RIGTC, Geodetic Observatory Pecný

The institute's mission is basic and applied research in geodesy and cadastreDesignated 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

Vojtech Pálinkáš and

Aleš BezděkResearch Institute of Geodesy, Topography

and Cartography

Astronomical Institute of the Czech Academy of

Sciences

DefinitionsIn 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)

PolAtmTF ggggg

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) , http://www.ggos.org/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 measurementsAbsolute 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 resultsMissions are successful, global gravity field modelling has improved considerably

CHAMP (2000–2010) GRACE A/B (2002–now) GOCE (2009–2013)

Satellite missions

Observation typesSatellite-to-satellite tracking: high-

low (SST-hl)satellites in high orbits: GPS (altitude: 20 000 km)positioning of satellites in low orbits (below 2 000 km)

Satellite to Satellite Tracking: low-low (SST-ll)observation of relative motion of two satellitesGRACE A/B: by means of microwave ranging

Satellite gravity gradiometry (SGG)to measure the gradient of gravity acceleration vectorspace gradiometer: composed of six accelerometers

(Fig: Rummel et al., 2002. J Geodyn 33, 3–20)

GOCE

GRACE A/B

CHAMP

Mission GRACEGravity Recovery And Climate Experiment First operational application of SST-ll to study the

gravity field: time-variable field and the static fieldmeasurement of relative motion of two satellites separated by 220±50 km using a microwave beamaccelerometer on each satellite: observation of nongravitational forceslow 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: www.csr.utexas.edu/grace/)

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 fieldMainly 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., 2006. 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 GRACEUnits: 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., 2013. J Geod 87:69–87)

GRACE, detection of trends

GRACE: measurementsQuantity: time-varying gravity field (observations of mass changes) from spaceStandard 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 regionE.g. much stronger gravity signal over land vs. weak signal over oceansMitigation 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

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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

Comparison of absolute gravimeters

ICAGs – at BIPM from 1981 to 2009

2003, 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 referenceFG5, FG5-X: most accurate absolute gravimeters based on laser interferometry, Standard uncertainty 2.5 Gal

INTERNATIONAL COMPARISONS – FG5s dominateFG5s / AGs: 13/21 (2009), 17/21 (2011), 19/25 (2013)Weights FG5s / other AGs : > 4 / 1

g(FG5s) < 10 GalReference 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

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 < 50-100 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 GRACETREND:

GRACE: 0.43 ± 0.09 Gal/year

PE : 0.39 ± 0.17 Gal/year

WGHM : 0.12 ± 0.11 Gal/year

PE_cor : -0.04 ± 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 projectENVIRONMENT 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 (2015-2017): http://www.copernicus.eu/projects/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