The GRACE-Continuity (GRACE-C) mission project is an element of the Earth System Observatory (ESO) within NASA’s Science Mission Directorate, and a follow-on project and a collaboration between NASA’s Jet Propulsion Lab and Deutsches Zentrum für Luft und Raumfahrt (DLR, the German space agency). The GRACE-C mission will ensure the continuity of the monthly record of changes in the Earth’s gravitational field with resolution and temporal sampling consistent with its predecessor missions GRACE1 and GRACE-FO2. The GRACE-C satellite system and mission design share significant heritage with GRACE-FO, with a few exceptions including the Laser Ranging Interferometer (LRI) instrument being the primary (and redundant) satellite-to-satellite (SST) ranging instrument, the absence of Microwave Instrument (MWI) and new lower-shock attitude control thrusters. A Podrix GNSS receiver from Beyond Gravity will also replace the former BlackJack GPS receiver which was onboard of both GRACE and GRACE-FO satellites. Each satellite of the GRACE-C pair also carries a set of attitude sensors identical to those of GRACE-FO which comprise three Star Tracker (STR) units and an angular rate-sensing Inertial Measurement Unit (IMU). At the center of mass of each satellite a precise accelerometer (ACC), identical to those of GRACE-FO, will measure linear and angular accelerations from non-gravitational forces. For SST gravity missions, it is important to consider that the satellites are the instrument. The extraction of a time-varying gravity from the intersatellite ranging observation entirely rest on the assumption that this range exactly represents the instantaneous distance between the two satellites’ centers of mass and that observations are done at their center of mass and in the direction of the line connecting them, or line-of-sight (LoS). The validation of the alignments and positions of the various sensors is therefore an essential consideration for science.
Positions and alignments of subsystems will be verified on ground as part of the pre-launch spacecraft assembly procedures. However, due to limitations of the on-ground procedures, vibrations during launch and the operational consumption of fuel which continuously modify the distribution of the mass within the spacecraft, the ground-calibrated positions and alignments need to be verified in orbit. The GRACE-C Science Data System (SDS) team has the responsibility to develop an in-orbit maneuver calibration plan and is tasked with performing the analysis of the in-orbit calibrations for the verifications of the necessary positions and alignments within the precision requirements defined by the project. Four types of calibration maneuvers will be implemented as ?part of this plan: a) the Quaternion from Star tracker to Accelerometer (QSA) calibration; b) the Center of Mass (CoM) calibration; c) the Tilt-To-Length (TTL) coupling factors calibration; and d) the thruster plateau calibration. The QSA-Cal will verify the alignment between STR units and  the ACC frame; the CoM-Cal will verify the position of the spacecraft CoM with respect to the center of the ACC instrument (where its proof mass is located); the TTL-Cal will verify the position of the LRI triple mirror assembly (TMA) vertex with respect to the spacecraft CoM; finally the thruster plateau calibration verifies the ACC response to the attitude thrusters. All four maneuvers are to be performed during the first 90 days following launch that make the In-Orbit Checkout (IOC) phase of the mission. After that, all but the QSA cal will be repeated at regular intervals during the mission: 4 times per year for the CoM-Cal and TTL-Cal, concurrently; once per year for the thruster plateau calibration.
All four maneuver types are entrenched in heritage from GRACE and GRACE-FO, though only the CoM-Cal and the QSA-Cal were part of the previous missions’ concepts of operation. Because the LRI – which flew for the first time on GRACE-FO – was a tech demo instrument and not a primary instrument, no in-orbit calibration (other than laser acquisition) was designed ahead of launch for the LRI. The calibration of the ACC response to the attitude thruster was made necessary for GRACE-FO, post-launch, when it became obvious that GRACE-FO ACC sampling and frequency response did not allow to accurately observe the acceleration imparted by the attitude thrust firings, introducing in the ACC observations fictitious non-gravitational accelerations. GRACE-C accelerometers being identical to those of GRACE-FO, it is expected that a calibration of their response will be necessary.