GRACE Follow-On Mission Research Articles

Multivariate Prediction of Total Water Storage Changes Over West Africa from Multi-Satellite Data

West African countries have been exposed to changes in rainfall patterns over the last decades, including a significant negative trend. This causes adverse effects on water resources of the region, for instance, reduced freshwater availability. Assessing and predicting large-scale total water storage (TWS) variations are necessary for West Africa, due to its environmental, social, and economical impacts. Hydrological models, however, may perform poorly over West Africa due to data scarcity. This study describes a new statistical, data-driven approach for predicting West African TWS changes from (past) gravity data obtained from the gravity recovery and climate experiment (GRACE), and (concurrent) rainfall data from the tropical rainfall measuring mission (TRMM) and sea surface temperature (SST) data over the Atlantic, Pacific, and Indian Oceans. The proposed method, therefore, capitalizes on the availability of remotely sensed observations for predicting monthly TWS, a quantity which is hard to observe in the field but important for measuring regional energy balance, as well as for agricultural, and water resource management. Major teleconnections within these data sets were identified using independent component analysis and linked via low-degree autoregressive models to build a predictive framework. After a learning phase of 72 months, our approach predicted TWS from rainfall and SST data alone that fitted to the observed GRACE-TWS better than that from a global hydrological model. Our results indicated a fit of 79 % and 67 % for the first-year prediction of the two dominant annual and inter-annual modes of TWS variations. This fit reduces to 62 % and 57 % for the second year of projection. The proposed approach, therefore, represents strong potential to predict the TWS over West Africa up to 2 years. It also has the potential to bridge the present GRACE data gaps of 1 month about each 162 days as well as a—hopefully—limited gap between GRACE and the GRACE follow-on mission over West Africa. The method presented could also be used to generate a near-real-time GRACE forecast over the regions that exhibit strong teleconnections.

Laser link acquisition demonstration for the GRACE Follow-On mission.

We experimentally demonstrate an inter-satellite laser link acquisition scheme for GRACE Follow-On. In this strategy, dedicated acquisition sensors are not required-instead we use the photodetectors and signal processing hardware already required for science operation. To establish the laser link, a search over five degrees of freedom must be conducted (± 3 mrad in pitch/yaw for each laser beam, and ± 1 GHz for the frequency difference between the two lasers). This search is combined with a FFT-based peak detection algorithm run on each satellite to find the heterodyne beat note resulting when the two beams are interfered. We experimentally demonstrate the two stages of our acquisition strategy: a ± 3 mrad commissioning scan and a ± 300 μrad reacquisition scan. The commissioning scan enables each beam to be pointed at the other satellite to within 142 μrad of its best alignment point with a frequency difference between lasers of less than 20 MHz. Scanning over the 4 alignment degrees of freedom in our commissioning scan takes 214 seconds, and when combined with sweeping the laser frequency difference at a rate of 88 kHz/s, the entire commissioning sequence completes within 6.3 hours. The reacquisition sequence takes 7 seconds to complete, and optimizes the alignment between beams to allow a smooth transition to differential wavefront sensing-based auto-alignment.

Precise Recovery of the Earth's Gravitational Field by Grace Follow‐On Satellite Gravity Gradiometry Method

AbstractBecause the twin GRACE Follow‐On satellites are equivalent to the one‐dimensional horizontal gravity gradiometer with a baseline length of the intersatellite range, the feasibility demonstrations on precisely and rapidly recovering the next‐generation Earth's gravitational field by the GRACE Follow‐On satellite gravity gradiometry method (GFO‐SGGM) are carried out in this study. The research results are as follows: Firstly, based on the GRACE Follow‐On satellite gravity gradiometry method, using the orbital parameters of the satellite (e.g. orbital altitude of 250 km, intersatellite range of 50 km, orbital inclination of 89° and orbital eccentricity of 0.001), the measurement precisions of key payloads (e.g. 10−6 m in intersatellite range, 10−7 m·s−1 in intersatellite range‐ rate and 10−10 m·s−2 in intersatellite range‐acceleration by the interferometric laser ranging system, 10−3 m in orbital position and 10−6 m·s−1 in orbital velocity by the GPS receiver, and 10−11 m·s−2 in non‐conservative force by the accelerometer), a 30‐day observation time and a sampling interval of 10 s, the Earth's gravitational field complete up to degree and order 120 is accurately recovered, and the cumulative geoid height error is 9.331×10−4 m at degree 120. Secondly, in 120 degrees, the accuracy of the gravity recovery from the future twin GRACE Follow‐On satellites is averagely improved by about 61 times than that from the current twin GRACE satellites. Therefore, the GRACE Follow‐On satellite gravity gradiometry method promisingly becomes a preferred approach for further improving the determination accuracy of the next‐generation Earth gravity field model. Finally, the advantages of the next‐generation GRACE Follow‐On mission as compared with the present GRACE mission principally consist of the lower orbital altitude of the satellite between 200 km and 300 km, the higher measurement accuracy of the space‐borne instruments between 10−7 m·s−1 and 10−9 m·s−1 and the shorter intersatellite range between 50 km and 100 km.

The design and construction of a prototype lateral-transfer retro-reflector for inter-satellite laser ranging

The Gravity Recovery and Climate Experiment (GRACE) mission, launched in 2002, is nearing an end, and a continuation mission (GRACE Follow-on) is on a fast-tracked development. GRACE Follow-on will include a laser ranging interferometer technology demonstrator, which will perform the first laser interferometric ranging measurement between separate spacecraft. This necessitates the development of lightweight precision optics that can operate in this demanding environment. In particular, this beam routing system, called the triple mirror assembly, for the GRACE Follow-on mission presents a significant manufacturing challenge. Here we report on the design and construction of a prototype triple mirror assembly for the GRACE Follow-on mission. Our constructed prototype has a co-alignment error between the incoming and outgoing beams of 9 μrad, which meets the requirement that this error must be less than 10 μrad.

Retroreflector for GRACE follow-on: Vertex vs. point of minimal coupling.

The GRACE Follow-On mission will monitor fluctuations in Earth's geoid using, for the first time, a Laser Ranging Interferometer to measure intersatellite distance changes. We have investigated the coupling between spacecraft rotation and the intersatellite range measurement that is incurred due to manufacturing and assembly tolerances of the Triple Mirror Assembly (TMA), a precision retroreflector to ensure alignment between in- and outgoing laser beams. The three TMA mirror planes intersect in a virtual vertex to which satellite displacements are referenced. TMA manufacturing tolerances degrade this ideal vertex, however, a Point of Minimal Coupling (PMC) between spacecraft rotation and displacement exists. This paper presents the experimental location of the PMC under pitch and yaw rotations for a prototype TMA. Rotations are performed using a hexapod, while displacements are monitored with heterodyne laser interferometry to verify the PMC position. Additionally, the vertex of the three TMA mirror planes is measured using a Coordinate Measuring Machine and compared to the PMC position. In the pitch and yaw axes, the biggest deviation between TMA vertex and PMC was 50 ± 64 μm. Thus, within the measurement uncertainties, no difference between TMA vertex and PMC could be observed. This is a key piece of information for integration of the TMA into the spacecraft: It is sufficient to use the readily-available TMA vertex location to ensure minimal rotation-to-displacement coupling during the mission.

Precise and Rapid Recovery of the Earth's Gravity Field from the Next‐Generation GRACE Follow‐On Mission using the Residual Intersatellite Range‐Rate Method

AbstractThe Earth's gravitational field from the Gravity Recovery and Climate Experiment (GRACE) Follow‐On mission up to degree and order 120 is recovered based on a new residual intersatellite range‐rate method (RIRM) by numerical simulation. Due to low‐precision orbit determination by the Global Positioning System (GPS), a new RIRM observation equation is created by introducing the residual intersatellite range‐rate with a precision of 10−7 m/s from the interferometric laser ranging (ILR) system into the line‐of‐sight (LOS) component of the residual orbital velocity difference vector from the twin satellites. The optimal number of interpolation points is comparatively demonstrated by the two‐point, four‐point, six‐point and eight‐point RIRM formulas, respectively. If the correlation coefficient and sampling interval are fixed, when the amount of signals from satellite observations is effectively enhanced with increasing the number of interpolation points, the satellite observation errors are simultaneously improved. Therefore, the six‐point RIRM formula is a preferred selection for recovering the Earth's gravitational field complete up to degree and order 120. The impact of the correlation coefficient on the accuracy of the Earth's gravitational field recovery exhibits different characteristics in different frequency bands. With the gradual increase of the correlation coefficients, the accuracy of the Earth's gravitational field recovery decreases in the long‐wavelength range, while it increases in the medium‐long‐wavelength range. The study on a comparison of influences of intersatellite range‐rate and residual intersatellite range‐rate measurements on the accuracies of the Earth's gravitational field recovery from GRACE Follow‐On mission is conducted, which indicates that cumulative geoid height errors from GRACE Follow‐On are 1.638×10−3 m and 1.396×10−3 m using the six‐point RIRM formula, an observation duration of 30 days and a sampling interval of 5 s at degree 120, respectively. The research results show that the sensitivity of satellite gravity recovery using residual intersatellite range‐rate measurements is greater than that using intersatellite range‐rate measurements, and the accuracy of the Earth's gravitational field recovery from GRACE Follow‐On is at least 10 times higher than that from GRACE.

Modeling and Simulation of a Laser Ranging Interferometer Acquisition and Guidance Algorithm

A novel acquisition algorithm sequence for the establishment of an optical intersatellite link is described. The acquisition algorithm is based on a differential-speed spatial domain scan and a frequency domain scan. Experimental results obtained using a digital simulator highlight the reliability of the algorithm always locking the signal, regardless of the initial spatial and frequency conditions. The results are tailored for the laser ranging instrument technology demonstration that is going to be implemented for the GRACE Follow-On mission as an example application. Nevertheless, with a proper tuning of the algorithm parameters, it is possible to study a general optical intersatellite acquisition process.

Ocean bottom pressure signals around Southern Africa from in situ measurements, satellite data, and modeling

[1] Ocean bottom pressure (OBP) variability in the region of the Agulhas Current off the South African coast is a crucial variable in the understanding of dynamic processes in the ocean, but measurements currently available lack either precision or spatial and temporal coverage. We provide a quantitative estimate of OBP variability throughout the region with the help of a setup of the ROMS regional ocean model. Driving the model with boundary conditions from a global ocean model and atmospheric reanalysis data and running it for 8 years, we are able to reproduce many characteristic properties of the regional ocean circulation visible in sea surface height and OBP fields. While the in situ pressure-inverted echo sounders (PIES) measuring local OBP variations on short time scales are sparse in the region, our model provides a comprehensive estimate of OBP variations throughout the region which reach values of up to 15 hPa when barotropic Agulhas rings reach the Cape Basin. These signals turn out to be difficult to measure with current gravimetry solutions from the GRACE satellites, but estimates of localized noise levels for a GRACE follow-on mission let the search for them in future satellite measurements appear viable.

Intersatellite laser ranging instrument for the GRACE follow-on mission

The Gravity Recovery and Climate Experiment (GRACE) has demonstrated that low–low satellite-to-satellite tracking enables monitoring the time variations of the Earth’s gravity field on a global scale, in particular those caused by mass-transport within the hydrosphere. Due to the importance of long-term continued monitoring of the variations of the Earth’s gravitational field and the limited lifetime of GRACE, a follow-on mission is currently planned to be launched in 2017. In order to minimise risk and the time to launch, the follow-on mission will be basically a rebuild of GRACE with microwave ranging as the primary instrument for measuring changes of the intersatellite distance. Laser interferometry has been proposed as a method to achieve improved ranging precision for future GRACE-like missions and is therefore foreseen to be included as demonstrator experiment in the follow-on mission now under development. This paper presents the top-level architecture of an interferometric laser ranging system designed to demonstrate the technology which can also operate in parallel with the microwave ranging system of the GRACE follow-on mission.

Understanding data noise in gravity field recovery on the basis of inter-satellite ranging measurements acquired by the satellite gravimetry mission GRACE

Spectral analysis of data noise is performed in the context of gravity field recovery from inter-satellite ranging measurements acquired by the satellite gravimetry mission GRACE. The motivation of the study is two-fold: (i) to promote a further improvement of GRACE data processing techniques and (ii) to assist designing GRACE follow-on missions. The analyzed noise realizations are produced as the difference between the actual GRACE inter-satellite range measurements and the predictions based on state-of-the-art forcemodels. The exploited functional model is based on the so-called “range combinations,” which can be understood as a finite-difference analog of inter-satellite accelerations projected onto the line-of-sight connecting the satellites. It is shown that low-frequency noise is caused by limited accuracy of the computed GRACE orbits. In the first instance, it leads to an inaccurate estimation of the radial component of the inter-satellite velocities. A large impact of this component stems from the fact that it is directly related to centrifugal accelerations, which have to be taken into account when the measured range-accelerations are linked with inter-satellite accelerations. Another effect of orbit inaccuracies is a miscalculation of forces acting on the satellites (particularly, the one described by the zero-degree term of the Earth’s gravitational field). The major contributors to the noise budget at high frequencies (above 9 mHz) are (i) ranging sensor errors and (ii) limited knowledge of the Earth’s static gravity field at high degrees. Importantly, we show that updating the model of the static field on the basis of the available data must be performed with a caution as the result may not be physical due to a non-unique recovery of high-degree coefficients. The source of noise in the range of intermediate frequencies (1–9 mHz), which is particularly critical for an accurate gravity field recovery, is not fully understood yet. We show, however, that it cannot be explained by inaccuracies in background models of time-varying gravity field. It is stressed that most of the obtained results can be treated as sufficiently general (i.e., applicable in the context of a statistically optimal estimation based on any functional model).

Simulation study of a follow-on gravity mission to GRACE

The gravity recovery and climate experiment (GRACE) has been providing monthly estimates of the Earth's time-variable gravity field since its launch in March 2002. The GRACE gravity estimates are used to study temporal mass variations on global and regional scales, which are largely caused by a redistribution of water mass in the Earth system. The accuracy of the GRACE gravity fields are primarily limited by the satellite-to-satellite range-rate measurement noise, accelerometer errors, attitude errors, orbit errors, and temporal aliasing caused by unmodeled high-frequency variations in the gravity signal. Recent work by Ball Aerospace and Technologies Corp., Boulder, CO has resulted in the successful development of an interferometric laser ranging system to specifically address the limitations of the K-band microwave ranging system that provides the satellite-to-satellite measurements for the GRACE mission. Full numerical simulations are performed for several possible configurations of a GRACE Follow-On (GFO) mission to determine if a future satellite gravity recovery mission equipped with a laser ranging system will provide better estimates of time-variable gravity, thus benefiting many areas of Earth systems research. The laser ranging system improves the range-rate measurement precision to approximately 0.6 nm/s as compared to approx. 0.2 micro-seconds for the GRACE K-band microwave ranging instrument. Four different mission scenarios are simulated to investigate the effect of the better instrument at two different altitudes. The first pair of simulated missions is flown at GRACE altitude (approx. 480 km) assuming on-board accelerometers with the same noise characteristics as those currently used for GRACE. The second pair of missions is flown at an altitude of approx. 250 km which requires a drag-free system to prevent satellite re-entry. In addition to allowing a lower satellite altitude, the drag-free system also reduces the errors associated with the accelerometer. All simulated mission scenarios assume a two satellite co-orbiting pair similar to GRACE in a near-polar, near-circular orbit. A method for local time variable gravity recovery through mass concentration blocks (mascons) is used to form simulated gravity estimates for Greenland and the Amazon region for three GFO configurations and GRACE. Simulation results show that the increased precision of the laser does not improve gravity estimation when flown with on-board accelerometers at the same altitude and spacecraft separation as GRACE, even when time-varying background models are not included. This study also shows that only modest improvement is realized for the best-case scenario (laser, low-altitude, drag-free) as compared to GRACE due to temporal aliasing errors. These errors are caused by high-frequency variations in the hydrology signal and imperfections in the atmospheric, oceanographic, and tidal models which are used to remove unwanted signal. This work concludes that applying the updated technologies alone will not immediately advance the accuracy of the gravity estimates. If the scientific objectives of a GFO mission require more accurate gravity estimates, then future work should focus on improvements in the geophysical models, and ways in which the mission design or data processing could reduce the effects of temporal aliasing.

Intersatellite laser ranging with homodyne optical phase locking for Space Advanced Gravity Measurements mission

In this paper, we present the scheme and the preliminary results of an intersatellite laser ranging system that is designed for the Earth's gravity recovery mission proposed in China, called Space Advanced Gravity Measurements (SAGM). The proposed intersatellite distance is about 100 km and the precision of inter-satellite range monitoring is 10 nm/Hz(1/2) at 0.1 Hz. To meet the needs, we designed a transponder-type intersatellite laser ranging system by using a homodyne optical phase locking technique, which is different from the heterodyne optical phase-locked loop used in GRACE follow-on mission. Since an ultrastable oscillator is unnecessary in the homodyne phase-locked loop, the measurement error caused by the frequency instability of the ultrastable oscillator need not be taken into account. In the preliminary study, a heterodyne interferometer with 10-m baseline (measurement arm-length) was built up to demonstrate the validity of the measurement scheme. The measurement results show that a resolution of displacement measurement of about 3.2 nm had been achieved.

Accurate and rapid error estimation on global gravitational field from current GRACE and future GRACE Follow-On missions

Firstly, the new combined error model of cumulative geoid height influenced by four error sources, including the inter-satellite range-rate of an interferometric laser (K-band) ranging system, the orbital position and velocity of a global positioning system (GPS) receiver and non-conservative force of an accelerometer, is established from the perspectives of the power spectrum principle in physics using the semi-analytical approach. Secondly, the accuracy of the global gravitational field is accurately and rapidly estimated based on the combined error model; the cumulative geoid height error is 1.985 × 10−1 m at degree 120 based on GRACE Level 1B measured observation errors of the year 2007 published by the US Jet Propulsion Laboratory (JPL), and the cumulative geoid height error is 5.825 × 10−2 m at degree 360 using GRACE Follow-On orbital altitude 250 km and inter-satellite range 50 km. The matching relationship of accuracy indexes from GRACE Follow-On key payloads is brought forward, and the dependability of the combined error model is validated. Finally, the feasibility of high-accuracy and high-resolution global gravitational field estimation from GRACE Follow-On is demonstrated based on different satellite orbital altitudes.

Laser interferometer for spaceborne mapping of the Earth's gravity field

The Gravity Recovery and Climate Experiment (GRACE) is one of the present missions to map the Earth's gravity field. The aim of a GRACE follow-on mission is to map the gravitational field of the Earth with higher resolution over at least 6 years. This should lead to a deeper insight into geophysical processes of the Earth's system. One suggested detector for this purpose consists of two identical spacecraft carrying drag-free test masses in a low Earth orbit at an altitude of the order of 300 km, following each other with a distance on the order of 50 to 100 km. Changes in the Earth's gravity field will induce distance fluctuations between two test masses on separate spacecraft. These variations in the frequency range 1 to 100 mHz are to be monitored by a laser interferometer with nanometer precision. We present preliminary results of a heterodyne interferometer configuration using polarising optics, demonstrating the required phase sensitivity.