 shows the baseline design of the physical assembly of the ICU architecture with its different modules, all-including internal redundancy for a high unit reliability over  
the mission's lifetime. The ICU consisted of three mechanical frames, housing the following electronics:  
**Back-End Electronics (BEE). ** It houses three major features, the optical bench electronics (OBEs), the cavity lock electronics and the phasemeter digital core electronics. The OBEs implemented the variable gain controllers and anti-aliasing filters to adapt the quadrant photo receiver output signals to the optimum voltage levels required by the analog-to-digital converters. The OBEs also implemented fast-steering mirror control electronics with a two-axis readout and control signals for motor actuation for beam pointing to the remote S/C. The cavity lock electronics implemented the signal conditioning electronics, digitization and actuation electronics for the optical cavity lock control. The phasemeter digital core electronics implemented the digital signal core processing of all fast-digital control loops with stringent requirements in latency based on an FPGA core (e.g., digital phase-locked loops for an interferometric amplitude and phase readout, Pound-Drever-Hall logic for cavity-lock control and fast Fourier transform to support laser link acquisition procedures). It also implemented diagnostic functions for temperature monitoring and debugging functions. **Data Handling and Laser Control Module. ** It housed two major boards, the instrument control module (ICM) and the laser monitoring and control electronics. The ICM implemented the microprocessor capabilities for unit operation, timing, synchronization and communication with the platform, as well as digital control for laser offset phase locking. The ICM board also managed diagnostic functions for temperature monitoring of the laser. The laser monitoring and control implemented functions such as an ON/OFF switch, optical power monitoring and diagnostics. **Power Supply Module (for power supply and internal distribution to ICU modules). This electronic device received a power supply from the satellite platform.