NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 1

Tasks of a new Working Group on Absolute Gravimetry

 

 Herbert WilmesFederal Agency for Cartography and Geodesy, Frankfurt (BKG)

NKG Working Group for Geodynamics Copenhagen, Denmark, 23 – 24 April 2008

NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 2

Motivation• Requests for reliable and time-dependent gravity data with global

coverage (Satellite gravity field missions, GGOS, Mass transports in the system Earth, etc.)

• At present, no agreed standards for AG observations, processing, results

• Instrument accuracy much better than the the valid gravity reference system

• Present working group on AG comparisons (BIPM, regional comparisons) can not cover additional tasks

NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 3

Parameter space for combination of gravity and geometry

X ... position, W ... Earth gravity potential, T ... disturbance potential, H ... phys. height, ... Variation of the geom. geom. heighth

Observations Geodetic Space

Techniques Satellite

radar SGM

Terrestrial Gravimetry

VLBI SLR GNSS I-

SAR ALT

SST Gradio

Lev. grav. rel.

AG SG Tide

Gauge

Parameter (t) 10-9 10-9 10-9 10-10 10-9 10-7 10-6 10-9 10-10 10-9

Datum

Geo center X0

Scale

Polar Motion

UT1 Te

rre

str

ial

Re

fere

nce

Sy

ste

m

(TR

F)

Stations Xp h· h

· h

·

Earth Surface DTM

Datum W0

Vert

ica

l R

efe

ren

e

Sys

tem

(VR

F)

Stations Hp

Datum g0

Gra

vit

y R

efe

ren

ce

sys

tem

(G

RF

)

Stations gp

Gravity Field GGM T

Reg. Geoid T

Atmosphere Hydrosphere Cryosphere Lithosphere (After Rottacher and Ihde)

NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 4

SG-Stationen BH,WE,MC - oberirdisch

-150

-100

-50

0

50

100

150

Jan 2002 Jan 2003 Jan 2004 Jan 2005 Jan 2006 Jan 2007

Zeit

Sch

wer

evar

iatio

nen

[nm

/s^2

]

Bad Homburg BH

Wettzell WE

Medicina MC

Gravimetric residual functions after combining AG and SG

Stations: Bad Homburg (D), Wettzell (D), Medicina (It)

NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 5

Motivation (cont.)• Combination of terrestrial observations with satellite gravity field

missions CHAMP, GRACE, GOCE

• Detection of periodic mass changes

water storage difference simulated with the global hydrological model WGHM

hydrological mass signal

Global continental pattern of differences between monthly GRACE gravity field solutions

Ref.: Ilk et al.: Mass transport and mass distribution in the Earth system

NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 6

Combination of SG and AG in Concepcion (Chile)

Example for strong gravity variations at a geodetic reference station

TIGO Concepcion: combined gravity signals SG-38 and FG5-227 (corrected for SG-drift, tides, air pressure, polar motion)

-25

-20

-15

-10

-5

0

5

10

15

20

2003 2004 2005 2006 2007

g -

979

923

641

[µ

gal

] p

ier

403@

125

cm

FG5-227 measured at 402, reduced to 403 by 9 µGal

FG5-227 measured at 403, corrected by - 2.2 µGal

FG5-227 measured at 403

FG5-227 measured at 403, corrected by 2.9,µGal

SG-38 observed at 401, drift 1.6 µGal/year

NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 7

Structure of AG-related working groups

BIPM IAG

IGFSInternational Gravity Field Service

R. Forsberg

Working Group on Absolute Gravimetry

H. Wilmes

Comm. 2 Gravity FieldY. Fukuda

Subcomm. 2.1 Gravimetry and Gravity Networks

L. Vituskin

CCM-W.G. on Gravimetry

IAG-SG 2.1 Comparison of AG

L. Vitushkin

NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 8

IGFS structure and relation to GGOS

IGFS

BGICNES Toulouse

IGeSPolimi Milano

ICETU. F. Polynesia

ICGEMGFZ Potsdam

Technical CentreNGA St. Louis

Advisory BoardAffiliate scientists

IDEMSDeMontfort UK

IGFS is a unified "umbrella" IAG service, which will- Coordinate collection, validation, archiving and dissemination of gravity field related data- Coordinate exchange of software of relevance for gravity field activities - Coordinate courses, information materials and general public outreach relating to the earth's gravity field

GGOS has e.g. identified a unified global absolute gravity network as a major element missing in the long-term basic GGOS network.

A special purpose of the IGFS will be to coordinate the gravity field services in relation to GGOS – the Global Geodetic Observing System – and take initiatives to cover missing elements, e.g. the provision of “combination” solutions” of gravity field related quantities, analogous to e.g. the IERS.

NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 9

Role of the new working group:

a) Standards and Conventions• Define standards for gravity data observation procedures• Define data reduction (tides, atmosphere, …), • Define data exchange format and archive• Define the standards for associated data (pressure, hydrology…)• Define data processing methods (adjustment,…)• Define gravity anomaly/disturbance computation (free-air, Bouguer, …)• Define the gravity reference system (replace IGSN71)• Define the geopotential reference system (W0, GRS80)• Evaluate and recommend relevant geophysical models

Requests from GGOS to the IGFS

NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 10

b) Networks and Communications• Design and realization of the global absolute gravity network• Design and realization of the global superconducting gravity network• Contribute to the definition and realization of the global height system• Contribute to the GGOS co-located site network (AG, SG, W,…)• Provide gravity field parameters at tide gauge stations • Integrate gravity and leveling networks• Contact with external networks (oceanic, atmospheric, hydrologic,…) • Making gravity data, gravity models and products available via IGFS Services and Central Bureau (web site)

NKG Working Group for Geodynamics Copenhagen, 23 –24 April, 2008 11

Objectives and Tasks of the Working Group

The proposed Working Group on Absolute Gravimetry will have following objectives and tasks:

1. Design and promote a worldwide AG network with repeated observations

2. Define standards for observation and processing of absolute gravity data

3. Set up a database for global AG observations

4. Support the combination of repeated AG measurements and SG time series

5. Enable the combination of gravity and geometric observations

6. Prepare an improved gravity reference system

7. Data policy for use of AG observations

Participants of the NGOS community are very welcome to contribute and actively support the aims of the working group!

Comments? Propositions? Ideas? Special areas of interest?

Which is most important?