Calibration and Verification of Jason-1 Using Global Along-Track Residuals with TOPEX

It is demonstrated that the Jason-1 measurements of sea surface height (SSH), wet path delay, and ionosphere path delay are within required accuracies, via a global cross-calibration with similar measurements made by TOPEX/Poseidon (T/P) over a 6-month period. Since the two satellites were on the same groundtrack separated in time by only 70 s, measurements were recorded at approximately the same location and time. The variations in the wet path delay measured by Jason-1 compared to T/P are only 5 mm RMS, well within the required performance of 1.2 cm RMS. The RMS of the ionosphere differences is also well within the expected values, with a mean RMS of 1.2 cm. The largest difference is that the Jason-1 SSH is biased high relative to T/P SSH by 144 mm after the T/P and Jason-1 data are both corrected with improved sea state bias (SSB) models. However, the bias will change if a different SSB model is used, so the user should be cautious that the bias used matches the SSB models. The bias is generally constant within ± 10 mm in the open ocean, but appears to be higher or lower in some regions. Additionally, the SSH has been verified by comparison with 36 island tide gauges over the same period. After removing the global relative bias, the Jason-1 SSH data agree with tide gauges within 3.7 cm RMS and with T/P data within about 3.5 cm RMS on average for 1-s measurements, meeting the required accuracy of 4.2 cm RMS.     

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 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.     

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.     

The Harvest Experiment: Monitoring Jason-1 and TOPEX/POSEIDON from a California Offshore Platform Special Issue: Jason-1 Calibration/Validation

We present calibration results from Jason-1 (2001–) and TOPEX/POSEIDON (1992–) overflights of a California offshore oil platform (Harvest). Data from Harvest indicate that current Jason-1 sea-surface height (SSH) measurements are high by 138 ± 18 mm. Excepting the bias, the high accuracy of the Jason-1 measurements is in evidence from the overflights. In orbit for over 10 years, the T/P measurement system is well calibrated, and the SSH bias is statistically indistinguishable from zero. Also reviewed are over 10 years of geodetic results from the Harvest experiment.     

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.     

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.     

The Harvest Experiment: Monitoring Jason-1 and TOPEX/POSEIDON from a California Offshore Platform

We present calibration results from Jason-1 (2001-) and TOPEX/POSEIDON (1992-) overflights of a California offshore oil platform (Harvest). Data from Harvest indicate that current Jason-1 sea-surface height (SSH) measurements are high by 138 ± 18 mm. Excepting the bias, the high accuracy of the Jason-1 measurements is in evidence from the overflights. In orbit for over 10 years, the T/P measurement system is well calibrated, and the SSH bias is statistically indistinguishable from zero. Also reviewed are over 10 years of geodetic results from the Harvest experiment.     

Leveling the Sea Surface Using a GPS-Catamaran Special Issue: Jason-1 Calibration/Validation

In the framework of the TOPEX/Poseidon and Jason-1 CNES-NASA missions, two probative experiments have been conducted at the Corsica absolute calibration site in order to determine the local marine geoid slope under the ascending TOPEX/Poseidon and Jason-1 ground track (No. 85). An improved determination of the geoid slope was needed to better extrapolate the offshore (open-ocean) altimetric data to on-shore tide-gauge locations. This in turn improves the overall precision of the calibration process. The first experiment, in 1998, used GPS buoys. Because the time required to cover the extended area with GPS buoys was thought to be prohibitive, we decided to build a catamaran with two GPS systems onboard. Tracked by a boat at a constant speed, this innovative system permitted us to cover an area of about 20 km long and 5.4 km wide centered on the satellites' ground track. Results from an experiment in 1999 show very good consistency between GPS receivers: filtered sea-surface height differences have a mean bias of ?0.2 cm and a standard deviation of 1.2 cm. No systematic error or distortions have been observed and crossover differences have a mean value of 0.2 cm with a standard deviation of 2.7 cm. Comparisons with tide gauges data show a bias of 1.9 cm with a standard deviation of less than 0.5 cm. However, this bias, attributable in large part to the effect of the catamaran speed on the waterline, does not affect the geoid slope determination which is used in the altimeter calibration process. The GPS-deduced geoid slope was then incorporated in the altimeter calibration process, yielding a significant improvement (from 4.9 to 3.3 cm RMS) in the agreement of altimeter bias determinations from repeated overflight measurements.     

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.     

Validation of Jason-1 Nadir Ionosphere TEC Using GEONET

The Jason-1 dual-frequency nadir ionosphere Total Electron Content (TEC) for 10-day cycles 1–67 is validated using absolute TEC measured by Japan's GPS Earth Observation Network (GEONET), or the GEONET Regional Ionosphere Map (RIM). The bias estimates (Jason–RIM) are small and statistically insignificant: 1.62 ± 9 TECu (TEC unit or 10¹⁶ electrons/m², 1 TECu = 2.2 mm delay at Ku-band) and 0.73 ± 0.05 TECu, using the along-track difference and Gaussian distribution method, respectively. The bias estimates are –3.05 ± 10.44 TECu during daytime passes, and 0.02 ± 8.05 TECu during nighttime passes, respectively. When global Jason-1 TEC is compared with the Global Ionosphere Map (GIM) from the Center for Orbit Determination in Europe (or CODE) TEC, the bias (Jason–GIM) estimate is 0.68 ± 1.00 TECu, indicating Jason-1 ionosphere delay at Ku-band is longer than GIM by 3.1 mm, which is at present statistically insignificant. Significant zonal distributions of biases are found when the differences are projected into a sun-fixed geomagnetic reference frame. The observed biases range from –7 TECu (GIM larger by 15.4 mm) in the equatorial region, to +2 TECu in the Arctic region, and to +7 TECu in the Antarctica region, indicating significant geographical variations. This phenomena is primarily attributed to the uneven and poorly distributed global GPS stations particularly over ocean and near polar regions. Finally, when the Jason-1 and TOPEX/Poseidon (T/P) TECs were compared during Jason-1 cycles 1–67 (where cycles 1–21 represent the formation flight with T/P, cycles 22–67 represent the interleave orbits), the estimated bias is 1.42 ± 0.04 TECu. It is concluded that the offset between Jason/TOPEX and GPS (RIM or GIM) TECs is < 4 mm at Ku-band, which at present is negligible.     

Calibration of JASON-1 Altimeter over Lake Erie Special Issue: Jason-1 Calibration/Validation

This article describes absolute calibration results for both JASON-1 and TOPEX Side B (TSB) altimeters obtained at the Lake Erie calibration site, Marblehead, Ohio, USA. Using 15 overflights, the estimated JASON altimeter bias at Marblehead is 58 ± 38 mm, with an uncertainty of 19 mm based on detailed error analysis. Assuming that the TSB bias is negligible, relative bias estimates using both data from the TSB-JASON formation flight period and data from 48 water level gauges around the entire Great Lakes confirmed the Marblehead results. Global analyses using both the formation flight data and dual-satellite (TSB and JASON) crossovers yield a similar relative bias estimate of 146 ± 59 mm, which agrees well with open ocean absolute calibration results obtained at Harvest, Corsica, and Bass Strait (e.g., Watson et al. 2003). We find that there is a strong dependence of bias estimates on the choice of sea state bias (SSB) models. Results indicate that the invariant JASON instrument bias estimated oceanwide is 71 mm, with additional biases of 76 mm or 28 mm contributed by the choice of Collecte Localisation Satellites (CLS) SSB or Center for Space Research (CSR) SSB model, respectively. Similar analysis in the Great Lakes yields the invariant JASON instrument bias at 19 mm, with the SSB contributed biases at 58 mm or 13 mm, respectively. The reason for the discrepancy is currently unknown and warrants further investigation. Finally, comparison of the TOPEX/POSEIDON mission (1992–2002) data with the Great Lakes water level gauge measurements yields a negligible TOPEX altimeter drift of 0.1 mm/yr.     

The Jason-1 Mission Special Issue: Jason-1 Calibration/Validation

On December 7, 2001, the Jason-1 satellite was successfully launched by a Boeing Delta II rocket from the Vandenberg site in California, USA. Its main mission was to maintain the high accuracy altimeter measurements, provided since 1992 by TOPEX/Poseidon (T/P), ensuring continuity in observing and monitoring the ocean for intraseasonal to interannual changes, mean sea level, tides, and so forth. Despite four times less mass and power, the Jason-1 system has been designed to have the same performances as T/P, measuring sea surface topography at the centimeter level. This new Centre National d'Etudes Spatiales/National Aeronautics and Space Administration (CNES/NASA) mission also provides near real-time data for sea state and ocean forecast. The first 10 months of the Jason mission were dedicated to the verification of the system performance and cross-calibration with T/P measurements. A complete CALVAL plan was conducted by the Science and Project Teams of the mission based on in situ and regional experiments, global statistical approaches, and multisatellite comparisons, taking advantage of the T/P-Jason overlap during the first months of the mission. CALVAL and first science results showed that the Jason-1 performances were compliant with prelaunch specifications. This was a needed preamble before starting the routine phase of the mission in July 2003 with generation and distribution of validated geophysical data records to the whole user community.     

Monitoring Measurements from the Jason-1 Microwave Radiometer and Independent Validation with GPS

The Jason-1 Microwave Radiometer (JMR) provides measurements of the wet troposphere content to correct the altimetric range measurement for the associated path delay. Various techniques are used to monitor the JMR wet troposphere path delays, with measurements of zenith troposphere content from terrestrial GPS sites used as an independent verification technique. Results indicate that an unexpected offset of approximately +4.1 ± 1.2 mm (drier) emerged in the JMR measurements of wet path delay between cycles 28–32 of the Jason-1 mission, and that the measurements may be drifting at a rate of approximately ?0.5 mm/year. These anomalies are shown to be caused by a ?0.7 K offset in 23.8 GHz brightness temperatures between cycles 28–32, and a 0.16 ± 0.04 and ?0.45 ± 0.08 K/year drift in the 18.7 and 34.0 GHz brightness temperatures, respectively. Intercomparison of the 3-Hz JMR brightness temperature measurements show that they have been drifting with respect to each other, and that a dependence on yaw-steering regime is present in these measurements. An offset of 0.5 m/s between cycles 28–32 and a drift of approximately 0.5 m/s/year in the JMR wind speed measurements is also associated with these anomalies in the 1-Hz brightness temperatures. These errors in JMR wind speeds presently have a negligible impact on the retrieved JMR path delays.     

Evidence That Sea State Bias is Different for Ascending and Descending Tracks

Jason-1 and TOPEX/Poseidon (T/P) measured sea-surface heights (SSHs) are compared for five regions during the verification tandem phase. The five regions are of similar latitude and spatial extent and include the Gulf of Mexico, Arabian Sea, Bay of Bengal, and locations in the Pacific and Atlantic Oceans away from land. In all five regions, a bias, defined as Jason SSH—TOPEX-B SSH, exists that is different for ascending and descending tracks. For example, in the Gulf of Mexico the bias for ascending tracks was ?0.13 cm and the bias for descending tracks was 2.19 cm. In the Arabian Sea the bias for ascending tracks was ?2.45 cm and the bias for descending tracks was ?1.31 cm. The bias was found to depend on track orientation and significant wave height (SWH), indicating an error in the sea state bias (SSB) model for one or both altimeters. The bias in all five regions can be significantly reduced by calculating separate corrections for ascending and descending tracks in each region as a function of SWH. The correction is calculated by fitting a second-order polynomial to the bias as a function of SWH separately for ascending and descending tracks. An additional constraint is required to properly apply the correction, and we chose to minimize the sum of the TOPEX-B and Jason-1 root-mean-square (rms) crossover differences to be consistent with present SSB models. Application of this constraint shows that the correction, though consistent within each region, is different for each region and that each satellite contributes to the bias. One potential source that may account for a portion of the difference in bias is the leakage in the wave forms in TOPEX-B due to differing altitude rates for ascending and descending tracks. Global SSB models could be improved by separating the tracks into ascenders and descenders and calculating a separate SSB model for each track.     

Monitoring Measurements from the Jason-1 Microwave Radiometer and Independent Validation with GPS

The Jason-1 Microwave Radiometer (JMR) provides measurements of the wet troposphere content to correct the altimetric range measurement for the associated path delay. Various techniques are used to monitor the JMR wet troposphere path delays, with measurements of zenith troposphere content from terrestrial GPS sites used as an independent verification technique. Results indicate that an unexpected offset of approximately +4.1 ± 1.2 mm (drier) emerged in the JMR measurements of wet path delay between cycles 28-32 of the Jason-1 mission, and that the measurements may be drifting at a rate of approximately -0.5 mm/year. These anomalies are shown to be caused by a -0.7 K offset in 23.8 GHz brightness temperatures between cycles 28-32, and a 0.16 ± 0.04 and -0.45 ± 0.08 K/year drift in the 18.7 and 34.0 GHz brightness temperatures, respectively. Intercomparison of the 3-Hz JMR brightness temperature measurements show that they have been drifting with respect to each other, and that a dependence on yaw-steering regime is present in these measurements. An offset of 0.5 m/s between cycles 28-32 and a drift of approximately 0.5 m/s/year in the JMR wind speed measurements is also associated with these anomalies in the 1-Hz brightness temperatures. These errors in JMR wind speeds presently have a negligible impact on the retrieved JMR path delays.     

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.     

Jason-1: Assessment of the System Performances Special Issue: Jason-1 Calibration/Validation

On 7 December 2001, Jason-1 was successfully launched by a Boeing Delta II rocket from the Vandenberg Air Force Base, California. The Jason-1 satellite will maintain the high accuracy altimeter service provided since 1992 by TOPEX/Poseidon (T/P), ensuring the continuity in observing and monitoring the Ocean Dynamics (intraseasonal to interannual changes, mean sea level, tides, etc.). Despite one-fourth the mass and power, the Jason-1 system has been designed to have basically the same performance as T/P, measuring sea surface topography at a centimetric level. This new CNES/NASA mission also provides near real-time data for sea state and ocean forecast. The first two months of the Jason-1 mission have been dedicated to the assessment of the overall system. The goals of this assessment phase were: 1. To assess the behavior of the spacecraft at the platform and payload levels (Jason-1 being the first program to call on the PROTEUS versatile multimission platform for Low and Medium Earth Orbit Missions developed in partnership between Alcatel Space and CNES); 2. To verify that platform performance requirements are met with respect to Jason-1 requirements; 3. To verify that payload instruments performance requirements evaluated at instrument level are met; 4. To assess the performance of the Jason-1 Ground System. This article will display the main outputs of the assessment of the system. It will demonstrate that all the elements of the onboard and ground systems are within the specifications. Provision of data to the Jason-1 Science Working Team started at the end of March 2002. This is the goal of a six-month phase after closure of the initial assessment phase to derive the error budget of the system in terms of altimetry user products.     

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.     

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.