The Laser Ranging Interferometer (LRI) onboard GRACE-FO has successfully demonstrated the technology that enables biased range observations with noise levels in the nanometer/sqrt(Hz) range or even below. The telemetry of this instrument has been analyzed in great detail and many lessons have been learned during instrument development and data analysis. The two upcoming gravimetric satellite missions, GRACE-C and NGGM, will both use laser-based systems as their primary and only ranging instruments.

We present the current status of the laser ranging interferometry of NGGM and GRACE-C, highlighting the redundancy implementation concept and some of the recent design changes and optimizations. We argue that with the new firmware already tested on GRACE-FO and the fact that a different type of thruster will be used on GRACE-C, the phase jump perturbations (seen on GRACE-FO) should not occur when using the attitude control thruster. The slightly modified timing architecture and the use of a different oscillator are also acceptable changes that will affect the data below other dominant noise levels, such as laser frequency noise. In addition, both future missions include a novel readout scheme for the absolute laser frequency, i.e. the scale factor of the LRI, since it can no longer be estimated by cross-correlation of KBR and LRI.

We will discuss some of the differences between the instruments of the US-German GRACE-C and the all-European NGGM baseline, with special emphasis on the instrument control unit of the NGGM laser tracking instrument currently under development.

How to cite: Müller, V., Misfeldt, M., Müller, L., Weberpals, M., Nicklaus, K., Voss, K., Bahr, J., and Heinzel, G.: Status of Laser Ranging Interferometry for GRACE-C and NGGM, GRACE/GRACE-FO Science Team Meeting, Potsdam, Germany, 8–10 Oct 2024, GSTM2024-78, https://doi.org/10.5194/gstm2024-78, 2024.

The Mass-Change and Geosciences International Constellation (MAGIC) is planned to consist of two satellite pairs measuring the variations in the Earth’s gravitational field. The first pair launched into a near-polar orbit is planned to be launched in 2028 followed by a second pair to be launched in an inclined orbit no later than 2032. Considering the planned lifetime of seven year, an overlap of 3 years is anticipated. However, it is possible that the first pair is failing due to unforeseen circumstances before the launch or during the early phase of the second pair. The second pair will thus the only source for gravity field recovery resulting in an incomplete coverage of the Earth.

The usual approach is to constrain the solutions, e.g. by Tikhonov or using a Kaula rule. This study investigates such a worst-case scenario but with a different approach. We consider a single GNSS-equipped satellite in a near-polar orbit that will contribute high-low satellite-to-satellite observations from GNSS to a common gravity field solution. We present the contribution of this observation type to the solution and quantify the degradation compared to the double pair and the improvement compared to a constrained solution.

How to cite: Weigelt, M.: MAGIC – investigating a solution for a worst-case scenario, GRACE/GRACE-FO Science Team Meeting, Potsdam, Germany, 8–10 Oct 2024, GSTM2024-7, https://doi.org/10.5194/gstm2024-7, 2024.

The Gravitational Reference Advanced Technology Test In Space (GRATTIS) mission will demonstrate the functionality and sensitivity performance of the Simplified Gravitational Reference Sensor (S-GRS), an ultra-precise inertial sensor for future Earth geodesy missions. These sensors are used to measure or compensate for all non-gravitational accelerations of the host spacecraft so that they can be removed in the data analysis to recover spacecraft motion due to Earth’s gravity field. Low-low satellite-to-satellite tracking missions like GRACE-FO that utilize laser ranging are technologically limited by the acceleration noise performance of their accelerometers, as well as temporal aliasing associated with Earth’s dynamic gravity field. The S-GRS is estimated to be 10 times more sensitive than the GRACE accelerometer requirement and more than 200 times more sensitive if operated drag-free.

The S-GRS concept is a simplified version of the LISA Pathfinder GRS. It consists of a free-falling cubic test mass (TM) inside an electrode housing that senses the position and orientation of the TM and electrostatically applies forces and torques to keep it centered at a nanometer-level. The applied forces and torques required are also used to determine the non-gravitational forces acting on the host spacecraft, as well as the spacecraft’s angular acceleration. The improved performance of the S-GRS relative to the accelerometers used on the GRACE missions is enabled by removing the small grounding wire used in the GRACE accelerometers, which limits its performance, and replacing it with a UV LED-based charge management system, increasing the mass of the sensor’s TM, and increasing the gap between the TM and its electrode housing.

GRATTIS will fly two identical S-GRS mounted next to one another near the center of mass of a 180 kg ESPA-class commercial microsatellite. The six-axis acceleration measurement capability of the S-GRS allows precision measurement of the spacecraft drag-induced translational acceleration, as well as the residual angular acceleration of the nominally inertially-pointed bus. By combining the outputs of each sensor and with the known relative position of the two TMs, we can recover the acceleration sensitivity (noise floor) of the S-GRS. Our mission goal is to demonstrate acceleration noise performance of ≤10^–11 m/s²Hz^1/2.

The PI and science team is led by the University of Florida (UF) and includes relevant experts from Texas A&M University (TAMU) and CrossTrac Engineering. The S-GRS mechanical sensor heads are provided by BAE Space & Mission Systems (formerly Ball Aerospace), while the S-GRS electronics units are provided by Fibertek, Inc. CrossTrac Engineering provides the S-GRS software and program management. The UF-led team will integrate the flight payload into a single thermal/mechanical enclosure and perform ground testing to the extent possible. Apex Space will provide the Aries microsatellite bus and launch services via a SpaceX F9 Transporter mission planned for Q1 2027. This presentation will describe the S-GRS technology development and planned GRATTIS demonstration mission.

How to cite: Conklin, J. and the The GRATTIS team: GRATTIS: The Gravitational Reference Advanced Technology Test In Space, GRACE/GRACE-FO Science Team Meeting, Potsdam, Germany, 8–10 Oct 2024, GSTM2024-20, https://doi.org/10.5194/gstm2024-20, 2024.

We present ODIN, an Optomechanical-Distributed instrument for Inertial sensing and Navigation system. ODIN is a novel instrument that has been funded under NASA’s InVEST program as technology demonstration program in space, to fly as a secondary payload on the GRATTIS mission led by the University of Florida. ODIN is an instrument of low cost, size, weight, and power (CSWaP), and utilizes arrays of in-plane dual-accelerometer systems that are capable of providing linear acceleration and angular measurements at levels of 10^-9 ms^-2/√Hz and 50 µrad/√Hz, respectively, which are relevant for mass change.

Accelerometry has become crucial for monitoring mass change within the Earth system.  Novel optomechanical inertial sensors provide an alternative instrument to existing ones, exhibiting lower cost, size, weight and power (CSWaP) with performances on par with GRACE. Reduced CSWaP makes these instruments suitable for enhancing mission reliability as redundant accelerometers, and can also improve science data quality by providing measurements of thruster firings and transient effects, among others.

Moreover, low CSWaP optomechanical instruments would enable cost-effective mission designs, spacecraft miniaturization, simplified architectures, as well as the deployment of constellations of satellite pairs flying at lower altitudes, and observations of transient phenomena that may impact mission performance.

We will discuss some of the potential science cases that can be addressed with this technology, as well as current status and development timeline of this flight instrument.

How to cite: Guzman, F. and Sanjuan, J.: ODIN – Optomechanical-Distributed instrument for Inertial sensing and Navigation, GRACE/GRACE-FO Science Team Meeting, Potsdam, Germany, 8–10 Oct 2024, GSTM2024-43, https://doi.org/10.5194/gstm2024-43, 2024.

Quantum Gravity Gradiometry (QGG), based on atom interferometric inertial sensing, offers the promise of accurate, high-resolution mass-change estimation from stable, well-calibrated, µE precision measurement of gravity gradients from orbit. Much of the technology for measurement and utilization of µE precision gradiometry, using a so-called "science grade instrument" (SGI) targeting ambitious next-generation Earth science community needs for mass change measurements, remains yet to be developed. However, current start of the art of atom interferometry allows testing and validation of key ideas behind the QGG concept in low-precision spaceflight experiments, as essential pathfinding to the SGI. We present the current status of the JPL-led QGG Pathfinder Concept Study from the viewpoint of the underlying science motivation of the SGI concept, the lessons to be learned from measurements drawn from a lower precision QGG pathfinder instrument, and the related mission design and concept of operations for the Pathfinder study.

How to cite: Bettadpur, S., Loomis, B., Wiese, D., Chiow, S., and Okino, C.: Status of development of the Quantum Gravity Gradiometer Pathfinder Concept, GRACE/GRACE-FO Science Team Meeting, Potsdam, Germany, 8–10 Oct 2024, GSTM2024-87, https://doi.org/10.5194/gstm2024-87, 2024.