# Towards NGGM: Laser Tracking Instrument for the Next Generation of Gravity Missions  
[PERSON]  
1 Spacetech GmbH, Seelbachstr. 13, D-88090 Immenstaad, Germany1  
[PERSON]  
1 Spacetech GmbH, Seelbachstr. 13, D-88090 Immenstaad, Germany1  
[PERSON]  
1 Spacetech GmbH, Seelbachstr. 13, D-88090 Immenstaad, Germany1  
[PERSON]  
1 Spacetech GmbH, Seelbachstr. 13, D-88090 Immenstaad, Germany1  
[PERSON]  
1 Spacetech GmbH, Seelbachstr. 13, D-88090 Immenstaad, Germany1  
[PERSON]  
1 Spacetech GmbH, Seelbachstr. 13, D-88090 Immenstaad, Germany1  
[PERSON]  
1 Spacetech GmbH, Seelbachstr. 13, D-88090 Immenstaad, Germany1  
[PERSON]  
1 Spacetech GmbH, Seelbachstr. 13, D-88090 Immenstaad, Germany1  
[PERSON]  
1 Spacetech GmbH, Seelbachstr. 13, D-88090 Immenstaad, Germany1  
[PERSON]  
1 Spacetech GmbH, Seelbachstr. 13, D-88090 Immenstaad, Germany1  
[PERSON]  
studied and developed in the context of a next-generation gravity mission (NGGM) activity under the lead of the European Space Agency (ESA) and in the context of the GRACE-I(carus)/mass change mission (MCM) as a continuation for GRACE-FO lead by a US-German collaboration. Studies are ongoing on the optimal constellation of these novel satellite pairs with discussions between the ESA, NASA and the German Aerospace Agency DLR in order to maximize the scientific benefit of flying at least two operational satellite pairs with some overlap time, with GRACE-I/MCM potentially being the first pair and the NGGM the second pair. High priority is assigned to a so-called 'Bender configuration' [10], which comprises a polar and an inclined pair. Such a configuration has shown an enhanced spatio-temporal resolution of the resulting gravity field maps in simulations [11; 12]. In accordance with gravity simulations, the satellite pairs of the upcoming missions are expected to have a satellite separation between 100 and 300 km and to be operated at an altitude between 300 and 500 km. The distance variations between both satellites in a NGGM or GRACE-like pair are precisely measured by means of a heterodyne laser interferometer, a laser tracking instrument (LTI), with nanometer ranging precision together with many other observations, such as attitude, non-gravitational accelerations, GNSS and others. Currently, the monthly gravity maps of GRACE-FO are mainly limited by accelerometer noise and background model errors, which alias short-term (<30 day) mass variations into the monthly solution. Hence, monthly data products derived from either MWI or LRI have shown only very few differences in GRACE-FO [13; 14], since the ranging data quality is not the limiting aspect. However, the low noise of the laser ranging systems allows for the use of post-fit residuals, i.e., the signal that cannot be absorbed by the monthly model, for the detection of short-term mass variations [15; 16; 17]. Exemplary post-fit residual maps derived with software from GeoForschungsZentrum (GFZ), Potsdam, using the RL06 processing standard [2], are shown in Figure 1, obtained as range accelerations by differentiating a time-series of range-rate post-fit values. The figure highlights the low-noise of the laser-based ranging compared to the MWI. This low-noise ranging could be exploited to a larger extent or might even be needed for gravity maps of the NGGM/future missions, considering that multiple pairs could mitigate temporal aliasing effects, background models could steadily continue to advance and the accelerometer noise and satellite platform stability could improve. Figure 1: The time derivative of range-rate post-fit residuals describe signals that cannot be explained with a monthly gravity field map, i.e., sub-monthly gravity variations, instrument noise (accelerometer, ranging instrument and others) and background model deficiencies. The left plot shows microwave-based residuals, which are more noisy compared to the LRI observations on the right side. The map is based on GRACE-FO data from January 2019.