School of Surveying and Spatial Information Systems

The University of New South Wales


Recent Developments in Satellite Missions

to Determine the Earth's Gravity Field

 

by Andrew Stubbs

 

Supervised by Assoc. Prof. A. H. W. Kearsley

Edited by J. M. Rüeger

October 2002


 

Introduction

With the advent space exploration and all of the subsequent technological advancements made, we now find ourselves in a situation where we are able to systematically study the Earth as a whole entity, the “machine” Earth. However, it is only now being realised that to comprehend the myriad of interactions between Earth systems, we must utilise a multidisciplinary approach within which the mapping of the Earth’s gravity, to a high degree of accuracy and resolution, plays a pivotal role (Lambeck, 1990). High-resolution models of the gravity and magnetic fields of the Earth help us to model the structure and the driving forces behind plate tectonics, mantle convection, lithospheric motions, etc. (Gravity Workshop, 1987). A high-resolution gravity field model will also help to establish a physically meaningful reference surface for the oceans, the geoid. With the extensive altimeter data sets of past missions like GEOSAT, GEOS-3, TOPEX/Poseidon and ERS-1, and future altimeter missions Grace, CHAMP and GOCE, ocean variations with respect to this geoid can be studied on different geometrical and temporal scales. In addition, by combining altimetry and gravity, ocean currents can be deduced and, possibly, long-term effects like global sea level change and El Nino cycles can be studied.

 

Recent Developments

In July 2000 the launch of the CHAMP satellite mission initiated a new era for gravity field study with an increased impetus upon the measurement of the gravity field as the sole objective of the satellite mission. Future missions like GRACE and GOCE reinforce the greater importance of long-medium wavelength gravity measurements from such dedicated missions. These missions will strengthen the resolution and accuracy of our current gravity field models by fulfilling the following fundamental criteria for gravity field measurements; (i) uninterrupted three dimensional tracking, (ii) accurate measurements of non-gravitational forces and compensation for their subsequent impacts, (iii) increased sensitivity of the satellites' sensors by achieving as low as possible an orbital altitude, and (iv) compensation for gravity field attenuation at altitude.

To achieve these criteria, certain procedures have been developed including: satellite-to-satellite tracking in the high-low mode (hl-SST) for all three missions (which benefit, with respect to the original concept, from the existing GPS spacecraft); satellite-to-satellite tracking in the low-low mode (ll-SST) for GRACE (with two co-orbiting satellites); satellite gravity gradiometry (SGG) for GOCE; plus micro-accelerometers of increasingly high sensitivity on all low Earth orbiters (LEOs) for the measurement of surface forces and, in the case of SGG, also for measuring the gravity gradients at satellite altitude (EGS XXVII General Assembly, 2002).

 

GEOS-3

GEOSAT

ERS1/2

TOPEX/

Poseidon

CHAMP

GRACE

GOCE

Proponent

NASA

US Navy

ESA

NASA/CNES

GFZ Potsdam

NASA/GFZ

ESA

Mission Duration

 

1975 - 1978

 

12th March 1985 ­

January 1990

 

17th July 1991 ­

June 1996

 

August 10th 1992 ­

Present

 

15th July 2000 ­

Present (5 yrs.)

 

17th March 2002 -

Present (5 yrs.)

 

2005

 

Altitude

 

843 km

 

785 km

 

780 km

 

1335 km

 

454 km

 

485 km

 

250 km

 

Inclination

 

114.98˚

 

108 ˚

 

98˚

 

66˚

 

87˚

 

89˚

 

96.5˚

 

 

 

Orbit

 

 

Circular Orbit

18 months of 3 day repeat

(Geodetic Mission)

17 day near repeat (Exact Repeat Mission)

Near polar-sun synchronous

3 day repeat

35 day repeat

168 day repeat

 

 

9.97 day repeat

 

 

Near Circular,

near polar Orbit

 

 

Polar Orbit

 

Near Circular

Sun Synchronous Orbit

Primary Application

Mapping of marine geoids

Mapping of the marine

geoid

Gravity Field

Monitoring

Marine Gravity Field Monitoring

Long-medium wavelength Gravity Field Monitoring

Long-Medium Wavelength Gravity Field Monitoring

Small Wavelength Gravity Field Monitoring

 

Tracking

C-Band Radar Transponders

 

SLR

 

SLR/PRARE

 

SLR/DORIS/GPs

 

GPs

 

GPs

 

GPs/GLONASS

 

Method

 

Radar Altimetry

 

Radar Altimetry

 

Radar Altimetry

 

Radar Altimetry

 

Precise Orbit

Determinations

2 Identical Satellites in Identical Orbits

(220 km separation)

 

Gravity Gradiometry

 

Resolution

 

10 km

 

2-3 km

 

17 km

 

300 km

 

500 km

 

500 km

 

100 km

 

Accuracy

 

50 cm (global)

20 cm (intensive

 

10-25 cm rising to 40-60 cm (solar activity)

 

10-20 cm

 

3-4 cm

 

1 cm for wavelengths

>1000 km

 

0.1 mm

 

2.5 mm and 0.08 mGal

 

The table documents the advances that have been achieved in relation to satellite missions for the gravity field over the last 27 years. The mission attributes most significantly modified have been the altitude, inclination and accuracy aspects of the missions. The atmosphere and the satellite's altitude act as dampeners and therefore attenuate the gravity field signature. This directly affects the gravity field features that are detectable at certain altitudes. As our knowledge of satellite technology in relation to Low Earth Orbits (LEOs) has evolved, so too has our ability to retrieve these long wavelength features accurately using technology to achieve sustained Low Earth Orbits. The inclination of the satellite orbits also indicates the transition of the primary applications of the satellite missions and thus the particular area of the Earth being mapped. For example, the TOPEX/Poseidon mission orbited at an inclination of 66˚ in order to map the Earth’s oceans. Whereas the higher inclinations of GEOS-3, GEOSAT and ERS1/2 were designed to penetrate the polar regions more effectively. The need for a unique gravity data set of a global nature has led to the recent CHAMP, GRACE and GOCE missions orbiting at more globally oriented inclination of between 87˚ to 96.5˚ thus providing a more homogeneous data set.

 

Conclusions

The three newest missions have and will continue to revolutionise our knowledge of the gravity field and the corresponding geoid. Benefits from these missions will be witnessed in many disciplines such as geodesy, oceanography and geophysics, the unification of reference height systems,  levelling by GPs, the use of inertial navigation systems, the precision of satellite orbit knowledge (e.g. for all altimetry missions), the study of the solid earth and especially of the continental lithosphere, the determination of the absolute mean ocean circulation and transports, the study of ice sheets and, an issue that is becoming increasingly vital,  a better understanding of sea level changes and El Nino weather events that affect us all.

 

 

Further Information

For more information, please contact:

 

Assoc. Prof. A. H. W. Kearsley

Email: W.Kearsley@unsw.edu.au

 

Mail:     

School of Surveying and Spatial Information Systems

University of New South Wales

UNSW SYDNEY NSW 2052

Australia

 

Phone: +61-2-9385-4188

Fax: +61-2- 9313-7493

WWW: http://www.gmat.unsw.edu.au

 

 

Mr. A. J Stubbs

Email: oihh@hotmail.com