Analysis of Altimeter Along Track SSHA Data from RADS, CTOH, AVISO and PISTACH,
off the Indian Coasts and Estimation of Biases
Aaron D’Souza
Department of Computer Science & Engineering
National Institute of Technology
Rourkela,Orissa
Mentor:
Dr. Y.K. Somayajulu,
Scientist, F
Data & Information Division
NATIONAL INSTITUTE OF OCEANOGRAPHY
DONA PAULA, GOA-403004
Project Duration:
10/05/2010 to 08/07/2010
Declaration
I hereby declare that the work incorporated in this project is original and carried out at the Data and Information Division of the National Institute of Oceanography, Dona Paula, Goa.
Aaron D’Souza
Bachelor of Technology, 6th Semester
Department of Computer Science & Engineering
National Institute of Technology
Rourkela, Orissa.
Certificate
This is to certify that this project is an authentic record of the research carried out by Mr. Aaron D’Souza under my supervision and guidance at the Data and Information Division of the National Institute of Oceanography, Dona Paula Goa.
Dr. Y. K. Somayajulu
Scientist F
Head Data and Information Division
National Institute of Oceanography
Acknowledgements
It is my pleasure and privilege to express my deep gratitude to Dr. Y. K. Somayajulu for giving me the opportunity to carry out the project under his guidance. I would like to express my thanks to The Director, National Institute of Oceanography, Goa for providing me with all the facilities needed for the successful Completion of the project work. Lastly, I thank my parents, and family members for their moral support and constant encouragement without which the work carried, would not have been possible.
Aaron D’Souza
Bachelor of Technology, 6th Semester
Department of Computer Science & Engineering
National Institute of Technology
Rourkela, Orissa.
About NIO
National Institute of Oceanography (NIO) is one of the forty research institutes under Council of Scientific and Industrial Research (CSIR) established in 1942. CSIR is the largest research and development organization in India providing S & T input to a diverse spectrum of economic, industrial and societal sectors. Its research and development activities cover fields from aerospace engineering to ocean sciences, from molecular biology to metallurgy, from chemical to mining, from food to petroleum and from leather to environment.
NIO was founded in Goa on 1st January 1966 with regional centers at Mumbai, Kochi and Vishakhapatnam. It was established as a natural outcome of the International Indian Ocean Expedition (1962-1965). From its small scientific and supporting staff, strength of the institute has grown to be largest marine research institute devoted to the studies in the Indian Ocean region.
Mission of NIO:
"To continuously improve our understanding of the seas around us and to translate this knowledge to benefit of all".
Major areas of research at NIO:
Minerals from the sea:Geological and geophysical surveys for exploration of minerals, organic chemicals from the sea, and identification of bioactive substances in the marine plants and animals for pharmaceuticals purposes.
Marine Biological Activities Survey of marine living resources, location of new fishing grounds through plankton and ocean colour studies, application of biotechnological methods to improve the growth of some important commercial fishes, feed organism in aquaculture.
Climate: To study various interactive processes between air and sea, sea and land and identifying the factors controlling the monsoon. Coastal zone management Studies on various physical, chemical and engineering parameters, control of coastal erosion and promotion of environmentally acceptable uses of the coastal zone, including the estuaries, Protection of marine environment, identification and monitoring of various pollutants, formulation of protective measures and location of suitable sites of the safe disposal of industrial waste and sewage.
Significant Achievements:
1. The institute has carried out surveys for polymetallic nodules in an area of about 4 million sq. km in the central Indian Ocean (water depth 4-6 km). This work has resulted in registration of India as the first pioneer investor country in the world and getting allocation of an area of 1,50,000 sq km for developing a mining site.
2. A survey by the Institute of the shallow offshore areas of Konkan coast has indicated ilmenite deposits of about 12.5 million tones.
3. A few models covering land Sea and air sea interactions and ocean dynamics have been developed to understand the variability of monsoon. Studies on the anticipated sea level rise due to greenhouse effect have shown some areas vulnerable to inundation.
4. A technology to prepare a bio fertilizer from marine benthic algae is developed which increases the growth of the terrestrial plant, fruit yield and resistance to pests. The product is being marketed under the trade name Pantozyme by a Goan firm.
5. More than 500 species of marine flora and fauna have been studied for antiviral, antifungal and CNS depressant activities. A few are found to be promising.
INDEX
· Declaration 2
· Certificate 3
· Acknowledgement 4
· About NIO 5
· Index 6
· Summary 7
· Basic principles of Satellite Altimetry 8
- The Coastal Altimetry Concept 10
◦ Primary Objectives
◦ Data Source Summary
◦ Applications to the Indian coast
◦ Influence of the Atmosphere
· Analysis and Results 14
1. Smoothening of RADS Ascii data using Loess filter and comparing with CTOH data to estimate the bias.
2. Applying corrections to AVISO Data and comparing results with processed RADS data and estimating bias between the two.
3. Comparing data sets from AVISO, RADS & CTOH for a given Jason -1 track and estimating biases between these three products.
4. Comparing data sets from AVISO, RADS & PISTACH for a sample Jason -2 track and estimating biases between the three products.
5. Comparing the variations in different correction terms closer to the coast.
· Codes 34
· List of Figures 48
· References 49
Summary
Sea Surface Height Anomaly (SSHA) data along satellite tracks in the Coastal regions for Topex/Poseidon, Jason-1, Jason-2 and Envisat satellites are available to the users (on internet) from various databases (RADS, CTOH, AVISO and PISTACH). As part of an ongoing study (Application of Sea Level Data from SARAL-AltiKa for monitoring the coastal regions of India) being funded by ISRO, we attempted to analyze and compare each of these products for few select tracks in the Arabian Sea and the Bay of Bengal. To start with, analysis of various correction terms and their variability towards the coast has been carried.
Processing of the GDR data from AVISO (PISTACH) involved applying various correction terms (dry and wet tropospheric, SSB, inverse barometric and tidal corrections) to the raw data before comparison. The 1 (20) Hz along-track SSHA data are spatially separated at 7 (.35) km intervals which required spatial smooting using a low-pass filter, considering 9 (151) points at a time, to remove the high frequency noise. The next step was to estimate the biases between various data products.
Comparison of Jason-1 SSHA reveal that the AVISO data have a significantly large bias compared to RADS with CTOH (7.77 vs. 3.55 cm for cycle 038), while in the case of Jason-2 data, AVISO and PISTACH closely match towards coast as compared to offshore and both showed a positive bias with RADS data (6.68 vs. 12.42 cm).
These results form the basis for the future analysis of SARAL/AltiKa (sheduled to be launched by ISRO, India towards the end of 2010) SSHA data product.
Basic principles of satellite altimetry
Radar altimeters on board the satellite permanently transmit signals at high frequency (Topex-Poseidon - over 1700 pulses per second) to Earth, and receive the echo from the sea surface. This is analyzed to derive a precise measurement of the round-trip time between the satellite and the sea surface. The time measurement, scaled by the speed of light (at which electromagnetic waves travel), yields a measurement of the satellite-to-ocean range.
Each signal pulse produced by a radar a atimeter consists of coherent electro- m magnetic energy.
E Receiving the echo signal, the sensor e measures not only the amplitude but l also the phase of the reflected signal
and the travel time of the pulse.
As electromagnetic waves travel through the atmosphere, they can be decelerated by water vapor or by ionization. Once these phenomena are corrected for, the final range R is estimated within 2 cm.
The ultimatthe ultimate aim is to measure the sea level. This requires independent measurements of the satellite orbital trajectory, i.e. exact latitude, longitude and altitude coordinates.
Typical Satellite path
The Coastal Altimetry Concept
Satellite altimeters are routinely used to map the variability of the surface of the Earth’s oceans. The methodology requires a sequence of processing steps to get usable information. These steps essentially consist in removing from the altimeter observations orbits and unwanted effects caused by the instrument and environment (atmosphere and ocean). The official altimetry products generally contain sensor measurements, orbit estimations and a full set of geophysical corrections. These data are normally sufficient in open ocean regions to compute altimetric estimates without any additional requirement.
The accuracy of the altimeter data decreases when approaching the coast for a number of reasons, including inadequacy of some corrections (e.g., the high frequency ocean response to tidal and atmospheric loading), difficulties in estimating the mean sea level around the shelf break and issues of land contamination in the altimeter and radiometer footprints very close to the coast.
As coastal altimetry we define altimetry over that domain close to land where standard processing is problematic. Satellite altimetry currently has an observational record of almost 15 years from a series of missions starting in the early 1990s. We want to recover that information and exploit it to improve our knowledge of coastal ocean processes.
Coastal ocean is difficult to observe due to the dynamics of these waters and the wide spectrum of temporal and spatial variability of the various physical processes. Both space and time scales decrease in the coastal ocean as one approaches land. Satellite altimeters provide sustained and spatial observations of the phenomena, often unachievable by other means.
Traditional altimeter data will not be able to resolve the scales (<25 km and <10 days) of the variability in coastal currents and SSH fields, given the distance between tracks and the time between repeats.
In recent years, the coastal altimetry community has put in good efforts to overcome these problems and extending the capability of the altimeters very close to the coast, where the hidden information can be recovered.
Primary Objectives:
· Removal of noise in coastal altimetry data using Loess filter and
· Estimate biases between the above products
Details of Satellite data handled by different processing Centres:
Processing site / SatellitesRADS / GS, E1, E2, GFO, N1, TP, J1, J2
CTOH / T/P, J1, J2, GFO, N1
PISTACH / J2 processed data (2007 - present)
AVISO / J1, J2, TP
COASTALT (ESA) / N1 processed data (2008 - present)
Data Sources:
1] RADS – Radar Altimeter Database System (Netherlands)
The Radar Altimeter Database System (RADS) is DEOS' effort in establishing a harmonised, validated and cross-calibrated sea level data base from satellite altimeter data. It operates within the framework of the Netherlands Earth Observation NETwork, an internet facility, funded by the Dutch government, for exploitation of remote-sensing expertise and data. The RADS data base aims at users at both expert level and entry level.
Website –http://rads.tudelft.nl/rads/rads.html
2] CTOH - Centre of Topography of the Oceans and the Hydrosphere (at Legos, France)
The CTOH is a French Observation Service dedicated to satellite altimetry studies (Centre of Topography of the Oceans and the Hydrosphere) . The CTOH aims to help scientific users develop new altimetric products and applications.
Within this framework, the CTOH maintains homogeneous altimetric data bases for the long-term monitoring of sea level and ocean currents, lake and river levels, the cryosphère, and the planet's climate.
Scientific users can extract :
· alongtrack GDR data with up-to-date corrections, over oceans and continental surfaces, for different altimetric missions (Topex/Poseidon, Jason-1, Jason-2, GFO, ENVISAT).
· coastal alongtrack GDR data with specific Xtrack processing
· global surface currents (Geostrophic and Ekman) from 1999-2008
The CTOH works in close collaboration with scientific research groups at LEGOS to develop new altimetric products, for monitoring lake and river levels (HYDROWEB), and over the cryosphere (OSCAR).
Website – http://www.legos.obs-mip.fr/en/soa/altimetry/ctoh/cotier/
3] AVISO - Archiving, Validation and Interpretation of Satellite Oceanographic Data (CLS, France)
Ssalto/Duacs Sea level anomalies
Contents: along-track sea surface heights, computed with respect to a seven-year mean.
Use: study of ocean variability (mesoscale circulation, seasonal variation, El Niño...), including operational oceanography (near-real time products)
Website – ftp://ftp.cls.fr/pub/oceano/aviso/ssh/duacs/global/nrt/msl/en/h/
4] PISTACH - Prototype Innovant de Système de Traitement pour l'Altimétrie et l’Hydrologie
Satellite altimeter techniques are unfortunately limited by the emerged lands leading to a growth of the error budget of the altimeter products. To fulfil the need of coastal studies, the French Spatial Agency CNES set up in November 2007 its PISTACH initiative for improving Jason-2 altimeter products over coastal areas and also inland waters.
Since November 2008, the PISTACH prototype have been generating coastal dedicated Level 2 (I)GDR altimeter products freely provided to users trough an anonymous FTP website. The evaluation of the actual improvements and data quality reached near the coasts with this new dataset is still under investigation by users. This contribution will present the main results of the PISTACH project about user needs. It will also define all the new algorithms developed and implemented into the prototype and exhibit some early results of comparison with standard products.
Website – ftp://ftpsedr.cls.fr/pub/oceano/pistach/
Applications to the Indian coast
Altimetry products, derived with sufficient accuracy would support studies on monsoon driven coastal current systems and their seasonal and interannual variability.
Further, filling gaps in the coastal zone would provide current and wave observations which will help in ship routing, monitoring surges and long-term coastal sea level changes.
The ultimate aim is to integrate altimeter derived measurements of sea level, wind speed and significant wave height into coastal ocean observing systems.