Gravitational Reference Sensor Technology Development Roadmap for the Mass Change Mission

Low-low satellite-to-satellite tracking missions like GRACE-FO that utilize laser ranging intereferometers are technologically limited by the acceleration noise performance of the electrostatic accelerometers, in addition to temporal aliasing issues associated with the dynamic gravity field measurements. The current accelerometers, used in the GRACE and GRACE-FO mission have a limited sensitivity of ~10 m/sHz around 1 mHz. Meanwhile, the LISA Pathfinder mission, which was a technology demonstrator for the future ESA/NASA LISA gravitational wave mission, demonstrated an acceleration noise performance of 2×10 m/sHz around 1 mHz. The results of LISA Pathfinder and extensive ground testing using precision torsion pendula indicate that a simplified version of the LISA Pathfinder gravitational reference sensor (GRS) could be used in future Earth geodesy missions beyond GRACE-FO. Such a sensor would have an acceleration noise below 10 m/sHz, which is understood to be the desired performance for future Earth geodesy missions utilizing laser interferometry for intersatellite ranging. This sensor could be directly integrated with the laser interferometer, potentially relaxing requirements on spacecraft attitude measurement and control, or it could be operated as a standalone instrument with a stable structural reference to the laser interferometer reference point. The improved performance is enabled by increasing the mass of the sensor’s test mass, increasing the gap between the test mass and its electrode housing, removing the small grounding wire used in the GRACE accelerometers and replacing them with a UV LED-based charge management system. The sensor’s performance would be optimized on a drag-compensated platform but could also operate on a traditional spacecraft utilizing control schemes tailored to the specific spacecraft platform with reduced performance. Here, we describe a GRS optimized for mass change missions, as well as a technology development effort needed to bring the instrument to TRL 6. We assume that this sensor would initially be implemented as a technology demonstration on a non-drag-free GRACE-like satellite, but could later be implemented on a drag-free platform. We therefore present the GRS’s performance under both scenarios. The instrument would have a volume of ~10 cm, a mass of ~15 kg, and a nominal power consumption of ~20 W per spacecraft. Development of this instrument to TRL 6 for incorporation as a technology demonstration on the next Mass Change Mission would require ~4 years of effort, and the delivery of flight units could be completed ~2 years later. Finally, it is important to note that LISA Pathfinder has demonstrated that the GRS architecture has no technical barriers to achieving even orders of magnitude better performance in the future. Thus, investment in the development of this technology opens up a path to continued mass change sensitivity improvements that could be pursued in parallel with laser ranging and other vital technology advancements. Conklin, et al., 2020 MCM Gravitational Reference Sensor Technology Development Roadmap Conklin, et al., 2020 2 1 Description of the Technology and its Benefits 1.1 Benefits of Improved Acceleration Noise The utility of the GRACE and GRACE-FO data have been substantial for climate-related research. Several studies have focused on understanding the full error budget of the GRACE and GRACE-FO missions, as well as how to lower the error levels for future missions. The gravity field retrieval errors fall into two categories: measurement system errors due to the performance of the onboard measurement system (the inter-satellite ranging instrument and accelerometer for GRACE-FO), and temporal aliasing errors due to undersampling of high frequency mass variations in the Earth system such as ocean tides and weather systems. Temporal aliasing errors are currently the largest source of error limiting the spatial resolution of the gravity fields (Figure 1). If the error associated with temporal aliasing can be reduced by either increasing the number of spacecraft employed or improving temporal aliasing models, then the gravity field retrieval becomes dominated by measurement system errors, the largest of which is the accelerometer (Figure 1). Hence, there is a need to improve upon the performance of the current electrostatic accelerometers flown on GRACE-FO. The simplified gravitational reference sensor (GRS) described here provides the same function as the GRACE accelerometers, but with at least a factor of 100 improved acceleration noise performance. It takes advantage of the flight heritage of LISA Pathfinder, which demonstrated a 10 improvement over GRACE-FO and ~10 improvement over GOCE (Armano, et al., 2018). Figure 1. Geoid height error for a simulated mission of a single pair of satellites at 500 km altitude. Shown is the impact of each individual source of error on the gravity retrieval. The black curve is the power in the hydrology and ice signal that we wish to recover. It is seen that the largest source of error on the gravity field recovery is due to temporal aliasing errors. The largest measurement system error is from the accelerometer. 1.2 The LISA Pathfinder Gravitational Reference Sensor The state of the art in ultra-precise inertial sensors (or accelerometers) is the LISA Pathfinder inertial sensor, shown in Figure 2. It uses a 2 kg, Au/Pt test mass (TM) inside a molybdenum electrode housing (EH). The housing contains 12 gold coated electrodes to differentially sense the position and orientation of the cube via capacitive sensing and to actuate it using electrostatic actuation. Six “injection electrodes” are driven with a 100 kHz AC bias voltage to frequency shift the capacitive measurement to high frequency. Readout of the sensing electrodes and driving of the actuation electrodes is performed by the front end electronics (FEE). The gap between the TM and housing is 4 mm and is a trade-off between reducing the effects of noise sources, e.g. from uncontrolled potentials on the electrodes, and being able to measure test mass displacement at the MCM Gravitational Reference Sensor Technology Development Roadmap Conklin, et al., 2020 3 nanometer level over the measurement bandwidth. The capacitive readout system is arranged such that electrodes facing opposing faces of the test mass are combined via a capacitive bridge. A change in the position of the test mass gives a differential, bi-polar, signal at the output of the bridge, which is used as an input to the drag-free control system. A caging and venting mechanism (CVM) uses a set of mechanical fingers to secure the TM during launch (Bortoluzzi, Conklin, et al., 2013). During science operations, the TM charge is controlled by a charge management system (CMS) based on UV photoemission using Hg vapor lamps as the UV light source (Wass, et al., 2018). The CMS eliminates the need for the small grounding wire used in the ONERA accelerometers that both limits their performance and potentially causes challenges during integration and test. LISA Pathfinder, launched in December of 2015, exceeded all expectations in terms of acceleration noise performance. Acceleration noise is caused by residual spurious forces acting on the TM, and it is the primary metric used to evaluate the performance of these instruments, as well as accelerometers. The goal of the LISA Pathfinder mission was to demonstrate a differential acceleration noise level between the two LISA Pathfinder test masses of 3×10 m/sHz above 1 mHz, the same frequency band that is important for Earth geodesy. Figure 3 shows the final results from the LISA Pathfinder mission, which exceeded its requirements by more than an order of magnitude. At 1 mHz the measured performance was 2×10 m/sHz, more than a factor of 10 over the performance of GRACE and GRACE-FO and 10 over that of GOCE. 1.3 A Simplified Gravitational Reference Sensor for Earth Geodesy The acceleration noise performance of inertial sensors, like that of LISA pathfinder, scales linearly with the mass of the test mass and at least linearly with the gap size between the test mass and the electrode housing. The results of LISA Pathfinder show that a similar, but scaled-down design could achieve 10 m/sHz for example with a TM on the order of a few 100 g and an electrode housing on the order of 5×5×5 cm. The lower TM mass in turn enables a simplified caging mechanism. Figure 3. Measured LISA Pathfinder acceleration noise performance and the requirements for the LISA gravitational wave observatory (Armano et al., 2018). The frequency band of interest for LISA is the same as that for Earth geodesy. MCM Gravitational Reference Sensor Technology Development Roadmap Conklin, et al., 2020 4 The key improvements of this simplified GRS over the accelerometers flown on GRACE and GRACE-FO are the following. 1. Increase TM-to-electrode housing gap size to ~1 mm and increase TM mass to 250-500 g These two key design parameters enable better acceleration noise performance by reducing the TM acceleration associated with several critical TM surface force noise contributions (see Box 1). 2. Remove the small TM grounding wire and replace it with UV LED charge control The thermal noise associated with the grounding wire limits the acceleration noise of the GRACE accelerometers to ~10 m/sHz around 1 mHz (Lebat et al., 2013). Eliminating this wire would (a) remove this performance limitation, (b) allow larger TM-EH gap sizes to be used, and (c) mitigate challenges associated with installing this wire during assembly, integration and test. In place of a grounding wire, a UV photoemmsion-based charge control would be employed. This technology was first demonstrated by Gravity Probe B and more recently by LISA Pathfinder. 3. Reduce the impact of environmental factors on the sensor’s performance Key to the performance of inertial sensors is isolating the test mass from its host platform. Larger gaps