## 4 Instrument Sensitivity, Accelerometer and Drag Compensation Assessment
### The Laser Tracking Instrument Preliminary Design  
The preliminary SST performances in Table 5 (top) can an achieved with a laser tracking instrument (LTI), which measures the intersatellite distance variation with a resolution of a few nanometers. The LTI will be a heterodyne Michelson interferometer, which is particularly suitable for measurements over very long distances and operates with continuous wave sources at 1064 nm wavelength (282 THz). As opposed to GRACE-FO, the LTI will be the primary payload of NGGM and specifically designed to match the laser metrology performance, which means that the ultimate performance is due to the instrument and not to other effects, such as a nonoptimal accommodation, environmental effects (e.g., temperature fluctuations) and the dynamic effects of residual air drag. At present, two interferometer schemes are under evaluation for MAGIC/NGGM. The first is a transponder scheme inherited from the GRACE-FO LRI [48; 49]. In such an interferometer (see Figure 11), the laser beam transmitted by the follower satellite (Satellite 2) is received by the leader satellite (Satellite 1) where it is \"regenerated\" by a second laser source, phase-locked with a frequency offset (heterodyne frequency) to the incoming beam, and retransmitted to Satellite 2. In the optical transponder scheme, a source with limited optical power output of approximately 25 mW, provided directly by the master oscillator, is sufficient to achieve the required signal-to-noise ratio on the photoreceiver. On the other hand, two laser sources must be active simultaneously, one on each of the satellites. In the second interferometer concept, the optical transponder is replaced by a passive retroreflector on Satellite 1, which intercepts the laser beam transmitted by Satellite 2 and reflects it back. Here, an acousto-optic modulator on Satellite 2 generates the heterodyne frequency, and also the photoreceivers detect the two beat signals produced by the interference of the laser beams: the combination of the photoreceiver outputs produces a sinusoidal signal with a phase proportional to the intersatellite distance variation. The retroreflector scheme (Figure 12) requires a source with a larger optical power output of approximately 500 mW, which is provided by a fiber amplifier stage after the master oscillator of the same power and quality as in the transponder scheme. In the retroreflector configuration, the acquisition of the optical link between the satellites is significantly simplified: it is sufficient to illuminate Satellite 1 with the laser to obtain the return beam, and no laser frequency scan is necessary to bring the beat signal within the photoreceiver bandwidth. Moreover, by replicating all the interferometer elements on both satellites, these can be made identical, thus realizing a functionally fully redundant system. In case of failure of the active part on Satellite 2, the position of the two satellites along the orbit can be swapped, keeping the same orientation, and the measurement can continue with the interferometer active on Satellite 1. All these features reduce the system complexity and increase its robustness, which are key aspects for an operational gravity mission and motivate a trade-off with the flight-proven optical transponder scheme [50]. For both concepts, the intersatellite distance variation measurement performance has been assessed via a bottom-up approach in order to verify the compliance to the top-level requirements in Table 5. For very low orbits of approximately 350 km and intersatellite distances of 100 km, the LTI has to reach an accuracy better than 20 nm/\(\surd\)Hz (threshold) and 10 nm/\(\surd\)Hz (goal) in the measurement bandwidth.