CGMS-41 NASA-WP-09
10 June 2013
Prepared by NASA
Agenda Item: WGII/10
Discussed in Working Group II
THE TERRESTRIAL REFERENCE FRAME
Richard S. Gross
NASA Jet Propulsion Laboratory, Pasadena, CA
Executive summary
The terrestrial reference frame (TRF) is the foundation for virtually all space-based, airborne
and ground-based Earth observations. Positions of objects are determined within an
underlying TRF and the accuracy with which objects can be positioned ultimately depends on
the accuracy of the reference frame. The TRF also allows different spatial information, such
as imagery from different space and airborne platforms, to be geo-referenced and aligned
with each other. Providing an accurate, stable, homogeneous, and maintainable terrestrial
reference frame to support numerous scientific and societal applications is one of the
essential goals of the International Association of Geodesy’s (IAG’s) Global Geodetic
Observing System (GGOS) and of NASA’s contribution to it, the Space Geodesy Project.
This working paper discusses the importance of geodetic measurements and the reference
systems determined from them to satellite observations of the climate system.
CGMS-41 NASA-WP-09
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The Terrestrial Reference Frame
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SPACE GEODESY
Geodesy is the science of the Earth’s shape, size, gravity and rotation, including their
evolution in time. A number of different techniques are used to observe the geodetic
properties of the Earth including the space-geodetic techniques of Very Long
Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Navigation
Satellite Systems (GNSSs) like the US Global Positioning System (GPS), and the
French Doppler Orbitography and Radio-Positioning by Integrated Satellite (DORIS)
system. These space-geodetic observations also provide the basis for the realization
of the reference systems that are needed in order to assign coordinates to points and
objects and thereby determine how those points and objects move over time.
Geodesy contributes to atmospheric science and hydrological studies by supporting
both observation and prediction of the weather by (1) geo-referencing meteorological
observing data, (2) providing atmospheric weather models with space- and timevarying gravity fields, and (3) globally tracking change in stratospheric mass and
lower tropospheric water vapor fields. Geodetic measurements of refraction profiles
derived from satellite occultation data are routinely assimilated into numerical
weather prediction models. To be useful, such measurements must be provided
rapidly to the meteorological community. This implies that highly accurate orbits of
the occulting satellites must be determined rapidly. Geodesy provides the satellite
tracking data that allows such near-real-time orbits to be determined. Geodesy also
provides a unique global reference system for measurements of: subseasonal,
seasonal and secular movements of continental and basin-scale water masses; the
wet part of the troposphere from atmospheric sounding; loading and unloading of the
land surface due to seasonal changes of groundwater and ice mass; local
measurement of the integral variation of groundwater from permanent gravimetric
tidal stations; measurement of water level of major lakes and rivers by satellite
altimetry; and improved digital terrain models as basis for flux modeling of surface
water and flood modeling.
Geodesy is at the heart of all present day ocean studies. Geodetic observations
uniquely produce accurate, quantitative, and integrated observations of gravity,
ocean circulation, sea surface height, ocean bottom pressure, and mass exchanges
among the ocean, cryosphere, and land. Geodetic observations have made
fundamental contributions to monitoring and understanding physical ocean
processes. In particular, geodesy is the basic technique used to determine an
accurate geoid model, allowing for the determination of absolute surface geostrophic
currents, which are necessary to quantify heat transport of the ocean. Geodesy also
provides the absolute reference for tide gauge measurements. Tide gauges monitor
sea level relative to the ground. Knowledge of the motion of the geodetic reference
point on land to which the tide gauge is attached is needed in order to relate the
relative tide gauge measurement to an absolute measurement of sea level change.
This knowledge is commonly provided by co-locating GNSS stations with tide gauges.
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Placing tide gauges in an absolute reference allows current rates of change of sea
level to be related to sea level change histories over much longer time spans,
thereby allowing better understanding of long-term signals and their impact on
regional and global change. Merging absolute tide gauge sea level change
measurements with satellite altimetric measurements provides a coherent worldwide
monitoring system for sea level change.
The present altimetric and gravimetric satellites can measure total sea level change
and its mass component, both of which are vital for understanding global climate
change. An important goal of IAG’s GGOS is to integrate the measurement
techniques that monitor Earth's time-variable surface geometry (including ocean and
ice surfaces), gravity field, and rotation into a consistent system for measuring ocean
surface topography, ocean currents, ocean mass, and ocean volume changes. This
system depends on both globally coordinated ground-based networks of satellite
tracking stations as well as an uninterrupted series of satellite missions. The groundbased networks of geodetic stations also provide the measurements used to
determine the terrestrial reference frame that is needed for studying regional and
global sea level change and ocean-climate cycles like El Niño, the North Atlantic
Oscillation, and the Pacific Decadal Oscillation [1]. Much of the future progress in
ocean observation will depend ultimately on the ability of the global geodetic
community to maintain the accurate and long-term reference frame required for Earth
observation. Continued refinements to the terrestrial reference frame depend on
adequate geographical coverage and co-location of the geodetic measurement
techniques of VLBI, SLR, GNSS, and DORIS. Implementing modern technology into
a network of core geodetic sites with better geographic distribution and more uniform
performance is therefore a high priority of GGOS.
2
TERRESTRIAL REFERENCE FRAME
The terrestrial reference frame is the foundation for virtually all space-based, airborne
and ground-based Earth observations. Through its tie to the celestial reference frame
(CRF) by time-dependent Earth orientation parameters, it is also fundamentally
important for interplanetary spacecraft tracking and navigation. The TRF determined
by geodetic measurements is the indispensable foundation for all geo-referenced
data used by science and society [2-4]. It plays a key role in modeling and estimation
of the motion of the Earth in space, in measuring change and deformation of all
components of the Earth system, and in providing the ability to connect
measurements made at the same place at different times, a critical requirement for
understanding global, regional and local change.
The most accurate global terrestrial reference frames currently available are the
International Terrestrial Reference Frames (ITRFs) provided by IAG’s GGOS and
International Earth Rotation and Reference Systems Service (IERS). The ITRFs are
determined by combining VLBI, SLR, GNSS, and DORIS station positions and
velocities and are produced every few years. The most recently determined ITRFs
are ITRF2000 [5], ITRF2005 [6] and ITRF2008 [7].
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One way of assessing the accuracy and stability of TRFs is to intercompare different
realizations of them. For example, when intercomparing ITRF2005 and ITRF2000,
significant differences are found: In the z-direction their origins differ by 5.8 ± 0.3
millimeters (mm) in position and 1.8 ± 0.3 mm/yr in rate [6]. This difference in the
origins is more than 5 times the accuracy in position and 18 times the accuracy in
rate that the World Climate Research Program (WCRP) and the IAG have
determined are needed in order to meet scientific and societal user requirements
(see below). In very simple terms, the most accurate global terrestrial reference
frames currently available are more than an order of magnitude less accurate than
required.
Errors like this in the terrestrial reference frame translate directly into errors in the
measurements taken by climate satellite missions. Beckley and colleagues [8]
studied the impact of errors in the terrestrial reference frame on TOPEX/POSEIDON
observations of sea level, finding that differences between the ITRF2005 and the
University of Texas CSR95 reference frames cause differences of up to ±1.5 mm/yr
in regional sea level trends (Figure 1), a substantial fraction of the global mean sea
level change of about 3 mm/yr. Thus, as noted by Beckley and colleagues [8], “the
change in reference frames directly impacts the interpretation of the regional
changes in mean sea level”.
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Figure 1. Differences in TOPEX-estimated sea surface trends caused by
differences in computing the TOPEX/POSEIDON orbit. The striping is caused by
differences in the assumed gravity field (JGM3 or Eigen-04), the north-south slope
by differences in the assumed terrestrial reference frame (ITRF2005 or CSR95).
These two terrestrial reference frames differ mainly by a relative drift in the zcoordinate of their origins, and are an example of how errors in the reference
frame can compromise understanding of sea level change. Source: [8].
3
SEA LEVEL CHANGE
Studies of sea level change pose the most clearly defined, most immediate, and
most stringent requirement on the terrestrial reference frame. Nearly a fourth of the
world’s population is vulnerable to the effects of a rising sea level. Although the longterm average rate of global sea level rise is only about 3 mm/yr, mitigation efforts
need to be planned well in advance. This globally averaged rate of sea level change
is modulated by local phenomena including crustal deformation, land subsidence,
glacial isostatic adjustment, steric effects and changes in ocean circulation that
impact the long-term average dynamic ocean topography. Such local effects can
substantially change the locally measured sea level and can only be unraveled and
understood by placing the local measurements in a global reference frame.
Great demands are placed on geodetic observations because the sea level change
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signal is so small. For example, the terrestrial reference frame, which should be at
least an order of magnitude more accurate than the amplitude of the signal being
measured, needs to be accurate to within about 1 mm and stable to within about 0.1
mm/yr to support studies of sea level change (e.g., [9-10]). The current uncertainty in
the TRF is an order of magnitude greater than this.
Over 160 scientists from 29 countries attended a Workshop on Understanding Sealevel Rise and Variability held in Paris June 6-9, 2006 [11]. The Workshop was
organized by the WCRP in order to bring together all relevant scientific expertise with
a view towards identifying the uncertainties associated with past and future sea level
rise and variability, as well as the research and observational activities needed for
narrowing these uncertainties. The Workshop was also conducted in support of the
Global Earth Observation System of Systems (GEOSS) 10-Year Implementation
Plan; as such, it helped develop international and interdisciplinary scientific
consensus for those observational requirements needed to address sea level rise
and its variability.
Workshop participants reviewed current knowledge concerning all aspects of sea
level change, determined uncertainties in knowledge of the contributing factors, and
generated a summary set of recommendations focused on reducing those
uncertainties. Workshop participants recognized that the development and
implementation of geodetic techniques has enabled a revolution in the Earth
sciences, providing the fundamental reference frame critical for the collection of all
satellite and many in situ observations addressing sea level change and variability.
However, to take advantage of those capabilities, they must be reliable and
consistent over the long term (i.e., decades). While these techniques collectively
define the terrestrial reference frame, they are at the same time losing support and
degrading in capability. In order to achieve the goal of determining sea level change
at the level of 0.1 mm/yr, the Workshop participants recommended [10] that:
“The International Terrestrial Reference Frame (ITRF) must be more
robust and stable over multi-decadal time scales. The target accuracy
is 0.1 mm/yr in the realization of the center of mass of the entire Earth
system (“geocenter stability”), and 0.01 ppb/yr in scale stability.”
4
ICE SHEET MASS CHANGE
Ice sheets, glaciers, and sea ice are intricately linked to the Earth’s climate system.
They store a record of past climate; they strongly affect surface energy budget,
global water cycle, and sea level change; and they are sensitive indicators of climate
change. To understand these requires long-term monitoring of changes in ice mass.
This is not possible at the required level of accuracy without improvements in both
geodetic measurement systems and the reference frames in which those
measurements are made.
Geodesy is crucial for these studies because of its ability to measure the motions of
ice masses and changes in their volumes: highly accurate measurements of ice
velocity from repeated GNSS surveys; spatially detailed measurements of glacier
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motion from interferometric SAR (InSAR); ice thickness measurements over glaciers
and ice sheets from low frequency radar; ice sheet thickening/thinning rates over vast
areas from laser and radar altimeters; estimates of rates of change of the mass of
entire ice sheets from satellite measurements of temporal changes in Earth’s gravity
field; time series of sea ice extent and motion from passive microwave and SAR
images; and routine mapping over almost entire ice sheets of characteristics such as
temperature, iciness and wetness of surface snow. These studies would not have
been possible without improvements in both geodetic measurement systems and
reference frames.
Improving the reference frame directly benefits glaciological research by, for example,
improving the accuracy of measured parameters such as ice surface elevation and
improving the detection of temporal changes in ice cover and ice surface velocities. It
also has indirect benefits such as improving the knowledge of rates of sea level
change and postglacial uplift. For example, errors in the time rate-of-change of the
origin of the z-coordinate of the reference frame (discerned in the difference between
the ITRF2000 and ITRF2005 reference frame realizations discussed above) maps
directly into the calculations of ice mass loss measured by laser or radar altimetry.
5
SUMMARY
We measure and monitor the Earth’s environmental system (its oceans, ice, land,
atmosphere) not only to understand the processes of global change, but also to
enable educated decisions on how to cope with these changes. Space agencies like
NASA are heavily investing in satellites to make these measurements. Many of these
depend on a highly accurate and stable terrestrial reference frame within which to
interpret the data and understand trends in the processes of change.
The terrestrial reference frame is maintained though a global network of ground sites
with co-located SLR, VLBI, GNSS, and DORIS stations and is realized as the
international standard through the ITRF. Requirements for the ITRF have increased
dramatically since the 1980s. Today, the most stringent requirement comes from
critical sea level programs: a global accuracy of 1.0 mm, and 0.1 mm/yr stability is
required. This is a factor of 10 to 20 beyond current capability. Current and future
satellites will have ever-increasing measurement capability and should lead to
increasingly sophisticated models to explain what is happening. The accuracy and
stability of the terrestrial reference frame needs to dramatically improve in order to
fully realize the measurement potential of the current and future generation of Earth
observing satellites.
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REFERENCES
[1] Chelton, D.C., Ries, J.C., Haines, B.J., Fu, L.-L., and Callahan, P.S., Satellite
Altimetry and Earth Sciences, edited by L.L. Fu and A. Cazenave, International
Geophysics Series, Volume 69, Academic Press, San Diego, 2001.
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[2] Solomon, S. C. & the Solid Earth Science Working Group, Living on a Restless
Planet, Jet Propulsion Laboratory, JPL 400-1040, 66 pp., Pasadena, Calif., 2002.
[3] Board on Earth Sciences and Resources, Living on an Active Earth: Perspectives
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