Acknowledgements

Content Developed By: Alan Ward

Designed By: Winnie Humberson

Edited By: Margaret Baguio, Bill Bandeen, Charlotte Griner, Michael King, Margaret Srinivasan.

Technical Input Provided By: Christoph Reigber, Rick Fitzgerald, Frank Flechtner, Bob Garnett, Marc Imhoff, Byron Tapley,

Srinivas Bettadpur, John Ries, Don Chambers,Ab Davis, Mike Watkins.

Front Cover Foreground: These images show three different views of the Earth’s geoid—a surface of equal 

gravitational potential that, over the ocean, closely follows the sea surface.The 

geoid was determined from data collected by previous satellite missions.Among 

these satellites is the Challenging Minisatellite Payload (CHAMP), the forerunner to 

GRACE, launched by the German Georesearch Center GeoForschungsZentrum Potsdam 

(GFZ). Similar products will be developed using data from GRACE with an expected 

improvement in accuracy of several orders of magnitude.These images were provided 

courtesy of GFZ. The illustration of the GRACE satellites was provided by Chris Meaney.

Front Cover Background: This day/night image of the Earth was produced using a combination of data from 

the Moderate Resolution Imaging Spectrometer (MODIS) sensor aboard NASA’s Terra 

satellite (daytime portion), and the city lights image produced using Operational Line Scan 

(OLS) data from the Defense Meteorological Satellite.The daytime side of the Earth in this 

image was made using surface reflectance data from MODIS.The data was composited 

over a period of several months during the Northern Hemisphere of summer 2001.

(Image courtesy of Robert Simmon and the MODIS Science Team.)

Back Cover: This is a photograph of the two identical spacecraft in the environmental test 

facility at Industrieanlagen-Betriebsgesellschaft GmbH (IABG) in Ottobrun, Germany. When

deployed into space, the two craft will be upright inside the launch vehicle. After launch,

they will fly 500 km above the Earth and will be separated by 220 km.The photograph was 

provided courtesy of Astrium GmbH.
What is it? 

You can’t see it! 

You can’t smell it! 

You can’t touch it! 

But it’s there. In fact, it’s everywhere.

Gravity!

While gravity is much weaker than other basic forces in nature, such as magnetism and

electricity, its effects are ubiquitous and dramatic. Gravity controls everything from the

motion of the ocean tides to the expansion of the entire Universe.To learn more about

the mysteries of gravity, twin satellites named GRACE—short for the Gravity Recovery

and Climate Experiment—are being launched to make detailed measurements of Earth’s

gravity field.This experiment could lead to discoveries about gravity and Earth’s natural

systems, which could have substantial benefits for society and the world’s population.

The GRACE mission will be the inaugural flight of NASA’s Earth System Science

Pathfinder Program (ESSP).A component of NASA’s Earth Science Enterprise (ESE), the

ESSP missions are intended to address unique, specific, highly focused scientific issues

and provide measurements required to support Earth science research.

The ESSP missions are an integral part of a dynamic and versatile program consisting of

multiple Earth system science space flights.The ESSP program is characterized by relatively low- to moderate-cost, small-to medium-sized missions that are capable of being

built, tested and launched in short-time intervals. Each mission is led by a Principal

Investigator (PI), who is responsible for all elements of the mission, from ensuring the

science accuracy to making sure the mission stays on budget and on time. ESSP missions are capable of supporting a variety of scientific objectives related to Earth science, including the atmosphere, oceans, land surface, polar ice regions and solid Earth.

Investigations include development and operation of remote sensing instruments and

conducting research using data returned from these missions. Subsequent satellite

launches are planned over the next few years, all of them focusing on the atmospheric

sciences.
If Earth were a smooth sphere composed of similar elements or ingredients, there

would be no need for a GRACE mission; the assumption made in most introductory

physics courses that the acceleration due to Earth’s gravitational field has a constant

value would indeed be correct—end of story. However, previous observations have

clearly demonstrated that our Earth isn’t smooth and homogeneous and it really isn’t

even a sphere! What’s more, the images shown above are just instantaneous snapshots

from one moment in time.The reality is that the gravity field is continually changing,

mostly due to variations in water content as it cycles between the atmosphere, oceans,

continents, glaciers, and polar ice caps. By far the largest “lump” is the flattening

observed at the poles—the Earth’s oblateness.The above profiles have removed the

portion of the response due to oblateness in order to focus on the smaller anomalies

that exist. GRACE will reveal the broad features of the Earth’s gravitational field over

land and sea; it will also allow for these smaller scale features to be identified and studied with unprecedented accuracy, and it will show how the Earth’s gravity field varies

with time.

“

It’

s a bumpy world”!!

This

is a three

dimensional rendering

of the Earth’s

geoid magnified a

thousand times, with the portion due to the Earth’ oblateness

removed.The magnification highlights smaller scale variations in

gravitational potential that are caused by the uneven distribution of mass

over the surface of the Earth.The GRACE mission will measure these fluctuations

with unprecedented precision. (Image courtesy of Jet Propusion Laboratory.)

2

Why

A Mission To Study Gravity?
Gravity is the invisible force that pulls two masses together.The branch of science dealing with obtaining precise measurements of the Earth, mapping points on the surface,

and studying its gravitational field is known as geodesy. Producing a precise model of

the fluctuations in gravity over the Earth’s surface has proven to be a formidable task.

Currently, data from several dozen satellites must be combined to produce a model of

Earth’s gravitational field.These models do a good job at replicating the large-scale features of Earth’s gravitational field but cannot resolve finer-scale features or accurately

describe the small month-to-month variations associated with the hydrologic cycle.The

unique design of the GRACE mission (twin satellites flying in formation) is expected to

lead to an improvement of several orders of magnitude in these gravity measurements

and allow much improved resolution of the broad-to finer-scale features of Earth’s

gravitational field over both land and sea.

This is a plot of the Earth’s geoid (surface of equal gravitational potential) produced by the Earth Gravitational

Model (EGM96), one of many models used for gravity studies. The Space Geodesy Branch of NASA Goddard

Space Flight Center worked in collaboration with the National Imagery and Mapping Agency (NIMA) and The

Ohio State University (OSU) to develop this model. Data from GRACE is expected to allow for a quantum

leap of several orders of magnitude in the precision and resolution of the geoid. The improved rendering of

the geoid will have benefits for many disciplines that rely directly or indirectly on precise measurements of the

gravity field, including disciplines that study the Earth’s climate.

3

180 90W 0 90E 180

60N 

30N 

0

30S 

60S 60S 

30S 

0

30N

60N 

mGal

-100 -50 0 50 100

180 90W 0 90E 180

GRACE 2002:

A Scientific Geodesy
The distribution of mass over the Earth is non-uniform. GRACE will determine this

uneven mass distribution by measuring changes in Earth’s gravity field.The term mass

refers to the amount of a substance in a given space, and is directly correlated to the

density of that substance. For example, a container filled with a more dense material,

like granite, has more mass than that same container filled with water. Because mass

and density are directly related, there is also a direct relationship between density and

gravity.An increase in density results in an increase in mass, and an increase in mass

results in an increase in the gravitational force exerted by an object. Density fluctuations on the surface of the Earth and in the underlying mantle are thus reflected in

variations in the gravity field.As the twin GRACE satellites orbit the Earth together,

these gravity field variations cause infinitesimal changes in the distance between the

two.These changes will be measured with unprecedented accuracy by the instruments

aboard GRACE leading to a more precise rendering of the gravitational field than has

ever been possible to date.

GRACE will do more than just produce a more accurate gravitational field plot, however.The measurements from GRACE have important implications for improving the

accuracy of many scientific measurements related to climate change. Substantive

advances in the interpretation of satellite altimetry, synthetic aperture radar interferometry, and digital terrain models, covering large land and ice areas used in remote

sensing applications and cartography, will result from the improved gravitational field

measurements provided by GRACE.These techniques provide critical input to many

scientific models used in oceanography, hydrology, geology and related disciplines, and,

for this reason, the Earth Science community eagerly anticipates the GRACE launch.

The next few pages present some of the expected scientific applications.

This schematic diagram illustrates the

different forces that redistribute mass

on the surface of the Earth. Since

mass changes and gravity changes are

correlated, the data retrieved by the

GRACE satellite should help scientists

better quantify these movements and

their impact on the Earth’s climate.

(Image provided by GFZ.)

Atmosphere

Lithosphere

Ice

Ocean

Mantle Continent

Global mass distribution and redistribution:

Spatial and temporal variations

in the Earth gravity field

(observation - interpretation)

4
Perhaps the most interesting and least well-measured cause of fluctuations in the

Earth’s gravitational field is movement of water over the surface of the Earth.The gravitational data collected by GRACE will be combined with data from other NASA satellites, aircraft and ground-based measurements to study the movement of liquid water

over our home planet with a level of detail never before possible.Water moves in significant quantities throughout the Earth’s hydrologic cycle (see diagram), and at a rapid

rate relative to other processes that redistribute mass over Earth’s surface.

The gravitational variations observed by GRACE are primarily attributable to the seasonal and interannual movement of water throughout the hydrologic cycle.This means

that by combining measurements from GRACE with measurements taken on the

ground, scientists will be able to improve their models of water exchange between the

ocean and land surfaces—through rainfall, deep soil moisture, and runoff.This can be

done from continental scales down to regional scales of a few hundred kilometers.

5

This diagram illustrates the hydrologic cycle and shows how water in solid, liquid and vapor forms,

circulates over, under and above the surface of the Earth. Gravity fluctuations correlate with variations in the density of the land surface below.These illustrations can be used to track water movement. In effect, gravity becomes a mechanism to track water movement our eyes cannot see.

GRACE data may lead to the identification of new sources of fresh water—which is of particular

interest to populations located in arid regions of the Earth.

Tracking water movement on and

beneath Earth’s surface
6

These figures illustrate the ability of GRACE to recover month-to-month variations in the water stored in individual

river basins as compared to more traditional methods. The figure on the left looks at the entire Mississippi River

basin while the figure on the right focuses on one sub-basin of the Mississippi. In each figure, the upper panel shows

an outline of the river basin, along with a color contour of a mathematical averaging function that GRACE might use

to estimate the water variations within that basin. The red line in the lower panel shows monthly variations in the

actual water storage signal, in units of cm of water thickness; the blue line shows the values that GRACE would

recover. In general, GRACE can recover water storage signals to about 1 cm or better for basins where the effective

radius is 250-300 km or larger. The larger the basin, the more data from GRACE will improve the accuracy of these

calculations. Hence it will do a better job looking at the entire Mississippi basin than it will do on individual subbasins. ( Image provided by John Wahr, University of Colorado.)
The question of whether ice caps are shrinking or growing is important for climate

change studies.As the area covered by ice sheets decreases, the surface area of the

Earth’s oceans increases, leading to increased heat absorption and rising temperatures.

This, in turn, melts more ice and contributes to global sea level rise. Data provided by

GRACE will help scientists better understand how ice sheet mass is changing and the

impact the changes are having on global sea level. GRACE data, when combined with

height variations measured with ground-based, aircraft, and satellite instruments, like

the Geoscience Laser Altimeter System (GLAS) on the Ice, Clouds, and Land Elevation

Satellite (ICESat), will allow for improved computations of ice sheet mass balance.Also

by combining the GRACE gravity field measurements with surface elevation measurements (such as from radar altimeters), scientists will be better equipped to distinguish

between the mean ocean level changes due to thermal expansion and those due to

actual redistribution of water.

7

GRACE will be able to precisely measure very small

changes in Earth’s gravitational field that result from

changes in the mass of ice sheets. Shown here is

computer generated image of Greenland that shows

how the ice mass is changing. Blues indicate areas

where the loss of ice is greatest, and yellows indicate

regions that are apparently thickening. Gray areas

indicate no significant change in ice thickness.

The issue of long-term changes in ice sheet mass is

an important one for climate studies, and data from

GRACE should help scientists quantify these changes.

The diagram illustrates the idea that as the ice sheet

melts, the increased surface area of open water

absorbs more heat, raising temperatures, melting

more ice, and also contributing to sea level rise.

Tracking changes in ice sheets

and changes in global sea level
8

Postglacial rebound accounts for the vertical

movement of land in many parts of the

world.These shifts, which have been continuing since the last Ice Age ended, affect relative

sea level at the coastline in a manner that

varies from place to place. Such movements

can confound tide gauge records obtained

from coastal sites and thus complicate efforts

to track the overall change in global sea level.

The data retrieved by GRACE will be combined with the previously mentioned altimeter readings to get a better handle on how

much of the perceived change in sea level is

attributable to the phenomenon of postglacial

rebound and how much might be attributed

to global warming.This particular image

depicts the predicted time rate of change in

geoid height expected to result from postglacial rebound as predicted by the ICE-4G

(VM2) Toronto model (Image provided by

W.R. Peltier, University of Toronto, Canada.)

For example, data from GRACE is expected to greatly improve scientists’ understanding of

how much global sea level is being impacted by a phenomenon known as postglacial

rebound. Postglacial rebound refers to the slow rebounding of the Earth’s crust that is

occurring now that the weight of the ice from the last Ice Age is no longer present.The

rebounding of the crust can affect relative sea level at the coastline in a manner that varies

from place to place.This can confound efforts to determine how much of the overall

observed change in sea level is actually caused by thermal expansion of the oceans resulting from global warming.

Satellite altimeters such as those on

TOPEX/Poseidon and Jason-1 can measure the total mean sea level change, as

illustrated here with TOPEX/Poseidon

data from 1993 to the middle of 2001.

However, the altimeter alone cannot distinguish between what portion of the

observed anomaly can be attributed to

warming of the ocean and what part is

due to water being added from melting

ice sheets. Data provided by GRACE,

used in conjunction with altimeter measurements like those shown here, should

help scientists better distinguish

between these two phenomena. (Image

courtesy of Don Chambers, University

of Texas, and Steve Nerem of the

University of Colorado.)

15

10

5

0

-5

-10

-15

-20

1994 1996 1998 2000 2002

Year

AMSL (mm)

-1 -.8 -.6 -.4 -.2 0 .2 .4 .6

Change in Geoid Height Over Time (mm/yr)
If the ocean were motionless, the sea surface would still have hills and valleys.This 

static topography of the ocean is caused by the fluctuations in the gravity field caused by

variations in density over the surface of the ocean. Because the oceans are always in

motion—swept by winds, seething with waves, and circulating in gigantic currents and

gyres driven by surface winds and by the rotation of the Earth—the shape of the

ocean’s topography cannot be determined by static gravity measurements alone.An

additional component, known as the dynamic topography must be accounted for to precisely measure the shape of the ocean surface at any given time.Although dynamic

topography only accounts for about one percent of the total variation of the ocean’s

topography, it is an extremely important component for it contains important information about the speed and direction of ocean currents. Consequently, scientists would

very much like to improve their ability to measure and track changes in dynamic topography.

GRACE data should help scientists

understand the role ocean currents

play in regulating Earth’s climate.This

image from the MODIS sensor

onboard the Terra satellite shows the

Gulf Stream (red), a surface ocean

current that plays a critical role in

redistributing heat from the tropics to

the poles. It has a moderating influence on the climate of Northwest

Europe, which would otherwise be

much colder—similar to other locations at the same latitude, such as

Greenland. This particular image was

obtained on May 2, 2001 and was

provided by Liam Gumley of the

MODIS Atmosphere Team, University

of Wisconsin-Madison Cooperative

Institute for Meteorological Satellite

Studies.

This image shows sea surface topography data from TOPEX/Poseidon. When such

data are combined with the precise geoid measurements expected from GRACE,

scientists should be able to study currents from satellites using a technique known

as dynamic topography. This image was acquired in March 2001 in the aftermath

of earlier El Niño/La Niña conditions in the tropical Pacific. Areas toward the

blue end of the spectrum represent anomalously low sea surface heights for

March and areas toward the red end represent anomalously high values for

March.

9

Studying ocean currents both near

the surface and far beneath the waves
10

Two pieces of information are needed to make a dynamic topography measurement,

sea surface height and a model for the Earth’s geoid. Since the impact of dynamic

topography on the shape of the ocean surface is quite small compared to the impact of

the static gravity field, the measurements must be extremely precise and of high resolution. Satellite altimeter measurements from TOPEX/Poseidon and Jason-1 can provide

this level of detail for sea surface height measurements, but to date, there are no geoid

measurements with sufficient precision and resolution to allow satellite-derived measurements of dynamic topography. Scientists expect the data returned by GRACE to

provide the precise, high resolution geoid measurements required to measure dynamic

topography affects.They hope to use these data to better understand the role that

ocean currents play in regulating the Earth’s climate.

Ocean currents regulate Earth’s climate by transporting heat from equatorial regions to

the poles.The Earth’s oceans essentially serve as huge heat reservoirs and currents

both at the ocean surface and far beneath the waves—known as deep ocean currents—act as massive “conveyor belts” that distribute this heat over the surface of the

Earth. Consequently, any changes in these equator-to-pole “conveyor belts” could significantly change the weather all around the globe. Scientists are particularly hopeful that

GRACE will help them better understand the role that deep ocean currents play in

regulating climate. Until now, deep ocean currents have remained shrouded in mystery

since they have been very difficult to measure. However, the design of the GRACE

satellites makes them ideally suited to examine deep ocean currents and should lead to

improved ability to track them and study their impact on global climate.

This map illustrates the heat exchange flux over Earth’s oceans for a five year period. A negative value indicates

an area where the ocean is losing heat while a positive value indicates an area where the ocean is absorbing heat.

Ocean heat exchange flux plays a crucial role in regulating the Earth’s climate and data from GRACE should help

improve the accuracy of these measurements

0 45 90 135 180 225 270 315 360

60

30

0

-30

-60

-15 -12 -9 -6 -3 0 3 6 9 12 15

Change in Ocean Heat Flux (W/m2)
GRACE data will also improve our understanding of Earth’s lithosphere—solid Earth.

More precise measurements of the Earth’s gravity field, such as those provided by

GRACE, should lead to an improved understanding of the dynamics of the inner structure of the Earth.The characteristics of the continental lithosphere are the subject of

varying opinions. Questions remain regarding its thermal and compositional nature,

thickness, and mechanical properties. GRACE can track variations in the density of the

lithosphere by looking at its impact on the Earth’s gravitational field.Thus, data from

GRACE will be used to compare competing models of the lower mantle viscosity and

contribute to the improved understanding of mantle convection.

In addition to the gravity measurement, the GRACE mission will

also employ an innovative new

technique that uses its extremely

precise Global Positioning System

(GPS) receivers as atmospheric

limb sounders. Limb sounding

measures the excess delay of

radio waves as they pass through

and are bent by the Earth’s

atmosphere. Just as light is

refracted or bends as it enters

water because of the slower

speed of light in the water, GPS

signals are refracted as they pass

through the atmosphere.The precise degree of bending (and hence

the precise excess delay) is highly

dependent on atmospheric pressure, temperature, and moisture

content.These can all be estimated from precise observations of

the changing signal delay as the

11

The graph shows a dry temperature profile derived by

GeoForschungsZentrum Potsdam (GFZ) from CHAMP occultation data observed on August, 27, 2001.The corresponding

meteorological analysis data from the European Centre for

Medium-Range Weather Forecasts (ECMWF) is plotted for comparison.

Occultation no. 12 on August 27, 2001

1:07 UTC 50.1°S 4.4°E

CHAMP

ECMWF

40

35

30 

25

20

15

10

5

0

200 220 240 260 280

altitude [km]

dry temperature [K]

Tracking changes in the solid Earth

Using GPS Receivers as Atmospheric

Limb Sounders
GPS Satellite rises or sets.The ionosphere, which is the name applied to the ionized

upper atmosphere above 100 km altitude, also affects the speed of radio waves

according to the density of electrons in the ionosphere. It turns out that GPS signal

distortion caused by the ionosphere varies as the radio frequency of the signal

changes but that atmospheric distortions remain constant with changing radio frequency.Thus, this technique not only measures the distortion of the GPS signal but

it can also distinguish how much of the observed distortion is caused by the atmosphere and how much is caused by the ionosphere.

As GRACE orbits the Earth, it will use forward and aft looking antennas to measure the signals from the slower orbiting GPS satellites as they appear to rise or set

behind the Earth's limb. GRACE will also measure GPS signals from higher elevation

GPS satellites that are not affected by the atmosphere (illustrated in the diagram)

as a standard of comparison.

By observing how the GPS path delays through the atmosphere change, GRACE

will create profiles of pressure, temperature, and humidity for the atmosphere and

measure the variability of the ionosphere down to 100 km altitude. It is anticipated

that the limb sounding technology aboard GRACE will extend and complement

other spaceborne atmospheric sensors. GRACE will provide about 500 limb sounding measurements per day, unaffected by weather conditions (such as clouds and

storms) that hinder or block other satellite sensors.The GRACE limb sounding

measurements will be used to determine the effectiveness of limb sounding in

improving numerical weather prediction in a cost effective manner.

12

This diagram,

originally provided by GFZ, illustrates how the

occultation

process works.

The GRACE

satellite tracks a

GPS satellite as it

“rises” and “sets”

through the limb

of the atmosphere, while a

second GPS

serves as a reference point.
13

GPS sounding of the continuously changing ionosphere aims to advance our understanding of the Sun’s influence upon the Earth’s environment, including its effects on climate, weather, radar and radio communications. Limb sounding will also be studied as a

means of detecting rapid vertical changes of the Earth’s surface such as volcanic explosions, earthquakes, tsunamis and other such phenomena, which are thought to cause

disturbances in the ionosphere.

System development, data processing and archiving are performed in a shared Science

Data System (SDS) between the Jet Propulsion Laboratory (JPL), the University of Texas

Center for Space Research (UTCSR), and the GeoForschungsZentrum Potsdam (GFZ).

Telemetry data are received by the GRACE Raw Data Center (RDC) at Deutsches

Zentrum für Luft und Raumfahrt (DLR) in Neustrelitz, Germany—illustrated in the

Instrumentation section.

The first level of data processing is performed at JPL, where sensor calibration factors

are applied, the data are correctly time-tagged, quality control flags are added, and the

data sample rate is reduced from the high rate data of previous levels. Data are then

sent to UTCSR and GFZ, where the mean and time variable gravity field is derived

from the calibrated and validated data. Data are archived for distribution at JPL’s

Physical Oceanography Distributed Active Archive Center (PO.DAAC) and at GFZ’s

Information System and Data Center (ISDC).15 GRACE data include 30-day estimates

of gravity fields, as well as profiles of air mass, density, pressure, temperature, water

vapor, and ionospheric electron content.

Data

Processing and Archiving
GRACE is different from most Earth observing satellite missions—Terra and Aqua

for example—because it will not carry a suite of independent scientific instruments on

board.The two GRACE satellites themselves act in unison as the primary instrument.

Instantaneous changes in the distance between the twin satellites are used to make an

extremely precise gravitational field measurement.

To measure gravity from space, the two identical GRACE satellites fly in the same orbit—

one 220 km (137 miles) ahead of the other.As the pair circles the Earth, areas of slightly

stronger gravity will affect the lead satellite first, pulling it away from the trailing satellite.

The uniquely designed Superstar Accelerometer is used to distinguish gravity influences

from those of air drag.The K-band ranging instrument is capable of measuring the distance

between the satellites with a precision better than the width of a human hair. By monitoring this distance, GRACE will be able to detect fluctuations in the gravitational field and,

therefore, differences in the density of the Earth’s surface beneath the satellites.The data

will be combined with GPS data to produce a map of the gravity field approximately once

a month.

14

A sneak peak at the innards of a spacecraft! For the GRACE mission, the spacecraft itself is the main instrument. This picture shows the GRACE spacecraft with the solar panels removed and gives a clear view of the

various components.A diagram of the interior components of the space craft appears on the inside back

cover.

This is

a schematic

diagram of the two

GRACE satellites in

orbit with the Kband microwave

beam connecting

them and precisely

tracking fluctuations

in the distance

between the two

satellites. The fluctuations are used to

infer changes in the

gravitational field on

the surface of the

Earth below.

Instrumentation

and Measurements
Abbreviations Used in GRACE Satellite Diagrams

ACC ICU Superstar Accelerometer Interface Control Unit

ACC SU Superstar Accelerometer Sensor Unit

ATH Attitude Control Thrusters

CESS Coarse Earth and Sun Sensor

GPS BKUP Antenna GPS Backup Antenna

GPS NAV Antenna GPS Navigational Antenna

GPS OCC Antenna GPS Occultation Antenna

IPU Instrument Processing Unit

KBR Assembly K-Band Ranging System Assembly

LRR Laser Retro Reflective Assembly

MTE Center of Mass Trim Assembly Electronics

MTM Center of Mass Trim Assembly Mechanism

MTQ Magnetorquers

OBDH On Board Data Handling

OTH-1, OTH-2 Orbit Control Thrusters

PCDU Power Control and Distribution Unit

RFEA Radio Frequency Electronics Assembly

SCA Baffle Star Camera Assembly Baffle

SCA +Y, SCA -Y Star Camera Assembly 

SZA – RX S-Band Zenith Antenna, Receive

SZA – TX S-Band Zenith Antenna, Transmit

Tank -X, Tank +X Cold Gas Tanks

USO-1 Ultra Stable Oscillator

GPS OCC

Antenna

CESS

OTH - 2

OTH - 1

SZA - TX

SZA - RX

GPS BKUP

Antenna

GPS NAV

Antenna

Top Panels

Global Star Silicon

Solar Cells

Side Panels

ATH

SCA Baffle

ATH

ATH

ATH

ATH

ATH

Shear Panel

OBDH

RFEA

GPS OCC

Antenna

GPS BKUP

Antenna

Rear Panel

GPS NAV

Antenna

GYRO

SCA - Y

MTM

MTQ

Sampler KBR

Assembly

K-Band

Horn

USO - 1

SCA + Y

Tank

- X

PCDU

Tank

+ X

ACC

ICU

ACC

SU

MTE

Main Equipment Platform

IPU

Front Panel

LRR

K Band Horn

X

S - Band Boom

(Magnetometer Inside) Launcher

Adapter

Battery

Y

Z

This diagram illustrates the flight configuration and ground support for the

GRACE mission. Fluctuations in density of the Earth’s surface result in very

small changes in the distance between

the two satellites, which are measured

with very high precision by the Kband ranging system.The S-band relay

(shown protruding from the bottom

of each satellite) allows for communication with surface tracking stations.

The GPS satellites are used as references to determine the precise location of the two satellites in orbit and

allow for the creation of gravity

maps—approximately once a month.

15
The GRACE mission benefits from

and will build upon the interest in

oceanography that has been growing in recent years with the recognition of the El Niño/La Niña phenomenon. GRACE provides a

strong basis to draw public and

student interest since new,

extremely accurate models of the

Earth’s gravity field will be generated.

The Texas Space Grant Consortium (TSGC)

works in partnership with the mission team to

develop and implement GRACE education and outreach.

GRACE master teachers were selected from across the nation through an application

and review process.These master teachers have created interdisciplinary K-12 educational materials that meet the National Educational Standards to support the GRACE

mission in the areas of Satellites, Gravity,Weather/Climate/Atmosphere, Oceans, and

GRACE general mission facts and objectives.These master teachers have trained hundreds of teachers about the GRACE mission impacting tens of thousands of students,

and the activities have been tested in classrooms across the nation.When GRACE

launches, the activities will be available to all educators on the TSGC web site,

http://www.csr.utexas.edu/grace/education/. In addition to classroom activities, an online GRACE activity guide, coloring book, and on-line quiz will be available for all interested.

Animations, photos and interviews with mission personnel will be available to all traditional media outlets—television, print, and radio. Computer animation, showing the

spacecraft deployments, orbit ground tracks, data collection, and real world applications

of the data, is available.The GRACE web site will also document, publicize, and expand

the mission’s science data including computer animations, downloadable educational

materials, a frequently asked questions (FAQ) archive, and evaluation/ suggestion forms

for feedback.This information will be updated regularly.

GRACE will revolutionize the way we look at Earth, providing new benefits for six billion people living on this beautiful, blue planet.Through youth development and education we will enhance our understanding of this dynamic and living world we call home.

16

and Public Outreach

Education
17

JPL

UTCSR

CU OSU KSC GSFC LaRC AMA

SS/L JHUAPL MIT UL ONERA GFZ UB TUM DLR

AGmbH EGmbH AWI

IABG KSRC

DLR/GSOC DTU

Participating Institutions

United States

AMA Analytical Mechanics Association, Inc. Hampton, VA

CU The University of Colorado Boulder, CO

GSFC Goddard Space Flight Center (NASA) Greenbelt, MD

JPL Jet Propulsion Laboratory (NASA) Pasadena, CA

JHUAPL Johns Hopkins University Applied Physics Lab. Laurel, MD

KSC Kennedy Space Center (NASA) Cocoa Beach, VA

LaRC Langley Research Center (NASA) Hampton, VA

MIT Massachusetts Institute of Technology Cambridge, MA

OSU The Ohio State University Columbus, OH

SS/L Space Systems/Loral Palo Alto, CA

UTCSR University of Texas Center for Space Research Austin, TX

European

AGmbH Astrium GmbH Friedrichshafen, Germany

AWI Alfred Wegener Institute for Polar and Marine Bremerhaven Germany

Research

DLR Deutsches Zentrum für Luft und Raumfahrt Bonn, Germany

DLR/GSOC DLR German Space Operations Center Oberpfaffenhofen, Germany

DTU The Technical University of Denmark Copenhagen, Denmark

EGmbH Eurockot GmbH Bremen, Germany

GFZ GeoForschungsZentrum Potsdam Potsdam, Germany

GRGS Groupe de Recherches de Géodésie Spatiale Toulouse, France

IABG Industrieanlagen-Betriebsgesellschaft GmbH Ottobrunn, Germany

KSG Kort & Matrikelstyrelsen Copenhagen, Denmark

KSRC Khrunichev State Research and Production Moscow, Russia

Space Center

ONERA Office National d’Etudes et de Recherches Paris, France

Aerospatiale

TUM Technical University of Munich Munich, Germany

UB University of Bonn Bonn, Germany

UL University of Lyon Lyon, France

Key Participants

B. Tapley UTCSR, Principal Investigator (PI)

C. Reigber GFZ, Co-PI

A. Davis JPL, Project Manager

M. Imhoff GSFC, ESSP Project Scientist

E. Koenemann DLR, GRACE Manager

R. Fitzgerald GSFC, GRACE Mission Manager

M.M. Watkins JPL, Project Scientist

U.S. Science Team

B. Tapley UTCSR, Lead

S. Bettadpur UTCSR

R. Gasparovic JHUAPL

R.S. Gross JPL

W.G. Melbourne JPL

D. Porter JHUAPL

C.K. Shum OSU

J. Wahr CU

M.M. Watkins JPL

C. Wunsch MIT

European Science Team

C. Reigber GFZ, Lead

G. Balmino GRGS

F. Flechtner GFZ

K.H. Ilk UB

N. Jakowski DLR

P. Knudsen KSG

Y. Ricard UL

R. Rummel TUM

J. Schroeter AWI

P. Schwintzer GFZ

KSG

GRGS
18

Specifications

General Spacecraft Information (Twin Satellites)

❖ Width: 1942 mm

❖ Length: 3123 mm

❖ Height: 720 mm

❖ Mass: 487 kg per satellite

❖ Design Life: 5 years

Orbit Characteristics

❖ Two satellites, co-orbiting at near polar inclinations at 300-500 km altitude.

❖ Separated along track by 220 km.

Key Components

❖ K-band Ranging System. Provides precise (within 10 μm) measurements of the distance change between 

 the two satellites and hence measures the fluctuations in gravity.

❖ S-band Boom. Used to send data from the satellites back to Earth for processing.

❖ SuperSTAR Accelorometers. Precisely measures the non-gravitational accelerations acting on the satellite.

❖ Star Camera Assembly. Precisely determines satellite orientation by tracking them relative to the position 

 of the stars.

❖ Ultra Stable Oscillator. Provides frequency generation for the K-band Ranging system.

❖ Coarse Earth and Sun Sensor. Provides omni-directional, reliable and robust, but fairly coarse Earth and 

 Sun tracking. To be used during initial acquisition and when GRACE is operating in “safe mode.”

❖ Center of Mass Trim Assembly. Precisely measures offset between the satellite’s center of mass and the 

“acceleration-proof” mass, and adjusts center of mass as needed during flight.

❖ Black-Jack GPS Receiver and Instrument Processing Unit. Provides digital signal processing; measures 

 the distance change relative to the GPS satellite constellation; and provides secondary atmospheric 

 occulation experiments.

❖ Laser Retro-Reflective Assembly. Provides measurements of the GRACE satellite orbits relative to 

 terrestrial tracking networks.

❖ Globalstar Silicon Solar Cell Arrays. Covers the outer shell of the spacecraft and generates power.

❖ Three-axis Stabilized Attitude Control System: Uses star camera and gyro sensors and a cold-gas

 nitrogen thruster system, with magnetorquers for fine corrections of spacecraft position.

❖ 1750-A Microprocessor for Flight Computer. Performs calculations for attitude adjustment and telemetry 

 processing.

* See photos and diagrams on page 14 and 15 for detailed layout of spacecraft.

Technical
19

The following table presents a concise summary of exactly what measurements GRACE

will obtain and the instruments that are used to obtain them, as well as how many spacecraft are involved in each measurement.

This science data in conjunction with ancillary data, will then be used to gain estimates of

spherical harmonic coefficients of the Earth’s gravitational potential.These coefficients are

necessary to construct the 30-day maps of gravitational potential that will be the endproduct from the primary GRACE measurements.

Gravity Measurement Error

Science Application Measurement Instrument # of spacecraft needed

 for measurements

Earth Gravity Field Inter-Satellite K-Band Microwave 2

 Range Change Link

Earth Gravity Field Non-Gravitational Accelerometer 2

 Accelerations

Earth Gravity Field Satellite Orbits Black Jack GPS Receiver 2

Atmospheric GPS-to-GRACE Black Jack GPS Receiver 1

Occultation Phase Change

Inter-satellite Range Measurement: < 4 μm @ twice/rev

 < 10 μm total @ 0.1 to 0.0001 Hz

Non-Gravitational Acceleration Measurement: < 10 -10 m/s2 noise (+1/f)

 < 4 x 10 -12 m/s2 tones.

Precision Orbit Measurement: 5 cm absolute

 < 0.2 mm, relative in plane

Science

Measurement Objectives
20

The estimates of the Earth gravity field from GRACE, in conjunction with other space-based

measurements, in situ data and geophysical models will be used to determine the time varying

changes in the mass of the Earth’s dynamical system due to different geophysical processes as

described in detail in this brochure.

Static Gravity Field Measurements

APPLICATION RESOLUTION

Oceanic Heat Flux >1000 km

Ocean Currents >1000 km

Solid Earth Sciences >300 km

Time Variable Gravity Field Measurements

APPLICATION RESOLUTION TIME SCALE COMMENTS

Oceanic Heat Flux >1000 km Seasonal 30 day estimate

Ocean Bottom Pressure >500 km Seasonal 30 day estimate

Deep Ocean Currents >500 km Seasonal 30 day estimate

Sea Level Rise >700 km Secular 5 year estimate

Evapo-Transpiration >300 km Seasonal 30 day estimate

Greenland/Antarctic Ice Secular 5 year estimate

 Seasonal 1 year estimate

Science

Applications Summary 

Goddard Space Flight Center

Greenbelt, Maryland

NP-2002-2-427-GSFC

GRACE

Mission

Twins in orbit fly

linked by microwaves reveal

Earth mass in motion

— Ab Davis, GRACE Project Manager, JPL