Absolute Calibration of the TOPEX/POSEIDON Altimeters using UK Tide Gauges,GPS, and Precise,Local Geoid-Differences

This article describes an "absolute" calibration of TOPEX/POSEIDON (T/P) altimeter bias using UK tide gauges equipped with Global Positioning System (GPS) receivers. The method is an extension of earlier work using the Newhaven tide gauge in the English Channel. However, the present analysis extends the research to a number of gauges around the UK and incorporates several improvements. The time-averaged TOPEX and POSEIDON biases are obtained with a precision of approximately 2 and 3 cm, respectively. The research complements work on bias determination by other groups in the T/P Science Working Team and can, in principle, be applied at other locations for which precise, local geoid-differences are available. The relatively sparse POSEIDON data set has been used as a test of our ability to perform an absolute calibration of upcoming Jason-1 altimetry as soon as possible after launch.     

Technical Issues and Recommendations Related to the Installation of Continuous GPS Stations at Tide Gauges

Geodesists around the world have begun installing continuous GPS (CGPS) stations at tide gauges in order to determine the exact position of these tide gauges and, in particular, the vertical velocity of the land or the seafloor underlying each tide gauge. The goal is to make these measurements in a well-defined global reference frame. The scientific applications of these measurements include the calibration of satellite altimeters and the removal of crustal motion signals from long time series of sea level change. In this article we focus on the technical issues associated with this agenda, including site selection, instrumentation, monumentation, ancillary measurements, and the tide gauge leveling program. There is no universally best approach to building CGPS stations at tide gauges. Therefore we emphasize the various trade-offs that typically occur, and give general recommendations and rules of thumb based on recent installations and experience. Additional information can be found at the CGPS@TG website.     

Technical Issues and Recommendations Related to the Installation of Continuous GPS Stations at Tide Gauges

Geodesists around the world have begun installing continuous GPS (CGPS) stations at tide gauges in order to determine the exact position of these tide gauges and, in particular, the vertical velocity of the land or the seafloor underlying each tide gauge. The goal is to make these measurements in a well-defined global reference frame. The scientific applications of these measurements include the calibration of satellite altimeters and the removal of crustal motion signals from long time series of sea level change. In this article we focus on the technical issues associated with this agenda, including site selection, instrumentation, monumentation, ancillary measurements, and the tide gauge leveling program. There is no universally best approach to building CGPS stations at tide gauges. Therefore we emphasize the various trade-offs that typically occur, and give general recommendations and rules of thumb based on recent installations and experience. Additional information can be found at the CGPS@TG website.     

Absolute Calibration of the Jason-1 Altimeter Using UK Tide Gauges

This article describes an “absolute” calibration of Jason-1 (J-1) altimeter sea surface height bias using a method developed for TOPEX/Poseidon (T/P) bias determination reported previously. The method makes use of U.K. tide gauges equipped with Global Positioning System (GPS) receivers to measure sea surface heights at the same time, and in the same geocentric reference frame, as Jason-1 altimetric heights recorded in the nearby ocean. The main time-dependent components of the observed altimeter-minus-gauge height-difference time series are due to the slightly different ocean tides at the gauge and in the ocean. The main harmonic coefficients of the tide differences are calculated from analysis of the copious TOPEX data set and then applied to the determination of T, P, and J-1 bias in turn. Datum connections between the tide gauge and altimetric sea surface heights are made by means of precise, local geoid differences from the EGG97 model. By these means, we have estimated Jason-1 altimeter bias determined from Geophysical Data Record (GDR) data for cycles 1–61 to be 12.9 cm, with an accuracy estimated to be approximately 3 cm on the basis of our earlier work. This J-1 bias value is in close agreement with those determined by other groups, which provides a further confirmation of the validity of our method and of its potential for application in other parts of the world where suitable tide gauge, GPS, and geoid information exist.     

Absolute Calibration of the Jason-1 Altimeter Using UK Tide Gauges

This article describes an “absolute” calibration of Jason-1 (J-1) altimeter sea surface height bias using a method developed for TOPEX/Poseidon (T/P) bias determination reported previously. The method makes use of U.K. tide gauges equipped with Global Positioning System (GPS) receivers to measure sea surface heights at the same time, and in the same geocentric reference frame, as Jason-1 altimetric heights recorded in the nearby ocean. The main time-dependent components of the observed altimeter-minus-gauge height-difference time series are due to the slightly different ocean tides at the gauge and in the ocean. The main harmonic coefficients of the tide differences are calculated from analysis of the copious TOPEX data set and then applied to the determination of T, P, and J-1 bias in turn. Datum connections between the tide gauge and altimetric sea surface heights are made by means of precise, local geoid differences from the EGG97 model. By these means, we have estimated Jason-1 altimeter bias determined from Geophysical Data Record (GDR) data for cycles 1-61 to be 12.9 cm, with an accuracy estimated to be approximately 3 cm on the basis of our earlier work. This J-1 bias value is in close agreement with those determined by other groups, which provides a further confirmation of the validity of our method and of its potential for application in other parts of the world where suitable tide gauge, GPS, and geoid information exist.     

Absolute Calibration of Jason-1 and TOPEX/Poseidon Altimeters in Corsica

The double geodetic Corsica site, which includes Ajaccio-Aspretto and Cape Senetosa (40 km south Ajaccio) in the western Mediterranean area, has been chosen to permit the absolute calibration of radar altimeters. It has been developed since 1998 at Cape Senetosa and, in addition to the use of classical tide gauges, a GPS buoy is deployed every 10 days under the satellites ground track (10 km off shore) since 2000. The 2002 absolute calibration campaign made from January to September in Corsica revealed the necessity of deploying different geodetic techniques on a dedicated site to reach an accuracy level of a few mm: in particular, the French Transportable Laser Ranging System (FTLRS) for accurate orbit determination, and various geodetic equipment as well as a local marine geoid, for monitoring the local sea level and mean sea level. TOPEX/Poseidon altimeter calibration has been performed from cycle 208 to 365 using M-GDR products, whereas Jason-1 altimeter calibration used cycles from 1 to 45 using I-GDR products. For Jason-1, improved estimates of sea-state bias and columnar atmospheric wet path delay as well as the most precise orbits available have been used. The goal of this article is to give synthetic results of the analysis of the different error sources for the tandem phase and for the whole studied period, as geophysical corrections, orbits and reference frame, sea level, and finally altimeter biases. Results are at the millimeter level when considering one year of continuous monitoring; they show a great consistency between both satellites with biases of 6 ± 3 mm (ALT-B) and 120 ± 7 mm, respectively, for TOPEX/Poseidon and Jason-1.     

Absolute Calibration of Jason-1 and TOPEX/Poseidon Altimeters in Corsica Special Issue: Jason-1 Calibration/Validation

The double geodetic Corsica site, which includes Ajaccio-Aspretto and Cape Senetosa (40 km south Ajaccio) in the western Mediterranean area, has been chosen to permit the absolute calibration of radar altimeters. It has been developed since 1998 at Cape Senetosa and, in addition to the use of classical tide gauges, a GPS buoy is deployed every 10 days under the satellites ground track (10 km off shore) since 2000. The 2002 absolute calibration campaign made from January to September in Corsica revealed the necessity of deploying different geodetic techniques on a dedicated site to reach an accuracy level of a few mm: in particular, the French Transportable Laser Ranging System (FTLRS) for accurate orbit determination, and various geodetic equipment as well as a local marine geoid, for monitoring the local sea level and mean sea level. TOPEX/Poseidon altimeter calibration has been performed from cycle 208 to 365 using M-GDR products, whereas Jason-1 altimeter calibration used cycles from 1 to 45 using I-GDR products. For Jason-1, improved estimates of sea-state bias and columnar atmospheric wet path delay as well as the most precise orbits available have been used. The goal of this article is to give synthetic results of the analysis of the different error sources for the tandem phase and for the whole studied period, as geophysical corrections, orbits and reference frame, sea level, and finally altimeter biases. Results are at the millimeter level when considering one year of continuous monitoring; they show a great consistency between both satellites with biases of 6 ± 3 mm (ALT-B) and 120 ± 7 mm, respectively, for TOPEX/Poseidon and Jason-1.     

SARAL/AltiKa Absolute Calibration from the Multi-Mission Corsica Facilities

The geodetic Corsica site was set up in 1998 in order to perform altimeter calibration of the TOPEX/Poseidon (T/P) mission and subsequently, Jason-1 and OSTM/Jason-2. The scope of the site was widened in 2005 in order to undertake the calibration of the Envisat mission and most recently of SARAL/AltiKa. Here we present the first results from the latter mission using both indirect and direct calibration/validation approaches. The indirect approach utilizes a coastal tide gauge and, as a consequence, the altimeter derived sea surface height (SSH) needs to be corrected for the geoid slope. The direct approach utilizes a novel GPS-based system deployed offshore under the satellite ground track that permits a direct comparison with the altimeter derived SSH. The advantages and disadvantages of both systems (GPS-based and tide gauges) and methods (direct or indirect) will be described and discussed. Our results for O/IGD-R data show a very good consistency for these three kinds of products: their derived absolute SSH biases are consistent within 17 mm and their associated standard deviation ranges from 31 to 35 mm. The AltiKa absolute SSH bias derived from GPS-zodiac measurement using the direct method is ?54 ±10 mm based on the first 13 cycles.     

Absolute Calibration of TOPEX/Poseidon and Jason-1 Using GPS Buoys in Bass Strait, Australia

An absolute calibration of the TOPEX/Poseidon (T/P) and Jason-1 altimeters has been undertaken during the dedicated calibration phase of the Jason-1 mission, in Bass Strait, Australia. The present study incorporates several improvements to the earlier calibration methodology used for Bass Strait, namely the use of GPS buoys and the determination of absolute bias in a purely geometrical sense, without the necessity of estimating a marine geoid. This article focuses on technical issues surrounding the GPS buoy methodology for use in altimeter calibration studies. We present absolute bias estimates computed solely from the GPS buoy deployments and derive formal uncertainty estimates for bias calculation from a single overflight at the 40-45 mm level. Estimates of the absolute bias derived from the GPS buoys is -10 ± 19 mm for T/P and +147 ± 21 mm for Jason-1 (MOE orbit) and +131 ± 21 mm for Jason-1 (GPS orbit). Considering the estimated error budget, our bias values are equivalent to other determinations from the dedicated NASA and CNES calibration sites.     

Modeling Local Tidal Constituents Using TOPEX/POSEIDON SSH for the Malaysian Marine Region

A new Local Ocean Tide Model, has been produced for the Exclusive Economic Zone (EEZ) of Malaysia, which incorporates some of the latest TOPEX/POSEIDON data for the years 1992 to 1998. Local tide gauge data are used as a comparison, along with another leading Global Ocean Tide Model, Ori96. The leading diurnal and semidiurnal constituents M₂, S₂, N₂, K₁, O₁, P₁ and Q₁ are reproduced using TOPEX/POSEIDON Sea Surface Heights (SSH) in a response analysis type least squares derivation following Munk and Cartwright (1966).     

TOPEX/Poseidon and Jason-1: Absolute Calibration in Bass Strait, Australia

Updated absolute calibration results from Bass Strait, Australia, are presented for the TOPEX/Poseidon (T/P) and Jason-1 altimeter missions. Data from an oceanographic mooring array and coastal tide gauge have been used in addition to the previously described episodic GPS buoy deployments. The results represent a significant improvement in absolute bias estimates for the Bass Strait site. The extended methodology has allowed comparison between the altimeter and in situ data on a cycle-by-cycle basis over the duration of the dedicated calibration phase (formation flight period) of the Jason-1 mission. In addition, it has allowed absolute bias results to be extended to include all cycles since the T/P launch, and all Jason-1 data up to cycle 60. Updated estimates and formal 1-sigma uncertainties of the absolute bias computed throughout the formation flight period are 0 ± 14 mm for T/P and +152 + 13 mm for Jason-1 (for the GDR POE orbits). When JPL GPS orbits are used for cycles 1 to 60, the Jason-1 bias estimate is 131 mm, virtually identical to the NASA estimate from the Harvest Platform off California calculated with the GPS orbits and not significantly different to the CNES estimate from Corsica. The inference of geographically correlated errors in the GDR POE orbits (estimated to be approximately 17 mm at Bass Strait) highlights the importance of maintaining globally distributed verification sites and makes it clear that further work is required to improve our understanding of the Jason-1 instrument and algorithm behavior.     

Calibration/Validation of an Altimeter Wave Period Model and Application to TOPEX/Poseidon and Jason-1 Altimeters

The altimeter radar backscatter cross-section is known to be related to the ocean surface wave mean square slope statistics, linked to the mean surface acceleration variance according to the surface wave dispersion relationship. Since altimeter measurements also provide significant wave height estimates, the precedent reasoning was used to derive empirical altimeter wave period models by combining both significant wave height and radar backscatter cross-section measurements. This article follows such attempts to propose new algorithms to derive an altimeter mean wave period parameter using neural networks method. Two versions depending on the required inputs are presented. The first one makes use of Ku-band measurements only as done in previous studies, and the second one exploits the dual-frequency capability of modern altimeters to better account for local environmental conditions. Comparison with in situ measurements show high correlations which give confidence in the derived altimeter wave period parameter. It is further shown that improved mean wave characteristics can be obtained at global and local scales by using an objective interpolation scheme to handle relatively coarse altimeter sampling and that TOPEX/Poseidon and Jason-1 altimeters can be merged to provide altimeter mean wave period fields with a better resolution. Finally, altimeter mean wave period estimates are compared with the WaveWatch-III numerical wave model to illustrate their usefulness for wave models tuning and validation.     

Calibration/Validation of an Altimeter Wave Period Model and Application to TOPEX/Poseidon and Jason-1 Altimeters

The altimeter radar backscatter cross-section is known to be related to the ocean surface wave mean square slope statistics, linked to the mean surface acceleration variance according to the surface wave dispersion relationship. Since altimeter measurements also provide significant wave height estimates, the precedent reasoning was used to derive empirical altimeter wave period models by combining both significant wave height and radar backscatter cross-section measurements. This article follows such attempts to propose new algorithms to derive an altimeter mean wave period parameter using neural networks method. Two versions depending on the required inputs are presented. The first one makes use of Ku-band measurements only as done in previous studies, and the second one exploits the dual-frequency capability of modern altimeters to better account for local environmental conditions. Comparison with in situ measurements show high correlations which give confidence in the derived altimeter wave period parameter. It is further shown that improved mean wave characteristics can be obtained at global and local scales by using an objective interpolation scheme to handle relatively coarse altimeter sampling and that TOPEX/Poseidon and Jason-1 altimeters can be merged to provide altimeter mean wave period fields with a better resolution. Finally, altimeter mean wave period estimates are compared with the WaveWatch-III numerical wave model to illustrate their usefulness for wave models tuning and validation.     

Detecting fluctuations of the Kuroshio axis south of Japan using TOPEX/POSEIDON altimeter data

Sea surface dynamic topography (SSDT) can be divided into temporal mean SSDT and fluctuation SSDT. The former is approximated with a climatological mean SSDT and the latter is derived from satellite altimetry data, to give an approximated total SSDT (called a composite SSDT). The method is applied to detecting fluctuations of the Kuroshio axis south of Japan using TOPEX/POSEIDON altimeter data from the first year mission in 1992–1993. The fluctuation SSDT averaged over a wide area south of Japan clearly shows an annual cycle with an amplitude of about 15 cm. Temporal changes of SSDT along a subsatellite track crossing the Kuroshio compare moderately well with those estimated from repeated hydrographic observations, although there is a discrepancy of unknown origin. The composite SSDT also compares well with SSDT estimated from the same hydrographic data. Horizontal distribution of the surface geostrophic velocity component normal to subsatellite tracks is derived every ten days from the composite SSDT. Most locations of estimated strong eastward geostrophic velocities coincide well with locations of the Kuroshio axis determined every 15 days fromin situ surface velocity measurements on various vessels; for example, a fairly large meander of the Kuroshio south of Honshu is clearly detected. It is concluded that the composite SSDT can be used reliably to detect fluctuations of the Kuroshio axis south of Japan.     

Accuracy Assessment of Jason-1 and TOPEX/POSEIDON Along-Track Sea Surface Slope Special Issue: Jason-1 Calibration/Validation

Sea surface slope computed from along-track Jason-1 and TOPEX/POSEIDON (T/P) altimeter data at ocean mesoscale wavelengths are compared to determine the equivalent 1 Hz instrument height noise of the Poseidon-2 and TOPEX altimeters. This geophysical evaluation shows that the Ku-band 1-Hz range noise for both instruments is better than 1.7 cm at 2 m significant wave heights (H_1/3), exceeding error budget requirements for both missions. Furthermore, we show that the quality of these instruments allows optimal filtering of the 1-Hz along-track sea surface height data for sea surface slopes that can be used to calculate cross track geostrophic velocity anomalies at the baroclinic Rossby radius of deformation to better than 5 cm/sec precision along 87.5% of the satellite ground track between 2 and 60 degrees absolute latitude over the deep abyssal ocean (depths greater than 1000 m). This level of precision will facilitate scientific studies of surface geostrophic velocity variability using data from the Jason-1 and T/P Tandem Mission.     

Accuracy Assessment of Jason-1 and TOPEX/POSEIDON Along-Track Sea Surface Slope

Sea surface slope computed from along-track Jason-1 and TOPEX/POSEIDON (T/P) altimeter data at ocean mesoscale wavelengths are compared to determine the equivalent 1 Hz instrument height noise of the Poseidon-2 and TOPEX altimeters. This geophysical evaluation shows that the Ku-band 1-Hz range noise for both instruments is better than 1.7 cm at 2 m significant wave heights (H_1/3), exceeding error budget requirements for both missions. Furthermore, we show that the quality of these instruments allows optimal filtering of the 1-Hz along-track sea surface height data for sea surface slopes that can be used to calculate cross track geostrophic velocity anomalies at the baroclinic Rossby radius of deformation to better than 5 cm/sec precision along 87.5% of the satellite ground track between 2 and 60 degrees absolute latitude over the deep abyssal ocean (depths greater than 1000 m). This level of precision will facilitate scientific studies of surface geostrophic velocity variability using data from the Jason-1 and T/P Tandem Mission.     

A GPS and Cold Ocean Brightness Temperature Calibration of the ERS-2 and TOPEX/Poseidon Microwave Radiometers

Sea-level change studies from altimetric satellites are reliant on range stability of the sea surface heights computed from orbital positioning and geophysically corrected data. One such correction, namely the wet tropospheric delay induced by the highly variable atmospheric water vapor content, is provided by radiometers onboard ERS-2 and TOPEX/Poseidon (T/P). In this study the long-term stability of the ERS-2 microwave radiometer (E2MR) and the T/P microwave radiometer (TMR) are investigated with the observed drift in the brightness temperatures approximated by reference to the coldest temperatures over the oceans. The E2MR stability is characterized by a gain anomaly fall in 1996 and a drift in the 23.8 GHz channel. For the TMR, investigations show that the dominant drift is about 0.2 K/year in the 18 GHz channel over the first 7-8 years but stabilizing and even decreasing slightly thereafter. In contrast, the 21 GHz and 37 GHz channels are comparatively stable. Utilizing correction formulae a modified wet tropospheric range is inferred from “small-change” analysis of the radiometric correction given on the altimetric Geophysical Data Records. The accuracy of this formulism is validated by independent comparison against GPS derived wet tropospheric delays inferred at 14 coastal IGS stations with near continuous data from September 1992 through to the present day. Comparisons between GPS results for ERS-2 and T/P show that the E2MR path delay is 14 mm short. For T/P, the spatial distribution of the wet tropospheric enhancement is further investigated to show that the nonuniformity can equate to a deviation in sea-level height change of about 0.1 mm/year compared with global average sea-level change. Finally, the altimetric range stability of T/P is revisited by comparison against time series from the global network of tide gauges. Analysis shows that the validated TMR drift correction results in a residual trend of -0.27 ± 0.11 mm/yr which is not significant at the 3σ level.     

Using Kolmogorov-Smirnov Statistics to Compare Geostrophic Velocities Measured by the Jason, TOPEX, and Poseidon Altimeters

The Kolmogorov-Smirnov (K-S) test is used to compare probability density functions (PDFs) of geostrophic velocities measured by the TOPEX, Poseidon, and Jason altimeters. Velocity PDFs are computed in 2.5° by 2.5° boxes for regions equatorward of 60° latitude. Although velocities measured by the TOPEX and Jason altimeters can differ, on the basis of the K-S test the velocities are statistically equivalent during the ∼200 day period when the satellites followed the same orbit. Full records from TOPEX, Poseidon, and Jason show less agreement, which can be attributed to temporal variability in ocean surface velocities and differing levels of measurement noise.     

Comparison of the Ku-Band Range Noise Level and the Relative Sea-State Bias of the Jason-1, TOPEX,and Poseidon-1 Radar Altimeters Special Issue: Jason-1 Calibration/Validation

TOPEX/Poseidon is a well known success, with the operational altimeter (TOPEX) and the experimental one (Poseidon-1), providing data of unprecedented quality. However, there are two major differences between the TOPEX and Poseidon-1 radar altimeters on board TOPEX/Poseidon. The first is related to the estimated range noise; the second is linked to the sea-state bias (SSB) model estimates. Since the recent launch of the Jason-1 radar altimeter (also called Poseidon-2), we have been cross-comparing these three systems to better characterize each of them. Analyzing standard user products, we have found that Jason-1 is behaving like Poseidon-1 and thus shows the same observed differences when compared with TOPEX. A comparative analysis of their features was performed, starting from the on-board acquisition of the ocean return and ending with the ground generation of the high level accuracy oceanographic product. The results lead us to believe that the sources for these differences lie in both the waveform tracking processing and the presence or abscence of a retracking procedure whether on-board or on ground. Because Poseidon-1 and Jason-1 waveforms are retracked while TOPEX waveforms are not in the products distributed to the users, we have applied the same ground retracking algorithm to the waveforms of the three radar altimeters to get consistent data sets. The analysis of the outputs has shown that: (a) the noise level for the three radar altimeters is definitively the same, and (b) the source of the relative SSB between Jason-1 and TOPEX lies in the different behavior of the on-board tracking softwares.     

Assimilation of TOPEX/POSEIDON Altimeter Data with a Reduced Gravity Model of the Japan Sea

Sea level data measured by TOPEX/POSEIDON over the Japan Sea from 1993 to 1994 is analyzed by assimilation using an approximate Kalman filter with a 1.5 layer (reduced-gravity) shallow water model. The study aims to extract signals associated with the first baroclinic mode and to determine the extent of its significance. The assimilation dramatically improves the model south of the Polar Front where as much as 20 cm² of the observed sea level variance can be accounted for. In comparison, little variability in the northern cold water region is found consistent with the model dynamics, possibly due to significant differences in stratification.