Earth System Science at
20 Oral History Project
Edited Oral History Transcript
Byron D.
Tapley
Interviewed by Rebecca Wright
Austin, Texas – 12 January 2010
Wright:
Today is January 12, 2010. This oral history with Dr. Byron Tapley is
being conducted in Austin, Texas for the NASA Headquarters Earth System
Science at 20 Oral History Project.
This interview is part of a series that is gathering experiences from
those who significantly were involved in the efforts to launch and foster
the concept of Earth System Science. Interviewer is Rebecca Wright,
assisted by Sandra Johnson. Thank you again for finding time in your
busy, busy schedule to talk with us today. We’d like for you to
start by telling us how you first got involved in your field of expertise.
Tapley:
My introduction into the space research field came as Sputnik was launched
in [October 4, 1957]. I had just finished my academic work, accepted
an appointment at the University of Texas [UT, Austin, Texas] in the
field of Engineering Mechanics, after performing my doctoral research
on the plastic deformations of materials under high strain rates.
When the Sputnik was launched, the university decided that it would
be appropriate to introduce a space-related course in aerospace engineering.
I was approached by the Chair of the Aeronautics Department about teaching
the course. I decided that, if I were going to make this change, I wanted
to develop a complete program, rather than just one course.
The university agreed that I would develop a program in the field of
astrodynamics, as a part of what became the aerospace engineering department.
It was a big change to leave an active and mature program of research
to initiate a program with a clean sheet of paper. This proved to be
a very big challenge. There was no academic capability on campus. No
curricula and no students at that point, and actually no one to have
an intellectual discussion about space issues. There was considerable
interest and excitement in the student body and after a couple years
the first set of Ph.D. candidates began to mature and the program began
to take on a life of its own, and a number of leading engineers and
scientists at various NASA and other government centers, academic institutions
and space related industrial firms passed through the academic program
on the way to their numerous accomplishments.
My early research was related to the theory of low thrust transfer trajectories,
which was of interest in the early design concepts for interplanetary
exploration missions The early research came out of a early meeting
with Dr. C.R. [John] Gates who was in charge of Section 312 at the Jet
Propulsion Laboratory. This section had the responsibility for developing
the guidance and orbit determination algorithms to support the unmanned
lunar missions in the early decade of space exploration. In addition
to encouraging NASA to provide the first research grant that I obtained,
Dr. Gates provided an introduction to a number of pioneers in the space
field including, Bill Melbourne, Carl Salloway, Tom Hamilton, Harry
Lass. I spent summer periods with this group during the first decade
of my career and I learned a great deal from their collective expertise.
The connection with JPL [Jet Propulsion Laboratory, Pasadena, California]
has been a long and close relation because of the intellectual interest
and the fact that so many students have initiated their careers there.
The first topic of low-thrust trajectory analysis proved an interesting
path. The technology for conducting the missions failed to mature and
the concept has never played a very large role in the missions for interplanetary
exploration. However, as the basis for initiating a space related curriculum,
it proved an excellent choice in that the topics of orbit determination,
guidance and navigation, and trajectory optimization were all encountered
as an integral part of the study. Courses in each of these disciplines
were added to the curricula and a good part of the period between 1965
and 1975 was spent studying various parts of this field. The first group
of very good students matured during this effort and most migrated to
JPL to take on increasing responsibilities during the subsequent decades.
Around 1968, we were approached by Gene [Eugene L.] Davis at the NASA
Johnson Space Center to assist with developing orbit determination capabilities
for the manned exploration program. We began working on the development
of batch estimation techniques and Kalman filter techniques for navigation
of Earth-orbiting satellites, and extended some of the effort into the
early Apollo program, with the navigation related to lunar exploration.
In the early part of the 1970s program, we initiated application of
the Precision Orbit Determination [POD] capability Geodynamics Program
at the NASA Goddard Space Flight Center [GSFC, Greenbelt, Maryland],
to participate in the analysis of some of the first satellite laser
ranging [SLR] being collected under a program put in place by NASA Goddard.
Dave Smith who was in charge of the geodynamics program at NASA GSFC
was interested in looking at how the sequential processing or the Kalman
filtering of the laser data would essentially compare with what would
be done with the traditional analysis results that NASA Goddard acquired
using their GEODYN [software] program.
This essentially led forward to our first exposure to satellite acquired
geodynamic-tracking data. We essentially began to develop our own software
systems to process that data. This activity led to developing a software
system called UTOPIA, the University of Texas Orbit Processor Incorporating
statistics Analysis, the first of a long line of software systems that
we developed to look at the solid Earth dynamics and the Earth System
Science type applications.
This capability allowed us to propose for a mission called GEOS-C [Geodynamics
Experimental Ocean Satellite]. It was one of the first satellite altimeter
missions. Our proposal was accepted and we joined the science team for
this mission. We began to analyze the altimeter data in combination
with the satellite laser ranging data. This data combination stimulated
the analysis that we performed at the University of Texas during the
subsequent decades. And it was through this analysis that we began to
have a significant input into the NASA Earth System Science program.
The first major step occurred in 1977. At this time, the Seasat mission
was being developed at JPL as the first microwave remote sensing satellite
and its primary focus was on studying the oceans. The objectives of
the mission were to make the first global measurements of the ocean
surface and the surface winds in the microwave frequency range. The
all weather and global nature of the altimeter, synthetic aperture radar
and scatterometer data promised a significant advancement in our understanding
of the ocean dynamics. Of these sensors, the radar altimeter required
an accurate orbit to utilize the measurements.
I was approached by George [H.] Born, who was in charge of the Seasat
data system with the question of whether I would take on the management
of a GPS [Global Positioning System] instrument team. The instrument
would have been the first GPS reviver to fly on a satellite. I was tasked
with assisting in the delivery of the accurate orbits required to apply
the Seasat altimeter measurements. I agreed to the assignment. Early
on it was apparent that there were a host of issues related to the technology
that required resolution.
The manufacturer fell behind schedule, overran the budget and the project
finally decided to eliminate the instrument. At that point there was
a problem with the Altimeter/Precision Orbit Determination team. Although
the implementation was through JPL, Seasat was a joint NASA-DoD [Department
of Defense] satellite. The Altimeter/POD Team was composed of a contingent
of NASA and DoD members with strongly differing opinions on the mission
implementation of the altimeter measurement. The nature of the team
interactions suggested that an individual that was not in either camp
should act as a leader of the team, so I was asked to take on that activity.
The decision to act as the Altimeter/POD team leader turned out to be
a very important step in setting the direction for the research that
I, and the Center that evolved from the research, conducted during the
next three decades. This activity was centered on a strong collaboration
with two of my early students George H. Born and Bob E. Schutz.
As mentioned earlier, George was a former student that completed his
graduate work in the late ’60s, and migrated through NASA JSC
to JPL. He was one of the early vanguards of the numerous students that
joined JPL after completing their graduate studies. Bob joined the faculty
at the University [of Texas] after completing his academic studies.
The problem of determining accurate orbits for altimeter satellites
provided a collaborative bond for our interactions during the next three
decades. The knowledge gained in these studies was the basis for our
manuscript on Statistical Orbit Determination.
The Seasat altimeter evolved and had another significant connection.
The altimeter instrument leader, who was the engineer in charge of the
altimeter fabrication, was Bill [William F.] Townsend. Bill joined NASA
HQ [Headquarters, Washington, DC] and was part of the management structure
that implemented the remarkable successful follow-on satellite altimeter
missions. He later advanced to Deputy Director of Goddard Space Flight
Center and served as [NASA] Acting Associate Administrator for Earth
Science. There was significant and enjoyable interaction with Bill throughout
each of these phases.
The Seasat activity actually was an extremely important mission in terms
of demonstrating the capability of the satellite laser ranging-radar
altimeter connection. The requirement for precise positioning of the
satellite, in order to be able to use radar measurement, was a requirement
to contend with. The effort that we made to satisfy this requirement
turned out to be a major factor in developing a capability that, over
the ensuing decades, has been a recognized standard of our program and
that’s the ability to compute orbits very accurately or the development
of the precision orbit determination area.
When I began involvement with the Seasat mission, one could make height
measurements with the altimeter at the sub-decicentimeter accuracy level,
but the best orbits had accuracies at five meters. With this level of
orbit accuracies, one could not use the use the altimeter measurements
of the ocean surface to meet the oceanographers needed.
The Seasat effort was relatively short lived. The Seasat launch placed
a really remarkable suite of instruments on orbit, but after an exciting
start, the satellite failed 90 days into orbit due to a significant
short in the power system. There was a solar power panel slip ring design
problem that had been identified in the military applications at Lockheed
Martin but the information hadn’t been passed to the civilian
applications area. Although the short caused failure of the satellite,
during the 90 days in orbit, we did get enough information on the altimeter
to know that we had a very powerful measurement technique. We immediately
set out on an effort to develop a program to fly a mission using an
altimeter and focused on an accurate measurement of the ocean surface
topography.
Stan Wilson joined NASA Headquarters to take over the oceanography program.
In one of his early actions, he convinced Bill Townsend to move from
the NASA Wallops Island Facility [Virginia] to the Headquarters program
to take responsibility for the mission that we were trying to initiate
to continue the altimeter measurements that we had started with Seasat.
This effort continued during a several year formative stage to define
a mission concept called TOPEX for Ocean Topography Experiment. Although
it was proposed during early budget preparation activities, it was successful.
For the 1983 NASA budget submission, an agreement to team with CNES
[Centre National dUEtudes Spatiales or National Space Study Center,
France] and make it a joint NASA-CNES mission was completed, and this
arrangement led to a mission start.
The effort associated with the TOPEX/Poisedon Mission, which was the
bi-lateral mission name, consumed most of my attention during the period
between 1983 and 1992. The Precision Orbit Determination Team that I
led was charged with delivering an orbit whose accuracy would not limit
the accuracy of the altimeter height measurement. The altimeter was
designed to measure the height with a precision less than three centimeters.
To be able to use these measurements for oceanographic studies, an orbit
accurate to five centimeters in the radial component was required. At
this point, the best orbit accuracies were on the order of five meters
in the radial component, and increasing the accuracy from five meters
to five centimeters required making advances to allow a two orders of
magnitude reduction in accuracy. We recognized this task as a significant
challenge.
After considerable initial study, we agreed to commit to a ten-centimeter
radial orbit accuracy. We had assembled an astrodynamics team composed
of members from JPL, NASA Goddard and UT to conduct the required effort.
So the better part of the 1980s decade was focused on defining and satisfying
the requirements for computing accurate satellites orbits.
Early in the investigation, errors in the Earth geopotential model were
identified as one of the limiting error sources. The better part of
the ten-centimeter error budget that we committed to satisfy was responsible
for errors in the gravity model. In an alternate effort, we had been
encouraging NASA to initiate an effort to improve the Earth’s
gravity model. While there was a recognized need for improved shortwave
length effects in the existing models, it was assumed that the long
wavelength content, which is of primary concern for satellite orbit
determination, was reasonably well known. While this was not correct
for our requirements, it was true for most of the other stated needs
and, since one could not measure the short wavelength gravity signals
from satellite altitude, most of the NASA funding for gravity model
development was being eliminated. This had a significant impact on the
significant space geodesy effort at NASA GSFC, where the NASA Gravity
Model development effort was centered.
In a highly serendipitous development, Bill Melbourne, Jim Marsh and
I attended a meeting in San Matteo, Italy, to propose that the GPS receiver
that we were developing for TOPEX be added to the ERS-1 instrument.
The European Remote Sensing, ERS-1, satellite would implement the first
European satellite altimeter. The GPS receiver that we proposed would
be the first satellite born high accuracy receiver. Our proposal was
not successful, because the German PRARE receiver had already been selected.
After the meeting was over, we offered to drop Stan Wilson, who was
the Oceanography program manager at Headquarters, off at the Milan airport.
Bill, Jim and I were going to drive overnight from Milan across to Toulouse
[France] where the four of us would meet the next day with members of
the French Space Agency, CNES, to discuss tracking systems for the proposed
TOPEX/Poseidon mission.
As
it turned out, we managed to miss Stan’s plane connection. He
noted, with some concern, that he had funded three of the world’s
best navigators to work on the POD problem, but they couldn’t
navigate to the airport in time for his plane connection. But in any
event, as the situation evolved, he had no choice but ride with us during
our overnight journey.
So during the overnight drive, we had a captive audience in which the
problems with the gravity model errors and the impact on the TOPEX mission
were discussed for an extended period. Stan sat through this lengthy
discourse without comment. I was not sure whether or not he was attentive
to the message, but shortly after we returned to the US, Stan asked
Bill Townsend to get with TOPEX project manager Charlie [Charles] Yamarone
[Jr.] at JPL to put in place funding to improve the gravity model. Although
TOPEX was an oceanography mission, the geodetic measurement requirements
associated with the gravity model and reference frame are fundamental
building blocks for an accurate measurement, and the gravity model development
initiated by Stan, Bill and Charlie as a result of this chance contact
had very broad importance to the Space Geodesy community. A significant
portion of our knowledge today can be traced to this event.
We implemented the gravity model improvement effort as a collaboration
between UT and Goddard. The plan was to develop a series of models that
we called the Joint Gravity Model, joint for UT-Goddard Gravity Model.
There were three models developed during this effort: JGM-1, JGM-2,
and JGM-3. The JGM-1 and JGM-2 model developments were executed at GSFC
with UT supporting the effort. UT took the lead in developing JGM-3.
The JGM-3 model incorporated the first of the satellite acquired GPS
tracking data. Although JPL completed the GPS receiver development and
we launched it, the Air Force was restricting use of the signals to
military applications only. After lengthy discussions we negotiated
an agreement by which the signal denial would be turned off during three
10-day periods. The satellite ground track covers the Earth’s
surface once every 10 days, so the 10-day interval for the GPS tracking
gave global coverage in each of the 10-day intervals.
We took the data from the three 10-day periods and combined it with
the information used to develop the JGM-2 gravity model to develop an
extremely good model for the TOPEX mission. In fact the JGM-3 proved
to be the best model for precision orbit determination for most satellites
until 2002, when we began to get the first results from the GRACE [Gravity
Recovery and Climate Experiment] mission. The gravity model effort started
for TOPEX led to our being in a good position to propose a gravity mission
as a response to the call for this first Earth System Science Pathfinder
mission.
In a follow-up effort, I collaborated with Goddard on a gravity mapping
mission called GAMES that had a pair of satellites orbiting, one behind
the other, in the same orbit plane. The gravity information was to be
inferred from accurate intersatellite measurements of the relative motion
of the two satellites. The intersatellite distance was measured using
a laser link between a passive trailing satellite and an active leading
satellite. This mission was given serious consideration, but as with
numerous other proposed missions in the two decades beginning around
1980, this mission was not implemented. A few years after the GAMES
mission was rejected, the call for the first Earth System Science Pathfinder
mission came out.
JPL approached me about essentially collaborating with them on a concept
similar to GAMES that involved an accurate microwave ranging measurement
with two active co-orbiting satellites. I also got a call from Goddard
about the same time about collaborating on a mission involving a gravity
gradiometer that had been under development. At this point, we’d
been trying to get a gravity mission since it was first recommended
in 1967. All of the missions proposed in the 1980s and 1990s were not
successful.
It is interesting to note that when TOPEX was finally selected in 1983,
there was a mission called Geopotential Research Mission, which was
in strong competition for a mission start. The Geopotential Research
Mission established measurement concept was proposed for GRACE, but
the mission was to fly at a much lower altitude and would be much more
expensive than the GRACE mission turned out to be.
Regarding the selection in 1983, we argued in a mission review at NASA
Headquarters that, since the ocean is changing with respect to time
and the gravity field is fixed, (conventional knowledge referred to
the gravity as a onetime measurement and you are done) we should do
the TOPEX mission first and then follow up in a few years with a gravity
mission. Shortly after TOPEX was accepted, we had the first Space Shuttle
disaster [January 28, 1986, STS 51-L, Challenger], which delayed most
mission implementations for several years. It delayed the TOPEX launch
until 1992 and it eliminated any chances of getting the GRM [Geopotential
Research Mission] gravity mission selected. The GRM team was finally
disbanded in 1986.
For the GRACE proposal, we took the base intersatellite measurement
approach from GRM and upgraded the concept by bringing the GPS receiver
on board to satisfy the orbit determination and time synchronization
requirements that were a challenge for GRM. We raised the altitude to
increase the mission life and we added an accurate accelerometer to
measure the surface forces due to drag and radiation pressure. This
eliminated the costly and mission life limitations associated with the
drag-free concept adopted for GRM. This allowed fairly low-cost mission
implementation mode that had the potential for a long mission life.
We proposed a teaming arrangement with German colleagues at the GeoForschungsZentrum
[GFZ] in Potsdam, Germany. Under the direction of my colleague, Christoph
Reigber, they had flown an earlier single satellite gravity and geomagnetics
mission called CHAMP [Challenging Minisatellite Payload] which would
provide most of the satellite technology and the accelerometer needed
for GRACE. We formed a team to develop the proposal for the GRACE mission.
We were successful in that proposal, and it led to a really remarkable
approach for measuring the Earth’s gravity field.
The mission concept proposed to measure the Earth’s gravity field
at monthly intervals. Since the gravity field is determined by the Earth’s
mass distribution, changes in the monthly gravity fields are caused
by changes in the Earth’s mass distribution. This realization
allowed the focus on measuring the mass exchange between the oceans,
atmosphere and land surface as a consequence of the Earth’s dynamic
system interactions. The major component of the signal observed by GRACE
is water moving about. Rather than focusing on only the fixed or stationary
gravity field, we proposed to look at the time-variable nature of gravity
also.
We had been measuring the long-wavelength components of the time variable
gravity using satellite laser ranging to a series of spherical satellites
called cannonball satellites—LAGEOS-1 [Laser Geodynamics Satellite],
LAGEOS-2—since the launch of LAGEOS-1 in 1978. LAGEOS and Starlette
[Satellite de Taille Adaptée avec Réflecteurs Laser pour
les Etudes de la Terre], which was launched by CNES, were round balls
with optical retro reflectors spread over their surface. We mostly focused
on the time-variable nature of the J2 coefficient, which is mostly related
to the oblate nature of the Earth (e.g., the polar diameter is less
than the equatorial diameter). We measured the annual variations and
observed that the annual variations appeared to be caused by both geophysical
and climate related effects.
We didn’t fully understand the climate connection at that point,
but we knew that there was annual variability in gravity field at the
long-wavelength components. This was one of the important topics for
study that we highlighted in this GRACE mission proposal. Not only would
we do the mean field, but we would study the time variable nature as
well. As we noted, gravity comes into play in a number of ways. The
mean field is important in the satellite altimeter missions such as
TOPEX and the Jason follow-on missions, both for computing the orbits
and to define the ocean surface geoid to which the altimeter measurement
is referenced.
Also the surface that one uses to reference the altimeter measurement
against, to get the quantity of interest to the oceanographers, is the
dynamic ocean topography. This quantity is the difference between the
sea surface height that the altimeter measures and the marine geoid.
The water would go to a surface that’s defined by the gravity
over the ocean (the marine geoid), if the effects of the Earth’s
rotation and the effects of atmospheric pressure and winds were not
present.
The altimeter measurement is extremely difficult because the dynamic
ocean topography has signals with amplitudes of about one meter, but
the actual shape of this ocean’s mean surface has variations with
amplitudes as large as 100 meters. That is, there are 100-meter highs
and lows at various points over the ocean surface, where the water departs
from the best fitting ellipsoid by as much as 100 meters below it and/or
100 meters above because of the internal mass distribution of the Earth.
So you’re looking at a one-meter dynamic ocean topography signal
and imbedded in a marine geoid with100-meter level variations. At the
time of launch of the TOPEX mission, the errors in the gravity field
were such that small errors in that marine geoid totally dominate the
dynamic topography signal.
At the time of the GRACE satellite launch, we had ten years of very
accurate altimeter measurements of the ocean surface, but they could
not be used to determine the general ocean circulation because the errors
in the gravity field hid the dynamic ocean topography signal. As we
noted, one of the objectives of the GRACE mission was to get a very
accurate mean sea surface to allow full use of the altimeter-defined
measurements. The other was to look at temporal variations in the gravity
field and relate that to mass flux going on in the Earth’s dynamic
system. That mass flux is mostly water moving around. Some of the signal
is related to long-term trends while other signals have a seasonal variation
that repeats from year to year at yearly intervals. The measured phenomena
with long term trends are related to ice mass loss in the polar regions
and the signals present in the rebound of the North American continent
after the unloading of the ice following the last ice age (e.g., the
glacial isostatic adjustment).
The more interesting activity is the ability to be able essentially
to look at the water in most of the major river basins in the world,
and look at the seasonal changes in this water. That’s both surface
water and subsurface water; the subsurface being the large-scale continental
aquifers, and the water changes in those are fairly interesting topics,
and of quite a bit of concern at the present time.
We also proposed some breakthrough measurements such as the ability
to use the mass measurements of the column including the ocean and the
atmosphere as an indicator of the ocean bottom pressure. By using these
measurements to infer change in the ocean bottom pressure, one deduces
information about the ocean bottom currents in the deep oceans.
There has been a number of really very interesting measurements that
have come out of the GRACE-related activities. It’s evolved from
the concept of a gravity mission into one of a mass flux mission, in
which the mass flux is mostly water, although, as we noted, phenomena
such as the glacial isostatic adjustment can be observed. You also see
large episodic changes. One gravity signal in this category is related
to the [2004 Indian Ocean] Andaman-Sumatra Earthquake. You see a very
sharp difference in the gravity field before and after that Earthquake
occurred.
Wright:
Speaking of GRACE, I believe it was selected in May of 1997 and it launched
in 2000. Can you share some of those interactions of getting it to that
selection process and then its launch?
Tapley:
The GRACE mission was the first one of the Earth System Science Pathfinder
[ESSP] missions accepted. It was submitted as a response to the first
call for ESSP mission. The Earth System Science Pathfinder Program was
to select innovative low-cost missions that could be placed on orbit
rapidly. Further, the mission manager of the program could be outside
NASA and, under this approach, an academician could be responsible for
the entire program.
In the teaming arrangement, we proposed what was to be demonstrated
as a very good concept. Under the teaming concept, JPL would be responsible
for the mission implementation, including the satellites and the instrument
compliment, UTCSR [University of Texas Center for Space Research] would
be responsible for the data system and for the overall mission management,
GFZ would be responsible for the German contributions to the mission,
which included the satellite launch and the mission operations. Ab [Edgar
S.] Davis, who ended up being the proposal project manager, and Mike
[Michael M.] Watkins, who later became the project scientist, were very
influential in maturing the concept. Mike Watkins was one of our students
who after completing his Ph.D. degree had joined JPL. While at CSR,
Mike had supported our effort on the proposed GSFC GAMES gravity mission
that I mentioned earlier. Mike had been involved in simulations that
we performed to support this proposal so he had a good understanding
of the nature of the mission concept. He was also involved in the SLR
studies of time variable gravity.
Ab had been involved in developing accurate GPS ranging systems, so
he understood the nature of intersatellite ranging measurement. As I
mentioned earlier, the Geopotential Research Mission had developed and
demonstrated the concept of using the accurate phase measurement to
do the “micron level ranging” between the two satellites.
We essentially adopted the intersatellite range measurement concept
that had been developed for the Geopotential Research Mission.
We had available all of the technology developed for all the missions
that were proposed, but were not successful in the 1970s and ’80s.
A concept called gradiometry, in which one measures the gradients directly,
had gone forward. ARISTOTELES was a joint ESA [European Space Agency]-NASA
mission that was given a great deal of consideration in the mid 1980s,
but wasn’t accepted. In the development effort for this mission,
the gradient measurement was obtained as the difference between accelerometers
located at different points on the same satellite. The differential
acceleration contains the signal associated with the gravity gradient
that one wants to measure. As a consequence of developments related
to this mission, the technology for accelerometers had been advanced
extensively in France at ONERA [Office National d'Etudes et Recherches
Aerospatiales, French Aerospace Lab].
Rather than use the GRM concept—in which they were going to put
a lot of propellant on board the satellites and fly the satellites so
that a proof mass in the center was shielded by the actual shape of
the satellite for any surface forces associated with radiation pressure
or atmospheric drag, the so-called pea in a pod version. That is a hard
requirement to satisfy. In addition to the difficult control requirements,
a great deal of propellant is required to maintain this condition at
the approximately 170 km altitude proposed for the mission. This fact
necessarily limits the life of mission. Rather than adopt this concept,
we chose to use a three-axis accelerometer to measure the surface forces
directly. We got a very accurate three-axis accelerometer from CNES,
and specified that it be located at the center of mass of the satellite
to eliminate the effects of the rotational accelerations. With that
accelerometer measurement sensitive to the surface forces only, we could
use the high accuracy intersatellite ranging measurements to focus on
the gravitational effect. That idea allowed us to design a concept with
a multi-year mission life and focus on long-term gravity changes.
With the POD requirements and timing requirements satisfied by tracking
with the GPS satellites, another major problem for GRM was eliminated.
The development of the GPS system, the development of the accelerometer,
the adoption of the formerly developed intersatellite ranging system
that had been developed for GRM, allowed us to apply existing technology
to implement a micron level intersatellite ranging system.
With the measurement concept in hand, we needed a satellite bus that
would satisfy a number of demands to be sure that the high accuracy
ranging measurement was not corrupted. The demanding requirements on
the satellite buses included high structural and thermal stability to
ensure that the micron level ranging accuracy is not influenced. A micron
is about a tenth the size of a human hair, and we’re measuring
at distances on the order of 200 kilometers. Anything that happens on
the satellites is a potentially troublesome source of error in measurement.
We leveraged some extremely difficult arrangements on requirements on
the actual satellites.
In the first ESSP proposal call, the dollar value of the missions was
really limited. You could either bid for the first mission with a $60
million cap or the second mission, which was to be launched at $90 million
cap. We clearly needed at least $90 million, so we bid for the second
mission. But to buy two satellites buses, build two paradigm-shifting
type intersatellite ranging measurements, provide the accelerometers
to measure the surface force measurements, and launch the two satellites
for $90 million was an extreme challenge. In the innovative teaming
arrangement we proposed, we would buy the satellites from Daimler Space
Systems (which later became Astrium) in exchange for the satellite launch
and the mission operations.
Astrium had demonstrated a satellite bus for the CHAMP [Challenging
Mini-Satellite Payload] mission, which could be modified to meet the
GRACE mission demands. It had accommodated an earlier version of the
ONERA accelerometer that we wanted to use. JPL had provided a GPS receiver
for the CHAMP mission, so this element had been accommodated on the
proposed satellite bus.
In deciding to buy the satellites from a foreign vendor (in this case
Astrium [EADS, European Aeronautic Defense and Space Company]), we proposed
that the German Space Agency [German Aerospace Center, DLR (Deutsches
Zentrum für Luft-und Raumfahrt)] agree to launch the satellites.
There was a strong interest at the German Space Operations Center in
operating the satellites, so we agreed to this element of the collaboration.
The important thing for us was the launch vehicle. That was a tall pole
in proposal “tent.” With that arrangement, we could submit
a proposal, which would allow us to stay under the cap, but just barely.
We proposed a cost of $87 million, but with essentially no reserves
in the budget.
In the first scenario that went forward on this, they essentially took
the initial proposal and screened those for possibilities to allow one
to go back and prepare a more definitive proposal. In that first proposal
screening, I understand that we were almost at the bottom of the ranking.
There were approximately 45 or 46 proposals submitted and we ranked
somewhere in the 30s. Some of the negative ranking was associated with
a lack of belief in the proposed teaming arrangement.
The scenario in Germany was uncertain, because a number of the DLR staff
that interacted directly with NASA was saying that DLR was not going
to do this mission. Other individuals in Germany were pushing the mission.
So we were involved with the ones that wanted to do the mission in preparing
the proposal.
There was also uncertainty associated with whether or not we could implement
what GRM had proposed for a 1983 cost that was an order of magnitude
larger. We did make the first cut. They did request that we prepare
the second version of the proposal. Early on in the rankings for the
second version, we advanced into the upper ten, and were ranked somewhere
around seven.
I was told later that in the final selection process that a fairly important
factor in our selection was the strong endorsement of Bill [William
M.] Kaula, who is one of the eminent names in satellite geodesy and
in gravity model development. Bill had been the project scientist for
the GRM, so he clearly understood the nature of the measurement and
the importance of the results if we were successful.
He also had chaired the highly important 1967 Williamstown Conference.
The report from this conference made the recommendations that provided
the basis for most of the geodetic and oceanographic missions that were
implemented in the 1970s and 1980s. The altimeter missions were recommended
in this report, and, to go along with the altimeter missions, a dedicated
gravity-mapping mission was proposed. So Bill clearly knew that among
the suite of missions recommended in the Williamstown Conference Report,
a gravity mission had not been implemented. He had chaired a couple
of other major studies and had been pushing NASA very strongly for the
entire timeframe to actually do a gravity mission. I think he saw this
as a chance to finally implement a credible gravity mission.
He was influential in arguing the importance of doing the mission, provided
that the technical story came together. After extensive deliberation,
we actually became one of the three that were selected. In that process
they selected two missions and one alternate or backup in case either
of the first two failed in the implementation process. If either of
the missions has problems with either cost or schedule, the plan was
to cancel the mission and look to implementing the third mission. An
interesting and perhaps important side note is the selecting official
for the first ESSP selection was Bill Townsend, with whom I had had
a number of years of interactions during the TOPEX mission and after
in his management role at GSFC. That Bill would be responsible for setting
our first gravity model effort in place under the TOPEX mission framework
and that he would be the official to set GRACE on its historic course
is a sense of personal pleasure.
At the time we were selected the actual feeling at Headquarters was
that we weren’t going to be successful, because the NASA selection
didn’t commit DLR. We were selected provided that DLR actually
agreed to provide the launch. In other words, we had a mission concept
that proposed elements that NASA would do and other elements that DLR
would accomplish, and if either one of those were not present, then
we had no mission.
The official stance of DLR indicated that we had difficulties. In the
mission concept we proposed, as the PI [principal investigator], I had
the ability to make all the final management decisions. I was responsible
to NASA for all elements of the mission. In the teaming arrangements,
a colleague Chris [Christopher] Reigber agreed to be the Co-PI and to
assume responsibility for the German elements of the proposal. Chris
was a very well established geodesist and geophysicist in Germany and
was the PI on the CHAMP mission. Chris, in addition to having outstanding
scientific and engineer credentials, was extremely astute in the political
ramifications in Germany. His capable efforts in the political community
were extremely important in the final success of our efforts.
In addition to Chris, the other individual that was very important in
getting the mission in place was Ab Davis. Ab had spent an extended
period in Germany working with Chris at GFZ in implementing the GPS
receiver on CHAMP. He used this period to establish contact with the
accelerometer group at ONERA. During this period, he also established
a friendly relation with the CHAMP satellite provider, which we turned
to for the GRACE satellites. As a consequence, he understood very well
the requirements for the teaming arrangements.
Through the combined efforts of Astrium [then Daimler Space Systems]
and Chris in approaching the ministry that funds DLR, DLR was encouraged
to go forward with the mission. Even with worst early prognostications,
the collaborative MoU [Memorandum of Understanding] between NASA and
DLR for the GRACE Mission was signed. As we found later, there were
two internal reasons for the ministry support in Germany. Astrium wanted
to build the satellites. They had a very good bus. They were trying
to get the bus established with NASA as a credible vehicle for future
business, so they gave us a very good price for building the satellites.
In a development that proved important, they agreed to build them at
a firm fixed price, which was fairly important to us since we had no
reserve, and if there were cost overruns, we ran the risk of cancelation.
As follow-on to the success of the GRACE mission, Astrium has been able
to get their Flexbus, as they named the bus used for GRACE, selected
for a number of subsequent missions. They accomplished their objectives.
But it is important to note that they did an incredible job in building
the GRACE satellites and delivered for the cost that they had agreed
to. There were a couple of design changes made late in the fabrication
phase, which added additional cost.
We were extremely lucky in that we actually negotiated the price in
terms of German marks, which later became Euros. Most of the payments
were made during a timeframe in which the dollar strengthened against
the euro, so that the cost in dollars was less than we anticipated.
We were able to cover some of the cost growth in other elements of the
development by this international fluctuation in the dollar. There was
some risk though, because the dollar value could have declined. We were
carrying some reserve for the dollar fluctuation, which we were able
to apply in other areas.
The other interaction involved the launch vehicle. We proposed the mission
expecting that we DLR would provide the Cosmos Russian launch vehicle,
since this vehicle had been used to launch CHAMP. We didn’t know
that another group inside Germany that was working on a commercial venture
with the Russians. This interaction led to the decision to launch the
GRACE satellites on a launch vehicle called the Rockot, which was provided
by the Eurockot Consortium.
When this was first announced, I indicated that I did not want to provide
the first satellites for launch on a new launch vehicle. I was assured
that there were other commercial customers and that the launch vehicle
would be used a number of times prior to the GRACE launch. It turned
out that their industrial customer was the Iridium [satellite constellation],
and shortly after making the announcement related to GRACE, Iridium
went bankrupt. All of a sudden, the GRACE satellites are first in line.
As preparation for the Iridium launches, Motorola [Inc.] had negotiated
a test flight, which was not conducted, and they turned over the actual
module that they were going to fly on the Rockot for a demonstration
test for GRACE. The first two stages of the Rockot were military missiles
that had a long very successful launch record. We weren’t worried
about the first two stages. We were worried about the third stage, referred
to as the Breeze, that was a new development and had never been flown.
It was developed for injecting commercial payloads into orbit. To demonstrate
the Breeze, the Rockot Corp. took the two Motorola demonstration payloads,
configured them to simulate the GRACE mission, and actually flew a preliminary
demonstration GRACE launch. In this demonstration, they launched the
Breeze into a GRACE orbit; the Breeze then injected the two payloads
into orbit, and finally the Breeze deorbited, effectively simulating
the requirements that we had for the GRACE mission.
The test was very successful and we got the actual loads and vibration
information that we could use to support our design and test program.
With that successful test, we agreed to the Rockot launch vehicle. As
a final point, the Rockot launch of the actual GRACE satellites was
perfect, and 45 minutes after the launch the two GRACE satellites and
the Breeze were mapped by the German military radar as they made their
first orbit over the German Space Operations Center in Oberpfaffenhofen,
Germany.
The mission cost growth exceeded the $90 million cap by approximately
$7 million. Most of this overrun was due to a set of Red Team Reviews
and additional testing required by the agency to move away from the
“faster better cheaper” implementation mode that evolved
as a consequence of the two Mars Mission failures around 2000. But we
were able to get the two satellites on orbit and get them in an operational
mode for cost on the order of $100 million NASA dollars. There was probably
another equivalent $50 million provided by the collaborative agreement
with DLR, so the overall mission cost for the two satellites on orbit
was approximately $150 million.
Present time now, we’re approaching 10 years in orbit. The last
Senior Review extended the mission out to 2015. There is concern as
to whether the components on the satellites will last that long. They’re
aged and the batteries are giving us problems. There’s a few other
things giving us problems, but the mission to date has provided a remarkable
dataset in place. The data has led to a paradigm shift in how we view
observations of the Earth system dynamics.
Wright:
Has it met your expectations?
Tapley:
We were pretty sure that the fundamental baseline requirement that the
mission had to satisfy, the determination of an accurate long wave mean
field, would be satisfied. We believed that if we collected global data
for a period of two to three months, we would meet this requirement.
That turned out to be correct. The first gravity model, based of 111
days of data, provided a gravity model that allowed determination of
the general ocean circulation features from the decades long sequence
of satellite altimeter measurements. So the first 111 days worth of
data in the mission essentially gave us that very significant dramatic
result.
The more difficult objectives associated with the mass flux measurement
was a more significant challenge. To validate these measurements interactions
with the oceanographic, cryospheric and hydrology communities was required.
The hydrology community was a new community in the gravity applications
area. They understood what we were talking about in general but they
didn’t understand how to use the global gravity coefficients that
we were distributing as the primary data product. After extensive interactions
a procedure for satisfying their requirements has been developed.
Recent investigations show applications of the data for seasonal river
basin water balance, changes in lake impoundment, change in underground
aquifers and drought monitor indices. After the slow start, the community
has just really embraced the measurements. There was a very interesting
AGU [American Geophysical Union] report that came out in December [2009]
showing the depletion of the water in the San Joaquin Valley Aquifer
in Central California. This water depletion is important, since a significant
portion of the agricultural produce consumed in the US is dependent
on the water from this aquifer.
There was another investigation that focused on an aquifer in India
that provides water for most of the Indian population. You have a very
large population where the underground water is going down very rapidly
due to agriculture applications. So there are a lot of these application-related
issues that are satisfied by the GRACE ability to sense underground
water change. These results, along with other important climate-related
measurements, suggest that the GRACE observations need to be continued.
There are plans for a GRACE Follow On Mission, but it is scheduled to
launch after the likely end of the current GRACE mission. One of the
things we’re working on now is trying to establish a bridge mission
to the next mission to keep the measurements going.
But, with regard to your question, I would say that the ability to accurately
observe a wide range of Earth System processes has been rewarding—to
see the wide ranges of communities utilizing the data for applications
that we didn’t originally anticipate is very rewarding. We knew
we could measure the global signal with unparalled accuracy, but we
didn’t fully understand all the ways the measurements would be
used.
We think we’re at the point now where the measurements from GRACE
are ready to be ingested into models to assist the prediction process.
That’s one of the more difficult challenges facing the Earth System
research. When you assimilate global measurements into the accurate
models for the Earth Dynamics processes and use those models for improving
the forecast, then you not only help the overall operational areas,
such as weather predictions, but the climate predictions where the long-term
forecast accuracy is under considerable scrutiny.
There have been some really nice additional results in the climate arena.
The altimeter measurement that we discussed above provides one example.
By using the global altimeter measurements for one 10-day ground track
repeat cycle, one can measure the average or mean global sea level.
This quantity is related to the volume of water in the ocean. By repeating
the measurements at ten-day intervals, you can observe a change in global
mean sea level. The global sea level change is currently recognized
as an important climate signal and has an important connection to the
GRACE mission.
We’ve been able to accurately measure the sea level change since
the beginning of the TOPEX mission. The original average of the global
altimeter measurements was used to calibrate the bias in the altimeter
measurement. If one can use other measurements to determine the bias,
then the global average of the altimeter measurements during a given
repeat cycle can be used to observe the mean seas level. This concept
was first proposed by Bob Stewart during a collaborative between Bob,
George Born, and I in determining procedures for calibrating the TOPEX
altimeter bias calibration.
One of the things that we were concerned with was understanding the
various error sources in the altimeter measurement. Bob noted that if
we successfully calibrated the altimeter measurement and accounted for
all the other error sources, then the remaining signal would be due
to sea level change, and that this could be an important signal for
study, in its own right. So we proposed in this 1983 paper, as an aside
comment, that one of the things we could do with global measurements
of a properly calibrated altimeter would be to measure the global sea
level and its changes. One did not have a set of altimeter measurements
to test this concept, so the idea lay dormant for a while. In 1987 there
was a call from NASA looking for climate related measurements. Wes [Wesley
T.] Huntress drafted the call and was the program manager for the effort.
I submitted a proposal to evaluate the use of the altimeter measurement
record as a means of sensing climate change.
This was the first study devoted to using satellite altimeter measurements
to observe global sea level change. The first test of this concept was
performed using GEOSAT altimeter measurements and the results were not
positive. The altimeter was a single frequency altimeter with uncertain
accuracy, and associated orbits were not accurate enough to allow a
credible measurement of sea level change. I initiated a study of the
problem with a few Ph.D. candidates. We conducted both simulated studies
to look at the issues that limited our ability to make this measurement
as well as attempts to use the data for recovery of the ocean circulation.
One of the students in this initial study was Steve Nerem, who has devoted
a significant part of his career to the question of Global Sea Level
Changes and is one of the current authorities on this effect. His work
is currently referenced as the NASA standard sea level measurement.
With the TOPEX/Poseidon mission, the accurate altimeter measurements
and the accurate orbits allowed an accurate measurement that has been
maintained for almost 20 years and is one of the fundamental climate
change measurements.
Although we could make the measurement, understanding the nature of
the temporal variations was a much bigger problem. We know that there
are two effects present in the sea level change. Temperature change
will cause sea level change due to the water expansion, and if you add
mass (water) to the ocean, the sea level will change. We know the polar
ice caps and continental glaciers are melting; the water released in
this melt ends up in the oceans. We also believe that the climate is
warming up and the water should be warming as a consequence. We know
that both of these effects are underway, but we do not know how much
of the sea level rise is due to ocean water heat increase and how much
is the effect of the addition of water from the melting glaciers.
The interesting thing is that GRACE will measure mass changes in the
ocean, but it’s not sensitive to temperature changes. The temperature
changes will not have an associated mass change and the mass change
is the gravity signal that GRACE can measure. So by using the altimeter
measurements of the global ocean surface topography, the total sea level
change can be observed. By flying GRACE, you observe the mass change
component. What’s left over is the temperature component, so those
two measurements allow you to separate the steric or the temperature-driven
component of sea level rise from the mass driven component. The mass
changes are due to water that’s actually being added, which is
fairly important in trying to understand from a climate point of view
what is influencing the sea level change. To help close the global mass
change budget, GRACE also measures the mass loss by the glaciers, which
should be most of the mass added to the ocean. Agreement with these
two GRACE measurements is a confirmation of the GRACE measurement accuracy.
I’ve been fortunate to participate in a number challenging missions
and it has been a great pleasure to see the successful application of
the measurements from these missions. It was very exciting in the 1970s
to begin the work with the LAGEOS laser ranging and it was more challenging
to address the requirements of the TOPEX mission. But GRACE I think
probably has been perhaps the most rewarding of all the missions that
I’ve been privileged to be associated with.
Wright:
Sounds like it keeps providing you more information to benefit from.
Tapley:
Yes. I think we’re still finding new ways that we can use the
measurements. It’s an extremely important interdisciplinary mission.
GRACE is the only mission with the ability to directly measure the regional
mass flux. Most of the other missions measure radiometric (reflectance)
or metric (height) properties in one form or another and, where required,
these measurements are used to make inferences about the mass flux.
But GRACE measures the effect of the mass itself. So it’s a very
good complement to most of the other measurements.
GRACE in combination with the SAR [Synthetic Aperture Radar] radar missions,
the altimeter missions and the satellite laser ranging missions, as
well as results from a number of the hydrology related missions, provides
the basis for a wide range of inter-disciplinary studies. One example
is found in the ICESat [Ice, Cloud,and land Elevation Satellite] mission,
which implements a laser altimeter to measure the ice sheet topography.
From these measurements one can determine the change in the ice sheet
volume. The SAR missions will measure surface velocity. GRACE will measure
the mass changes, so together they give a complete picture. There will
be missions to measure soil moisture, which along with the total subsurface
water change observed by GRACE will provide essential information on
the water budget.
Wright:
You used the word interdisciplinary. Let’s talk about the whole
concept of Earth System Science. How are the benefits that GRACE is
providing for us working with the other concepts, how are you able to
help the other disciplines within Earth System Science with the work
that you’re doing?
Tapley:
In the GRACE proposal we described an interdisciplinary climate-related
mission. The name GRACE is an acronym for Gravity Recovery and Climate
Experiment. We actually proposed several paradigm shifting climate related
measurements for the GRACE mission. The ability to infer mass change
below the Earth’s surface was a paradigm shifting capability that
had not been provided by any other mission.
In response to the interdisciplinary related capabilities, the mass
flux measurement concept evolved from an extension of a program initiated
under the Earth Observation System, the EOS program. I led an interdisciplinary
EOS science investigation proposal, which was selected to look at the
integration of data from the EOS measurement suite with the objective
of focusing on the Earth system dynamics. I proposed an investigation
that would study a number of the topics that GRACE is addressing.
The EOS implementation was delayed and the data needed to accomplish
the investigations was never provided, but we did perform a number of
simulated investigations and we did use the time variable gravity measurements
observed by the LAGEOS satellites to begin initial studies that were
very beneficial to the GRACE mission. We actually understood a lot of
the inter-disciplinary applications that GRACE addressed when we proposed
the GRACE mission. In the GRACE proposal, we outlined contributions
to oceanography, hydrology, cryology and contributions to geophysics.
We also proposed some paradigm shifting measurements, such as inferring
the deep ocean currents and the change in underground continental aquifers.
In oceanography we focused on providing the mean ocean geoid to allow
determination of the general ocean circulation from the satellite altimeter
measurements, we described changes in the mean sea level, and we proposed
inferring the ocean bottom pressure changes as a means of inferring
deep ocean currents. The GRACE measurement component was viewed as an
essential augmentation to other measurements and, without GRACE, an
important part of the overall puzzle would not be measured. So in the
initial context, GRACE was always viewed as having a strong interdisciplinary
thrust in the Earth System Science context. Early on in the GRACE mission,
we argued that GRACE is an essential member of the satellite suite that
NASA provides to observe the Earth’s dynamic system. In all of
the base objectives of the Earth science program, there is a place where
the mass and the mass flux provided by GRACE are essential to the scientific
interpretation. The mass flux taken by itself usually won’t solve
the problems, but it is a very important piece of the puzzle. You usually
can’t solve the problem without understanding the associated mass
and mass flux.
So the measurement of gravity has evolved from what was viewed in a
fairly narrow context as a geodetic measurement, the mean gravity (or
static) gravity field, into one that’s really central to in the
climate change considerations. It is being recognized as one of the
significant climate parameters that we should to be measuring.
Wright:
What do you see that needs to happen in the next 20 years in the field
of Earth science?
Tapley:
The transition in the NASA mode of operation is undergoing some interesting
perturbations. NASA, from the beginning, has had a mission of developing
new technology and providing new proof of concepts. It uses the missions
as a rationale for the technology development. The idea of repeating
a measurement that you’ve already demonstrated has been a big
problem for them. It’s has been a problem for management in deciding
what NASA should do, and it’s been a problem in terms of resource
allocation since they are always budget limited, and repeating a previous
measurement means that you will not be able to do some new measurement.
However, we find ourselves at the present time with a serious need of
having observations of climate related quantities that extend over multi-decade
time frames. The satellite role in making many of these measurements
is crucial, because the satellite measurements are the only acceptable
way of getting global near-synoptic measurements. The accuracy of these
measurements and the global nature of those measurements are extremely
important for climate change studies. NASA is the only agency that has
demonstrated the capability and the will to this role.
NOAA [National Oceanic and Atmospheric Administration] could improve
the operational-related measurements to meet the climate needs, but
they have not delivered the measurements with the precision and accuracy
associated with NASA products. So at the present, the issue of maintaining
continuity of some important measurements has not been resolved and
some of the quantities that we’ve talked about such as the sea
level measurement has become a global climate change indicator, and
maintaining a continuous measurement is fairly important.
The sequence of mass flux measurements coming out of GRACE has the potential
for becoming such an important data record, if we can continue the measurement
sequence after the current GRACE mission. But the issue of how NASA
responds to the need for measurement continuity to support climate change
studies is a difficult one to address. Either the NASA mission needs
to be enlarged to allow the agency to address these issues, or their
needs to be another agency put in place and charged task.
On another front, the missions themselves are getting extremely expensive.
All of them are in the few hundreds of millions of dollars to billions
of dollars. We can’t do very many missions under this cost profile.
In the technology development mode, NASA needs to develop the ability
to get the critical measurements in a cheaper way. One proposed technology
that may come into play is associated with the smaller satellite implementation.
The nanosatellites have been fabricated and orbited, but the requisite
technology base to use them is not in place. Actuators, thrusters, instruments
and power supplies are needed for the nanosatellite regime. If these
technology demands can be met, then clusters of satellites that allow
you to distribute the required measurement functions can be discharged
in a more cost friendly implementation. Development along these lines
is one way in which we have the potential for essentially making the
measurement systems more robust and to provide them at a lower cost.
I think there will be considerable effort in this direction in the future.
The one measurement sequence where the US seems to be lagging is in
the radar measurement area. We demonstrated the first satellite radar
capability on Seasat in 1977, but we haven’t had another dedicated
polar orbiting radar on orbit since that time. We’ve done short-term
radar demonstrations such as the SRTM [Shuttle Radar Topography Mission].
However, none of the proposed dedicated radar missions have been successful.
All of the other nations have. Canada, Germany, Japan, and ESA all have
flown dedicated satellite radar missions. I believe that this situation
will be remedied in the current decade.
Looking down 20 years and trying to use the history to project forward
20 years is a risky venture. But if I looked at where we are now, one
of the key problems that we need to solve is how we maintain, hand off,
operate satellites in a way to continue some of the high-quality measurements
sequences. Future requirements will require that we use cluster and
constellations of satellites to satisfy increasing demands for higher
spatial and temporal resolution (or coverage). Development of the nanosats
may be one way of satisfying these requirements, so I see development
in this area.
Wright:
Since you looked forward, let me ask you to look back. What do you believe
to be some of the greatest accomplishments of the last 20 years since
Earth System Science has developed and evolved?
Tapley:
The development of the metric range measurement accuracy, which allowed
us to define the shape of the Earth, the reference frame used to describe
changes, the dynamic properties on and inside the Earth, is one of major
accomplishments. The measurement accuracy, the metric/measurement accuracy,
has gone down from the five-to-ten-meter level in the mid-1970s to the
micrometer level today, with the nanometer level accuracy just over
the horizon.
The ability to define positions in a geocentric reference frame, to
be able to observe changes in this reference frame, allows the ability
to study tectonic deformations, land subsidence, and the ability to
observe the millimeter scale movement of the Earth’s center of
mass as various dynamic processes occur, represents one of the great
achievements of the past few decades. The development of laser and microwave
ranging systems with the measurement accuracy required to perform these
studies has been one of the biggest accomplishments in our ability to
study the Earth, and an extremely important point in being able to figure
out how you’re going to conduct studies.
The idea of making micron-level measurements over a distance of 200
kilometers was a concept that was proposed in the ’70s and early
80s timeframe. We are demonstrating these measurements on GRACE today.
Another success lies in our ability to put these measurements together
and to look at the whole Earth system, at one time, with this level
of precision, and it gives you a new way to view the Earth and to understand
what’s going on both in scientific and in application terms. This
global, near synoptic measurement capability brought forward by the
satellite platform has allowed Earth system studies to be conducted
in a completely different context.
Some of the unique investigations include the ability to measure the
mean sea level change with the millimeter level precision, to use the
ocean surface topography measurements to infer the general circulation,
to infer changes in the deep ocean bottom currents, to observe changes
in the mass of the polar ice caps, and to measure changes in ground
water aquifers throughout the world. These are all views of the Earth
that are completely new, very important, confirm studies that people
have conjectured about for long periods, and allows us to quantify the
processes that are underway.
Out of all this we begin to get both the database and the confidence
in the database to think about assimilation of the measurements into
the models. I fail to include this area in the accomplishments of the
next 20 years. I do think that during the next 20 years we’re
going to achieve the capability to assimilate the satellite data into
the models, improve the model fidelity and use the improved predictions
to understand multi-decadal climate trends. That’s the next significant
step in using the satellite data. In addition to the predictions of
future trends, ingesting the satellite data into models allows the models
to extrapolate the satellite information to a higher spatial and temporal
resolution.
Satellites are limited to observing phenomena only when they overfly
it. But the models allow you to assimilate the measurements and then
extrapolate spatially and temporally between the subsequent views so
that you can “observe” the phenomena at more frequent intervals.
I think evolving our current capabilities could be one of the biggest
steps forwards in being able to understand the Earth. It’ll help
improve the physical principles on which the models are based. Then
once the physics is right, the initialization and steering provided
by the satellite observations will allow the prediction modes to be
conducted with the requisite accuracy.
Wright:
Let me switch subjects as our time starts to close, because I wanted
you to have an opportunity to talk to us for a few minutes about the
fact that you have worked 50 years in your field. During that time period
you founded the Center for Space Research for the University of Texas
at Austin. Share with us why you felt that was a good thing for the
world, for us to have the center.
Tapley:
For most of my early career, I operated in the individual faculty member,
graduate student mode. This is the way most faculty members want to
work. That’s the best way to conduct a teaching-research relation.
Although I did not want to get into administration, I did agree to serve
as the chair of the Aerospace Engineering and Engineering Mechanics
Department for the 11-year period between 1966 and 1977. During that
time period, we organized an informal institute for advanced studies
in orbital mechanics. The institute was organized primarily because
the Air Force was willing to provide funding for an institute to study
astrodynamics. A colleague that we had hired by the name of Professor
Victor Szebehely had brought the Air Force funding with him. We put
reports out under the institute name for about 10 years, but it had
no management structure within the university.
In 1982 or 1983 there was a move on campus to form a space-based research
center. I worried about the direction that the proposed management was
going, and what impact it might have on what we were doing. At that
point, we had a pretty healthy program underway. We’d already
done the Seasat mission and were involved in the formative stages of
the TOPEX mission.
To protect the thrust that we had developed, I decided it would be best
to propose that we become an organized research unit. So we put the
proposal in place and formally organized it at that point. We were assigned
to the Bureau of Engineering Research, primarily because most of the
faculty came from the Aerospace Engineering Department. From the beginning,
the center has evolved with a strong interdisciplinary focus. We’ve
had good collaboration with astronomy, collaboration with physics, with
the natural sciences including the geography and geophysics group, and
more recently with the Jackson School [of Geosciences, The University
of Texas] in terms of the geophysical-related areas.. It’s evolved
into an internationally recognized an interdisciplinary research unit.
Because of the success of the LAGEOS efforts, the TOPEX mission, this
EOS interdisciplinary research grant, the ICESat mission and the GRACE
mission, we have had a very productive three decades of activity. The
research effort has allowed us essentially to establish collaborative
relations with a number of internationally recognized research groups,
such as GFZ Potsdam, Shanghai Observatory, etc.
The organized research unit also provided a basis for larger student
involvement and a place of employment, once they’d finished their
academic work. A major factor in our success has been our ability to
keep some of our best graduates active to allow them to continue their
research. So it turned out that forming the organized research unit
was an important step in the evolution of our program. We’ve extended
the center not only in the space geodesy area, but also into a number
of other satellite remote sensing areas that we have not discussed.
We have established the capability for receiving satellite data in a
direct broadcast mode. In addition to supporting research, we use the
data in teaching and in a number of other areas such as regional hazard
monitoring.
Gordon Wells, who is one of the key individuals in this effort, is a
lead member of the governor’s Division of Emergency Management.
He plays an important role in the states response to natural and manmade
disasters such as hurricanes, floods, fires, etc. We also are the home
for the multi-university Texas Space Grant Consortium. It’s an
outreach type program that NASA funds. Under the center's effort, we
prepared the proposal for this program in 1980 and have been involved
with its efforts since that time frame. I was the PI and Steve Nichols
was the Co-PI on the proposal. Steve was influential in establishing
and actually chairing the first national space grant organization.
So the Center for Space Research has been a good way to combine our
interest in space research and exploration with our interest in teaching
in one unit. The general thrust has been a campus-wide focus for both
space research and space applications, and the academic components that
are associated with this effort.
Wright:
In your spare time you currently serve on the NASA Advisory Council
[NAC].
Tapley:
Yes.
Wright:
Is that a relatively new role for you or is that something you’ve
been doing for a while?
Tapley:
No, I think I went on this—time gets by on that. I don’t
actually remember. It must have been two years ago in January. I’ve
been involved in a number of advisory positions over the years. I’ve
bumped around a couple times. At one point I looked fairly carefully
at taking the Associate Administrator role for Earth Science when Charlie
[Charles F.] Kennel left. In fact Bill Townsend actually moved into
the position. GRACE was at a point where it was critical for me to not
make this move. I really did want to participate in the GRACE mission.
This fact, coupled with some family medical problems, prevented me from
making this move.
But the NAC role required a smaller time commitment and it does give
you the chance to provide advice that can have an impact. Although you
do not have the ability to make decisions, you can have an input to
put the thought process. It’s rewarding to be able to work at
that level.
I did a fair amount of alternate advisory work in the late ’80s
and up through the middle of the ’90s for the National Academy
[of Sciences] in which I was a member of the Space Science Board and
Chaired the Committee on Earth Science. During this time, the EOS mission
suite was going forward and we were able to provide advisory oversight
to this process. It’s been interesting to see how the NASA side
of the advisory process evolves. Both activities are rewarding as long
as you feel that your efforts are making a contribution.
[End
of interview]
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