SESolid EarthSESolid Earth1869-9529Copernicus PublicationsGöttingen, Germany10.5194/se-10-293-2019Impact of terrestrial reference frame realizations on altimetry satellite orbit quality
and global and regional sea level trends: a switch from ITRF2008 to ITRF2014Assessment of ITRF2014 for precise orbit determination of altimetry satellitesRudenkoSergeisergei.rudenko@tum.dehttps://orcid.org/0000-0001-5149-3827EsselbornSaskiahttps://orcid.org/0000-0002-1924-4449SchöneTilohttps://orcid.org/0000-0003-4118-9578DettmeringDenisehttps://orcid.org/0000-0002-8940-4639Helmholtz Centre Potsdam – GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, GermanyDeutsches Geodätisches Forschungsinstitut der Technischen Universität München (DGFI-TUM), Arcisstr. 21, 80333 Munich, GermanySergei Rudenko (sergei.rudenko@tum.de)6February20191012933059July201825July201814December20189January2019This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://se.copernicus.org/articles/10/293/2019/se-10-293-2019.htmlThe full text article is available as a PDF file from https://se.copernicus.org/articles/10/293/2019/se-10-293-2019.pdf
A terrestrial reference frame (TRF) is a basis for precise orbit
determination of Earth-orbiting satellites, since it defines positions and
velocities of stations, the tracking data of which are used to derive satellite
positions. In this paper, we investigate the impact of the International
Terrestrial Reference Frame realization ITRF2014, as compared to its
predecessor ITRF2008, on the quality of orbits, namely, on root-mean-square
(rms) fits of observations and orbital arc overlaps of three altimetry
satellites (TOPEX/Poseidon, Jason-1, and Jason-2) in the time
interval from August 1992 to April 2015 and on altimetry products computed
using these orbits, such as single-satellite altimeter crossover differences,
radial and geographically correlated mean sea surface height (SSH) errors and regional and global mean sea level trends. The satellite orbits are computed
using satellite laser ranging (SLR) and Doppler Orbitography and
Radiopositioning Integrated by Satellite (DORIS) observations of a global
network of stations.
We have found that using ITRF2014 generally improves the orbit quality as
compared to using ITRF2008. Thus, the mean values of the rms fits of SLR
observations decreased (improved) by 2.4 % and 8.8 % for
Jason-1 and Jason-2, respectively, but are almost not impacted
for TOPEX/Poseidon when using ITRF2014 instead of ITRF2008. The internal
orbit consistency in the radial direction (as derived from arc overlaps) is
reduced (improved) by 6.6 %, 2.3 %, and 5.9 % for TOPEX/Poseidon,
Jason-1, and Jason-2, respectively.
Single-satellite altimetry crossover analyses indicate reduction
(improvement) in the absolute mean crossover differences by 0.2 mm
(8.1 %) for TOPEX, 0.4 mm (17.7 %) for Jason-1, and 0.6 mm
(30.9 %) for Jason-2 with ITRF2014 instead of ITRF2008. The major
improvement of the mean values of the rms of crossover differences (0.13 mm;
0.3 %) has been found for Jason-2.
Multi-mission crossover analysis shows slight improvements in the standard
deviations of radial errors: 0.1 %, 0.2 %, and 1.6 % for TOPEX,
Jason-1, and Jason-2, respectively. The standard deviations of
geographically correlated mean SSH errors improved by 1.1 % for
Jason-1 and 5.4 % for Jason-2 and degraded by 1.3 % for
TOPEX.
The change from ITRF2008 to ITRF2014 orbits only has minor effects on the
estimation of regional and global sea level trends over the 22-year time
series from 1993 to 2015. However, on interannual timescales (3–8 years)
large-scale coherent trend patterns are observed that seem to be connected to
drifts between the origins of the tracking station networks.
This leads to the changes in interannual global mean sea level of up to
0.06 mm yr-1 for TOPEX,
0.05 mm yr-1 for Jason-1, and up to 0.12 mm yr-1 for
Jason-2, i.e., up to 4 % of the corresponding sea level signal
based on altimetry for timescales of 3 to 8 years. The respective changes in
the regional sea level trend on these timescales are up to
0.4 mm yr-1 in the time span from April 1993 to July 2008 and up to
1.0 mm yr-1 in the time span from July 2008 to April 2015.
Introduction
Precise information on positions and the motion of points located on the Earth's
surface is important for practical applications, such as positioning and
navigation, and scientific investigations, such as Earth's rotation, plate
tectonics, seismological deformations, and precise orbit determination (POD). Precise positions and velocities of geodetic stations are
provided by International Terrestrial Reference System (ITRS) realizations
created by ITRS Combination Centres based on solutions provided by
International DORIS Service (IDS), International GNSS (Global Navigation Satellite System) Service (IGS),
International Laser Ranging Service (ILRS), and International VLBI Service
for Geodesy and Astrometry (IVS). These solutions are derived from the
analysis of Doppler Orbitography and Radiopositioning Integrated by Satellite
(DORIS), Global Positioning System (GPS), satellite laser ranging (SLR), and
very long-baseline interferometry (VLBI) observations. Three new recently
released ITRS realizations are ITRF2014 , DTRF2014
, and JTRF2014 .
A precise and stable terrestrial reference frame (TRF) is important for
the long-term consistency of altimetry measurements, since it provides the basis
for mapping sea level change to an accurate and stable coordinate system for
calibration and validation and improved long-term monitoring of sea level
changes . The realization of a terrestrial
reference system has been shown to have a detectable impact
on the regional and global mean sea levels trends derived from altimetry.
Thus, they found changes of up to ±1.5 mm yr-1 in the regional sea
level rates related to the change from ITRF2000 to ITRF2005.
showed that a 10 mm error in the TZ realization of a
terrestrial reference frame can cause a systematic error of -1.2 mm in the
derived mean sea level. Using TOPEX/Poseidon altimetry data, they estimated a
precision of 3.0 mm in sea level and 0.37 mm yr-1 in sea level trend
in the case of ITRF97. From the analysis of Jason-1 and Jason-2
orbits derived using GPS and SLR and DORIS observations in 2002–2014 in ITRF2008
instead of ITRF2005, estimated a sea level trend error
caused by these ITRF realizations of 0.05 mm yr-1 (globally) and
0.3 mm yr-1 (regionally) at the decadal timescale. From their study,
the sea level trend error caused by these ITRF realizations can reach
0.03 mm yr-1 (globally) and 1 mm yr-1 (regionally) at the
interannual timescale. More recently, from the analysis of TOPEX altimetry
observations for the period 1993 to 2004, have
estimated a sea level trend error caused by ITRF2008 realization, as compared
to ITRF2014 of 0.01 mm yr-1 (globally) and 0.2 mm yr-1
(regionally) at the decadal timescale and 0.03 mm yr-1 (globally) and
0.2 mm yr-1 (regionally) at the interannual timescale. Recently,
investigated the impact of ITRF2014, DTRF2014, and
JTRF2014 on orbits of altimetry satellites TOPEX/Poseidon, Jason-1,
Jason-2, and Jason-3 derived using SLR and DORIS measurements
over 1992–2016. They found that the replacement of ITRF2008 by ITRF2014 impacts
the regional sea level trend within ±0.2 mm yr-1.
The most widely used ITRS realizations are derived by the International Earth
Rotation and Reference Systems Service (IERS) ITRS Product Center at Institut
National de l'Information Géographique et Forestière, France.
Therefore, in this paper we assess the impact of the new (ITRF2014)
realization, as compared to its predecessor, ITRF2008 ,
on the orbits of three altimetry satellites, namely, TOPEX/Poseidon (from
23 September 1992 to 9 October 2005), Jason-1 (from 13 January 2002
to 5 July 2013), and Jason-2 (from 5 July 2008 to 6 April 2015),
since these are the reference missions for sea level investigation
. We have computed orbits of these satellites (called GFZ
VER13; ) using the ITRF2014 reference frame. We analyze
these orbits, as compared to the GFZ VER11 orbits of the
same satellites derived within the second phase of the Sea Level project of the European Space Agency (ESA) Climate Change Initiative (CCI) program using the ITRF2008 reference frame. For both sets of
orbits, all other background models for precise orbit determination and
estimated parameters are the same. Both VER11 and VER13 orbits are derived
using SLR and DORIS observations available from ILRS and
IDS , respectively. We investigate the impact of ITRF2014,
as compared to ITRF2008, on the root-mean-square (rms) of residuals of SLR
and DORIS observations used for orbit determination; on the rms
and mean of single-satellite altimetry crossover differences and radial
errors; and on the global and regional mean sea level trends (between 66∘ S
and 66∘ N in latitude). We performed similar investigations for
studying the impact of geopotential models and ocean and
atmospheric de-aliasing products . By contrast with the
paper of , we investigate the impact of the replacement
of ITRF2008 by ITRF2014 also on 2-day arc overlaps in radial, cross-track,
and along-track directions and the geographically correlated mean sea surface
height (SSH) errors. Moreover, we use 3.7- and 5-year longer time intervals
for TOPEX/Poseidon and Jason-1, respectively, than those used by
. Additionally, we use another software and models for
orbit computation and altimetry analysis. All this leads to slightly
different results than those obtained by .
The rest of the paper is organized as follows. A short description of the
ITRF2014 and its differences with respect to ITRF2008 is given in
Sect. . A description of the models used for POD of altimetry
satellites and the impact of the ITRF2014, as compared to ITRF2008, on the
rms fits of SLR and DORIS observations used for orbit determination as well
as on the 2-day arc overlaps in the radial, cross-track, and along-track
directions is discussed in Sect. . The impact of ITRF
realizations on the rms and mean of single-satellite altimetry crossover
differences for the three satellites is presented in Sect. . The
influence of the change from ITRF2008 to ITRF2014 on the radial orbit errors
and geographically correlated mean sea surface height errors as well as on
the global and regional mean sea level trends is presented in
Sects. and , respectively. The main results
of our study are summarized in Sect. .
ITRF realizations used for precise orbit determination
The detailed description of ITRF2008 and ITRF2014 is given in
and , respectively. The main differences
in ITRF2014 with respect to ITRF2008 consist of
6-year longer time span (2009.0–2015.0) used for the generation of the
reference frame and, therefore, an increased number of stations and their
occupations,
using information from 36 new surveys performed since the release of
ITRF2008, which resulted in employing 139 local ties for ITRF2014 instead of
104 for ITRF2008,
enhanced modeling of nonlinear station motions, provided by annual and
semiannual variations in station positions that were excluded prior to the
determination of station positions and velocities and by post-seismic
deformations made available for stations affected by major earthquakes.
Since only observations from DORIS and SLR stations are used in our study,
the following description concerns only these stations. Since an additional
time span was used in ITRF2014 for SLR and DORIS stations, ITRF2014 contains
30 additional DOMES (Directory Of MERIT (Monitoring Earth Rotation and Intercomparison of Techniques) Sites) numbers for DORIS stations and 12 for SLR stations as
compared to ITRF2008 (Table ). ITRF2014 includes 22
additional discontinuities for DORIS stations and 17 for SLR stations as
compared to ITRF2008. Moreover, ITRF2014 provides post-seismic deformation
models for 13 DORIS and 10 SLR stations that have been used by us for this
reference frame. No annual and semiannual signals were applied by us for ITRF2008 or for ITRF2014, since they are not a part of these
realizations and were estimated internally for ITRF2014 by its authors for
enhancing the velocity field estimation of the secular frame.
The number of DOMES numbers and discontinuities and data span for DORIS and SLR stations in the ITRF2008 and ITRF2014.
ITRF2008 ITRF2014 ObservationNumber ofNumber ofDataNumber ofNumber ofDatatypeDOMES numbersdiscontinuitiesspanDOMES numbersdiscontinuitiesspanDORIS130401993.0–2009.0160621993.0–2015.0SLR128241983.0–2009.0140411983.0–2015.0Impact of ITRF2008 and ITRF2014 realizations on the orbit quality
To perform our study, we have derived orbits of TOPEX/Poseidon (from
23 September 1992 to 9 October 2005), Jason-1 (from 13 January 2002
to 5 July 2013), and Jason-2 (from 5 July 2008 to 6 April 2015)
at 12-day arcs with 2-day arc overlaps. Orbit computations were performed
using the “Earth Parameter and Orbit System – Orbit Computation (EPOS-OC)”
software developed at GeoForschungsZentrum (GFZ) Potsdam. We use SLR and DORIS observations
for all three satellites. To derive the satellite orbits, we use the same
models, procedures, and parameterization as described in
but use two different ITRF realizations – ITRF2008 and
ITRF2014. The main models used for orbit determination are given in
Table . For the details on the models and procedures used
for the POD, the reader is referred to and
. The orbits of these satellites derived using ITRF2008
and ITRF2014 are called VER11 (version 11) and VER13 (version 13) orbits,
respectively. Some of the models used by us for POD correspond to Geophysical
Data Records (GDR)-E POD standards; some of them, like ITRF2014,
correspond to Precise Orbit Ephemeris (POE)-F standards. At the same time,
some of the models used by us, e.g., the EOT11a ocean tide model, the EIGEN-6S4 Earth
gravity field model, and the GFZ AOD1B RL05 nontidal atmospheric and oceanic gravity
model, differ from those defined in GDR-E and POE-F POD standards, details on
which can be found at
ftp://ftp.ids-doris.org/pub/ids/data/POD_configuration_POEF.pdf (last access: 23 January 2019).
The main models used for orbit determination (for the details and
references for the models, see ).
ItemThe model usedEarth gravity field modelEIGEN-6S4 (up to n=m=90)Solid Earth and pole tidesIERS Conventions (2010)Ocean tide modelEOT11aAtmospheric tidesBiancale and Bode (2006)Nontidal atmospheric and oceanic gravityGFZ AOD1B RL05Third bodies (Sun, Moon, and seven major planets)DE-421 ephemeridesRadiation pressure modelCNES/IDS box/wing modelsEarth radiationKnocke modelAtmospheric density modelMSIS-86Polar motion and UT1IERS EOP 08 C04 (IAU2000A) with IERS daily and sub-daily correctionsPrecession and nutationIERS Conventions (2010)Tropospheric correction for DORIS dataVienna Mapping Function 1
The rms fits of observations are an indicator of the accuracy of
observations, models, reference frame realizations, and parameterization used
for POD. Since we use the same observations, models, and parameterization to
compute the VER11 and VER13 orbits and replace only ITRF realizations, the
changes in rms fits of observations indicate the impact of the change in ITRF
realizations on the rms fits of observations. We have found that a switch from
ITRF2008 to ITRF2014 did not change the rms fits of SLR
observations of TOPEX/Poseidon significantly. Their mean value slightly increased from 1.96
to 1.97 cm, i.e., by 0.3 %. However, the mean values of SLR rms fits
decreased (improved) from 1.19 to 1.16 cm, i.e., by about 2.4 %, for
Jason-1 and from 1.23 to 1.13 cm, i.e., by 8.1 %, for
Jason-2 when using ITRF2014 instead of ITRF2008. The major reduction in the SLR rms fits is obtained in 2009–2015
(Figs. –), since ITRF2014
was derived using additional observations for this time span allowing a more
precise determination of station positions for this time span and station
velocity over the whole time interval. In these figures, we use a 52-week
running mean in order to get rid of short-periodic variations in the rms
fits.
The mean values of DORIS rms fits are reduced (improved) for Jason-2
from 0.3490 to 0.3484 mm s-1, i.e., by about 0.2 % when using
ITRF2014 instead of ITRF2008. A larger improvement of 0.3 %–1.0 % is
observed in 2012–2015. A rather small impact on DORIS rms fits of
Jason-1 is found. Thus, a small improvement (about 0.2 %) of DORIS
rms fits is observed in 2010–2011 and a small degradation (about 0.3 %)
of DORIS rms fits is observed in 2012–2013 for this satellite. The mean
values of DORIS rms fits are almost unchanged for TOPEX/Poseidon when
using ITRF2014 instead of ITRF2008. However, an improvement of DORIS rms fits
of 1 %–3 % is observed at about 20 arcs in 1993–1998 when using
ITRF2014 instead of ITRF2008. The number of accepted DORIS observations at
these arcs is 1.2–2.5 times larger when using ITRF2014 due to a better fitting
of observations.
Satellite orbit and adjusted parameters are computed at different arcs using
a different observations and, in some cases, using different parameterization
depending on the amount of available observations. Therefore, though the
background models used for orbit computations at orbit overlaps are the same,
non-zero differences in satellite coordinates at overlaps are obtained. We
call the differences in satellite coordinates of overlaps internal
consistency, since the orbits are computed using the same software and the
same background models. We have found from our analysis that the internal
consistency of satellite orbits derived using ITRF2014 has improved, as
compared to that obtained using ITRF2008 (Table ). The
most significant reduction (improvement) is found for the along-track arc
overlap.
Mean values of the rms fits of SLR and DORIS measurements, 2-day
arc overlaps, and the number of arcs used to compute these values for
TOPEX/Poseidon (from 23 September 1992 to 9 October 2005), Jason-1
(from 13 January 2002 to 5 July 2013), and Jason-2 (from 5 July 2008 to 6 April 2015) orbits derived at the time intervals specified using
ITRF2008 and ITRF2014. The percentage of the parameter change by switching
from ITRF2008 to ITRF2014 is given in parentheses (positive value indicates
an improvement).
Fifty-two-week running mean of the rms fits of Jason-1 SLR
observations obtained using ITRF2014 (VER13 orbit) and ITRF2008 (VER11 orbit)
from 13 January 2002 to 16 February 2012. SD denotes standard deviation.
Fifty-two-week running mean of the rms fits of Jason-2 SLR
observations obtained using ITRF2014 (VER13 orbit) and ITRF2008 (VER11 orbit)
from 5 July 2008 to 6 April 2015. SD denotes standard deviation.
Impact of the change from ITRF2008 to ITRF2014 on the single-satellite altimetry crossover differences
Single crossover analyses for all three missions have been performed based on
ESA CCI Sea Level v2 ECV data . The data are available with
a 1 Hz sampling rate, and all corrections for instrumental and geophysical
effects by the state-of-the-art models are provided with the data. For
consistency reasons, we replaced some internal correction models, in
particular, the ocean tide and loading correction with the EOT11a tide model
and the solid Earth tides following the IERS Conventions 2003 . The altimeter crossover differences
(ascending pass minus descending pass) are calculated at a 10-day step with
GFZ's Altimeter Database and Processing System (ADS; )
for each test orbit (VER11 and VER13) separately. The global mean crossover
difference and the associated rms values are calculated after applying a
3σ test. On average, about 5000 valid crossover points are found with
some annual changes due to hemispheric change in sea ice coverage. To find
valid crossover points, all internal quality criteria, which are part of the
GDRs, are checked. In addition, we use only those points which meet the
following criteria: deep water (with depth deeper than 200 m), wind speed
less than 15 m s-1, significant wave height less than 12 m, and crossover difference less
than 1.5 m. The latter is especially valid in areas with sea ice occurrence.
For all three missions, the rms of the crossover differences is around 5 cm. The non-zero mean of the crossover differences which
indicates a constant offset in sea level heights between ascending and
descending tracks is also notable. Comparing VER11 and VER13 results, the mean of the
crossover differences becomes smaller for VER13, indicating smaller
discrepancies between ascending and descending tracks. Also, a slight
improvement of 0.01–0.13 mm in the rms of crossover differences is observed
(Table ) when replacing the VER11 orbit by the VER13 orbit. The
quality of Jason-2 crossovers improves noticeably, indicating a better
performance of the ITRF2014-based VER13 orbit.
Statistics of crossover differences for orbits derived using
ITRF2008 and ITRF2014. For Jason-1 the geodetic phase is not
considered due to the change in crossover point distribution. The values are
means over all 10-day analyses in millimeters.
The evolution of the difference in the mean crossover difference per 10-day periods of VER11 minus VER13 orbits (Fig. 3), which emphasizes the changes in discrepancies between the ascending and descending tracks, is also of interest. The differences for TOPEX are stable over
the full mission period with only minor variations. They can be attributed to
very small changes in the radial orbit component. For Jason-1 a small,
negligible drift of 0.06 mm yr-1 can be observed. This (positive)
drift indicates that for the VER13 orbits, the discrepancy between ascending
and descending tracks becomes smaller with time. The drift in the difference
for Jason-2 is much larger and reaches 0.31 mm yr-1. It is also notable that starting around the middle of 2009, the differences for Jason-1 and
Jason-2 show a sinusoidal signal with a period of about 120 days (the
period is fixed to 10 days as per the analyses period) with amplitudes of
around 4 mm. The larger scatter of crossover differences for the periods
after the middle of 2009 shown in Fig. are due to the fact that
station positions used for the computation of satellite positions are
computed in the case of using ITRF2008 by the extrapolation of station velocities
derived by 2009.0 beyond this instant, while station velocities in the case of
using ITRF2014 are derived by 2015.0. This is an indication of an error,
introduced by an older ITRF realization used beyond the time span at which it
was derived.
Differences of the crossover (XO) differences for the TOPEX (TP),
Jason-1 (J1), and Jason-2 (J2) missions between VER11 and VER13
orbits. Values are in meters. The x axis shows time from 1 April 1993 to
25 February 2015.
Impact of the change from ITRF2008 to ITRF2014 on the radial orbit errors and geographically correlated mean sea surface height errors
In order to investigate the influence of using satellite orbits based on
different realizations of the reference system on the precision and
consistency of altimetry-derived sea level products, SSH crossover
differences with a maximum time limit of 2 days are analyzed. For this
purpose, a global multi-mission crossover analysis (MMXO) as described by
is applied to derive radial errors as well as
geographically correlated error patterns for all three missions and for both
orbit solutions. The comparison of the results obtained using ITRF2008 and
ITRF2014 reveals valuable information on the product quality and consistency
for different periods.
Relative difference (VER11-VER13) in the standard deviation of
radial errors per year for three missions: TOPEX (green), Jason-1
(blue), and Jason-2 (red). Positive values indicate improvements for
orbits based on ITRF2014.
For all three missions, slight improvements in the standard deviations of
radial errors are obtained through the use of ITRF2014 orbits as can be
seen in Table . The choice of the reference system only has a
small impact on the overall scatter of the radial errors and changes the
standard deviations by less than 1 mm for all missions. However,
whereas for TOPEX and Jason-1, the improvement is less than 1 %,
for Jason-2 an improvement of 1.6 % is visible. This larger
relative improvement is partly related to the smaller scatter of radial
errors. However, it is expected that the different behavior is also related
to the measurement period of the missions. Thus, in order to access the
temporal evolution of these values, standard deviations for each calendar
year are computed. These values are plotted in Fig. and
reveal clear trends for Jason-1 and Jason-2. After 2010,
observable improvements for all missions are visible reaching a maximum of
nearly 3 % for Jason-2 in 2014.
Standard deviations of the radial errors obtained using orbits of
three satellites based on ITRF2008 and ITRF20014 and their differences
(positive values indicate improvements for orbits computed using ITRF2014).
For many sea level applications, the most harmful errors are those with a
fixed geographical pattern. Following the theory of ,
the MMXO provides geographically correlated mean SSH errors (GCEs) for all
missions involved . The change from
ITRF2008 to ITRF2014 for orbit computation also influences the GCE.
Figure displays the GCE for VER13 orbits (based on ITRF2014) as
well as the GCE differences to VER11 orbit solutions. One can see
that for all three missions, the GCEs remain below about 1 cm and the change
in ITRF accounts for less than 2 mm differences (positive as well as
negative). Over the entire globe, the improvement yields 1.1 % for
Jason-1 and 5.4 % for Jason-2 and a degradation by 1.3 % for TOPEX (in terms of reduction in standard deviation as can be
seen from Table ).
Standard deviations of geographically correlated mean SSH errors
obtained using orbits of three satellites based on ITRF2008 and ITRF20014 and
their differences (positive values indicate improvements obtained for the
orbits computed using ITRF2014).
Geographically correlated mean SSH errors for three missions based
on ITRF2014 orbits (a, c, e) and VER11-VER13 differences (b, d, f) for TOPEX (a, b), Jason-1 (c, d), and
Jason-2 (e, f); wrt.: with respect to, diff.: difference.
Impact of the change from ITRF2008 to ITRF2014 on regional and global mean sea level
We investigate the interannual signals and long-term trends of the regional
and global mean sea level from altimetry related to the change from the
ITRF2008 to ITRF2014. Since the radial orbit component maps directly onto the
sea level measurement, it is possible to study the effect of the improved ITRF
on global and regional sea level from altimetry by analyzing orbit data only.
The main focus of this analysis is on timescales of more than 1 year.
Rms per cycle, rms, and trend of the global mean over the ocean and
maximum regional (absolute) trend values from VER11 minus VER13 radial orbit
differences for the combined TOPEX, Jason-1, and Jason-2 series
and for subseries. The percentage of the ITRF-related changes relative to
the total signal measured by altimetry is given in brackets for comparison.
Mission, time spanRms per cycleRms of globalTrend of globalRegional trend(mm)mean differencemean differenceup to(mm)(mm yr-1)(mm yr-1)TOPEX/Jason-1/Jason-21.83 (3 %)0.33 (2 %)-0.00±0.00 (0 %)0.14TOPEX I (April 1993 to May 1997)1.82 (3 %)0.19 (3 %)-0.06±0.01 (2 %)0.38TOPEX II (June 1997 to September 2005)1.89 (3 %)0.23 (3 %)0.04±0.01 (1 %)0.31Jason-1 I (January 2002 to October 2007)1.74 (3 %)0.25 (4 %)0.05±0.01 (2 %)0.34Jason-1 II (October 2007 to February 2012)2.04 (3 %)0.38 (6 %)-0.05±0.02 (2 %)0.50Jason-2 I (July 2008 to March 2012)2.09 (4 %)0.49 (9 %)-0.02±0.04 (1 %)1.01Jason-2 II (March 2012 to April 2015)1.97 (4 %)0.57 (11 %)0.12±0.06 (4 %)0.81
We evaluate the VER11 minus VER13 radial orbit differences sampled over the
oceans. The orbits calculated in ITRF2014 are converted to the ITRF2008
system by a Helmert transformation by applying the transformation parameters
from . Jason-1 data from the geodetic phase (starting in
May 2012) are excluded from the analysis. The orbit differences are mapped
cycle by cycle along-track and are interpolated on a 1∘×1∘ grid. From these data a global mean time series over the ocean is
inferred. From the global mean and the 1∘×1∘ mapped
time series, rms and trend values are estimated, as described by
.
A measure of the amount by which the radial components of the two orbits differ is the rms value per cycle (Fig. ). Its
mean value is 1.8 mm for the combined TOPEX, Jason-1, and
Jason-2 VER11 minus VER13 series which corresponds to 3 % of the
rms value per cycle of the corresponding sea level data from altimetry
(Table ). While the values for most of the TOPEX time
series and also for the first few years of the Jason-1 series are
below the mean, they are increased for the interleaved orbits of the TOPEX
series (after the middle of 2002) and after the middle of 2006 for the Jason-1 and
also the Jason-2 series.
The impact of the change in the ITRF solution on the estimated global mean
sea level is minor. The rms of the global mean radial differences over the
ocean is 0.3 mm, which corresponds to 2 % of the rms of the global mean
sea level from altimetry over the corresponding period
(Table ). The rms of the global mean radial orbit
differences over the ocean is slightly higher for Jason-2 than for
the TOPEX and Jason-1 missions. The global mean radial orbit
differences are of the order of 0.5 mm for TOPEX and of 0.3 mm for
Jason-1 and Jason-2. This offset between the two ITRFs is
consistent with a slight shift (a few mm) in the VER13 origin from the South
Pacific in the direction of Eurasia relative to the origin of the VER11
orbits. Such a shift is, most probably, related to slight changes in the
positions of the tracking station network.
The spectral analysis of the global mean radial orbit differences over the
ocean shows that most of the energy can be found for periods of less than
110 days; however, this analysis focuses on the interannual to decadal timescales. The low-pass filtered time series of the global mean VER11 minus
VER13 radial orbit differences over the ocean is shown in
Fig. . The global mean sea level trend over
the oceans is not affected by the switch from the ITRF2008 to the ITRF2014
realization for POD (Table ). However, the global mean of
the VER11 minus VER13 series (Fig. )
exhibits interannual- to decadal-scale variability. In order to study these
effects in detail we define the following four periods, which are covered by
the denoted missions from the inflection points of the time series:
April 1993 to May 1997 (TOPEX I),
June 1997 to October 2007 (TOPEX II, Jason-1 I),
October 2007 to March 2012 (Jason-1 II, Jason-2 I),
March 2012 to April 2015 (Jason-2 II).
For these periods, the trends of the global mean radial orbit differences
range between -0.06 and 0.12 mm yr-1, which corresponds
to up to 4 % of the sea level trend from altimetry over the corresponding
periods (Table ). The geographical patterns of the trends
for these four periods are given in Fig. . Note that the
trend patterns for Jason-1 I and Jason-2 I (not shown) resemble
closely the patterns for TOPEX II and Jason-1 II (shown). The changes in the global and regional trends strongly depend on the analyzed period of
time (see Table and
Fig. ). For the first two periods (up to
2007), the trend patterns are consistent with relative drifts of the
Z components of the origins with a change in direction in 1997. The
regional trends in this period reach maximum values of 0.3 to
0.4 mm yr-1. For the last two periods, relative drifts of the origin
in the horizontal plane are dominant. The regional trends after 2007 reach
maximum values of 0.5 to 1 mm yr-1. The changes in the regional sea
level trend (up to 0.14 mm yr-1) found by us from the analysis of
TOPEX, Jason-1, and Jason-2 for the period 1993 to 2015 agree
with those (up to 0.2 mm yr-1) obtained by by
using the same altimetry missions for the period 1992 to 2016.
The uncertainties in global mean sea level trends due to the TRF realization
have decreased considerably during the last decades.
reported global mean sea level changes of 0.44 mm yr-1 related to the
change from ITRF2000 to ITRF2005. The change from ITRF2005 to ITRF2008 still
led to apparent global sea level drifts of 0.05 mm yr-1 at the decadal timescale. For the change from ITRF2008
to 2014, – based on five GSFC orbits - estimated an
uncertainty in the global mean sea level of 0.03 mm yr-1. According to
our studies the global mean sea level trend for the 22-year series is hardly
impacted at all. This is a major improvement with respect to the previous TRF
realizations. The uncertainties in the global mean sea level trends have been
dominated by uncertainties in the z coordinate of the origin
. From our analyses we observe corresponding patterns until
2008, even though the amplitudes decreased by a factor of 4. After 2008 the
uncertainties in the horizontal coordinates of the origin increasingly impact
regional sea level drifts.
Global mean rms per cycle of gridded radial orbit differences
(VER11-VER13) for TOPEX (blue), Jason-1 (cyan), and Jason-2
(red). The mean value is marked by the dashed line.
Mean radial height differences (VER11-VER13) over the global ocean
low pass filtered by 1-year boxcar filter for TOPEX (blue), Jason-1
(cyan), and Jason-2 (red). The sub-periods used for the calculation of
trends are marked by dashed lines.
Trend differences in radial orbit components for VER11-VER13 for
four periods. TOPEX I: April 1993–May 1997; TOPEX II:
June 1997–September 2005; Jason-1 II: October 2007–February 2012; Jason-2 II: March 2012–April 2015. Regions with formal errors
larger than the fitted value are masked out (white). The global mean trend
difference is given in Table .
Conclusions
From the analysis of TOPEX/Poseidon (September 1992 to October 2005),
Jason-1 (January 2002 to July 2013), and Jason-2 (July 2008 to
April 2015) orbits computed by us using the ITRF2008 and ITRF2014
realizations, we have found that using ITRF2014 generally improves the orbit
quality as compared to using ITRF2008. Thus, the mean values of the rms fits
of SLR observations are reduced (improved) by 2.4 % and 8.8 % for
Jason-1 and Jason-2, respectively, and are almost not impacted
for TOPEX/Poseidon when using ITRF2014 instead of ITRF2008. At the same
time, the replacement of ITRF2008 by ITRF2014 has a minor impact (less than
0.1 %) on the rms fits of DORIS observations of TOPEX/Poseidon and
Jason-1. A slightly larger impact has been found for Jason-2, for
which the mean values of DORIS rms fits are reduced (improved) by about
0.2 % over the whole time span (2008–2015) and a larger improvement of
0.3–1.0 % is observed in 2012–2015.
The internal orbit consistency in the radial direction being important for
altimetric applications and being characterized by the satellite position
differences in this direction at 2-day arc overlaps is reduced (improved)
by 7.1 %, 2.4 %, and 5.1 % for TOPEX/Poseidon, Jason-1,
and Jason-2, respectively. The internal orbit consistency in the
cross-track direction improved by 1.1 % for TOPEX/Poseidon, 1.7 % for
Jason-1, and 3.4 % for Jason-2 in the time spans analyzed
when using ITRF2014 instead of ITRF2008. Even more significant improvement of
the internal orbit consistency has been obtained in the along-track
direction: by 22.0 % for TOPEX/Poseidon, 7.9 % for Jason-1,
and 12.4 % for Jason-2.
Single-satellite altimetry crossover analyses indicate a reduction
(improvement) in the absolute mean crossover differences by 0.2 mm
(8.1 %) for TOPEX, 0.4 mm (17.7 %) for Jason-1, and 0.6 mm
(30.9 %) for Jason-2 with ITRF2014 instead of ITRF2008. The
reduction in the mean of crossover differences indicates reduction in the
discrepancies between ascending and descending tracks when using ITRF2014
instead of ITRF2008. The mean values of the rms of crossover differences also show a reduction (improvement) when using ITRF2014 instead of ITRF2008 but
to a lesser extent: by 0.05 mm (0.1 %) for TOPEX, 0.01 mm (0.02 %) for
Jason-1, and 0.13 mm (0.3 %) for Jason-2.
Multi-mission crossover analysis shows slight improvements in the standard
deviations of radial errors through the switch from ITRF2008 to ITRF2014 for
POD: 0.1 % for TOPEX, 0.2 % for Jason-1, and 1.6 % for
Jason-2. The standard deviations of geographically correlated
mean SSH errors improved by 1.1 % for Jason-1 and 5.4 % for
Jason-2 but degraded by 1.3 % for TOPEX.
The change from ITRF2008 to ITRF2014 orbits only has minor effects on the
estimation of regional and global sea level trends over the 22-year time
series from 1993 to 2015. However, on interannual timescales (3–8 years)
large-scale coherent trend patterns are observed that seem to be connected to
drifts between the origins of the tracking station networks. This leads to
changes in the global interannual trends of up to 0.06 mm yr-1 for
TOPEX, 0.05 mm yr-1 for Jason-1, and up to 0.12 mm yr-1
for Jason-2, which corresponds to changes of up to 4 % in the
actual sea level trends from altimetry. The respective changes in the
regional sea level trend reach maximum values of 0.4 mm yr-1 for
TOPEX, of 0.5 mm yr-1 for Jason-1, and of 1.0 mm yr-1 for
Jason-2. While until 2008 regional sea level drifts are mainly related
to uncertainties in the z coordinate of the origin, recently uncertainties in
the horizontal coordinates have become increasingly important. This shows the
effects of the increasing uncertainties in the tracking station positions and
velocities in ITRF2008 after 2009.0 on the estimated regional sea level
trends. For this period the user requirements for the error of the regional
mean sea level cannot be met everywhere when using
ITRF2008 orbits.
Our analyses show that the use of ITRF2014 instead of ITRF2008 slightly
improves the satellite orbits as well as the derived sea level values since
1993. The analyses and statistics for TOPEX/Poseidon show the differences
between the two ITRF realizations until 2005. More evident improvements are
found from 2009.0 for Jason-1 and, in particular, for Jason-2.
This is in agreement with the results obtained by and
. To minimize errors caused by the extrapolation of
station velocities beyond the time span at which they had been derived, ITRS
realizations should be regularly (at least every 5–6 years) updated.
Therefore, it is strongly recommended to use the new ITRS realization for
precise orbit determination for the time span beyond the time instant of the
end of the time interval used for the generation of a previous ITRS
realization. This also stresses a need for the periodical reprocessing of
altimetry satellite orbits using a new ITRS realization to get reliable and
high-accuracy altimetry products.
GFZ VER13 SLCCI orbits of ERS-1, ERS-2,
Envisat, TOPEX/Poseidon, Jason-1, and Jason-2 are available
from the GFZ Data Services at 10.5880/GFZ.1.2.2018.003 (Rudenko et al.,
2018b).
SR initiated this research and wrote
Sects. –. SE wrote Sect. . TS wrote Sect. . DD wrote Sect. . All authors
contributed to Sect. . All authors read and approved the
final manuscript.
The authors declare that they have no conflict of interest.
Acknowledgements
This research was partly supported by the European Space Agency (ESA) within
the Climate Change Initiative Sea Level Phase 2 project, by the German
Research Foundation (DFG) within the DGFI-project “Consistent dynamic
satellite reference frames and terrestrial geodetic datum parameters” of the
DFG Research Unit “Space-Time Reference Systems for Monitoring Global Change
and for Precise Navigation in Space”, through grant CoRSEA as a part of
the Special Priority Program (SPP) 1889 “Regional Sea Level Change and
Society” (SeaLevel), and by the International Office of the BMBF under the
grant 01DO17017 “Sea Level Change and its Hazardous Potential in the East
China Sea and Adjacent Waters” (SEAHAP). The authors are grateful to
Karl Hans Neumayer and Jean-Claude Raimondo for preparing some input data used in
this study. The authors thank the editor and two referees for their comments
that allowed them to improve this paper.
This work was supported by the German Research Foundation (DFG) and the Technical University of Munich (TUM) in the framework of the Open Access Publishing Program.
Edited by: Simon McClusky
Reviewed by: Erricos C. Pavlis and one anonymous referee
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