RIGTC, Geodetic Observatory Pecný The institute's mission is basic and applied research in geodesy and cadastre Designated institute of Czech Metrology Institute National Standards for lengths 25 m to 1450 m and gravity acceleration ACTIVITIES: Earth’s gravity field: theory and geoid determination GPS analysis center: EUREF, GPS meteorology, CZEPOS DORIS analysis center of IDS METROLOGY, national gravity standard, GPS test baseline, time and frequency standard GRAVIMETRY

Contribution of gravity measurements for monitoring environmental impacts of climate changes

Definitions In geodesy, the Earth’s Gravity Field is described by non-inertial system – geocentric terrestrial reference system (including oceans and atmosphere) : gravity potential W = gravitational. p. U + Q centrifugal p. gravity acceleration grad W = g = gravitation b + z centrifugal acceleration g=|g| (magnitude of gravity acceleration) gravity acceleration = free-fall acceleration corrected for tides, polar motion and atmospheric effects

Gravity changes Temporal gravity changes due to mass transport and redistribution within the Earth system: ice melting, sea level changes, terrestrial water storage changes, crustal deformations, postglacial rebound, earthquakes, center of mass variations … The Global Geodetic Observing System (GGOS), Measurement accuracy: 1E-9; g  1  Gal (1E-8 m/s^2)  2-5 mm Consistent reference is needed (at 1  Gal level). Ilk et al., 2004 … some of them are closely related to impacts of climate changes

In situ measurements Absolute gravimetry Relative gravimetry Satellite gravimetry Two types of „g“ measurements

Aim: improved knowledge of Earth gravity field These projects have brought new types of observations → thanks to recent technological achievements → new inversion methods have been developed Each mission has a different kind of observations → mutual complementarity of results Missions are successful, global gravity field modelling has improved considerably CHAMP (2000–2010)GRACE A/B (2002–now)GOCE (2009–2013) Satellite missions

Observation types Satellite-to-satellite tracking: high-low (SST-hl) satellites in high orbits: GPS (altitude: km) positioning of satellites in low orbits (below km) Satellite to Satellite Tracking: low-low (SST-ll) observation of relative motion of two satellites GRACE A/B: by means of microwave ranging Satellite gravity gradiometry (SGG) to measure the gradient of gravity acceleration vector space gradiometer: composed of six accelerometers (Fig: Rummel et al., J Geodyn 33, 3–20) GOCE GRACE A/B CHAMP

Mission GRACE Gravity Recovery And Climate Experiment First operational application of SST-ll to study the gravity field: time-variable field and the static field measurement of relative motion of two satellites separated by 220±50 km using a microwave beam accelerometer on each satellite: observation of nongravitational forces low polar orbit: initial altitude 500 km, inclination 89° German/US project (DLR/NASA) two GRACE satellites launched in 2002, still working Currently GRACE mission provides best satellite gravity field models in the long and medium wavelengths (resolution 350–40000 km). (Fig:

GRACE,detection of time-variable “g” How masses move within Earth’s subsystems (land, ocean, ice, solid Earth) GRACE gravity fields computed from microwave observations every month. First detection by satellite (on regional scale ≈500 km): seasonal time- varying gravity field Mainly caused by water movement on the Earth’s surface in the atmosphere - global hydrological phenomena Variation in the geoid height due to seasonal hydrology ± 8 mm

GRACE, detection of time-variable “g” For major river basins, gravity field variations detected by GRACE correspond to those measured on the ground. Detection of the depletion of large groundwater aquifers: northern India, Central Valley of California (Figs: Schmidt et al., Glob. Planet. Change 50 (1–2), 112–126)

Changes of ice mass in polar areas are essential for climate change and for possible increase in mean ocean level. In situ observations are difficult. GRACE data have enabled direct estimate of ice mass changes. Figure: Mass changes provided by GRACE Units: mass change transformed to the sea level change (m/yr). Correction due to postglacial rebound: important influence on the interpretation of secular trends. (Fig: Siemes et al., J Geod 87:69–87) GRACE, detection of trends

GRACE: measurements Quantity: time-varying gravity field (observations of mass changes) from space Standard products: monthly gravity fields Optional products: gravity fields with shorter time resolution (10-day fields, 1-day fields) Spatial resolution of standard solutions: 400–500 km (globally) Time series of observations covers 13 years by now (2002–2015) Special filtering of standard solutions necessary to reduce correlated errors (stripes): Enhancement of spatial resolution down to ≈300 km Drawback: impact on observations (magnitude, uncertainty) Aliasing of gravity signal from one region to the neighbouring region E.g. much stronger gravity signal over land vs. weak signal over oceans Mitigation procedures affect the observations (magnitude, uncertainty) Smoothing filters usually applied: impact on the observations (magnitude, uncertainty) Quite complex processing of satellite data is an important aspect in obtaining particular quantitative results and their subsequent interpretation. Although there are publications about accuracy of GRACE products etc., no general consensus has been reached yet

In-situ measurements Absolute gravimeter: based upon physical standards no drift uncertainty: ± 2.5 µGal Long-term reproducibility : ± 1.5 µGal observation epochs Absolute gravimeter: based upon physical standards no drift uncertainty: ± 2.5 µGal Long-term reproducibility : ± 1.5 µGal observation epochs Superconducting gravimetr: relative values calibrated by absolute gravimeters precision < 0.1 µGal continuous registration high temporal resolution Superconducting gravimetr: relative values calibrated by absolute gravimeters precision < 0.1 µGal continuous registration high temporal resolution

Comparison of absolute gravimeters ICAGs – at BIPM from 1981 to , 2007, 2011, 2013 – Walferdange in Luxembourg 2 CIPM_KC + PS: 2009 and 2013; 2009: 11 KC + 10 PS; 2013: 10 KC + 15 PS 2 EURAMET_KC + PS: 2011, 2015

Accuracy of the reference FG5, FG5-X: most accurate absolute gravimeters based on laser interferometry, Standard uncertainty  2.5  Gal INTERNATIONAL COMPARISONS – FG5s dominate FG5s / AGs: 13/21 (2009), 17/21 (2011), 19/25 (2013) Weights FG5s / other AGs : > 4 / 1  g(FG5s) < 10  Gal Reference gravity values are strongly “FG5 dependent” !!! Systematic effects have to be captured: DIFFRACTION TEST MASS ROTATION FLOOR RECOIL RESIDUAL AIR PRESSURE ELECTRONICS COLLIMATION SELT ATTRACTION VERTICALITY ……..

Combination of AG and SG Superconducting gravimetr: relative values precision < 0.1 µGal continuous registration high temporal resolution Superconducting gravimetr: relative values precision < 0.1 µGal continuous registration high temporal resolution Absolute gravimeter: based upon physical standards no drift uncertainty: ± 2.5 µGal observation epochs Absolute gravimeter: based upon physical standards no drift uncertainty: ± 2.5 µGal observation epochs Reproducibility (Instrumental) : ± 0.72  Gal

“ g ” variations at Pecný Temporal gravity variations are caused mainly by hydrological effects

Hydrological effects on „g“ Global water storage variations : - direct effect, important signal d >1000 km, - loading effect, important signal 100 – 3000 km Loading effect, Newtonian effect and the Total effect caused by a centered homogeneous shell of water (of angular distance) and thickness of 1 m. Local water storage variations : Direct effect, important signal <1 km, 80% of the effect comes from d < m from the station Newtonian effect of the 10 m thick cylinder with 10% porosity and variable radius Variable distribution of the water within the Earth system affects “g”.

Corrected gravity series Roughly 50% of gravity variations at SG stations are caused by local hydrology SG measurements at reference stations should be available and the reference AG station should be close to the SG

Comparison with WGHM and GRACE TREND: GRACE: 0.43 ± 0.09  Gal/year PE : 0.39 ± 0.17  Gal/year WGHM : 0.12 ± 0.11  Gal/year PE_cor : ± 0.12  Gal/year AGREEMENT (std of differences): PE_cor vs. WGHM: ± 0.63  Gal PE_cor vs. GRACE_920 : ± 0.87  Gal PE_cor vs. GRACE_400 : ± 1.11  Gal WGHM vs. GRACE_400 : ± 1.32  Gal

IAG Resolution (No. 2) for the establishment of a global absolute gravity reference system The International Association of Geodesy,... resolves,  to adopt the Strategy Paper as the metrological basis for absolute gravimetry,  to initiate a working group to compile standards for the definition of a geodetic gravity reference system based upon the international comparisons of absolute gravimeters,  to establish a gravity reference frame by globally distributed reference stations linked to the international comparisons of absolute gravimeters where precise gravity reference is available at any time,  to link the reference stations to the International Terrestrial Reference System by co- location with space-geodetic techniques,  to initiate the replacement of the International Gravimetric Standardization Network 1971 (IGSN71) and latest International Absolute Gravity Basestation Network (IAGBN) by the new Global Absolute Gravity Reference System.

Proposed Gravity Reference Sites

Idea of an EMPIR project ENVIRONMENT TP: GEODESY & METROLOGY for achieving robust estimates of climate changes from satellite and terrestrial gravity data Tasks: -Robust estimates (magnitude and uncertainties) from satellite gravimetry on climate changes – new approaches for processing satellite data, robust uncertainty estimates, Data source: daily solutions from Horizon2020 project Egsiem ( ): - Improvement of technologies in absolute gravimetry (systematic effects) to ensure consistent results through several decades - Establishment of reference gravity station (AG+SG) including estimation of local hydrology signal - Validation of trends in gravity data (satellite vs. in-situ) - Comparison at regional scale, e.g. large groundwater aquifers

Thank you for your attention! New gravity lab and gravimeter at the Pecný station