Satellite Altimeter Validation and Data Management
Interim EOC Project Report
John Church and Neil White, CSIRO Division of Marine Research (DMR)
Richard Coleman, University of Tasmania
1.1 Background
Satellite altimetry is at present the only practical method for synoptically measuring the height field of the global ocean surface and its temporal evolution. While the initail focus was on geoid studies, it allows the measurement of global sea-level change and the global estimation of near-surface geostrophic currents. The first satellite altimeters were carried on the GEOS-3 (launched in 1975) and SEASAT satellite missions. For a limited period of time (only 3 months for SEASAT), these missions demonstrated the potential of satellite altimetry for oceanographic research. However, it was not until the USA Department of Defence Geosat mission (launched in 1985) was moved into an exact repeat orbit in 1986 that this potential started to become realised. The Geosat Exact Repeat Mission lasted for less than three years. Recently (early 1998), a Geosat Follow On (GFO) mission has been launched.
The European Space Agency has included satellite altimeters on its ERS-1 (launched in 1991) and ERS-2 Missions and plans to carry a more accurate altimeter system on Envisat.
The value of satellite altimeter missions for climate related oceanographic research depends critically on the quality of the orbits and the various corrections necessary for determining sea-level height. The NASA and CNES TOPEX/POSEIDON satellite altimeter mission, launched in August 1992, was optimised for oceanographic research. It provides global sea surface height measurements for determining surface ocean currents and their variability, and for the determination of global sea-level change. Because of its high accuracy (point sea level height measurements to about 3 cm) and optimally designed orbit parameters TOPEX/POSEIDON is particularly valuable for climate research and is an essential component of the World Ocean Circulation Experiment of the World Climate Research Programme.
As a result of the outstanding success of the TOPEX/POSEIDON mission and its demonstrated importance for Climate Research, a follow-on to the TOPEX/POSEIDON Mission is being planned. This mission is to be called JASON-1 and the anticipated launch date is May 2000. The TOPEX/POSEIDON satellite is expected to continue to operate normally until at least 2000 and thus there will be overlap with the JASON-1 Mission allowing for cross-calibration of the altimeters systems and a greater spatial/temporal resolution for a limited period.
Greater spatial/temporal resolution is also available by combining the data from the ERS (and potentially the GFO and Envisat) altimeter data sets with the higher accuracy TOPEX/POSEIDON altimeter data. However, care must be tasked not to degrade the accuracy of the TOPEX/POSEIDON data in this process.
1.2 Australian Involvement
Early involvement in GEOS-3 and SEASAT, satellite altimetry was led by Professors Ron Mather and Kurt Lambeck, and Drs Richard Coleman and Chris Rizos. Australian interest in satellite altimetry was revived with acquisition of the Geosat altimeter data in 1988. The success of this work led to the current Australian involvement in satellite altimetry.
Following a NASA and CNES Announcement of Opportunity, we were selected by NASA to join the TOPEX/POSEIDON Science Definition Team in 1987 and subsequently were confirmed as members of the TOPEX/POSEIDON Science Working Team. Early in 1996, NASA called for proposals for the TOPEX/POSEIDON Extended Mission (TPEM). We submitted a detailed proposal in June 1996 and in late 1996, we were selected by NASA to be part of the TPEM Science Working Team (one of 36 successful NASA proposals from a total of 62 submitted). Membership of the TPEM Working Team supersedes our earlier membership of the TOPEX/POSEIDON Science Working Team.
In 1998, we submitted a proposal to be a member of the JASON team and are awaiting the results of this decision.
As a result of our activities, a significant number of additional scientists have recognised the value of the satellite altimetry, and the TOPEX/POSEIDON altimeter data set in particular. Many of these scientists are part of the TPEM Team and our Jason-1 proposal. The Jason-1 Team includes a number of non-Hobart based and international investigators. In addition, there are a number of users of TOPEX/POSEIDON data who are not a member of our team. We have actively supported these groups where applicable.
As a result of the breadth of out team, both our TPEM and Jason-1 proposals have a range of objectives covering areas of Satellite Altimeter Verification, Understanding and Predicting ENSO and Indian Ocean Impacts on Australian Climate Variability, Understanding Basin to Global Scale Ocean Circulation and Variability, Dynamics of the Antarctic Circumpolar Current, Understanding and modelling the Australian Exclusive Economic Zone and Evaluation, Analysis and Development of the OCCAM global ocean model using satellite altimeter data. Note that there are a number of international collaborations established in this proposal.
The EOC Project objectives are:
Data Management
Satellite verification
Note that these objectives are much narrower than the objectives of the TPEM and Jason-1 objectives. However, they are important underpinning activities for those broader objectives.
The data management aspects are relevant to Section 3.5 (Data base for Environmental Time Series Data) of the EOC Primary Threads and Tasks. The validation aspects of this task are relevant to Section 3.4 (Validation Missions) of the EOC Primary Threads and Tasks.
The milestones as outlined in the EOC Proposal were
Milestones for the 1996/97 financial year:-
Milestones for the 1997/98 financial year:-
3.1 Results against Milestones
Data management
Our primary focus is in the TOPEX/POSEIDON data set. While there is no recognised "standard" software for managing satellite altimeter data, we have made significant use of existing software where appropriate; eg our software has evolved from software originally supplied by NOAA.
The TOPEX/POSEIDON data set we rely on (the Merged TOPEX/POSEIDON Geophysical Data Records) is provided through the NASA Oceanographic Data System (NODS) at the Jet Propulsion Laboratory. It consists of one CD-ROM for every 3 cycles (30 days). This becomes about 400 MB for the 6 year extended mission.
On receipt, this data set is preprocessed through three main stages. The first stage removes data over land, obviously bad data and rarely used flags, the second does all corrections and edits (e.g. path delay corrections and tides) and the third takes out means and reformats to a convenient form for reading. This results in the compression of the data set by a factor of about 100. The resultant data are stored in files of residual heights. These files are stored on line and are the starting point for all scientific projects by a number of users.
A revised version of the Merged GDR's was released in late 1997 and we have been actively updating our data sets. We now have all of the available reprocessed TOPEX/POSEIDON data set on line. This data set commences in September 1992 and the most recently available data is cycle 183 (September 1997). This is the only data available because of delays in reprocessing the merged GDRs.
We also have historical holdings of the Geosat Exact Repeat Mission data on line.
The ERS altimeter data sets have been of lower overall standard than the TOPEX/POSEIDON data set. Two ERS-1, phase C data sets have been available on line for some time. One data set is processed on site, using GDR tapes from ESA, orbits from TU-Delft and software from NOAA. The other is an early version of AVISO (CNES) CORSSH data set.
Recently, in conjunction with mapping work in the Southern Ocean, we evaluated value added-data sets for ERS-1 phases C and G, and ERS-2 phase A. These are:
After this evaluation, we have decided to use the SLA data sets in future. They will be available on line soon. An additional advantage of these data sets over the ESA data sets is that they can be made freely available to all users (regardless of whether or not users are ERS1/2 Principal Investigators).
As a result of these activities, the following data sets are (or will shortly be) held on line
These data sets are available to all Marine Research staff and can be made more widely available if required. We have made our data sets and software freely available in the past.
The volume of these data sets is much smaller than for many remote sensing data sets - the above data sets are basically complete, useable science products.
We also make available associated data sets. An example of these is the CDROM of tidal models resulting from the TOPEX/POSEIDON data stream. These tidal models are currently being used in a project examining tidal dissipation and its importance for ocean mixing.
Regular communication is maintained with users of the altimeter data through e-mail. On average there are a few communications/month advising users of new data availability as well as polling users on possible enhancements to storage formats, etc.
There is a complete duplicate set of the CD-ROMS at the University, all of the processing software and the processed data sets are regularly backed up as part of the site backups. There are also occasional additional backups of the on-line data sets.
Satellite verification
This project is the only Australian (and only southern hemisphere) activity contributing to the validation of TOPEX/POSEIDON satellite altimeter data. It is an important contribution to the international calibration/validation activities and maintains a high profile for Australia in the international satellite community. Likelihood of continued access to the TOPEX/POSEIDON data stream is enhanced by continued support for verification activities.
The insitu equipment at our primary verification site at Burnie (the instrumentation consisting of a Sutron Aquatrak tide gauge, atmospheric pressure and temperature sensors, and wind speed sensors with all data are available in near-real time) has been maintained through our strong connections with the National Tidal Facility. We have also identified several other sites which also give useful satellite altimeter verification data. The most useful locations are the gauges at Lorne in Victoria, Cocos Island in the Indian Ocean, and Cook Island in the Pacific.
The results from the calibration site at Burnie have been valuable for the T/P mission (White et al., 1994). The data from the Burnie site and from the three additional sites have been reported at the annual TOPEX/POSEIDON Science Working Team Meetings.
The value of the calibration sites has been clearly demonstrated. Along with other investigators, we identified an apparent drift in the bias in the TOPEX altimeter data. This bias was subsequently shown to be an artifact of an error in one of the NASA algorithms. This error had also been included in previous satellite missions but was only revealed by the high accuracy TOPEX data, its careful comparison with insitu data and the POSEIDON altimeter data. (Some investigators mistakenly reported the uncorrected data sets as revealing an apparent rapid rise in global sea level.)
One of the present limitations of the Burnie site is the lack of an adequate geoid model, leading to a less precise estimate of the absolute altimeter bias compared to other calibration sites. However, this uncertainty does not affect estimates of the altimeter bias drift. We have attempted to improve the local geoid through a student project to measure the deflection of the local vertical. The analysis of these results has not made a significant improvement to the local geoid model.
Geodetic surveys of the datum of the Burnie gauge have been completed and the data is being processed. These results have not yet revealed any significant change in the datum. However, given the high accuracy required, we plan to use funds recently made available (March 1998) through this project to install a permanent GPS receiver at the Burnie site. To date we have quotes for the necessary instrumentation and informal agreement with the National Tidal Facility to allow the installation of the equipment and for NTF to download the data daily with the existing data stream.
Since correction of the NASA processing algorithm, the Burnie data indicates no detectable drift in the altimeter bias over the five years of data currently available. For the other three sites, the most recent comparisons indicate that there may be problems with the insitu data sets. These data sets are now being checked by the National Tidal Facility. This indicates a remarkable progress - the satellite data is being used to quality control the insitu verification data. A manuscript describing the latest verification results is being prepared for one of the altimeter newsletters.
3.2 Future Directions
We plan to continue with acquiring satellite altimeter data and making it available in a readily accessible form to our collaborators and the broader research community. The central focus will remain the TOPEX/POSEIDON data sets and related products.
We are also now more actively pursuing ERS altimeter data sets and making these available to users.
We anticipate acquiring GFO data when it becomes available is a useful format.
Up until recently we were copying and making available TOPEX quick look data. This activity has now been taken over by another group in the Division.
We plan to maintain the Burnie site during the lifetime of the TOPEX/POSEIDON Extended Mission and through the Jason-1 mission. This will allow us to continue to monitor the TOPEX/POSEIDON altimeter bias to determine if any drift is occurring in the altimeter (or associated corrections) and to allow cross-calibration with the Jason altimeter data.
The funding for most of the TOPEX/POSEIDON satellite altimeter work is covered by appropriation funding (Climate and Atmosphere and Marine Sectors), funding received from DEST through CSIRO's Climate Change Research Program and funding received from LWRRDC for the development of a prototype climate variability prediction model.
The CSIRO Division of Marine Research also supports much of the data management and verification activities.
In 1996/97, EOC funding was used to improve the computing facilities
available by the purchase of a workstation dedicated to the altimeter work.
Using funding recently made available (March 1998), we are purchasing a
PC to use as an X-Windows terminal to facilitate the altimeter activities,
maintaining and upgrading the workstation purchased in 1996/97, and we are
in the process of establishing a GPS receiver and data transmission facilities
for the Burnie calibration site.
We will need continuing support for maintenance/upgrade of the Workstation and Software. This includes extra disk space ($6K), a DLT Tape drive ($10K), A CPU Upgrade ($16K) and miscellaneous items ($6k/year). We may need an extra $2K for a PC to log the GPS data and an additional PC for use as an X-terminal. Suggested timing is
1998/99 - miscellaneous items. We may need an extra $2K for a PC to log the GPS data and an additional PC for use as an X-terminal - Total $11K
1999/2000 - miscellaneous items, Disk Space and DLT Tape - Total $22K
2000/2001 - miscellaneous items, CPU Upgrade - Total $22K
In the future, more emphasis will be focused on exploiting the progress to date in data management. This will mean analysing the altimeter data sets in combination with other satellite data sets (winds, sea surface temperatures) in combination with insitu data sets and model data sets. The scientific directions are outlined in the Jason-1 proposal. Exploiting the opportunities offered by progress to date and meeting national and international commitments outlined in the Jason-1 proposal would be greatly facilitated by the appointment of a postdoc.
Altimeter data is the ONLY present observational method capable of synoptically observing the global-ocean, near-surface circulation. It is therefore of high value for the development of ocean models and the determination of sea-level rise. High-quality (verified) and easily accessible altimeter data is required in the Climate and Atmosphere and Marine Sectors. For the determination of sea-level rise, high accuracy verified data is absolutely essential. Altimeter data is one of the most important satellite data streams for climate related oceanographic research. Altimeter data is being increasingly used to initialise the ocean component of climate models used for predicting interannual climate variability.
Current users of the processed data sets include, at the CSIRO DMR:
at the Antarctic CRC:
national and international users of the data set are
Most recently, the data has been used for planning of Aurora Australis (Trull Antarctic CRC) and Southern Surveyor (Rintoul DMR) research cruises in the Southern Ocean. Prior to the cruises and each week during the cruise, estimates of the surface dynamic height field and composite sea surface temperature images were emailed to both ships.
As described above, the data management and verification tasks will continue. However, these are activities are well in hand. In the future, more emphasis will be focused on exploiting the progress to date in data management. This will mean analysing the altimeter data sets in combination with other satellite data sets (winds, sea surface temperatures) in combination with insitu data sets and model data sets. The scientific directions are outlined in the Jason-1 proposal. Exploiting the opportunities offered by progress to date and meeting national and international commitments outlined in the Jason-1 proposal would be greatly facilitated by the appointment of a postdoc.
Papers:
Morrow, R., J.A. Church, R. Coleman, D. Chelton and N.J. White, 1992. Eddy momentum flux and its contribution to the Southern Ocean momentum balance. Nature, 357, 482-484.
van Gysen, H., Coleman, R., Morrow, R.A., Rizos, C. and Hirsch, B. 1992. Analysis of Collinear Passes of Satellite Altimeter Data. J. Geophys. Res., 97(C2), 2265-2277.
Coleman, R. and van Gysen, H. 1993. Estimating Sea Surface Height Variability from Collinear and Crossover Analysis of Satellite Altimetry Data. (In) Satellite Remote Sensing of the Oceanic Environment, I.S.F. Jones, Y. Sugimori and R.W. Stewart, (eds), Seibutsu Kenkyusha, Tokyo, 143-152.
Church, John A., 1993. Ocean Currents (p349-398), in Satellite Remote Sensing of the Oceanic Environment, Ed I. S.F. Jones, Y Sugimori and R.W. Stewart. Seibutsu Kenkyusha, Japan, 528pp.
Morrow, R., R. Coleman, J.A. Church, and D. Chelton, 1994. Surface eddy momentum flux and velocity variances in the Southern Ocean from Geosat altimetry. Journal of Physical Oceanography 24, 2050-2071.
White, N.J., R. Coleman, John A Church, P.J. Morgan and S.J. Walker, 1994. A southern hemisphere verification for the Topex/Poseidon satellite altimeter mission. J. Geophys. Res., 99, 24, 505-24516.
Ananga, N., Coleman, R. and Rizos, C. 1995. Geodetic Monitoring of Tide Gauge Bench Marks with GPS. Journal of Geodetic Society of Japan, 41(1): 91-97.
van Gysen, H. and Coleman, R. 1995. A Uniform Approach to the Estimation Problems of Satellite Altimetry. (In) "Gravity and Geoid", (eds.) H. S¸nkel and I. Marson, Springer-Verlag, pp 307-316.
van Gysen, H. and Coleman, R. 1996. Deformations of the Ocean Surface Determined From Satellite Altimetry. Artificial Satellites, 31(1), (Planetary Geodesy No. 26): 25-36.
van Gysen, H. and Coleman, R. 1997. On the satellite altimeter crossover problem. Journal of Geodesy, 71(2): 83-96.
van Gysen, H., Coleman, R. and Hirsch, B. 1997. Local Crossover Analysis of Exactly Repeating Satellite Altimeter Data. Journal of Geodesy, 72(1): 31-43.
van Gysen, H. and Coleman, R. 1997b. On the analysis of repeated geodetic experiments, with application to satellite altimetry. Journal of Geodesy, (under review).
Phillips, H.A., Allison, I., Coleman, R., Hyland, G., Morgan, P.J., Young,
N.W. 1997. Comparison of ERS Satellite Radar Altimeter Heights with GPS-derived
Heights on the Amery Ice Shelf, East Antarctica. Annals Glaciology,
(in press).
Papers submitted:
van Gysen, H. and Coleman, R. 1997. On the analysis of repeated geodetic experiments, with application to satellite altimetry. reviewed for Journal of Geodesy
Reports and Conference publications:
Morrow, R., R. Coleman, J.A. Church, and D. Chelton, 1993. Surface eddy
momentum flux and velocity variances in the Southern Ocean from Geosat altimetry.
In Preprints 4th Intl. Conference on Southern Hemisphere Meteorolgy and
Oceanography, Hobart, March 29-April 2. AMS Boston, MA, 54-55.
White, N.J., R. Coleman and John A Church (1994) A southern hemisphere verification for the Topex/Poseidon satellite mission. Proceedings of the Pacific Ocean Remote Sensing Conference, Melbourne Australia, 1-4 March 1994, 27-34.
Coleman, R., White, N.J. and Church, J.A. (1996). Progress in estimating ocean circulation from satellite altimetry. In: Paltridge, G and Platt, C.M.R. (eds.). Proceedings of the 3rd National AMOS Conference: incorporating the 4th Australasian Conference on the Physics of Remote Sensing of Atmosphere and Ocean and the 1st Australian ERS-1 Symposium (Hobart, Tas: February 5-7, 1996).
Gould, W.J. and J.A. Church, 1996. Oceans and Climate. Physics World, 9(12), 33-37.
Church, J.A., Breck Owens and John Gould, 1996. WOCE: The World Ocean Circulation Experiment, World Climate Research Programme Newsletter, 1, 14-16.
Church, J.A. and Steve Rintoul, 1997. The Southern Ocean's role in the climate system, Proceeding of the Southern Ocean Workshop, National Tidal Facility, Adelaide, 1996
Church, J.A., 1997. Where has WOCE got to? International WOCE Newsletter, No. 26, April 1997, 2.
Church, J.A., 1977. The Role of the Southern Ocean in the climate system and in global change. IOC Special Publication (in preparation).
Church, J.A., W. John Gould, and Breck Owens and, 1996. The World Ocean Circulation Experiment - Developing ocean models for predicting climate variability and change, World Climate Research Programme Conference Proceedings, accepted.
Meyers, G., Barton, I.J. and Coleman, R. 1996. Ocean structure observed by satellite, and Australian droughts in 1993 and 1994. Proceedings of the First Australian ERS Symposium, University of Tasmania, Hobart, February 6, 1996, (ed) J. Kingwell, COSSA Publication 037, pp 85-93.
Bindoff, N.L., Coleman, R., van Gysen, H. and Wolff, J.-O. 1997. The role of heat fluxes and winds on seasonal sea-level signals. Proceedings of ORI symposium `Global Geodynamics Coupled with Variations of Atmosphere and Ocean', (Ed.) Masatugu Ooe, November 26-27, 1996, Tokyo, Japan, pp. 45-57.
Phillips, H.A., Hyland, G., Morgan, P.J., Coleman, R., and Young, N.W. 1997. Comparison of ERS altimeter and GPS heights on the Amery Ice Shelf, East Antarctica. Proceedings of the 3rd ERS Symposium, March 17-21, 1997, Florence, Italy, ESA Special Publication SP-414, pp. 899-904.
Conference Presentations
Western Pacific American Geophysical Union meeting, July 23-27, 1996 Brisbane, Australia. Co-convenors: John Church and Richard Coleman on session "Ocean Dynamics Using Altimetry and Radar"
Paper presented: Herman van Gysen, Richard Coleman, Nathan Bindoff, Joerg Wolff, "The contribution of the Oceans to the Dynamics of the Earth as determined by satellite altimetry".
TOPEX/Poseidon SWT meeting, October 23-26, 1996 Southampton, UK Paper presented: NJ White, Richard Coleman, JA Church, "Southern Hemisphere Monitoring of the TOPEX/Poseidon Altimeter Bias"
American Geophysical Union Spring meeting, May 27-30, 1997 Baltimore, USA Co-convenors: Richard Coleman and Reinhard Deitrich on session "Antarctic Geodynamics"
Paper presented: Helen A Phillips et al., "Comparison of ERS altimeter and GPS heights on the Amery Ice Shelf, East Antarctica"
IAMAS/IAPSO 1997 Joint Assemblies, "Earth-Ocean-Atmosphere, Forces for Change", July 1-9, 1997, Melbourne, Australia IAG Representative on Local Organising Committee: Richard Coleman, Co-convenors: Steve Nerem, Gary Lagerloef, Richard Coleman on session "Geoid and ocean circulation"
Papers presented: Church et al., "Monitoring the Location, Structure and Transport of the Antarctic Circumpolar Current Fronts south of Australia"; Schroeter et al., "Assimilation of large scale satellite altimeter data in a PE model of the Southern Ocean"; van Gysen et al., "Oceanic Excitation of Length-of-Day Fluctuations from TOPEX/Poseidon"
"Monitoring the Oceans in the 2000's: An Integrated Approach", October 15-17, 1997, Biarritz, France Papers presented:
Church et al., "Monitoring the Location, Structure and Transport of the Antarctic Circumpolar Current Fronts south of Australia"; Schroeter et al., "Assimilation of long wavelength altimetry"
Last updated 22 May 1998
Copyright 1998, CSIRO Australia
Use of this web site and information available
from it
is subject to our Legal
Notice and Disclaimer