Live display program

In February 2020 CNES/GRGS published its 5th reprocessing (called "RL05") of the GRACE data, from August 2002 to August 2016. The extension of this series covering the span of the GRACE-FO data, 2018-now, will be released in October. This new times series comes, as for the previous releases, in a monthly and a 10-day time resolution. 

The main differences of the new release with respect to the previous one are :
- the use of the most recent version of the JPL KBR data (version 3),
- the use of the IGS orbits and clocks in replacement of the GRGS ones,
- a completely homogenous processing over the full time span,
- the use of AOD1B-RL06 for the dealiasing data in replacement of the ERA-Interim+TUGO products.

For the processing of the GRACE-FO data we have used the TUGRAZ transplant accelerometer data instead of the JPL one, resulting in a great stabilization of the accelerometer scale factors, now close to 1 over the full time span, in a reduction of the range-rate and GPS residuals and in an improvement of the gravity field solutions.

This presentation will focus on the processing details and on the comparison of this new series with the Release 06 from JPL / GFZ / CSR, the ITSG-Grace2018 time series from TUGRAZ, and the combined time series from the new international combination service COST-G.

How to cite: Lemoine, J.-M. and Bourgogne, S.: RL05 monthly and 10-day gravity field solutions from CNES/GRGS, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-51, https://doi.org/10.5194/gstm2020-51, 2020.

The Laser Ranging Interferometer onboard GRACE-FO is a novel instrument, that measures  the inter-spacecraft distance up to nanometer precision with high reliability. An important quantity for this precise measurement is the absolute laser frequency (or scale factor), which is used to convert the raw phase measurements into physical biased ranges with unit of meters. The current processing scheme of LRI1B data products employed by SDS estimates the LRI scale factor by means of daily cross-correlations of LRI range with respect to KBR range. However, in potential future missions, where the LRI becomes the primary and only ranging instrument, such a cross-calibration would not be available. We therefore present a method to estimate the LRI laser frequency from telemetry and evaluate the model accuracy.

The laser frequency is determined and known to some extend from the settings of the lasers. We derived a simple linear model to relate the LRI level 1A telemetry to the absolute laser frequency, which is purely based on pre-flight ground measurements. This telemetry-derived frequency matches to the in-flight LRI-KBR cross-calibration value to approx. 100 MHz, which has to be compared to the absolute frequency of 281.615 THz of the laser light. The fractional accuracy of 3*10^-7 translates to a range scale factor uncertainty of 1.5*10^-7 due to the conversion from round-trip to one-way range.  The remaining residuals, given by the difference of our model with respect to cross-calibration values, are described in our analysis as an additional empirical model, which is dominated by a linear drift of 1.5 Hz/s. Some recurring features, likely with yearly periodicity, and of the order of a few 10 MHz are present and will be addressed as well.

With our presented model, based on pre-flight characterizations and in-flight LRI-KBR comparisons from approximately two years in orbit, one can compute the LRI laser frequency or scale factor continuously to a few parts in 10⁸. By applying the continuous scale factor in the LRI1B data generation, one can avoid discontinuities in the LRI range at the day bounds currently present in LRI1B RL04 data. We conclude with implications of our analysis to future mission developments utilizing LRI-like instruments.

How to cite: Misfeldt, M., Müller, V., Wegener, H., Müller, L., and Heinzel, G.: Scale Factor of the Laser Ranging Interferometer in GRACE Follow-On, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-13, https://doi.org/10.5194/gstm2020-13, 2020.

GRACE-FO carries a Laser Ranging Interferometer (LRI) as a technology demonstration to provide measurements of inter-satellite range changes. This additional measurement technology provides supplementary observations, which improve the reliability of the range rate measurements and allow for a cross-instrument diagnostics and calibration with the K-band ranging (KBR) system.

We present a two-step approach used for LRI1B data calibration within the ITSG-Grace2018 scheme, which is compatible with the entire v04 release timespan. The aim of this study is to mitigate the remaining systematics due to the LRI datation time offset and LRI scale factor. We discuss the implementation of calibration parameters and the contribution of the calibration approach to the overall accuracy of gravity field solutions.

How to cite: Behzadpour, S., Kvas, A., and Mayer-Gürr, T.: Two-step LRI1B data calibration approach, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-15, https://doi.org/10.5194/gstm2020-15, 2020.

The test Laser ranging interferometer (LRI) on the GRACE Follow-On satellites provides complementary inter-satellite ranging measurements to the baseline K-band microwave ranging (KBR) system that can be used to examine standard, and create novel, GRACE-FO data products.  We first calculated the KBR and LRI inter-satellite ranging residuals using dynamic orbits computed from non-gravitational accelerations, a static gravity field model and other background geophysical models like ocean tides. To accurately quantify the improvement by LRI, we directly examined the inter-satellite ranging residuals in the time and frequency domains. The frequency-domain analysis reveals that LRI enhances the accuracy of gravity measurements by ~1 order of magnitude over 60-200 CPR (10-37 mHz) frequencies with the signal dominated by static gravity field of the Earth. The time-domain analysis shows that LRI is capable of detecting static gravity signals as small as a few 0.1 nm/s² in 100-200 CPR frequency band. We made use of such LRI data acquired in 2019 to validate the state-of-the-art gravity field models GGM05S, GGM05C, GOCE-TIM-R6e, EIGEN-6C4, ITSG-Grace2018s and GOCO06s. We found that LRI data can identify subtle un-/mis-modeled static gravity signals in these models in the spectral as well as spatial domains, and thus, suggest how the next generation of gravity field models could be improved. We also examined the high‐frequency (sub-monthly) variations of the Argentine Gyre using LRI measurements along with satellite altimetry data. Through comparison of measured gravity change by LRI with synthetic gravity change from altimetry sea surface data (evaluated at GRACE Follow-On altitude), we clearly demonstrate how the high-frequency Argentine Gyre signal is fully captured by instantaneous LRI measurements by individual data arcs, but not in the monthly mean Level-2 data. Such along-orbit analyses of LRI data could be employed for, among others, validation of high-frequency non-tidal ocean models used in GRACE and GRACE Follow-On de-aliasing products.

How to cite: Ghobadi Far, K., Han, S.-C., Sauber, J., Ray, R., McCullough, C. M., Wiese, D. N., Yuan, D.-N., and Landerer, F. W.: Application of ultra‐precise GRACE Follow-On LRI measurements for validation of the state-of-the-art static gravity field models and examination of sub-monthly time-variable gravity signals, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-41, https://doi.org/10.5194/gstm2020-41, 2020.

At our Institute we compute monthly gravity potential solutions from GRACE/GRACE-FO level 1B data by using the variational equations approach. The gravity field is recovered with our own MATLAB software "GRACE-SIGMA" that was recently updated in order to reduce the calculation time with parallel computing approach by approx. 80%. Also the processing chain has changed to update the background modeling and we made tests with different orbit types and different parametrizations. We discuss progress to include laser ranging interferometer data in gravity field solutions. We present validation results and analyze the properties of postfit range-rate residuals.

How to cite: Duwe, M., Koch, I., Flury, J., and Shabanloui, A.: Processing of GRACE FO satellite to satellite tracking data using the GRACE SIGMA software, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-3, https://doi.org/10.5194/gstm2020-3, 2020.

We present the operational GRACE-FO combined time-series of monthly gravity fields of the Combination Service for Time-variable Gravity fields (COST-G) of the International Association of Geodesy (IAG). COST-G_GRACE-FO_RL01_operational is combined at AIUB and relies on operational monthly solutions of the COST-G Analysis Centers GFZ, GRGS, IfG, LUH and AIUB and the associated Analysis Centers CSR and JPL. All COST-G Analysis Centers have passed a benchmark test to ensure consistency between the different processing approaches and all of the contributing time-series undergo a strict quality control focusing on the signal content in river basins and polar regions with pronounced changes in ice mass to uncover any regularization that may bias the combination.

The combination is performed by variance component estimation on the solution level, the relative monthly weights thus providing valuable and independent insight into the consistency and noise levels of the individual monthly contributions. The combined products then are validated internally in terms of noise, approximated by the non-secular, non-seasonal variability over the oceans. Once they have passed this quality control the combined gravity fields are assessed by an external board of experts who evaluate them in terms of orbit predictions, lake altimetry, river hydrology or oceanography.

How to cite: Meyer, U., Lasser, M., Jäggi, A., Flechtner, F., Dahle, C., Mayer-Gürr, T., Kvas, A., Behzadpour, S., Lemoine, J.-M., Bourgogne, S., Koch, I., Groh, A., Förste, C., Eicker, A., and Meyssignac, B.: Combination Service for Time-variable Gravity Field Solutions (COST-G) – GRACE-FO operational combination, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-16, https://doi.org/10.5194/gstm2020-16, 2020.

Ocean tides produce significant gravitational perturbations that affect near-Earth orbiting spacecraft.  The gravitational potential induced by tidal mass redistribution is routinely modelled for global gravity analysis and orbit determination, although generally by assuming a spherical Earth and a uniform seawater density.  The inadequacy of these simplifications is here addressed.  We have developed an accurate yet efficient algorithm to compute the ocean tidal geopotential, allowing for Earth’s elliptical shape and variable seawater density.  Using this new computation, we find that (1) the effect of ellipticity is several percent of the tide signal over mid to high latitude regions, which is comparable to elevation error in the state-of-the-art ocean tide models; (2) the effect of seawater density variations on the potential is as large as 2-3 cm in water-height equivalent, primarily in deep water where density increases 2-3% from compressibility.  Our analysis of new GRACE Follow-On (GRACE-FO) laser ranging interferometer measurements reveals evident errors when ellipticity and density variations are ignored.  When accounted for, the GRACE-FO residual tidal gravity perturbations are reduced by half, depending on the adopted tide model; only the remaining half likely represents actual model elevation error.  The use of a spherical surface and a uniform seawater density is no longer tenable given the precision of gravity measurements from GRACE and GRACE-FO satellites.

How to cite: Han, S.-C., Ghobadi-Far, K., Ray, R., and Papanikolaou, T.: Tidal geopotential dependence on Earth ellipticity and seawater density and its detection with the GRACE Follow-On laser ranging interferometer, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-59, https://doi.org/10.5194/gstm2020-59, 2020.

The GRACE-FO’s mission primary goal is the precise mapping of variations within the Earth’s hydrology by observing changes in gravitational attraction. This particular signal, however, is widely superimposed by other signals of higher amplitudes and shorter time scales like mass redistributions within atmosphere and ocean (AO). The hereby induced temporal aliasing is treated by reducing tidal background models during the data processing. The residual signal which is caused by imperfections of the background models has so far been left untreated.

We present the DMD approach for further treatment of the residual AO-signal which is somewhat similar to the so-called Wiese parametrization proposed for multi-pair missions (Wiese et al., 2011). However, contrary to Wiese, the DMD directly affects the right-hand side of the least squares adjustment. The method consists of (a) estimating short-term low-degree fields based on the full observation vector in a first step, (b) reducing it by the observations computed from the short-term solutions (thus effectively having applied an entirely data-based de-aliasing model), and finally, (c) estimating a high-degree field over the entire observation period (e.g. a month). Resubstituting the mean of (a) to (c) then yields a gravity solution for the observation interval. The functionality of the DMD is verified by simulations and allows for an improved retrieval of the residual signal above the maximum degree of the short-term fields over the observation interval. In order to counteract potential over-estimation within the low-degree part of the solution additional conditions are introduced and discussed.

How to cite: Abrykosov, P., Pail, R., and Gruber, T.: Data-driven multi-step AO de-aliasing (DMD) for GRACE/GRACE-FO, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-66, https://doi.org/10.5194/gstm2020-66, 2020.

Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) carries the Laser Ranging Interferometer (LRI) as a technology demonstrator that measures the inter-satellite range with nanometer precision. For the precise laser beam pointing, LRI uses the beam steering method where the fast steering mirror is actuated to correct for the misalignment between the incoming and outgoing laser beams. From the fast steering mirror commands, we can compute the inter-satellite pitch and yaw angles. These angles are provided as LSM1B product and represent spacecraft's relative orientation with respect to line-of-sight (LOS). The SCA1B data, which is computed by combining the data of three star cameras and IMU in the Kalman filter, represents the absolute orientation of the spacecrafts. Currently, this SCA1B product is used in the gravity field determination.  

Here we present first results of a new attitude product which is computed by the combination of fast steering mirror data with the star cameras and IMU in the attitude kalman filter.  We also present the impact of this new combined attitude data on the gravity field solutions.

How to cite: Goswami, S., Bandikova, T., Yuan, D.-N., Francis, S., and Spero, B.: Results of fusion of LRI based attitude data in the attitude Kalman filter, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-18, https://doi.org/10.5194/gstm2020-18, 2020.

We study gravity field determination from GRACE-FO satellite-to-satellite tracking using the inter-satellite K-band link and kinematic positions of the satellites as observations and pseudo-observations respectively. We employ 10-second kinematic positions from a precise point positioning where the undifferenced carrier phase ambiguities are fixed to integer values using observation-specific phase bias products from the CODE analysis centre.
We present an up-to-date GRACE-FO time series computed with the Celestial Mechanics Approach extended by empirical noise models derived from the post-fit residuals to better characterise the stochastic behaviour of both observation types. We investigate the interplay between the empirical noise model and the co-estimation of stochastic parameters set-up to absorb unmodelled signal (short term mass variations, accelerometer errors etc.).
We validate our results of GRACE-FO gravity field determination by analysing the residuals of combined orbits calculated using both kinematic positions and K-band data, and by analysing the quality of co-estimated gravity field solutions.

How to cite: Lasser, M., Meyer, U., Arnold, D., and Jäggi, A.: Time-variable gravity field determination from GRACE-FO data using the Celestial Mechanics Approach, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-28, https://doi.org/10.5194/gstm2020-28, 2020.

We present a summary of our recent work on time-variable gravity estimates derived from GRACE/GRACE-FO and satellite laser ranging (SLR). We show the latest results of our monthly mascon solution, with special attention paid to the selection and impact of the damping parameter applied to the regularization matrices. Additionally, we present a new method of regularized mascon estimation from spherical harmonics. Provided that the full normal equations for these coefficients are available, this method allows for a mathematically equivalent mascon estimate to those determined from a single iteration solution from Level-1B observations, but at a fraction of the computational cost. For this we use the ITSG-Grace2018 solution and full normal equations to produce a new mascon solution of comparable quality to current mascon products, and we are in the process of using this rapid approach to perform a trade study of various regularization strategies. Lastly, we present low degree SLR solutions that form the basis of the C20 and C30 solutions provided in Technical Note 14, and present preliminary results of large SLR-derived mascons from 1993 – Present.

How to cite: Loomis, B., Croteau, M., Sabaka, T., Luthcke, S., and Rachlin, K.: GRACE/GRACE-FO mascons and satellite laser ranging: Updates from GSFC, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-32, https://doi.org/10.5194/gstm2020-32, 2020.

The unique instrumentation of GRACE Follow-On (GRACE-FO) offers two independent inter-satellite ranging systems with concurrent observations. Next to a K-Band Ranging System (KBR), which has already been proved during the highly-successfully GRACE mission, the GRACE-FO satellites are equipped with an experimental Laser Ranging Interferometer (LRI), which features a drastically increased measurement precision compared to the KBR. Having two simultaneous ranging observations available allows for cross-calibration between the instruments and, to some degree, the separation of ranging noise from other sources such as noise in the on-board accelerometer (ACC) measurements.  

In this contribution we present a stochastic description of the two ranging observation types provided by GRACE-FO, which also takes cross-correlations between the two observables into account. We determine the unknown noise covariance functions through variance component estimation and show that this method is, to some extent, capable of separating between KBR, LRI, and ACC noise. A side effect of this stochastic modelling is that the formal errors of the spherical harmonic coefficients fit very well to empirical estimates, which is key for combination with other data types and uncertainty propagation. We further compare the gravity field solutions obtained from a combined least-squares adjustment to KBR-only and LRI-only results and discuss the differences between the time series with a focus on gravity field and calibration parameters. Even though, at the moment, global statistics only show a minor improvement when using LRI ranging measurements instead of KBR observations, some parts of the spectrum and geographic regions benefit significantly from the increased measurement accuracy of the LRI. Specifically, we see a higher signal-to-noise ratio in low spherical harmonic orders and the polar regions.

How to cite: Kvas, A., Behzadpour, S., and Mayer-Guerr, T.: Gravity Field Recovery and Observation Noise Separation from Simultaneous Laser Ranging Interferometer and K-Band Ranging System Measurements, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-14, https://doi.org/10.5194/gstm2020-14, 2020.

The Laser Ranging Interferometer (LRI) instrument on-board of the GRACE Follow-On (GRACE-FO) satellites has been collecting science data since a June 2018, a few weeks after launch. Though the LRI instrument, based on design concepts developed for a future LISA mission, was intended mostly as a demonstration instrument, it has far-exceeded its mission requirements and has provided intersatellite ranging observations with improved signal-to-noise ratio compared to the K-Band Ranging (KBR) instrument. The exploitation of the LRI observations has led to a set of monthly gravity field solutions comparable in many ways to the ones obtained from KBR. Though the ranging observations from the LRI have a much lower high-frequency noise content, this has not so far led to an improvement of the time-varying gravity estimates in the spatial domain, while stark differences are visible in the spectral domain between the KBR and LRI fields. We present the series of LRI gravity models in comparison to its KBR counterpart, as well as regional intercomparisons of the gravity solutions against hydrology models.

How to cite: Pie, N., Tamisiea, M., Krichman, B., Nagel, P., Poole, S., Save, H., and Bettadpur, S.: Evaluation of the Time Varying Gravity Fields from GRACE Follow-On LRI Instrument, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-34, https://doi.org/10.5194/gstm2020-34, 2020.

The International Combination Service for Time-variable Gravity Fields (COST-G), a product center of IAG’s International Gravity Field Service, aims on the combination of monthly global gravity field models. A consolidated gravity field series is derived from individual releases provided by different analysis centres (AC). COST-G’s Product Evaluation Group (PEG) assesses the combined gravity field series as well as the series provided by the ACs regarding their suitability for studying mass changes in the Earth’s subsystems (e.g. oceans, cryosphere, continental hydrosphere). Here we present results from the PEG’s assessment regarding mass changes of the ice sheets. Our study focuses on the COST-G RL01 series and the contributing releases AIUB RL02, GFZ RL06, ITSG-Grace2018, CSR RL06 and an unconstrained variant of GRGS RL05.

Based on residual variations of the spherical harmonic (SH) coefficients with respect to a long-term and seasonal model, we quantify the noise level of the latest GRACE/GRACE-FO solutions series provided by COST-G and the contributing ACs. This assessment is performed both in the SH domain and in the space domain, focusing on the polar regions. A regional integration approach using tailored sensitivity kernels is applied to derive mass change time series for individual drainage basins and the entire ice sheets in Greenland (GIS) and Antarctica (AIS). A measure for the noise level of the different mass change time series is inferred from the residuals with respect to a climatology, corrected for remaining inter-annual mass changes. In this way we are able to assess if and to which extent mass change products for GIS and AIS can benefit from the combination of different solution series. Furthermore, we also quantify the signal content inherent to the individual mass change time series in terms of the seasonal signal and the linear trend (i.e. mass balance). Differences in the signal content between the releases are further investigated with respect to contributions from different parts of the SH spectrum. In addition to selected SH coefficients (e.g. C₂₁, S₂₁), we systematically attribute the differences to each SH degree and the corresponding SH orders.

How to cite: Groh, A., Meyer, U., Lasser, M., Dahle, C., Kvas, A., Lemoine, J.-M., Jäggi, A., Flechtner, F., Mayer-Gürr, T., and Horwath, M.: Intercomparison of signals and noise in time variable gravity field releases from an ice sheet perspective, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-37, https://doi.org/10.5194/gstm2020-37, 2020.

To assess the quality of the CSR solutions, we compare results against external data sets that have contemporaneous availability.  These evaluations fall into three categories: changes in terrestrial water storage against data from the North American and Global Land Data Assimilation Systems, variations in ocean bottom pressure against data from the Deep Ocean Assessment of Tsunami Network, and estimates of the low degree and order Stokes coefficients compared against those inferred from satellite laser ranging observations (i.e. the CSR monthly 5x5 gravity harmonics from the MEaSUREs project).   As the mission provides a unique measurement of mass changes in the Earth system, evaluation of the new solutions against other data sets and models is challenging.  Thus, we primarily focus on relative agreement with these data set with the GRACE-FO solutions in relation to the historic agreement of the data sets with the GRACE solutions.

How to cite: Tamisiea, M., Krichman, B., Save, H., Bettadpur, S., Kang, Z., Ries, J., Pie, N., Poole, S., and Nagel, P.: Intercomparisons of CSR GRACE-FO Solutions with External Data Sets, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-62, https://doi.org/10.5194/gstm2020-62, 2020.

The satellite gravimetry missions GRACE (Gravity Recovery and Climate Experiment) and GRACE Follow-On provide the global and monthly gravity field for almost 17 years, which plays an irreplaceable role in understanding the mass transport of the Earth system. The key observation is the biased inter-satellite range, which is measured primarily by a K-Band Ranging system (KBR) in GRACE and GRACE Follow-On. The GRACE Follow-On satellites are additionally equipped with a Laser Ranging Interferometer (LRI), which provides measurements with lower noise compared to the KBR. However, the measured biased range which is directly measured by the inter-satellite ranging systems differs from the instantaneous biased range which is usually required for gravity field recovery. The difference is called the Light Time Correction (LTC) and arises from the non-zero travel time of electromagnetic waves between the spacecraft. We re-analyzed the LTC calculation from first principles considering general relativistic effects and state-of-the-art models of Earth’s potential field, and different types of orbital data. By analyzing the iterative equations in the LTC calculation of KBR and LRI, a novel analytical expression method is obtained to avoid the numerical limitation of the classical method. The dependency of the LTC on geopotential models and on the parameterization is further studied, and afterwards the results are compared against the LTC provided in the official datasets of GRACE and GRACE Follow-On. It is shown that the new approach has significantly lower noise, well below the instrument noise of current instruments, especially relevant for the LRI, and even if used with kinematic orbit products. This allows calculating the LTC accurate enough even for the next generation of gravimetric missions.

How to cite: Yan, Y., Müller, V., Heinzel, G., and Zhong, M.: Revisiting the Light Time Correction in Gravimetric Missions Like GRACE and GRACE Follow-On, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-4, https://doi.org/10.5194/gstm2020-4, 2020.