Satellite Altimetry for Geodesy, Geophysics and Oceanography

Inhaltsverzeichnis

1. Frontmatter

2. Keynote Speeches

   1. Frontmatter

   2. Geodetic Application of Satellite Altimetry

      Wolfgang Bosch

      Abstract

      Satellite altimetry has developed to an operational remote sensing technique with important applications to many geosciences. In the present paper we consider in particular geodetic applications of satellite altimetry. It is shown that this space technique not only allows mapping and monitoring a major part of the Earth surface, but also did contribute to essential improvements for the Earth gravity field. Even with the new dedicated gravity field missions of CHAMP, GRACE and GOCE satellite altimetry will be needed for the determination of the high-resolution gravity field. The mean sea level can be mapped and monitored for seasonal and secular changes through the combination of altimeter mission with different space-time sampling. The realisation of the global mean sea surface will help to unify national height systems, to clarify local sea level trends visible in tide gauge records and contribute to studies on the global sea level rise, the most prominent indicator of global change.

   3. Present-day sea level rise: from satellite and in situ observations to physical causes

      A. Cazenave, K. Do Minh, M. C. Gennero

      Abstract

      We investigate climate-related processes causing variations of the global mean sea level on interannual to decadal time scale. We focus on thermal expansion of the oceans and continental water mass balance. We show that during the 1990s where global mean sea level change has been measured by Topex/Poseidon satellite altimetry, thermal expansion is the dominant contribution to the observed 3 mm/yr sea level rise. For the past decade, exchange of water between continental reservoirs and oceans as well as glaciers and ice sheet melting may account to up 1 mm/yr. This suggests that there is a missing contribution which counterbalances the 1 mm/yr eustatic contribution. Dam building and irrigation associated with anthropogenic activities appear able to cancel out the effect of ice melting and land water change. For the last four decades, thermal contribution is estimated to about 0.5 mm/yr, with a possible accelerated rate of steric sea level rise during the 1990s.

   4. Advances in Large-Scale Ocean Dynamics From a Decade of Satellite Altimetric Measurement of Ocean Surface Topography

      Lee-Lueng Fu, Yves Menard

      Abstract

      The past decade has seen the most intensive observations of the global ocean surface topography from satellite altimeters. The Joint U.S./France TOPEX/POSEJDON (T/P) Mission has become the longest radar mission ever flown in space, providing the most accurate measurements for the study of ocean dynamics since October, 1992. The European Space Agency’s ERS-1 and -2 Mission also provided altimetric observations from 1991–2000. The combined data from T/P and ERS have higher spatial resolution and greater coverage than the individual missions. Major advances in large-scale ocean dynamics from these observations are reviewed in the paper, including the ocean general circulation and its variability, the evolution of the El Nino Southern Oscillation cycles as well as the emerging decadal variability, the response of the ocean to wind forcing, assimilation of altimeter data by ocean general circulation models and the estimation of deep ocean circulation, global sea level rise, and tidal models and mixing.

3. Geodetic Applications

   1. Frontmatter

   2. Calibration Results of GFO

      Changyin Zhao, C. K. Shum, Yuchan Yi

      Abstract

      GFO has been operational since November 2000 and it’s data products are available to the scientific community. This paper provides a summary of the GFO calibration and sensor validation results, and presents results of sensor performance evaluations and accuracy of both the near-real time and offline Geophysical Data Record data products.

   3. Determination of 30′x30′ marine gravity anomalies from multisatellite altimetry

      Ruihua Zhang, Pan Shi, Fuqing Peng, Zheren Xia, Guangming Liu

      Abstract

      This paper presents the satellite altimeter data pre-processing and recovery of marine gravity anomalies from the vertical deflections using the inverse Vening Meinesz formula. The formulae for corrections of dynamic oceanic topography (DOT) to the vertical deflections and the model values of mean residual vertical deflections are derived. It shows that the standard deviation (STD) is about ±3.7∼±4.4 mGal by comparing the 30′x30′ gridded gravity anomalies over sea area 82°S∼82°N, 0°E∼360°E with that measured by marine gravimeter.

   4. Recovering Deflections of Vertical from A Tangent Plane of Gridded Geoidal Heights from Altimetry

      Lifeng Bao, Yang Lu

      Abstract

      To improve the space resolutions of vertical deflections and gravity anomaly recovering from altimetry data, a new method for computing vertical deflections is presented. At first, we eliminate the influence of sea surface topography from the mean sea surface height of altimetry data along ground tracks, and regard the results as geoidal heights. The data coordinates are then transformed into new Cartesian coordinates, which consist of a osculating tangent plane and a normal of the reference ellipsoid. Then, we fit a osculating tangent plane of the local geoid using a least-squares approach by minimizing the squared summation of the total distance between the discrete points and the fitting plane; this is to determinate the mean value of each component of the vertical deflections. An experiment was made in the area of the South China Sea. The comparisons between the vertical deflections determined by this method and those by other methods show that the precision of this method agrees well with those from other methods on 5′x5′ grid.

   5. Marine Gravity Anomaly from Satellite Altimetry: a Comparison of Methods over Shallow Waters

      Cheinway Hwang, Hsin-Ying Hsu, Xiaoli Deng

      Abstract

      Gravity anomalies over shallow waters are useful in many geodetic and geophysical applications. This work compares three methods of gravity anomaly derivation from altimetry over shallow waters near Taiwan: (1) compute gravity anomalies by LSC using along-track, differenced geoidal heights and height slopes, (2) compute gravity anomalies by least-squares collocation (LSC) using altimeter-derived along-track deflections of vertical (DOV), and (3) grid along-track deflections of vertical by LSC and then compute gravity anomalies by the inverse Vening Meinesz formula. A nonlinear filter with outlier rejection is applied to along-track data. We used altimeter data from Seasat, Geosat, ERS-1, ERS-2 and TOPEX/POSEDION missions. Retracked ERS-1 waveforms are shown to improve the accuracy of estimated gravity anomalies. For the three methods, the RMS differences between altimetry-derived gravity anomalies and shipborne gravity anomalies are 9.96 (differenced height) and 10.26 (height slope), 10.44 and 10.73 mgals, respectively. The RMS differences between shipborne gravity anomalies with gravity anomalies from retracked and non-retracked ERS-1 waveforms are 11.63 and 14.74 mgals, indicating retracking can improve the accuracy.

   6. Enhanced Free-air Gravity Anomalies by Satellite Radar Altimetry

      Jeong Woo Kim, Daniel R. Roman, Ralph R. B. von Frese, Chan Hong Park

      Abstract

      Ocean satellite altimetry-implied free-air gravity anomalies have had the shortest wavelengths removed during the processing to generate the optimal solution between multiple radar altimeter missions. ERS-1 168-day mission altimetry was residualized to a reference geoid surface generated by integrating Anderson and Knudsen’s free-air gravity anomalies for the Barents Sea. The altimetry tracks were reduced and filtered to extract the shortest wavelengths (between 4 and 111 km) from both ascending and descending tracks, respectively. These data were recombined using existing quadrant-swapping techniques in the wavenumber domain to generate a correlated, high frequency gravity field related to the local geologic sources. This added-value surface adjusted the reference free-air gravity anomalies to better reflect features in the gravity field at a wavelength related to the distance between altimetry ground tracks.

   7. Topex Altimetric Mean Sea Level and Gravimetric Geoid in the North of Algeria

      S. Kahlouche, A. Rami, S. A. Benhamed Daho

      Abstract

      The mean sea level is processed from Topex altimetric measurements collected on the Algerian coasts (on the South West of the Mediterranean Sea) over a three months period (April 20 to July 28, 2000). The model used, based on the Barrick-85 model, determine the instantaneous sea surface heights for 80 cycles distributed on 8 orbital arcs. The pre-processing of the raw data permit the selection about 75 % of the measurements. The processing of the fitted data, by polynomial adjustment, is done after the correction of the main disturbed effects (instrumental, atmospheric,...) and the reduction of the radial component of the orbital error. The analysis of the mean arcs in the crossing points by the least squares adjustment allows a centimetric accuracy of the mean sea level. The surface obtained was extrapolated to the North of Algeria (surrounding the coasts) and compared to a gravimetric geoid calculated with a set of BGI free air gravity anomalies and GETECH data using the FFT techniques implemented in the Geofour program (Tscherning, 1994). The metric difference obtained is due to the insufficient altimetric and gravimetric data distributions, and to the extrapolation method used. The availability of an exhaustive gravity measurement, particularly the gravity cruise data (sea data), and a long period altimetric measurement, will allow a strong comparison between the surfaces.

   8. A Modified Method for Recovering Bathymetry from Altimeter Data

      Zhai Guojun, Huang Motao, Ouyang Yongzhong, Bian Shaofeng, Liu Yanchun, Liu Chuanyong

      Abstract

      At present, there exist two types of methods to recover the bathymetry from altimeter data, i.e. the deterministic method and the stochastic one. This paper first reviews the general principles of the two aforementioned methods in order to form the basis for the development of the new approach. Then, based on the theory of least-squares collocation, a modified statistical model for recovering bathymetry from altimeter data is proposed. The new model is used to compute the ocean depth in the South China Sea from altimetry-derived gravity anomalies. Finally the predicted depths are compared to the shipborne ones. The results show that the achievable agreement is very good. Taking into account the existence of errors in the shipborne depths, it can be believed that the relative error of altimetry-derived depths reaches the level of about 7%.

   9. Absolute Calibration of the ERS-2 Altimeter using UK Tide Gauges

      X. Dong, C. Huang, P. Woodworth, P. Moore, R. Bingley

      Abstract

      This paper describes the use of UK tide gauges, the sea level data from which have been located in the same geocentric reference frame as altimeter data from ERS2 using Global Positioning System (GPS) receivers, to provide an ongoing absolute calibration of the altimeter bias. The method is an extension of earlier work for TOPEX/Poseidon. However, the present analysis extends to the ERS-2 altimeter, which flies on a sun-synchronous orbit. The S₂ tide error of ERS-2 will be aliased to a permanent local bias, so it cannot be detided by the harmonic method which was used in TOPEX/Poseidon calibration. The calibration sites were chose at the TOPEX/Poseidon and ERS-2 dual-satellite crossover points, in order to use TOPEX altimeter data to extract the tidal parameters. These tidal parameters were applied to ERS-2 altimeter data to reduce the affect of tide differences between data from the ERS-2 calibration sites and tidal gauges. The timeaveraged ERS-2 biases are derived over 24 crossover points in 6 tidal gauges, and the mean is 44.0cm with a 6.6cm RMS. When using first 1000 days TOPEX/Poseidon data which is not overlay with ERS-2 data, the time-averaged biases is only a few millimetres different at each site. It suggest that other sun-synchronous satellites (with or without an overlap with TOPEX/Poseidon) can be similarly evaluated.

   10. An Intercomparison of Parametric Models of Sea State Bias for the TOPEX, Poseidon and Jason-1 Altimeters

       E. L. Mathers

       Abstract

       The ‘BM4’ four-parameter (Gaspar et al., Journal of Geophysical Research, 1994) empirical model of sea state bias based on wind stress and significant wave height (SWH) is applied in both crossover and collinear analyses of all available TOPEX, Poseidon and Jason-1 data provided by the Radar Altimeter Database System at Delft University of Technology. The relative biases are - 2%, -5% and -4% of SWH for TOPEX, Poseidon and Jason-1 respectively. Estimates of the uncertainties on the lower-order coefficients are large and unhelpful. For each altimeter, crossover analyses resulted in a greater proportion of the variance in sea surface height differences explained by the calculated model than collinear analyses. From the available Jason-1 data, the calculated crossover model performs as well as the sea state bias model provided with the data.

   11. Determination of Global Mean Sea Surface Using Multi-satellite Altimetric Data

       Jiang Weiping, Li Jiancheng, Wang Zhengtao

       Abstract

       In this paper, the overall editing criteria for altimetric data are considered and the coordinate datums of various altimetric data are unified to an unique one. A method of full-combined crossover adjustment for different altimetric tracks is used to improve the radial orbits of Geosat, ERS-1 and ERS-2 data. In addition, the method for determining Mean Sea Surface (MSS) by using multi-altimetric data is developed. The data used to compute WHU2000 MSS includes 7 years of Topex/ Poseidon data (from cycle 11 to 249), 2 years of Geosat ERM data (from cycle 1 to 44), 5 years of ERS2 data (from cycle 1 to 52) and all ERS-1 168day data. The WHU2000 MSS is determined with a grid resolution of 2′x2′ within the ±82° latitude and its precision is better than 0.05m. For an external check, WHU2000 MSS is compared with CLS_SHOM98.2 MSS, GFZ MSS95A and OSU MSS95, and the corresponding STandard Deviation (STD) of the comparing differences are ±0.090m, ±0.21 lm and ±0,079m respectively. The results of the comparisons show that the precision of WHU2000 MSS is in the same level with CLS_SHOW98.2 and OSU MSS95 but its resolution is higher than the latter ones.

   12. Pseudo-Harmonic Representation of Gravity Field over South China Sea

       Yang Lu, Hou-Tse Hsu

       Abstract

       Recently, the marine mean gravity anomaly dataset are available to us on a 2 arc minute grid obtained from satellite altimeter data. To formulate gravity field model based on this dataset, in this work we refer to use the method named Pseudo-Harmonic Regional Analysis (PHRA) instead of the conventional Spherical Harmonic Analysis (SHA) due to its high resolution for describing the regional gravity field. The result show that even if taking factor f_sc=5 in PHRA, a model to degree 1080 will be enough to represent mean gravity anomaly dataset with the accuracy ±2.4 mGal on 2 arc minute grid, the RMS of the differences between the PHRA model and the geoid undulations altimeter derived are ±0.095m. The PHRA model offers significant improvements compared with any existing models with the same degree and order.