MIDTERM REPORT OF SPECIAL STUDY GROUP 3.186:

 ALTIMETRY DATA PROCESSING FOR GRAVITY, GEOID AND SEA SURFACE TOPOGRAPHY DETERMINATION

Cheinway Hwang

Department of Civil Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan, ROC

1. Introduction

This midterm report summarizes the background, research goals, members and current results of IAG SSG 3.186. Since the Seasat mission of 1978, satellite altimetry has found its wide applications in geodesy, geophysics and oceanography. As new satellite missions such as GFO-1, ENVISAT JASON-1, and CRYOSAT will contribute more to the existing data sets of Seasat, Geosat, ERS-1/2, and TOPEX/POSEDION, these applications will continue to grow. But there are still many applications to be explored, many problems to be solved, and many data processing techniques to be improved. For example, coastal geoids, gravity anomalies tide models and bathymetry models derived from satellite altimetry have important engineering applications, which did not receive much attention in the past. But exploiting satellite altimetry in coastal areas requires much more sophisticated correction models and data processing techniques than in the open oceans. The data and coordinate systems of different satellite missions should be properly weighted/corrected and unified in order to obtain an optimal multi-satellite data set for subsequent analyses. Shipborne gravity data are abundant in many areas of the oceans, and have high quality and good spatial resolution. They should be combined with altimetry data for global gravity and geoid computation and estimation of high-degree geopotential model. Bathymetry model is an important element in, e.g., the general circulation model of the world oceans and the hydrodynamic tide model, and should be optimally derived with altimetry and other data. Eddies in coastal areas are associated with coastal upwellings, which are extremely important for marine production.

SSG 3.186 encourages members to tackle the following problems:

(1) improving the quality of coastal altimeter data by improving geophysical corrections, retracking waveforms and "tuning" altimeter measurements.
(2) promoting engineering applications of coastal altimetry with high quality coastal geoid, gravity anomaly, bathymetry, ocean tide and sea surface topography models from altimetry.
(3) investigating the best method and the best altimeter data type for computing gravity anomalies, mean sea surface heights from multi-satellite altimeter data
(4) developing a best technique to compute bathymetry from altimeter-derived geoids or gravity anomalies, with emphasis on the downward continuation and filtering problems.
(5) finding a best strategy and data sources to combine shipborne gravity/airborne gravity and altimeter data for generating global and regional gravity anomalies and geoids.
(6) improving orbit accuracies of altimetric satellites and accuracies of the long wavelength gravity field by crossover and other methods.
(7) unifying the coordinate systems between two or more satellite missions for determining long-term time series of oceanographic parameters.

2. Members

Currently there are 21 members from 12 countries in SSG 3.186. They are mostly university professors, doctoral students and research scientists. For doctoral students, their research topics more or less fit the recommended research topics of SSG 3.186 (see above). A list of members and their email addresses is shown in the following table.

3. A summary of current activities and results of members

The geodesy group in the Civil Engineering, National Chiao Tung University (lead by C. Hwang) and the group in the Ohio State University (lead by CK Shum and Y. Yi) are jointly testing algorithms for retracking ERS-1 waveforms over the continental shelf of east Asia. This is an attempt to see the effect of retracked altimeter data in improving accuracy and resolution of geoid and gravity anomaly determination in the shallow waters. Furthermore, Hwang and Hsu (2001) derive global mean sea surface heights (SSHs) on a 2´×2´ grid using Seasat, Geosat (ERM and GM), ERS-1 (1.5-year mean of 35-day, and GM) and TOPEX/POSEIDON (T/P) (5.6-year mean), ERS-2 (2-year mean) and Geosat-Follow-On (GFO) (18-month mean) altimeter data over the regions 0°-360° longitude and -82°-82° latitude. Hwang and Chen (2000a) use TOPEX/Poseidon (T/P) altimeter data to compute time-varying circulation and eddies over the South China Sea (SCS) for 1993-1999. Hwang and Chen (2000b) use 5.6 year of T/P sea level time series to identify important signals of the South China Sea by Fourier and wavelet analyses.

Deng and Featherstone (2000) analyze Poseidon (January 1998 to January 1999) and ERS-2 (March 1999 to April 1999) altimeter data in an area extending up to 350km from the Australian coasts (Deng and Featherstone, 2000). They investigate the behavior of the altimeter data in coastal regions and estimate a possible boundary around Australia’s coasts in which the altimeter range may be estimated poorly. Using the standard deviation of the mean power of the returned waveforms as an indication of the general variability of the altimeter returns, shows obvious coastal contamination out to ~4km, and less obvious contamination out to ~8km. The results from individual waveforms indicate that the data contamination varies with the type of shoreline topography, which in turn leads to a distance-varying contamination around Australia.

Vergos and Sideris (2001) investigate the possibility of improving the estimation of the bottom topography of the Earth’s oceans using gravity data in two extended test areas. The first area is located in the Mediterranean Sea, and the other one is across the mid Atlantic ridge bounded by 40^o £ f £ 50 ^o and 330 ^o £ l £ 340 ^o.

The Danish group (lead by Andersen and Knudsen) is continuing to improve the accuracy of gravity and mean sea surface determination, as well as the accuracy of global ocean tide model. Their recent results can be found in Andersen et al (2000) and Andersen and Knudsen (2000).

The Danish group (lead by Andersen and Knudsen) is continuing to improve the accuracy of gravity and mean sea surface determination, as well as the accuracy of global ocean tide model. Their recent results can be found in Andersen et al (2000) and Andersen and Knudsen (2000).

Rentsch et al. (2000) generate a global 2' by 2' high-resolution grid of marine gravity anomalies by processing upgraded altimeter data from the Geodetic Missions of Geosat and ERS-1. They also retrack ERS-2 waveforms in the Chinese Sea. A much higher along-track resolution is achieved from the retracked altimeter profiles and has improved the accuracy of the marine gravity field model. However, new problems arise by using such data, e.g. a higher noise level and the absence of convenient corrections like ocean tide and wet tropospheric path delay.

Klokonick et al. (2000) investigate the single- and dual-satellite Crossover (SSC and DSC) residuals between and among Geosat, T/P, and ERS 1 or 2. They present the theory and give various examples of certain combinations of SSC and DSC that test for residual altimetry data errors.

Wang (2000) compute a global set of mean SSH using TOPEX, ERS-1 and Geosat data. Inter-comparisons show that the root mean square values of the difference in mean SSH are 6.8, 6.8 and 7.2 between GSFC98/OSU95, GSFC98/CRS95, and OSU95/CSR95.

Andritsanos and Tziavos (2000) investigated the method of multiple input and output for gravity parameter recovery.

4. Challenges and future works

One challenge is in the shallow waters, where altimeter data quality is seriously degraded. Here waveform retracking can improve the situation, but more work is still needed. In particular, tide model accuracy must be significantly improved in order to have the possibility of coastal applications of satellite altimetry. Another challenge is the combination of data from multi-sensors, such as satellite/air-borne altimeters, ship/air-borne gravimeters, for marine geoid/gravity determination. Different sensors have different noise levels and spatial resolutions, which make the combination a difficult task. To the SSG3.186 members, the determination of oceanic dynamic topography, which is important for determining ocean circulations, is a subject not well studied at the current stage, especially in the coastal areas. It is indeed very desired to see if coastal oceanography can benefit from satellite altimetry. Finally, many of the groups have computed global sets of marine gravity and mean SSH, so it will be necessary to perform an inter-comparison of these results and compute an optimal set from these various sets using a weighted average method, something like the method for combing the IGS orbit of GPS. SSG3.186 may then presents this optimal set of marine gravity and mean SSH to the world scientific community for various applications.

5. SSG3.186-related publications of members  

Andritsanos, V.D., and I.N. Tziavos, 2000. Estimation of gravity field parameters by a multiple input/output system. Phys. Chem. Earth (A), 25 (1), 39-46.

Andersen, O.B., and P. Knudsen, 2000. The role of satellite altimetry in gravity field modeling in coastal areas, Phys. Chem. Earth, 25 (10), 17-24.

Andersen, O.B., P. Knudsen and R. Trimmer, 2000. The KMS99 global gravity field from ERS and Geosat altimetry, Proc. ERS-Envisat Symp. 2000, Göteborg, Sweden.

Deng, X. and W. Featherstone, 2000. Analysis of ERS-2 satellite altimeter waveform data around Australian coasts, paper presented to the Annual Research Seminar, The University of New South Wales, Sydney, Australia, 20-21 November, 2000.

Hwang, C, and S.-A. Chen, 2000a. Circulations and eddies over the South China Sea derived from TOPEX/Poseidon altimetry, J. Geophys. Res.,105, 23,943-23,965,

Hwang, C., and S.-A Chen, 2000b. Fourier and wavelet analyses of TOPEX/Poseidon-derived sea level anomaly over the South China Sea: A contribution to the South China Sea Monsoon Experiment, J. Geophys. Res., 105, 28,785-28,804.

Hwang, C, and H.-Y. Hsu, 2001. A global mean sea surface grid from Seasat, Geosat, ERS-1, and TOPEX/POSEIDON altimetry: application of deflection-geoid formula, abstract submitted to the IAG Scientific Assembly, Budapest, 2-9 September, 2001.

Klokonick, J., C.A. Wagner and J. Kostelecky, 2000. Residual errors in altimetry data detected by combinations of single- and dual-satellite crossovers, J. Geod., 73, 671-683.

Medvedev, P., 2001. The use of the satellite altimetry data for Sea of Okhotsk and Caspian Sea studies and the plans of GPS and GLONASS applications, abstract submitted to the IAG Scientific Assembly, Budapest, Sep 2-9, 2001.

Rentsch, M., A. Braun, T. Schöne, T. Gruber, and P. Schwintzer, 2000. Recent results and applications from GFZ marine gravity grids, EGS XXV General Assembly, Nice, France, 26 April, 2000.

Vergos, G.S., and M.G. Sideris, 2001. Improving the estimation of bottom ocean topography with satellite altimetry derived gravity data using the integrated inverse method, abstract submitted to the IAG Scientific Assembly, Budapest, 2-9 September, 2001.

Wang, Y., 2000. The satellite altimeter data derived mean sea surface GSFC98, Geophys. Res. Lett., 27 (5), 701-704, 2000.

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