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

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

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 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

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.

The detailed description of ITRF2008 and ITRF2014 is given in

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.

The number of DOMES numbers and discontinuities and data span for DORIS and SLR stations in the ITRF2008 and ITRF2014.

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

The main models used for orbit determination (for the details and
references for the models, see

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.

The mean values of DORIS rms fits are reduced (improved) for Jason-2
from 0.3490 to 0.3484 mm s

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

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.

Single crossover analyses for all three missions have been performed based on
ESA CCI Sea Level v2 ECV data

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

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

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

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

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

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

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.

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

A measure of the amount by which the radial components of the two orbits differ is the rms value per cycle (Fig.

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 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.

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).

The uncertainties in global mean sea level trends due to the TRF realization
have decreased considerably during the last decades.

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

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

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

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

SR initiated this research and wrote
Sects.

The authors declare that they have no conflict of interest.

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