Over the past year several studies to possible improvement of high resolution gravity field modelling in coastal region has been carried out. One is a corporation between SHOM, DTU Space and Noveltis on updating altimetric data and on the use of these for gravity field modelling in the Northeast European shelf. The second is a corporation between DTU Space and CNES to contribute to the GEOMED2 project and compute high resolution gravity fields in the eastern part of the Mediterranean Sea. These joint investigations have in various ways particularly focused on studying the effect on the use of SAR altimetry from the Cryosat-2 mission which is operating in SAR mode in the Northeast part of the Atlantic Ocean. In this presentation we investigate the effect of using SAR altimetry for marine gravity field modelling in comparison with RDSAR altimetry.
In the Aegean Sea in the Mediterranean we were fortunately to get the mode mask changed from SAR to SAR-in with Mode mask v3.8 and 3.9. Hence in this region we can compare various Cryosat-2 data and the implication of the mode mask selection for marine gravity field determination.
We select CryoSat-2 altimetry products over a time interval of 6-years from October 2010 to December 2016 along the Northeastern coasts of the Atlantic Ocean. The ESA GPOD processors SARVatore were applied to retrack the 20 Hz Cryosat-2 SAR. The SAMOSA-2 (Ray et al., 2014) and SAMOSA+ SAR products have been evaluated in the presentation. They initially differ in the way the SAR waveforms are build (i.e. Hamming weighting window on the burst data prior to the azimuth FFT, zero-padding prior to the range FFT, doubling of the extension for the radar range swath) and in the retracking methodology.
The Cryosat-2 waveforms were turned into conventional LRM like data and the RDSAR waveforms have been retracked by the RADS altimetry system and this dataset were also introduced into the validation in order to investigating the effect of using SAR altimetry using two near-similar datasets.
In order to ensure that the most recent up to date ocean tide model the FES2014 have been applied to the various datasets by Noveltis as the ocean tide signal is huge in large part of the Northeast European shelf.
In order to evaluate these first data we compared with marine gravity field data along the Northeast European shelf and in the Aegean Sea we compare with an updated GEOMED2 marine gravity dataset.

One of the main strengths of InSAR is that it can provide trillions of geodetic observations without any advanced planning and installation of markers in the field. Unfortunately, the geodetic signal provided in each radar satellite track, is defined in its own arbitrary reference system. It remains a big challenge to combine different radar tracks into an internationally agreed upon reference system (such as the ITRS), unless well-defined co-locations between InSAR and other geodetic techniques (such as GNSS), can be established.

We argue that this must be a real mechanical connection; just selecting a couple of InSAR persistent scatterers (PS) nearby a GNSS station will not do as they generally experience different autonomous motion. For InSAR there is the choice between using trusty but bulky radar corner reflectors, or slim but not yet so well established radar transponders. We believe there is a use for both. Either attach a radar transponder to an existing GNSS station, or purposely design a co-located device with two radar reflectors, GNSS antenna, levelling markers, etc. Having co-located GNSS and InSAR reference points serves two main purposes: (i) connection of the InSAR reference frame to ITRS, (ii) monitoring of the autonomous motion of the GNSS antenna (or actually integrated device) with respect to selected PS, which will increase the value of the GNSS observations for space-geodesy, geodynamics and reference frame maintenance.

In this presentation we present the state of the art in passive reflectors and active transponders. In particular, we focus on experimental results of the so-called Integrated Geodetic Reference Stations (IGRS), of which 25 are being deployed in the Netherlands, and testing of active transponders. Most of the results have been obtained at a dedicated test site in Wassenaar, the Netherlands, but we will also show results from equipment deployed in the field. We will show that the co-location of corner reflectors and GNSS do not interfere with each other or degrade the individual accuracy. Also we will show that, by clever design, some disadvantages of corner reflectors can be overcome. We will present our experiences with testing of active transponders of different manufacturers, and show that the phase precision of the active transponders is in the order of an equivalent displacement of 0.5-1 mm, comparable to that of a corner reflector with a inner leg length of 90 cm.

Finally, we will present our plans and ideas for a community-driven effort for the realisation of an InSAR infrastructure.

Launched on May-22, 2018, the GRACE Follow-On mission is set to continue the data record of mass transport in the Earth system that was initiated by the GRACE mission in 2002, and which ended in mid-2017 after more than 15 years of successful operations and science discoveries. GRACE provided a unique data record of monthly to decadal observations of global mass transport in the Earth system from observations of temporal variations in the Earth’s gravity field. These observations have become indispensable for climate-related studies and provide critical measurements of Earth's time varying mass distribution. Tracking mass changes through gravity measurements provides unique observations of the evolution of the global water cycle, including ocean dynamics, polar ice mass changes, and global ground water changes.

The GRACE Follow-On mission is the result of a partnership between NASA (US) and GFZ (Germany) to ensure continuity of this important climate record using a microwave ranging instrument, similar to the one used on GRACE. Additionally, GRACE Follow-On is demonstrating improved inter-satellite ranging from a Laser-Ranging Interferometer that promises to improve the spacecraft-to-spacecraft ranging measurements for the next-generation of mass change missions.

In this presentation, we will provide status update of the post-launch, in-orbit checkout and early operations phases, and instrument and science payload performance. We will discuss data processing and calibration approaches, as well as provide an assessment of the first GRACE-FO gravity fields and the continuity with GRACE observations. We will conclude with an outlook on mission lifetime, and possible future mass change missions.

Several mountain ranges as Alps, Himalaya and Tibet are presently subject to uplift, as documented by GNSS vertical movement rates. Uplift occurs in response to climatic mass loss (deglaciation or hydrologic mass loss) or due to the dynamic forces (crustal compression or mantle inflow below uplifting crust). The uplift generates a mass change, which produces a time variation of the gravity field. The deglaciation and changes in the subsurface hydrologic budget, also generate a mass change, which sums to the tectonic change. The Sentinels can be used to determine the shrinking outlines of glaciers, using both multispectral imaging as well as Radar observations, thus allowing to determine the surface geometry change. The essential value for climate change and estimate of the hydrologic budget is though the total volume budget estimate, which requires also the thickness variation. Remote sensing catches the surface height changes, but these must be corrected for the crustal uplift. The geodetic measurements of the crustal dynamics of the Alpine and Himalayan mountain ranges in terms of height and gravity changes, are therefore in close relation to the estimate of the climatic changes inducing glacier and hydrologic budget changes. We estimate the hydrologic and glacier signal for the Alps and Himalaya-Tibet, using results from remote sensing and subsurface hydrologic observations, where available. We estimate the contribution of the dynamic uplift by direct observations of GNSS. We find that the hydrologic and glacier gravity signal calculated at satellite heights of GRACE and GOCE are superposed to the tectonic signal, and discuss to which amount the signals can be resolved by gravity measurements. We compare the predicted signals with the satellite observations of GRACE and GOCE, finding that the tectonic uplift signal is small relative to the expected glacier/hydrologic signals, but that it cannot be neglected. We define the requirements to future gravity satellites in order to make a significate contribution to the detection of hydro-glacial mass change detection and the separation of the tectonic signal.
Reference:
Chen W., Braitenberg, C., Serpelloni, E. (2018) Interference of tectonic signals in subsurface hydrologic monitoring through gravity and GPS due to mountain building, Global and Planetary Change, Volume 167, August 2018, Pages 148-159.

We perform several NGGM simulations over a 12-year operational period by including in the background gravity field the time-dependent gravity anomalies caused by different earthquake scenarios and considering, separately and jointly, the different sources of error on 28-day mean gravity field solutions: the instrumental errors of the interferometer and accelerometers (modelled by a white noise or a more realistic colored noise), the background model (static, EIGEN-6C4, and time dependent, ESA-ESM) and the atmosphere-ocean dealiasing. In order to assess whether the observational errors mask or not the earthquake-induced gravity signals, we assume known the background gravity field and the spatial and temporal pattern of the earthquake-induced gravity anomalies. Then, for each earthquake, we estimate the amplitude of its gravity anomaly by inverting the NGGM synthetic data time series and we check its consistency with the expected amplitude, as well as with the null hypothesis. In order to investigate case studies representative of the main earthquake characteristics and their compliance with the NGGM specifications, we have considered normal, inverse and strike-slip focal mechanisms striking with different angles with respect to the polar orbit, reaching the Earth surface and in depth, occurring inland, off-shore and close to the coastlines and at the beginning (2-4 years), at the middle (5-7 years) and at the end (8-10 years) of the 12-year operational period. The fault dimensions and slip distribution vary with the seismic moment magnitude and are prescribed according to the circular fault model by Eshelby (1957). Furthermore, we also consider two different rheological stratifications with asthenospheric viscosity of 10¹⁸ and 10¹⁹ Pa s. In order to discuss whether the earthquake signal can be discriminated from other geophysical processes (like atmosphere, ocean, hydrology and glacial isostatic adjustment), we also perform the same inversion but, this time, its amplitude is estimated jointly with the time dependent background gravity field, which we simply model using static values, trends and periodical functions.

In the past decades, 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 typically suffer from high noise at low frequencies, cold atom interferometers (CAI) are highly accurate over the entire frequency range and 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 (GOCE-like and GRACE-like) and including validation activities, e.g. a first successful airborne survey with a CAI gravimeter. We will present the first conclusions of these different mission and instrument studies:
• The airborne gravity survey during the ESA Cryovex/KAREN 2017 campaign using this technology was conducted by DTU and ONERA. The survey started with a return flight along a profile from Akureyri to Snæfellsnes, for which the reproducibility of the instrument’s measurements was better than 4 mGal in terms of standard deviation. A gravity survey over the glacier Vatnajokull was then realized during three days flying. The measurements did not show any drift and their accuracy was found to be less than 6 mGal, leading to the conclusion that the world’s first airborne campaign using an absolute CAI gravimeter was a full success.
• A first space quantum gravity mission is based on a gravity gradiometer concept that has a high common mode rejection, which greatly relaxes the drag-free control requirements compared to the GOCE mission.
• The second is to deploy quantum accelerometers for correcting low frequency errors of electrostatic accelerometers that are used in a low-low satellite-to-satellite ranging concept for measuring non-gravitational accelerations.
We will present for both concepts the expected improvement in measurement accuracy and the expected improvement of 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 such as attitude control, altitude of the satellite, lifetime of the mission, etc.