Displays

At the beginning of the 20th century the theories of special and general relativity were developed by Einstein and his contemporaries. These physical theories revolutionize our conceptions of time and of the measurement of time. The atomic clocks, which appeared in the 1950s, are so accurate and stable that it is now essential to take into account many relativistic effects. The development and worldwide comparisons of such atomic clocks allowed for some of the most stringent of fundamental physics, as well as new ideas for the search of dark matter. On a more applied level, when taking general relativity for granted, distant comparisons of atomic clocks can be used for navigation and positioning, as well as the determination of the geopotential. I will show how the chronometric observables can fit and be used within the context of classical geodesy and geophysics, presenting various applications: determination of the geopotential with high spatial resolution, vertical reference system, and discussing the possible applications associated to the geodynamic processes related to mass transfers.

How to cite: Delva, P. and Lion, G.: Chronometric measurements in geodesy and geophysics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21892, https://doi.org/10.5194/egusphere-egu2020-21892, 2020.

The realization of International Height Reference System (IHRS) is one of the major tasks of the International Association of Geodesy (IAG). Here we formulate a framework for connecting two local VHSs using ultra-precise frequency signal transmission links between satellites and ground stations, which is referred to as satellite frequency signal transmission (SFST) approach. The SFST approach can directly determine the geopotential difference between two ground datum stations without location restrictions, and consequently determine the height difference of the two VHSs. Simulation results show that the China’s VHS and the US’s VHS can be unified at the accuracy of several centimeters, provided that the stability of atomic clocks used on board the satellite and on ground datum stations reach the highest level of current technology, about 4.8×10^-18 in 100 s. The SFST approach is promising to unify the global vertical height datum in centimeter level in future, providing a new way for the IHRS realization. This study is supported by NSFCs (grant Nos. 41721003, 41631072, 41874023, 41804012, 41429401, 41574007) and Natural Science Foundation of Hubei Province of China (grant No. 2019CFB611).

How to cite: Shen, Z., Shen, W.-B., and Zhang, S.: Unification of global vertical height system using precise frequency signal links, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3831, https://doi.org/10.5194/egusphere-egu2020-3831, 2020.

Stationary optical clocks show fractional instabilities below 10^-18 when averaged over an hour, and continue to be improved in terms of precision and accuracy, uptime and transportability. The frequency of a clock is affected by the gravitational redshift, and thus depends on the local geopotential; a relative frequency change of 10^-18 corresponds to a geoid height change of about 1 cm. This effect could be exploited for sensing large-scale temporal geopotential changes via a network of clocks distributed at the Earth's surface. The CLOck NETwork Services (CLONETS) project aims to create an ensemble of optical clocks connected across Europe via optical fibre links.
A station network spread over Europe, which is already installed in parts, would enable us to determine temporal variations of the Earth's gravity field at time scales of days  and thus provide a new means for validating satellite missions such as GRACE-FO or potential Next Generation Gravity Missions. However, mass changes at the surface of an elastic Earth are accompanied by load-induced height changes, and clocks are sensitive to non-loading e.g. tectonic height changes as well. As a result, local and global mass redistribution as well as local height change will be entangled in clock readings, and very precise  GNSS measurements will be required to separate them.
Here, we show through simulations how ice (glacier mass imbalance), hydrology (water storage) and atmosphere (dry and wet air mass) signals over Europe could be observed with the currently proposed/established clock network geometry and how potential extensions can benefit this observability. The importance of collocated GNSS receivers is demonstrated for the sake of signal separation.

How to cite: Schröder, S., Springer, A., Kusche, J., and Stellmer, S.: Validating future gravity missions via optical clock networks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1998, https://doi.org/10.5194/egusphere-egu2020-1998, 2020.

In the past three decades, optical clocks and frequency transfer techniques have experienced a rapid development. They are approaching a fractional frequency uncertainty of 1.0x10^-18, corresponding to about 1.0 cm in height. This makes them promising to realize “relativistic geodesy”, and it opens a new door to directly obtain gravity potential values by the comparison of clock frequencies. Clocks are thus considered as a novel candidate for determining the Earth’s gravity field. We propose to use a spaceborne clock to obtain gravity potential values along a satellite orbit through its comparison with reference clocks on ground or with a co-orbital clock. The sensitivity of clock measurements is mapped to gravity field coefficients through closed-loop simulations.

In addition, clocks are investigated for other geodetic applications. Since they are powerful in providing the height difference between distant sites, clocks can be applied for the unification of local/regional height systems, by estimating the offsets between different height datums and the systematic errors within levelling networks. In some regions like Greenland, clocks might be a complementary tool to GRACE(-FO) for detecting temporal gravity signals. They can be operated at locations of interest and continuously track changes w.r.t. reference clock stations. The resulting time-series of gravity potential values reveal the temporal gravity signals at these points. Moreover, as the equipotential surface at a high satellite altitude is more regular than that on the Earth’s surface, a couple of clocks in geostationary orbits can realize a space-based reference for the determination of physical heights at any point on the Earth through clock comparisons.

We gratefully acknowledge the financial support by the Deutsche Forschungsgemeinschaft (DFG) under Germany’s Excellence Strategy EXC-2123/1 (Project-ID: 390837967).

How to cite: Wu, H., Müller, J., and Knabe, A.: Optical clocks for gravity field observation and further geodetic applications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10476, https://doi.org/10.5194/egusphere-egu2020-10476, 2020.

The GRACE Follow-On satellites were launched on 22^nd May 2018 to continue the measurement of Earth’s gravity field from the GRACE satellites (2002-2017). A few weeks later, an inter-satellite laser link was established with the novel Laser Ranging Interferometer (LRI), which offers an additional measurement of the inter-satellite range next to the one provided by the conventional microwave ranging instrument. The LRI is the first optical interferometer in space between orbiters, which has demonstrated to measure distance variations with a noise below 1 nm/rtHz at Fourier frequencies around 1 Hz, well below the requirement of 80 nm/rtHz.

In this talk, we provide an overview on the LRI and present the current status and results regarding the characterization of the instrument. We will address the scale factor, which is needed to convert the phase measurements to a displacement, and the removal of phase jumps that are correlated to attitude thruster activations. Moreover, the results comprise the coupling of attitude variations into the measured range, which is determined by means of the center-of-mass calibration maneuvers. This coupling is expected to be one of the major error sources at low frequencies, however, it is not directly apparent due to the large gravity signal.

We conclude with some learned lessons and potential modifications of the interferometry for future geodetic missions.

How to cite: Müller, V. and the GRACE Follow-On LRI Team: Laser Ranging Interferometer on GRACE Follow-On: Current Status, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10566, https://doi.org/10.5194/egusphere-egu2020-10566, 2020.

In the past decade, it has been shown that atomic quantum sensors are a newly emerging technology that can be used for measuring the Earth’s gravity field. Whereas classical accelerometers, based e.g. on capacitive sensing and electrostatic actuation, are limited by relatively high noise at low frequencies, Cold Atom Interferometers (CAI) can be highly accurate over the entire frequency range, which also has the benefit that they do not need any calibration phase. Several studies related to these new sensor concepts were initiated at ESA, mainly focusing on technology development for different instrument configurations (gravity gradiometers and  satellite-to-satellite ranging systems) and including validation activities, e.g. two successful airborne surveys with a CAI gravimeter. We will present the first conclusions of these different mission and instrument studies:

• The first airborne gravity survey during the ESA Cryovex/KAREN 2017 campaign using this technology was conducted by DTU and ONERA. The measurements did not show any drift and the accuracy was found to be less than 4 mGal at 11 km resolution. A second campaign has been conducted by ONERA and CNES in 2019 in the South of France and improved the accuracy by a factor 4, reaching classical airborne survey state-of the art performance.
• A first space quantum gravity mission concept is based on a gravity gradiometer that delivers a very high common mode rejection, greatly relaxing the drag-compensation requirements.
• The second concept is based on quantum accelerometers for correcting low frequency errors of electrostatic accelerometers that are used in a low-low satellite-to-satellite ranging concept in order to measure non-gravitational accelerations.

For both concepts we will present the expected improvement in measurement accuracy and in the derived Earth gravity field models, taking into account the different types of measurement (e.g. single axis vs. three axis, integration time, etc.) and different mission parameters (e.g. attitude control, altitude of the satellite, lifetime of the mission, etc.). A technology roadmap will be outlined for potential implementation of a quantum inertial sensor geodesy mission within 10-15 years.

How to cite: Massotti, L., Carraz, O., Bensi, P., Haagmans, R., Martimort, P., and Silvestrin, P.: Cold Atom Interferometer activities for measuring the Earth’s gravity field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8924, https://doi.org/10.5194/egusphere-egu2020-8924, 2020.

After three years of development in collaboration with LNE-SYRTE, we report on the development, the integration and the preliminary operation of an industry-grade absolute differential gravimeter. This new generation of instrument goes beyond the possibilities offered by existing gravity gradiometers, as one differential gravimeter measures simultaneously g and the vertical gradient of g [1]. Relying on atom interferometry with cold 87 Rb atoms, a single vertical laser beam simultaneously measures the vertical acceleration experienced by two sets of laser-cooled atoms free-falling from different heights. For each drop, the half-sum of the two vertical accelerations gives access to g and the half-difference to dg / dz. As far as technology is concerned, our differential gravimeter relies on a physical principle and a set of technologies that have already been validated for absolute quantum gravimeters [2].

Our demonstrator is operational since November 2019 and has shown the ability to run continuously for more 18 days without any human attendance.  We will present in detail the experimental results for the measurement of g and dg / dz. Regarding the measurement of the vertical gradient of g, we obtain a short-term sensitivity of 76 E/√t (1E = 10 -9 s -2 = 0.1 µGal/m) and a resolution of a 4 E when data is averaged over 1000 s. Regarding the measurement of g itself, we obtain a short-term sensitivity of 36 µGal/√t and a resolution of a few µGal when data is averaged over 500 s. These are preliminary results and options and future plan to improve the sensitivity and the stability of the measurements will be discussed.

Such quantum differential gravimeter is to our knowledge the only technology that allows for an absolute continuous drift-free monitoring of simultaneously gravity and gravity gradient over timescales from a few minutes to several months.

This work has been supported by the DGA, the French Department of Defense.

[1] R. Caldani et al., "Simultaneous accurate determination of both gravity and its vertical gradient", Phys. Rev. A 99, 033601 (2019)

[2] V. Ménoret et al., "Gravity measurements below 10−9 g with a transportable absolute quantum gravimeter", Nature Scientific Reports, vol. 8, 12300 (2018)

How to cite: Janvier, C., Ménoret, V., Lautier, J., Desruelle, B., Merlet, S., Pereira dos Santos, F., and Landragin, A.: Operating an industry-grade quantum differential gravimeter, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9185, https://doi.org/10.5194/egusphere-egu2020-9185, 2020.

Determination of Earth’s gravity field in a high accuracy needs different complementary data and also methods to combine these data in an optimized procedure. Newly invented resources such as GPS, GRACE, and GOCE provide various data with different distribution which makes it possible to reach this aim. Least Squares Collocation (LSC) is one of the methods that help to mix different data types via covariance function which correlates the different involved parameters within the procedure. One way to construct such covariance functions is involving two steps within the remove-compute-restore (RCR) procedure: first, calculation of an empirical covariance function from observations which the gravitational effects of global gravity field (Long-wavelength) and topography/bathymetry have been subtracted from it and then fitting the Tscherning–Rapp analytical covariance model to the empirical one. According to the corresponding studies, the accuracy of LSC is directly related to the ability to localize the covariance function which itself depends on the data distribution. In this study, we have analyzed the data distribution and geometrically fitting factors, on GPS/Leveling and GOCE gradient data by considering the various case studies with different data distributions. To make the assessment of the covariance determination possible, the residual observations were divided into two datasets namely, observations and control points. The observations point served as input data within the LSC procedure using the Tscherning – Rapp covariance model and the control points used to evaluate the accuracy of the LSC in gravity gradient, gravity anomaly, and geoid predicting and then the covariance estimation. The results of this study show that the Tscherning-Rapp (1974) covariance has different outcomes over different quantities. For example, it models accurate enough the empirical covariance of gradient gravity but requires more analysis for gravity anomalies and GPS/Leveling quantities to reach the optimized results in terms of STD of difference between the computed and control points.

How to cite: Heydarizadeh Shali, H., Ramouz, S., Safari, A., and Barzaghi, R.: Assessment of Tscherning-Rapp covariance in Earth gravity modeling using gravity gradient and GPS/leveling observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1059, https://doi.org/10.5194/egusphere-egu2020-1059, 2020.

The Earth’s geoid is one of the most important fundamental concepts to provide a gravity field- related height reference in geodesy and associated sciences. To keep up with the ever-increasing experimental capabilities and to consistently interpret high-precision measurements without any doubt, a relativistic treatment of geodetic notions within Einstein’s theory of General Relativity is inevitable. 

Building on the theoretical construction of isochronometric surfaces we define a relativistic gravity potential as a generalization of known (post-)Newtonian notions. It exists for any stationary configuration and rigidly co-rotating observers; it is the same as realized by local plumb lines and determined by the norm of a timelike Killing vector. In a second step, we define the relativistic geoid in terms of this gravity potential in direct analogy to the Newtonian understanding. In the respective limits, it allows to recover well-known results. Comparing the Earth’s Newtonian geoid to its relativistic generalization is a very subtle problem. However, an isometric embedding into Euclidean three-dimensional space can solve it and allows an intrinsic comparison. We show that the leading-order differences are at the mm-level. In the next step, the framework is extended to generalize the normal gravity field as well. We argue that an exact spacetime can be constructed, which allows to recover the Newtonian result in the weak-field limit. Moreover, we comment on the relativistic definition of chronometric height and related concepts.

In a stationary spacetime related to the rotating Earth, the aforementioned gravity potential is of course not enough to cover all information on the gravitational field. To obtain more insight, a second scalar function can be constructed, which is genuinely related to gravitomagnetic contributions and vanishes in the static case. Using the kinematic decomposition of an isometric observer congruence, we suggest a potential related to the twist of the worldlines therein. Whilst the first potential is related to clock comparison and the acceleration of freely falling corner cubes, the twist potential is related to the outcome of Sagnac interferometric measurements. The combination of both potentials allows to determine the Earth’s geoid and equip this surface with coordinates in an operational way. Therefore, relativistic geodesy is intimately related to the physics of timelike Killing vector fields.

How to cite: Philipp, D., Laemmerzahl, C., Hackmann, E., Perlick, V., Puetzfeld, D., and Mueller, J.: Fundamental Notions in Relativistic Geodesy - physics of a timelike Killing vector field, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16528, https://doi.org/10.5194/egusphere-egu2020-16528, 2020.

Gravity field measurement by free-falling atoms has the potential for very high stability
over time as the measurement exposes a direct, fundamental relationship between mass
and acceleration. However, the measurement rate of the current state-of-the-art limits
the performance at short timescales (greater than 1 Hz). Classical inertial sensors operate
at much faster response times and are thus natural companions for free-falling atom
sensors. Such a hybrid device would gain the ultra-high stability of the free-falling atom
sensor while greatly extending the bandwidth to higher frequency using the classical
sensor. This requires the stable bandwidth of both devices to overlap sufficiently. We
have developed opto-mechanical inertial sensors (OMIS) with good long term stability for
just this purpose. The sensors are made of highly stable fused silica material, feature a
monolithic optical cavity for displacement readout, and utilize a laser diode stabilized to
a molecular reference. With no temperature control and only the thermal shielding
provided by the vacuum chamber, this device is stable down to 0.1 Hz which overlaps
with the bandwidth of free-falling atom sensors. The OMIS are self-calibrating by
converting the fundamental resonances of a molecular gas into length using the
free-spectral range of the optical cavity,  FSR = c/2nL,  and then sampling the OMIS
mechanical damping rate and resonance frequency using a nearby piezo. This
acceleration calibration is potentially transferable to a companion free-falling atom
sensor. Readout is performed by modulating the cavity length of the OMIS with one
cavity mirror being the OMIS itself and the other being a high frequency resonator. The
high frequency resonator is driven by a nearby piezo well above the response rate of the
OMIS and acts like an ultrastable quartz clock. The resulting highly stable tone is
demodulated by the readout electronics. For the low finesse optical cavity used here, this
yields a displacement resolution of 2x10^-13 m/√Hz and a high frequency acceleration
resolution of 400 ng /√Hz. At 0.1 Hz the acceleration resolution is 1.5 μg /√Hz limited by
the stability of our vibration isolation stage. The OMIS dimensions are about 30 mm x 30
mm x 5 mm and can be fiber coupled to enable co-location with other sensors or as
standalone devices for future gravimetry both on Earth and in space

How to cite: Kumanchik, L., Guzman, F., and Braxmaier, C.: Opto-Mechanical Inertial Sensors (OMIS) for High Temporal Resolution Gravimetry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22614, https://doi.org/10.5194/egusphere-egu2020-22614, 2020.

ONERA (the French Aerospace Lab) is developing, manufacturing and testing ultra-sensitive electrostatic accelerometer for space application. Accelerometers have been successfully developed for the Earth-orbiting gravity missions CHAMP, GRACE, GOCE and GRACE-FO and for Earth-orbiting Fundamental Physics mission MICROSCOPE.

In ONERA accelerometer design, the proof mass was levitated and was maintained at the center of an electrode cage by electrostatic forces. Moreover this proof mass was connected by a thin conductive wire (typically 5, 7 or 10 µm diameter wire). This wire allows us to polarize the proof mass and to evacuate the random charges induced by space radiation.

By removing this polarization wire, there will be positive impacts on the accelerometer defaults such as the removal of the parasitic dumping noise at low frequencies created by wire or its bias contribution; but it is important to verify that there are not also negative impacts such as noisy charging process.

After studying the evolution of the space radiation energy distribution on interesting orbits for earth missions, an evaluation of implemented current on the proof mass has been performed. A UV LED had been tested; the set-up and first measurements will be presented. Moreover a prototype is developed by ONERA to characterized charge management capabilities of such a system on a representative environment.

How to cite: Boulanger, D., Christophe, B., Rodrigues, M., Liorzou, F., Lebat, V., Dalin, M., and Huynh, P.-A.: Future Electrostatic Accelerometer without Polarization Wire , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4640, https://doi.org/10.5194/egusphere-egu2020-4640, 2020.

The transportable Quantum Gravimeter QG-1 will perform absolute measurements of local gravitational acceleration with an unrivalled uncertainty below 3 nm/s² by utilising collimated Bose-Einstein-Condensates for atom interferometry in a compact setup. To permit this performance, leading order error sources of today’s cold atom gravimeters, predominantly stemming from the horizontal velocity of the interrogated atoms, will be minimised by this novel approach.
This contribution elaborates on the design and implementation of the interferometry setup into the atom chip based experimental system. We discuss their impact on the targeted uncertainty of 3 nm/s² and present recent developments for further miniaturisation and further reduction of next-generation instrument's complexities.

We acknowledge financial support from "Niedersächsisches Vorab" through "Förderung von Wissenschaft und Technik in Forschung und Lehre" for the initial funding of research in the new DLR-SI Institute and by the Deutsche Forschungsgemeinschaft (DFG) in the project A01 of the SFB 1128 geo-Q and under Germany's Excellence Strategy - EXC 2123 QuantumFrontiers, Project-ID 390837967.

How to cite: Herr, W., Heine, N., Matthias, J., Abend, S., Timmen, L., Müller, J., and Rasel, E. M.: A transportable absolute Quantum Gravimeter employing collimated Bose-Einstein condensates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21986, https://doi.org/10.5194/egusphere-egu2020-21986, 2020.

How to cite: Knabe, A., Wu, H., Schilling, M., and Müller, J.: Hybridization of atomic and electrostatic accelerometers for satellite control and gravity field recovery, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9893, https://doi.org/10.5194/egusphere-egu2020-9893, 2020.

In recent years, an innovative mission concept has been proposed for gravity measurements with the aim of continuously monitoring the Earth gravity and its changes. The concept is based on a satellite-borne interferometer exploiting ultra-cold atom technology. Among other studies, a team of researchers from Italian universities and research institutions proposed and carried out the MOCASS project, to investigate the performance of a cold atom interferometer flying on a low Earth orbiter and its impact on the modeling of different geophysical phenomena.

In this study, the basic idea was that of a GOCE follow-on mission, with a unique spacecraft carrying an instrument capable of measuring functionals of the Earth gravitational potential. The geodetic data analysis of the gravity gradient data attainable by such a mission was carried out following the space-wise approach developed at Politecnico di Milano. The mathematical model for the processing of the MOCASS data was formulated, including the filtering strategy applied to take into account the cold atom interferometer transfer function. Numerical simulations were performed, with different configurations of the satellite orbit and pointing mode of the interferometer; data were simulated for two cases: (i) a single-arm gradiometer observing T_xx or T_yy or T_zz gradients; (ii) a double-arm gradiometer observing T_xx and T_zz gradients or T_yy and T_zz gradients. The results of the simulations will be illustrated, showing the applicability of the proposed concept and the neat improvement in modeling the static gravity field with respect to GOCE.

Moreover, a new study called MOCAST+ has been lately started proposing an enhanced cold atom interferometer which can deliver not only gravity gradients but also time measurements. The study will investigate whether this could give the possibility of improving the estimation of gravity models even at low harmonic degrees, with inherent advantages in the modeling of mass transport and its global variations: this will represent fundamental information, e.g. in the study of variations in the hydrological cycle and relative mass exchange between atmosphere, oceans, cryosphere and solid Earth.

How to cite: Migliaccio, F., Reguzzoni, M., and Batsukh, K.: The space-wise approach for cold-atom interferometry geodetic data analysis: the MOCASS study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21546, https://doi.org/10.5194/egusphere-egu2020-21546, 2020.

In the summer of 2019 the Deutsches Zentrum für Luft- und Raumfahrt (German Aerospace Center) Institute for Satellite Geodesy and Inertial Sensing (DLR-SI) was founded. This new institute is located in Hannover with one group in Bremen. It benefits from strong ties to different institutes from the Leibniz University in Hannover (Institute for Quantum Optics, Institute of Geodesy, Institute for Gravitational Physics), the Center of Applied Space Technology and Microgravity (ZARM) in Bremen and the National Metrology Institute (PTB) in Brunswick.

In this talk I will give an overview of the focus of the working group for Laser Interferometric Sensing. The overarching goal of our group is the technology development for laser interferometric satellite geodesy missions, like a possible Next Generation Gravity Mission (NGGM). Due to the technological overlap, collaboration with the LISA community is also possible. Specifically, the currently planned work packages include, but are not limited to the development of novel optical bench topologies for laser interferometric ranging instruments for geodesy missions and the design of low noise photoreceivers and laser link acquisition sensors. Furthermore, interferometric readout of monolithic accelerometers will be studied and flight data from the GRACE Follow-On Laser Ranging Interferometer (LRI) will be evaluated within our group. In my talk I will give an overview of these planned work packages and will point out the expected benefit of these novel technologies to the geodesy community.

How to cite: Koch, A., Heinzel, G., Wanner, A., and Ertmer, W.: Advancing intersatellite laser interferometry for geodesy applications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14996, https://doi.org/10.5194/egusphere-egu2020-14996, 2020.

The Laser Ranging Interferometer (LRI) on-board GRACE Follow-On, which was launched in May 2018, provides ranging data between two satellites with previously unknown precision. The low noise level of approximately 200 pm/rtHz at Fourier frequencies around 10 Hz allows us to investigate features, that have not been seen before in the ranging data.  

Due to this high sensitivity of the LRI, we are able to assess spurious linear non-gravitational accelerations in direction of the line-of-sight caused by attitude thruster activation, which should ideally produce only angular motion. This analysis may help to refine the models used in the Calibrated Accelerometer Data (ACT) product. The ACT product is derived from raw accelerometer data and corrects artefacts present in the raw accelerometer (ACC) product. However, linear non-gravitational accelerations can only be measured in narrow frequency ranges by the LRI, where the gravity ranging signal decayed below other contributors.

The conversion of LRI Level-1A to 1B is a complex task that comprises non-trivial removal of phase jumps, scaling, filtering and interpolation of data. In order to access the high-quality ranging data and have low post-fit residuals, the LRI instrument team at the Albert-Einstein Institute (AEI) in Hanover, Germany derived an alternative LRI Level-1B data product for January 2019 with some improvements compared to the official SDS RL04 data. The data can be downloaded at https://wolke7.aei.mpg.de/s/AYza4wrFjYBxHHQ.

In this poster we compare the AEI release with RL04 and explain the differences in the preprocessing of the data, which mainly originate from a more sophisticated estimation of the scale factor (i.e. the absolute laser frequency or wavelength), a continuous data stream without biases at day bounds and a light time correction with less noise from numerical inaccuracies.

How to cite: Misfeldt, M., Müller, V., Heinzel, G., and Danzmann, K.: Alternative Level 1A to 1B Processing of GRACE Follow-On LRI data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15569, https://doi.org/10.5194/egusphere-egu2020-15569, 2020.

According to the general theory of relativity, two clocks placed at two different positions with different geopotential run at different rates. Thus one can determine the geopotential difference between these two points by comparing the running rates of the two clocks. In this paper, we propose, design and describe in detail an approach for determining the geopotential difference between The Atomic Clocks Ensemble in Space (ACES/PHARAO mission) and a ground station based upon a simulation experiment. The correction due to Ionosphere, troposphere and Sagnac effect will be taken into account. Our team is working on a wide range of problems that need to be solved in order to achieve high accuracy in (almost) real-time. In this paper, we will present some key aspects of the measurement, as well as the current status of the software's development. the proposed approach may have prospective applications in geoscience, and especially, based on this approach a unified world height system could be realized with one-centimetre level accuracy in the near future.

How to cite: Ashry, M., Shen, W., and Sun, X.: An Approach for Determining the Geopotential Difference between The Atomic Clocks Ensemble in Space (ACES) and a ground station, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13154, https://doi.org/10.5194/egusphere-egu2020-13154, 2020.

Atomic Clock Ensemble in Space (ACES) is an ESA mission designed mainly to test gravitational redshift with high-performance atomic clocks in space and on the ground. Here we develop tri-frequency combination (TFC) method based on the measurements of frequency shifts of three independent microwave links between ACES and a ground station. The potential scientific object requires an accuracy of at least 3×10^-16, thus we need to consider various effects including Doppler effect, second-order Doppler effect, atmospheric frequency shift, tidal effects, refraction caused by atmosphere, Shapiro effect, with accuracy level of tens of centimeters. The ACES payload will be launched in middle of 2021, and the formulation proposed in this study will enable us to test gravitational redshift at an accuracy level at least 2×10^-6 level, one order more higher than the present accuracy level. This study is supported by NSFCs (grant Nos. 41721003, 41631072, 41874023, 41804012, 41429401, 41574007) and Natural Science Foundation of Hubei Province of China (grant No. 2019CFB611).

How to cite: Sun, X., Shen, W.-B., Shen, Z., Cai, C., Xu, W., Zhang, P., and Ashry, M.: Test of gravitational redshift based on tri-frequency combination of frequency links between Atomic Clock Ensemble in Space and a ground station, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2349, https://doi.org/10.5194/egusphere-egu2020-2349, 2020.

Recovering the gravity field with the satellite’s frequency signal might be an alternative measuring mode in the future when the accuracy of the onboard clock was good enough. On the one hand, we analyze the performance of recovering gravity field model from the gravitational potentials with different accuracies on different satellite altitudes (from 200 km to 350 km) based on semi-analytical (SA) method. On the other hand, we analyze the performance based on the numerical analysis. First, the gravitational potentials along the satellite orbit are computed from the clock observations based on the method of satellite’s frequency signal with the accuracies of 10^-16 and 10^-18s. Then, based on the derived gravitational potentials, we recovered the gravity field models up to degree and order 200 (corresponding to 100 km spatial resolution). At last, the errors of recovered models are validated by comparing with the reference model.

How to cite: Xu, X., Shen, Z., Shen, W., and Zhao, Y.: Simulated experiment of recovering gravity field from the observations of satellite’s frequency signal, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20120, https://doi.org/10.5194/egusphere-egu2020-20120, 2020.

Recent advancements in atomic clocks have enabled us to measure gravitational potential differences with a precision which is applicable to geodetic uses, based on the gravitational red shift. In Europe, international fiber networks linking optical clocks have been developed for promoting the unification of height reference systems across countries, and 10 cm-level agreements in terms of the equivalent height difference to the gravitational potential have been achieved in the comparisons between chronometric and classical geodetic methods. In Japan, similar comparisons using two optical lattice clocks were carried out for i) a 15-km fiber connecting RIKEN in Wako city and the Hongo campus of the University of Tokyo and ii) a 450-m fiber link which vertically connected the observatory and the ground at the Tokyo Skytree. For the former comparison, agreement between chronometric and geodetic results was better than 10 cm, and for the latter, data are under analysis. A new clock site has been developed at the NTT Basic Research Laboratories in Atsugi City. Clocks in Wako, Hongo and Atsugi constitute an approximately 100-km-scale network. In this presentation, we report a preliminary result on the geodetic leveling survey to determine the gravitational potential difference between these three sites. To estimate uncertainties in the potential difference, we will compare the result partially with those determined from the geoid model and the GNSS ellipsoidal height. We will also consider the effects of crustal vertical motion, in addition to measurement errors.

How to cite: Tanaka, Y., Aoki, Y., and Nishiyama, R.: Geodetic determination of the gravitational potential difference for an optical lattice clock comparison in the Kanto region in Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6220, https://doi.org/10.5194/egusphere-egu2020-6220, 2020.

Abstract In this study, we carried out experiments of the geopotential difference determination at CASIC, Beijing with the help of two hydrogen atomic clocks, using the two-way satellite time and frequency transfe technique. Here the ensemble empirical mode decomposition method is adopted to extract geopotential-related time-elapse signals from the original observations. The clock-comparison-determined geopotential difference in the experiments is determined, which is compared to the previously known results determined by conventional approach. Results show that the geopotential difference determined by time comparison deviates from that determined by conventional approach by about 1589 m²s^-2, which is equivalent to 162 m in height, in consistence with the stability of the hydrogen atomic clocks applied in the experiments (at the level of 10^-15/day). Since the stability of the optical clocks achieve 10^-18 level, the geopotential determination by accurate clocks is prospective, and it is prospective to realize the unification of the world vertical height system. This study is supported by NSFCs (grant Nos. 41721003, 41631072, 41874023, 41804012, 41429401, 41574007) and Natural Science Foundation of Hubei Province of China (grant No. 2019CFB611).

How to cite: Ning, A., Wu, K., Shen, W.-B., Shen, Z., Cai, C., and Sun, X.: Experiments of determining the geopotential difference using two hydrogen atomic clocks and two-way satellite time and frequency transfer technique, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3388, https://doi.org/10.5194/egusphere-egu2020-3388, 2020.

According to general relativity theory, one may determine the geopotential difference between two arbitrary stations by comparing there-located clocks’ running rates. In this study, we provide experimental results of the geopotential determination based on the time elapse comparison between two hydrogen atomic clocks, one fixed clock  and one portable clock , using the common view satellite time transfer (CVSTT) technique. We compared the portable clock  located at Jiugongshan Time Frequency Station (JTFS) with the fixed clock  located at Luojiashan Time Frequency Station (LTFS) for 30 days. The two stations are separated by a geographic distance of around 240 km with height difference around 1230 m. Then the clock  was transported (without stopping its running status) to LTFS and compared with clock  for zero-baseline calibration for 15 days. The clock-comparison-determined geopotential difference between JTFS and LTFS is determined. Results show that the clock-comparison-determined result deviates from the EGM20080-determined result by about 2322±1609 m²s^-2, equivalent to 237±164  m in height, in consistence with the stability of the hydrogen atomic clocks applied in the experiments (at the level of 10^-15/day).

This study is supported by NSFCs (grant Nos. 41721003, 41631072, 41874023, 41804012, 41429401, 41574007) and Natural Science Foundation of Hubei Province of China (grant No. 2019CFB611).

How to cite: Wu, K., Shen, W.-B., Shen, Z., Cai, C., Sun, X., Ning, A., and Wu, Y.: An experiment of determining the geopotential difference using two hydrogen atomic clocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1716, https://doi.org/10.5194/egusphere-egu2020-1716, 2020.