Elsevier

Advances in Space Research

Volume 51, Issue 8, 15 April 2013, Pages 1284-1300
Advances in Space Research

The challenges in long-term altimetry calibration for addressing the problem of global sea level change

https://doi.org/10.1016/j.asr.2012.06.005Get rights and content

Abstract

Long-term change of the global sea level resulting from climate change has become an issue of great societal interest. The advent of the technology of satellite altimetry has modernized the study of sea level on both global and regional scales. In combination with in situ observations of the ocean density and space observations of Earth’s gravity variations, satellite altimetry has become an essential component of a global observing system for monitoring and understanding sea level change. The challenge of making sea level measurements with sufficient accuracy to discern long-term trends and allow the patterns of natural variability to be distinguished from those linked to anthropogenic forcing rests largely on the long-term efforts of altimeter calibration and validation. The issues of long-term calibration for the various components of the altimeter measurement system are reviewed in the paper. The topics include radar altimetry, the effects of tropospheric water vapor, orbit determination, gravity field, tide gauges, and the terrestrial reference frame. The necessity for maintaining a complete calibration effort and the challenges of sustaining it into the future are discussed.

Introduction

The concept of satellite altimetry, using a radar altimeter onboard an artificial satellite orbiting the Earth to measure the height of the sea surface for geodetic and oceanographic studies, was quickly developed after the launch of artificial satellites in the late 1950s. The technology for precision ranging and orbit determination was first demonstrated by the GEOS-3 Mission in the mid 1970s (Stanley, 1979) and drastically improved by the Seasat Mission in the late 1970s (Born et al., 1979). Despite its short life of just over 100 days, Seasat established the foundation of modern satellite altimetry. With 10-cm accuracy in radar ranging and 1-m accuracy in orbit determination, a wealth of discoveries was made in geodesy and oceanography, shedding light on the potential of satellite altimetry for long-term monitoring and studying Earth.

After the demise of Seasat, the world oceanographic community began planning for an ambitious campaign to observe and understand the global ocean circulation using evolving observational and modeling tools. The campaign was dubbed the World Ocean Circulation Experiment (WOCE). WOCE was motivated by the promise of satellite observations demonstrated by Seasat. Satellite altimetry was perceived as the means for determining the upper boundary condition of the global ocean general circulation. However, the challenge was to make an order-of-magnitude improvement in the measurement accuracy to meet the stringent requirement for making fundamental advances in the knowledge of ocean circulation.

The concept of an optimally designed altimetry mission for ocean circulation was then conceived and called TOPEX (The Ocean Topography Experiment) by a group of oceanographers commissioned by NASA for a mission study in 1980. An independent study of an advanced altimetry mission, called Poseidon, was also underway in Europe led by CNES. TOPEX/Poseidon became a joint mission of NASA and CNES in 1983. The challenge for the mission presented itself on two fronts: the measurement of the range between the altimeter and the sea surface, and the determination of the location of the satellite in orbit. Both fronts demanded significant improvement over the performance of Seasat and Geosat, the latter of which provided observations of quality similar to Seasat in the late 1980s (Douglas and Cheney, 1990).

Among many improvements in the design of the TOPEX/Poseidon radar altimeter, the key was the addition of a second (C-band) channel to complement the primary (Ku-band) one. The two frequencies allow determination of the total electron content along the radar path for correcting the range delay from the ionosphere. This dual-frequency design set the standard for future precision altimetry missions. The approach to the correction for the tropospheric range delay due to water vapor was the same as Seasat, which used a 3-frequency microwave radiometer. The accuracy of the range measurement of TOPEX/Poseidon was about 3.2 cm (Fu et al., 1994), about three-fold improvement over Seasat. Another key development was the French contribution of an experimental solid-state altimeter, called Poseidon, which became the foundation for more robust altimetry missions in the future.

The more difficult challenge for TOPEX/Poseidon was precision orbit determination (POD). The accuracy of Seasat POD was on the order of 1 m. The range of the ocean dynamic topography associated with the ocean general circulation is only 2 m, with a wide range of spatial and temporal variability of a few cm in amplitude but important for understanding ocean circulation in relation to climate change – the primary goal of WOCE. The charge for the mission development was to make at least an order of magnitude improvement in POD accuracy to a level close to 10 cm. A three-tiered approach was adopted to address the challenge. In addition to the traditional laser ranging, the newly developed technique by the French, called DORIS (Doppler Orbit determination and Radio-positioning Integrated on Satellite), was added to the payload for measuring the velocity of the satellite using the Doppler technique. The reader is referred to a special issue of Advances in Space Research devoted to DORIS: Volume 45, Issue 12, 15 June 2010. Secondly, a long-lead effort of improving the Earth’s gravity field was established, involving multiple international teams of geodesists working collaboratively. Lastly, a revolutionary approach using the newly available GPS satellite system was developed as an experiment for the mission. Remarkably, these efforts delivered a POD accuracy of 3–4 cm for the mission (Tapley et al., 1994, Nöuel et al., 1994, Bertiger et al., 1994), about two orders of magnitude better than the GEOS-3 performance.

The success of TOPEX/Poseidon revolutionized the way the oceans are studied. Never before had the expanse of the global oceans been sampled uniformly in both space and time with a measurement accuracy that could reveal the signals of ocean circulation on a wide range of scales. The exceptional performance of TOPEX/Poseidon has also been exploited to enhance the accuracy of other altimetry missions that are not specifically designed for ocean circulation studies. The data from the altimeters onboard the ERS-1, ERS-2, and Envisat series of missions have been reprocessed through cross-calibration and merging with those from TOPEX/Poseidon, leading to a data set of higher quality with enhanced spatial and temporal resolution (Ducet et al., 2000). It is beyond the scope of the paper to review the enormous progress made from satellite altimetry. The reader is referred to Fu and Cazenave, 2001, Fu et al., 2010 for comprehensive reviews.

As the accuracy of satellite altimetry measurements of sea level reached centimetric level, a new observational tool for the problem of global sea level change was established. Global sea level rise is among the most unambiguous consequences of climate change and it has become a priority subject for both research and societal applications using altimetry observations (Church et al., 2011, Cazenave et al., 2009). A major thrust in the future is exploitation of the unique role of satellite altimetry in monitoring the change of global sea level and detecting trends that may impact the world’s coastal zones. This thrust is posing the next challenge in satellite altimetry: to ensure long-term consistency of measurements from successive multiple missions (Nerem et al., 2010) to detect early signals of potentially larger long-term sea-level changes and associated geographic patterns (Willis and Church, 2012). This paper provides a brief overview of the contributions of altimetry to the sea level problem and issues facing long-term calibration. The problem of global sea level change is discussed in the next section, followed by an overview of altimetry calibration. Specific discussions on the various components of the altimetry measurement system are given in the ensuing sections including radiometer, altimeter, orbit determination, gravity, tide gauges, and terrestrial reference frame. A summary and remarks on future prospects are provided at the end.

Section snippets

The problem of global sea level change

The idea of measuring the global mean sea level in relation to climate change using satellite altimetry was first discussed by Born et al. (1986). Using only 24 days of Seasat altimeter data, they found the global mean sea level varied over a range of ±7 cm, which was obviously dominated by the time-varying component of the 1-m orbit error. However, they anticipated an accuracy of 1–2 cm for global mean sea level measurements from the TOPEX/Poseidon mission. Given the exceptional performance of

Calibration

Despite the exceptional accuracies supported by current altimetric missions, the reliable identification of sea-level signals linked to long-term climate changes remains a daunting challenge. An essential prerequisite to tackling any questions of physical attribution (natural or anthropogenic? changes in mass or heat content?) is the basic validation of the signals themselves: Are they real? And to what extent do they represent errors in the observations?

From the vantage point of Earth orbit,

Radiometer

A three-frequency microwave radiometer has been an essential component of each of the three reference missions (T/P, Jason-1 and -2) used in producing the characteristic global sea-level curve from altimetry. The measurements taken by these passive systems are used to correct the altimeter ranges for path delays from water vapor and cloud liquid water in the troposphere. These systems are well suited for monitoring the highly variable water-vapor signal at a variety of scales, but ensuring

Altimeter

The altimeter is the primary observation system, and provides measurements of significant wave height, and wind speed as well as range to the sea surface. These active microwave systems can be expected to degrade with age and cumulative radiation exposure. Aging of the altimeter will generally manifest as a loss of signal strength in proportion to the signal (e.g., Quartly, 2000). As was the case with T/P, the Jason altimeters undergo regular on-orbit internal calibrations to monitor

Orbit errors, gravity and the terrestrial reference frame

The reference system for altimetric sea-level measurements is provided by precise orbit determination (POD). In the POD process, accurate and diverse tracking measurements (e.g., from GPS, DORIS and SLR) are used to constrain the orbit computation based on high-fidelity models of the many forces underlying the satellites’ motions (Lemoine et al., 2010). The terrestrial reference frame realized in this process is dictated mainly by: (1) the dynamical constraints imparted on the computed orbit,

Tide gauges and the terrestrial reference frame

Tide gauge calibration has become an essential ingredient for developing the long-term sea-level record from altimetry. One of the limiting error sources in this technique, whether it is performed at dedicated calibration sites or using the global network is the uncertainty in measuring vertical land motion.

Tide gauges measure the local sea level relative to a nearby geodetic marker fixed to the Earth’s crust. They cannot distinguish whether rising [falling] sea levels are due to land

Summary and future prospects

A recent assessment of future challenges for measuring sea level change can be found in Cazenave et al. (2009), including recommendations on satellite altimetry and calibration. We provide herein an updated summary of the status of research on the problem and our views on the key issues in future development.

The canonical time series of global sea level change from altimetry is now approaching 20 yr in duration. The consensus estimates for the global trend of sea level rise over this period

Acknowledgements

The research presented in the paper was carried out at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under contract with the National Aeronautic and Space Administration. Support from the Jason-1 and OSTM/Jason-2 Projects is acknowledged. We are grateful for input from three anonymous reviewers, as well as Xiaoping Wu, Shannon Brown and Shailen Desai at JPL.

© 2012 California Institute of Technology. Government sponsorship acknowledged.

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