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TOPEX/Poseidon

TOPEX/POSEIDON SEA SURFACE MAPPING SATELLITE


THE OCEAN PLANET

Earth is the Ocean Planet: ocean waters, vital to all life, cover more than 70
percent of its surface.  Stirred and mixed by mighty currents, the oceans
distribute heat across the globe and regulate our climate.  The Earth's climate
has changed in the past--and it may soon change again.  Rising atmospheric
concentrations of carbon dioxide and other "greenhouse gases" produced as a
result of human activities could generate a global warming, followed by an
associated rise in sea level.  But in order to make reliable predictions, we
must first gain a quantitative understanding of the role of ocean currents in
climate change.  This understanding requires comprehensive new observations of
the Ocean Planet from space.  The challenge will be met by TOPEX/POSEIDON, a
joint mission of the United States and France.


THE TOPEX/POSEIDON MISSION

The United States and France celebrate the International Space Year, 1992, with
the launch of TOPEX/POSEIDON--the most advanced space mission ever designed to
study ocean currents.  The mission is sponsored by the U.S. National
Aeronautics and Space Administration (NASA) and France's space agency, the
Centre National d'Etudes Spatiales (CNES).

Sea-Surface Mapping from Space

TOPEX/POSEIDON will use radar altimetry to measure sea-surface height over 90
percent of the world's ice-free oceans.  Circling the Earth every 112 minutes,
the satellite will gather data for three to five years, carrying enough fuel
for a full decade of operation.  In combination with a precise determination of
the spacecraft orbit, the altimetry data will yield global maps of ocean
topography--the barely perceptible hills and valleys of the sea surface.  From
a knowledge of ocean topography, scientists can calculate the speed and
direction of ocean currents worldwide.

United States/France Partnership

Joint planning was initiated in 1983.  NASA, responsible for mission
operations, is supplying the spacecraft and four instruments, including the
primary radar altimeter.  CNES is providing launch aboard an Ariane 42P
expendable launch vehicle as well as two instruments, including an
experimental altimeter.  Management is provided by NASA's Jet
Propulsion Laboratory and the CNES Centre Spatial de Toulouse.

Unprecedented Accuracy

From an altitude of 1,336 km (830 miles), TOPEX/POSEIDON will measure the
distance from the satellite to the sea surface within 3 cm (1.2 inches).  Three
independent tracking systems will determine the position of the spacecraft
within 10 cm (4 inches).  These measurements will together yield accurate
topographic maps over the dimensions of entire ocean basins--the primary
mission objective.  These data will permit quantitative studies of ocean
circulation and its time variability that are crucial to an understanding of
climate change.

Forecasts of Climate Change

TOPEX/POSEIDON is closely coordinated with the Tropical Ocean and Global
Atmosphere and World Ocean Circulation Experiment seagoing measurement programs
sponsored by the World Climate Research Programme. The satellite data, together
with TOGA and WOCE measurements, will be analyzed by an international
scientific team.  This team will develop and refine computer models of the
global ocean that can be used to investigate natural climate variability and
assess the impact of human activities on climate--laying the groundwork for
forecasts of future climate change.

FROM ANTIQUITY TO THE SPACE AGE

The mystery and power of the seas have always drawn us to voyage across
them--to explore, to understand, and to take the measure of the Ocean Planet.
This timeless and universal need to define ourselves and our world is now being
reshaped by the Space Age.

Early Oceanographers

Navigational needs prompted the first attempts to measure the speed and course
of ocean currents.  Fragmentary records of tidal and current patterns along the
Red Sea and Mediterranean coasts, compiled by early sailors and traders as an
aid to commerce, have come down to us from antiquity.  Driven by necessity,
these intrepid seafarers were the first oceanographers.  Viking expeditions
crossed the North Atlantic, reaching Vinland (Labrador) around the year 1000.
However, tales of a New World across the sea remained locked in Nordic lore.
But early in the Renaissance, other Europeans began to fan out across the
world's oceans in search of riches and new lands.  By 1600, Portuguese,
Spanish, French, and English expeditions had mapped most of the oceans and
continents upon a globe that, in Europe, had lain half unknown scarcely a
century before.  These bold adventurers, bridging the Old World and the New,
initiated the systematic exploration of the seas and revealed the dimensions of
the Ocean Planet.

Beginnings of Research

As Deputy Postmaster General for the American colonies, Benjamin Franklin
sought to shorten the London-New York shipping time by charting Atlantic Ocean
currents.  In 1775, while sailing from London to Philadelphia, Franklin
delineated the edges of the warm Gulf Stream through water-temperature
measurements; his later map, constructed with the help of Nantucket sea captain
Timothy Folger, is a classic of oceanography.  The first office dedicated to
ocean mapping had been established in France only five years earlier.  Shortly
afterward, the English captain James Cook carried out his famous voyage to
explore and chart the Pacific (1776-1779).  Marine expeditions quickly
multiplied.  In 1849, drawing upon data collected from ships' logs, American
naval officer M.F. Maury published the first worldwide wind and current charts.
The year 1872 marked the beginning of the first purely scientific oceanography
expedition: the 42-month voyage of H.M.S. Challenger, sponsored by the British
Royal Society. Covering 113,000 km (70,000 miles), Challenger systematically
surveyed the deep ocean, collecting data on ocean depth, temperature, currents,
and other properties.  These revelations were organized and compiled over the
following 20 years to lay the foundations of modern oceanography.

Oceanography Matures

The 20th century has seen major advances in oceanographic theory and
technology, the widespread deployment of instruments in the sea, international
scientific collaborations-- and a host of new findings.  The beginning of the
century (1900-1930) extended the studies of Challenger, particularly through
the work of analogous national survey expeditions.  Highlights include
invention of the first reliable surface-to-bottom sampling bottle, development
of the theory of wind-driven currents, and completion of the first
trans-Atlantic physical, chemical, and biological measurements.  The period
1930-1960 saw rapid technical progress, intensive testing of theories of
oceanic processes, and the first global research collaborations.  Highlights
include use of efficient new instruments for measuring temperature and tracking
deep currents, models of large-scale ocean circulation, and recognition of the
importance of eddies in global ocean circulation.  The last three decades
(1960-present) have seen dramatic strides in ocean-measurement and computing
technology, detailed regional studies, and a growing appreciation of the role
of the oceans in climate.  Highlights include the Indian Ocean Expedition,
mooring of deep-ocean current meters, computer models of global oceanic and
atmospheric circulation, the International Decade of Ocean Exploration, the
beginnings of satellite oceanography, and the start of TOGA and WOCE. Most
importantly, scientists have recognized the necessity of studying the Earth as
a unified system of coupled components: land, oceans, atmosphere, and
biosphere.

The Space Age

The beginning of the Space Age in 1957 heralded a technological revolution in
Earth studies.  In 1960, a U.S. weather satellite returned the first images of
the Earth's cloud cover.  By the 1970s, satellites were routinely gathering
information on the physics, chemistry, and dynamics of the atmosphere and
features of the land surface.  The 1970s also saw the first use of satellite
altimetry for measurements of sea- surface height.  Measurements carried out by
ships and by instruments deployed in the sea are essential for research.
However, these techniques are limited in both duration and geographic coverage;
they cannot provide an understanding of global ocean circulation, which
requires frequent, long-term observations of currents over ocean-basin scales.
The measurement of ocean topography by satellite radar is the only way to
obtain these observations.  The promise of two decades of progress in radar
altimetry from space will be fulfilled by the launch of TOPEX/POSEIDON in 1992.


THE OCEAN-CLIMATE CONNECTION

What drives ocean currents?  What controls their movement?  How much heat do
they distribute around the globe?  Are ocean processes coupled to climate?
These questions take us to the heart of the TOPEX/POSEIDON mission and the
ocean-climate connection.  Both the oceans and the atmosphere transport
roughly equal amounts of heat from the Earth's equatorial regions--which are
intensely heated by the Sun--toward the icy poles, which receive relatively
little solar radiation.  The atmosphere transports heat through a complex,
worldwide pattern of winds; blowing across the sea surface, these winds drive
corresponding patterns of ocean currents.  But the ocean currents move more
slowly than the winds and have a much higher heat storage capacity.  Like a
massive flywheel that regulates the speed of an engine, the vast amount of heat
stored in the oceans regulates the temperature of the Earth. The oceans are the
thermal memory of the climate system.

Ocean Circulation

Atmospheric winds sweep the ocean surface layer along with them, raising
sea-level height downwind.  The surface of the tropical Pacific Ocean, for
example, is normally piled about 50 cm (20 inches) higher off Asia than off
South America because of the steady westward sweep of the tropical trade winds.
The ocean also responds to the Earth's rotation.  Ocean currents are deflected
to the right (in the Northern Hemisphere) or to the left (in the Southern
Hemisphere) by the "Coriolis force," a rotational effect first explained by the
French mathematician Gaspard Gustave de Coriolis (1792-1843).  Driven by the
winds and directed by the Coriolis effect, ocean surface currents circulate in
enormous "gyres" around regions of raised or lowered sea level--the hills and
valleys of ocean topography--just as winds blow around the extensive highs and
lows of atmospheric surface pressure.  Observations of ocean topography,
together with our knowledge of the Coriolis force, thus permit the speed and
direction of surface currents to be calculated.

A Global Conveyor Belt for Heat

Along the western margins of the oceans, swift and narrow currents carry warm
tropical surface waters toward the polar seas, where they lose heat to the
atmosphere and then sink into the ocean depths.  This sinking is most
pronounced in the North Atlantic Ocean. Migrating back toward the Southern
Hemisphere above the sea floor, the cold water eventually wells up to the
surface layers of the Indian and Pacific oceans.  This massive, global circuit
takes almost 1,000 years.  The steady oceanic transport of tropical heat to the
cold polar seas moderates our climate.  Without the warming effect of the Gulf
Stream, for example, the climate of Europe would resemble that of northern
Canada. Conversely, human activities have the potential to influence this
circulation pattern and thus alter our climate.

Climate Change

The oceans also regulate climate by absorbing carbon dioxide and other
greenhouse gases.  Analysis of ocean carbonate sediments and polar ice-cap
deposits shows that atmospheric composition and ocean circulation have both
varied dramatically in the past: 18,000 years ago, during the last Ice Age,
carbon-dioxide levels were 40 percent below those of today and sea level stood
more than 100 m (330 feet) lower.  A better understanding of ocean processes is
needed to interpret this history and to forecast long- term climate trends.
The oceans play a role in short-term climate shifts as well.  At intervals of
three to seven years, the weakening of tropical trade winds allows warm Asian
surface waters to surge eastward across the Pacific to South America.
Accompanied by major shifts in atmospheric circulation and rainfall, these "El
Nino" events disturb climate worldwide.  The El Nino of 1982-83, the worst of
this century, triggered flooding and landslides that claimed 600 lives in
Ecuador and Peru and devastated the U.S. West Coast; cyclones left 25,000
homeless in Tahiti, and severe droughts struck Australia, Indonesia, the
Philippines, and South Africa. Improved knowledge of upper-ocean circulation in
the tropical Pacific is essential for the reliable prediction of such events.

THE OCEAN DECADE

How do ocean-atmosphere interactions shape El Nino climate events?  What is the
global pattern of ocean circulation?  What additional observations from space
are required?  These questions are now being addressed through an
international, coordinated research effort, including NASA's Mission to Planet
Earth. During the decade of the 1990s, oceanography involving seagoing field
experiments and space missions is being intensified.  TOPEX/POSEIDON is a core
element of this research.

 TOGA

The international Tropical Ocean and Global Atmosphere (TOGA) program was begun
by the World Climate Research Programme (WCRP) in 1985 to study the
year-to-year variability of the tropical oceans and their coupling to the
global atmosphere.  This decade- long effort involves extensive observations
and modeling studies.  The results will help to improve the predicability of
ocean-atmosphere interactions on timescales ranging from months to
years--particularly El Nino events in the Pacific Ocean. During the flight of
TOPEX/POSEIDON, an intensive TOGA field program will study the warm-water pool
in the western tropical Pacific--the Earth's largest reservoir of warm surface
water--and its role in the El Nino phenomenon.  The altimeter measurements of
sea-level height will provide a Pacific-wide perspective on this field study.
Conversely, the TOGA measurements will help to validate the observations from
space.

WOCE

The World Ocean Circulation Experiment (WOCE) was begun by WCRP in 1990 to
better describe and understand global ocean circulation and its relationship to
climate changes over decades or longer.  During the 1990s, scientists from 40
nations will carry out an unprecedented series of oceanographic observations
and measurements through a worldwide network of sea-level stations and a fleet
of research vessels covering all the oceans.  The WOCE network will permit
calibration of TOPEX/POSEIDON observations--which, in turn, will provide a
global reference framework for the integration of the WOCE seagoing
measurements.  WOCE will provide the data necessary to make major improvements
in the accuracy of computer models of ocean circulation.  As these models
become more precise, they will be coupled to models of atmospheric circulation
to simulate--and ultimately to help predict--how the ocean and the atmosphere
will together determine our future climate.

Space Missions

TOPEX/POSEIDON will be complemented by other important space missions over the
next decade.  The multipurpose European Remote-sensing Satellite (ERS-1)
satellite, launched in 1991, carries a radar altimeter together with five other
instruments.  Although its altimetric measurements are less accurate than those
of TOPEX/POSEIDON, its high-latitude coverage allows ERS-1 to study polar ocean
and ice topography inaccessible to the U.S./France mission.  The two missions
are complementary: their altimeter data will be merged to yield a single data
set of greater coverage than either can provide alone.  The NASA Scatterometer
(NSCAT), to be flown on the Japanese Advanced Earth Observing Satellite (ADEOS)
in 1995, will measure the speed and direction of sea-surface winds with high
accuracy over 95 percent of the global ocean every two days.  The potential
overlap of the TOPEX/POSEIDON and NSCAT missions provides an exciting
opportunity to study the ocean's response to wind forcing.  The ARISTOTELES
gravity-measurement satellite being considered by the European Space Agency
(ESA) for flight in the late 1990s would carry a gravity gradiometer to measure
in detail the spatial variations of the Earth's gravity field.  These data
would greatly improve the accuracy of ocean-circulation calculations.  The
Earth Observing System (EOS) is a series of satellites that will be a central
component of Mission to Planet Earth beginning in the late 1990s.  EOS will
provide long-term (15-year) data sets to further our understanding of the
interactions among the Earth's land surfaces, oceans, atmosphere, and
biosphere, and how these are being influenced by human activities.  As well as
providing new data on the global hydrologic and biogeochemical cycles, EOS will
extend altimetric measurements of global ocean circulation into the next
century.


A GLOBAL VIEW FROM SPACE

Oceanography is no longer tethered to the sea.  Satellite technology has now
enabled scientists to place sensitive sensors in space, far above the sea
surface, to take a new measure of the Ocean Planet. Two Decades of Progress
Although an experimental radar altimeter was flown in 1973 aboard NASA's Skylab
mission, NASA's Geophysical Satellite-3 (Geos-3, 1975-1978) carried the first
instrument to yield useful measurements of sea level and its variability with
time.  The Geos-3 map of Gulf Stream variability was in good agreement with
historical ship observations.  NASA's 1978 Seasat carried the first altimeter
designed for oceanography and returned the first global sea-surface
measurements.  Although the mission operated for only 100 days, it collected
more ocean-topography data than the previous 100 years of shipboard research.
An altimeter on the U.S. Navy's Geodetic Satellite (Geosat, 1985-1989) gathered
the first multiyear global data set on sea-level height, producing the most
accurate variability measurements yet obtained.  The pioneering Geos-3, Seasat,
and Geosat missions reflect two decades of progress in instrument technology
and scientific analysis.  Improvements in altimetric precision reduced the
uncertainty in satellite height above the sea surface from Skylab's 60 cm (24
inches) to Geosat's 4 cm (1.6 inches).  However, all were limited by
uncertainties in the satellite orbit, which ranged up to 10 m (33 feet) for
Geos-3 and up to 2 m (6.6 feet) for Seasat and Geosat. Consequently, these
missions could not provide definitive studies of ocean circulation, which
require topographic measurements with an error of 14 cm (6 inches) or less
across an entire ocean basin.

TOPEX/POSEIDON: Meeting the Challenge

TOPEX/POSEIDON is the first mission designed to bring both high altimetric
precision and high orbital accuracy to bear upon the challenge of ocean
topographic mapping.  It will provide a detailed, global snapshot of the ocean
surface every 10 days over a period of three to five years.  A high orbital
altitude of 1,336 km (830 miles), chosen to minimize atmospheric drag and the
effect of spatial variations in the Earth's gravity, will permit TOPEX/POSEIDON
to be tracked with unprecedented accuracy.  The orbital inclination will carry
the spacecraft to 66 degrees north and south latitude, allowing a thorough
sampling of tidal signals.  The satellite's measurement tracks across the Earth
will be reproduced within 1 km (0.6 mile) over every 10-day repeat cycle, so
that the altimeters can repeatedly measure nearly identical points on the sea
surface.  The widest spacing between tracks, at the equator, will be only 315
km (195 miles).

MAPPING THE OCEAN SURFACE

What determines the shape of the ocean surface?  How is this shape measured
from space?  How can we minimize the uncertainty of this measurement?  The
answers to these questions draw upon fundamental physics, advanced technology,
and the inspired perseverance of a generation of satellite oceanographers.

Static Gravity, Dynamic Oceans

If the ocean were motionless, the shape of its surface would be determined
entirely by the gravitational attraction of the Earth. Even in that case,
however, the sea surface would have hills and valleys.  Because matter is
distributed unevenly within the Earth's crust-- densely packed within mountain
ranges, thinned by valleys--the Earth's gravity field varies substantially over
both the continents and the seas.  This spatial variation of gravity itself
generates sea-surface topography with a global altitude range of nearly 160 m
(525 feet) relative to the center of the Earth. Swept by winds and seething
with waves, the ocean is actually in ceaseless motion, flowing in gigantic
currents and gyres directed by the Coriolis effect of the Earth's rotation.
The shape of the sea surface is dynamic, and it therefore departs from the
static topography determined by gravity alone.  This departure, called dynamic
ocean topography, has a global range in altitude of about 2 m (6.6
feet)--barely one percent of the altitude range of the gravitational
topography.  But only the dynamic topography contains information about the
speed and direction of ocean currents.  The challenge is to measure this slight
topographic variability over the vast dimensions of the ocean basins.

How Altimetry Works

Dynamic ocean topography is mapped through a three-step process: (1) The radar
altimeter sends short pulses of microwave energy toward the ocean below; the
round-trip travel times of the reflected pulses yield the distance between the
spacecraft and the sea surface. (In addition, the shape of the reflected pulse
is used to determine wave height and sea-surface wind speed.) (2) A precise
determination of the satellite orbit then permits these distance measurements
to be translated into a global map of sea level relative to the center of the
Earth. (3) Finally, the map of sea-surface topography is compared with a map of
gravitational topography, which must be obtained independently; the difference
between the two is dynamic ocean topography, from which ocean-current
velocities can be calculated.  Step (1) requires careful correction for
atmospheric effects.  Water vapor absorbs microwave radiation and delays the
radar pulses during their round trip to the sea surface.  Following the example
of Seasat, TOPEX/POSEIDON therefore carries a microwave radiometer for
concurrent measurements of atmospheric water- vapor concentrations; the
water-vapor effect can then be calculated and eliminated from the data.
Electrons released by the ionization of gases in the upper atmosphere
(ionosphere) by sunlight also introduce a pulse-arrival delay.  Because this
delay depends on the radar frequency, observations at two different frequencies
permit correction for this effect as well.  The most important uncertainty,
however, enters at step (2): determination of the satellite's orbital altitude.
TOPEX/POSEIDON will therefore be tracked with unprecedented accuracy by three
independent and complementary systems.

The Tides

Regular sea-level oscillations caused by the tides present another challenge to
accurate topographic measurements.  However, the TOPEX/POSEIDON orbit has been
chosen to permit separation of the primary lunar and solar tidal signals from
the dynamic ocean topography, so that tidal effects can be eliminated from the
ocean- circulation calculations.  TOPEX/POSEIDON will in fact provide the
observations needed to compute definitive global tide models, which can then be
applied to data from all future oceanographic missions.


INSTRUMENTS: UNPRECEDENTED ACCURACY

The TOPEX/POSEIDON spacecraft, constructed by the Fairchild Space Company, is a
modification of the Multimission Modular Spacecraft (MMS) used for NASA's 1980
Solar Maximum Mission and the later Landsat-4 and -5 missions.  The 2,400-kg
(5,300-lb) satellite carries a suite of six instruments, provided by the United
States and France, housed in a special instrument module attached to the MMS
satellite bus.  Power is supplied by a single large solar-cell array.  In
addition to the dish-shaped antenna used for radar altimetry, the spacecraft
has a variety of communications antennas to link the mission with the NASA
Tracking and Data Relay Satellite System (TDRSS), the CNES Doppler Orbitography
and Radiopositioning Integrated by Satellite (DORIS) tracking system, and the
U.S. Global Positioning System (GPS) of high-altitude navigational satellites.

Two Altimeters

The primary instrument is the NASA dual-frequency TOPEX altimeter, operating at
5.3 and 13.6 GHz, which draws upon a long heritage of single-frequency
instruments extending back to Seasat. Dual-frequency operation permits
correction for the ionospheric-electron delay effect.  This instrument, managed
by NASA's Goddard Space Flight Center (GSFC) and built by The Johns Hopkins
University's Applied Physics Laboratory (JHU/APL), is fully redundant and
incorporates well understood, flight-tested technology.  The companion TOPEX
microwave radiometer, developed by NASA's Jet Propulsion Laboratory (JPL),
operates at frequencies of 18, 21, and 37 GHz to provide estimates of total
atmospheric water-vapor content.  The 21-GHz channel is the primary measurement
channel; the 18-GHz and 37-GHz channels are used to remove the effects of wind
speed and cloud cover, respectively.  These data allow reduction of the
water-vapor delay error to 1 cm (0.4 inch), permitting an overall altimetric
precision of 3 cm (1.2 inches).  The mission also carries an advanced,
experimental solid-state POSEIDON altimeter, designed by CNES and built by
Alcatel Espace, which uses the same antenna as the NASA altimeter but operates
at a single frequency of 13.6 GHz. The ionospheric-electron correction is
provided by a model that makes use of the simultaneous dual-frequency
measurements of the DORIS tracking system.  Both the operating principles and
the expected performance of the two altimeters are similar.  However, the
single-frequency POSEIDON altimeter has only one-fourth the mass, volume, and
power consumption of the NASA instrument.  Moreover, the telemetry data rate is
reduced by a factor of 7 because of more extensive on-board processing.  The
CNES instrument is thus a prototype for the satellite altimeters of the future.

Three Tracking Systems

The NASA tracking system, managed by GSFC, operates by laser ranging to
reflectors (built by JHU/APL) arrayed around the altimeter antenna, which
permit the satellite to be intermittently tracked by a worldwide, ground-based
network of 12 laser stations within about 2 cm (less than 1 inch).  These data
will be used with computer models of the Earth's global gravity field
(developed at GSFC, at the University of Texas at Austin, and in France) for
precision orbit determination and calibration of the altimeters.  The DORIS
system determines the satellite's velocity by measuring the Doppler shifts of
two ultrastable microwave frequencies (2,036 MHz and 401 MHz) transmitted by a
global network of some 50 ground-based beacons whose positions are known within
a few centimeters (several inches).  Validated by a prototype receiver launched
in 1990 on the French SPOT-2 Earth-observing mission, DORIS has already
provided more than two million measurements; these have been used to refine
data-processing methods and improve gravity models.  DORIS is manufactured
under CNES management by Dassault Electronique (receiver), CEIS Espace
(beacons), and CEPE and OSA (quartz oscillators).  The mission also carries an
experimental GPS receiver, developed by Motorola under contract to JPL, in
order to demonstrate GPS capabilities.  The GPS system provides continuous
spacecraft tracking with a potential accuracy of 10 cm (4 inches) or better and
promises to revolutionize orbit determination for future satellites.


UNITED STATES/FRANCE PARTNERSHIP

The Jet Propulsion Laboratory in Pasadena, California is responsible for
TOPEX/POSEIDON project management, including prelaunch mission planning,
development of the U.S. sensors, design and development of the U.S. ground data
system, and post-launch control and communication with the satellite.  Within
CNES, the Centre Spatial de Toulouse is responsible for participation in
mission design and management, development of the French sensors and ground
data system, and provision of the Ariane launch- vehicle system and launch
services.  The mission schedule is divided into five phases: launch, assessment
(35 days), initial verification (six months), observation (three years), and
extended observation (two more years, with even longer operation possible).

Launch and Assessment

TOPEX/POSEIDON will be launched by an Ariane 42P expendable launch vehicle from
the European Space Agency's Guiana Space Center in Kourou, French Guiana. JPL
will conduct mission operations, data acquisition, and data processing; the
Centre Spatial de Toulouse will process data from the CNES payload.  The
critically important assessment phase officially begins 18 minutes after launch
with the separation of the satellite from Ariane and ends 35 days later.
During this period, the satellite and sensor systems will be deployed,
activated, and functionally certified, and the satellite will achieve its
operational orbit.

Triple Tracking

The NASA/GSFC laser-ranging system will furnish the baseline tracking data
needed for precise orbit determination and verification of the altimeter
measurements.  However, the laser beams cannot penetrate cloud cover.  The
recently demonstrated CNES DORIS system, used in addition to the NASA system,
will provide all- weather tracking through radio beacons to the onboard DORIS
receiver.  Since neither system provides constant coverage of the satellite,
computer models of the Earth's gravity field will assist in high-precision
orbit determination; TOPEX/POSEIDON tracking data will, in turn, help to
provide still further improvements to these models.  The GPS system will
supplement the two primary tracking systems as a demonstration of its
capability for continuous satellite tracking.

Instrument Verification

Verification of the performance of the satellite and its instruments is
necessary to ensure the validity and integrity of the scientific data.
Although this is a continuous task, an intensive verification campaign will be
conducted jointly by NASA and CNES during the first six months of the mission
to calibrate and verify the satellite data through comparison with measurements
made at two specially chosen verification sites.  This work will be carried out
in parallel with additional, longer-term programs of data validation conducted
through TOGA and WOCE.

NASA Data Validation

JPL is instrumenting an oil-drilling platform, provided by Texaco Corporation,
12 km (7 miles) west of Point Conception, California, to obtain data on sea
level and related parameters.  Sea-level measurements will be made by an
acoustical device and pressure gauges mounted on the platform.  These data,
together with data from nearby laser tracking sites, will be used to determine
the distance from the satellite to the sea surface, which will then be compared
with the altimeter ranging measurements to calibrate their performance.  Other
instruments will include a GPS receiver to measure total ionospheric electron
content, together with a surface pressure gauge and an upward- looking
radiometer to check the altimeter ranging correction.

CNES Data Validation

CNES is instrumenting two islands, Lampione and Lampedusa, in the Mediterranean
Sea. The instrument configuration will include a laser, tidal gauges, deep- sea
pressure gauges, a DORIS station, a radiometer, a meteorological station, and
wind and wave buoys.  These instruments will verify sea level, atmospheric
pressure, wind speed, wave height, the water-vapor correction, orbit
determination, and other sources of ocean-topography error.  Ionospheric
corrections will be verified by comparison of the DORIS and GPS measurements
with those made by the NASA dual-frequency altimeter, as well as through data
furnished independently by a European ground-based radar system.

Later Mission Phases

The observational phase begins at the completion of initial verification and
extends until the nominal end of the mission, three years after launch.  The
priorities during this phase are the recording of observational data and
maintenance of the scientific capability of the satellite.  If good performance
continues and funding is provided, the TOPEX/POSEIDON mission will be extended
for an additional two years or more.  The spacecraft will carry sufficient fuel
for a full decade of operation.  Data Distribution and Archiving The primary
mission product is the Geophysical Data Record (GDR).  During the initial
verification phase, activity will focus on data validation; altimetry data will
be processed into interim GDRs and distributed to the science team for study.
By the end of this phase, all the geophysical measurements will be calibrated
and verified, and the parameters necessary for the production of the final GDR
will be approved.  The final GDR will be distributed both to the mission
scientists and the broader scientific community.  As the complete record of
TOPEX/POSEIDON observations, the GDR will include sea-height measurements and
all the corrections applied to them, as well as data on wave height, wind
speed, and satellite altitude and location.  Both interim and final GDRs will
be available through NASA's EOS Physical Oceanography Distributed Active
Archive Center (PO-DAAC) at JPL and the French data center, AVISO.


SCIENTIFIC UNDERSTANDING

Through the work of an international science team, the results of
TOPEX/POSEIDON will be widely shared, analyzed, and interpreted to increase our
knowledge of global ocean circulation and its role in climate change.
Selection and Role When TOPEX/POSEIDON was still in its early planning stages,
NASA and CNES selected thirty-eight scientists through a competitive
Announcement of Opportunity to play leadership roles in the analysis and
interpretation of mission data.  They represent nine countries: sixteen are
from the United States, thirteen are from France, and the remainder are from
Japan, Australia, South Africa, Germany, Norway, the Netherlands, and the
United Kingdom. These men and women, together with their more than 160 research
collaborators, form the TOPEX/POSEIDON science team, which holds primary
responsibility for the achievement of mission science goals.  However, the
entire TOPEX/POSEIDON data set will be made available to the international
scientific community to support additional investigations, opening a new
spectrum of research efforts directed at an understanding of global climate
change.

Science-Team Investigations

TOPEX/POSEIDON will return the first accurate topographic data on ocean-basin
scales, permitting thorough investigations of circulation in the Pacific,
Atlantic, Indian, and Southern oceans.  When combined with the results of
seagoing measurement programs, these data will also permit global-scale studies
of the interaction of active surface circulation with slow-moving deep
waters--the key to understanding long-term climate variability.  Investigation
of the Southern Ocean will be of parti- cular interest.  The Antarctic
Circumpolar Current plays a fundamental role in the heat balance of the West
Antarctic Ice Shelf; if this enormous ice mass should melt and break away from
the underlying bedrock as a result of global warming, sea level would rise by 4
to 5 m (about 15 feet).  TOPEX/POSEIDON will greatly advance our understanding
of this remote region and improve our ability to predict the fate of the ice
shelf.  Tropical-ocean studies will focus on the causes of Pacific sea-level
variations and the disastrous El Nino climate events.  Other investigations
will refine our knowledge of the Earth's gravity field and its effect on ocean
topography, permitting more accurate models of both gravity and ocean
Ocean Circulation Experiment. By merging the satellite observations with TOGA
and WOCE findings, they will establish the extensive data base needed for the
quantitative description and computer modeling of ocean circulation.  The ocean
models will eventually be coupled with atmospheric models to lay the foundation
for predictions of global climate change.


THE FUTURE OF THE OCEAN PLANET

When the United States/France TOPEX/POSEIDON mission is complete,
oceanographers will have carried out the most accurate mapping of ocean
topography ever attempted.  For the first time, scientists will have the
detailed description of ocean circulation necessary for an understanding of the
role of the ocean in climate change.  In combination with the findings of
seagoing measurement programs and other complementary space missions, the
TOPEX/POSEIDON results will be used to refine models of global ocean
circulation and to improve our understanding of the ocean's influence on
climate.  Armed with this new knowledge, oceanographers will collaborate with
other Earth scientists to develop forecasts of global change, enabling
governments around the world to assess the future of the Ocean Planet.
GPS system provides continuous spacecraft tracking with a potential accuracy of 10 cm (4 inches) or better and promises to revolutionize orbit determination for future satellites. UNITED STATES/FRANCE PARTNERSHIP The Jet Propulsion Laboratory in Pasadena, California is responsible for TOPEX/POSEIDOʖU=$sACEacJ5$n / 1 S U 1 u   [ C % 6 8 U W C$qS*{dL,z!aE&waH613yfB'xaI2 y 7!!!"n"" #R###:$$$$$$$I%%%,&y&&'a'''P(w(y(((($)q))*]***L+++5,,,,,,(-t--.!c.R...C/// 0 000g001M111;222 3p33 4U444?5A5S5U5558666&7A7C7J7L777-8{88 9\999K:::::::G;;;5<<<=b===M>>>>>>-?w??@j@@AXAAADBBB%CvCCD]DDD