Sea Level Anomaly altimeter products (SLAs) are                               
generated from AVISO GDR-M products for TOPEX/POSEIDON (T/P)                  
or from CERSAT OPRs for ERS altimeter data (phases C and G                    
for ERS-1, phase A for ERS-2). They are derived from                          
the CORSSH products.                                                          
                                                                              
Elementary records are one per second along-track data.                       
Software to access SLA data is provided on the CD ROM.                        
The SLA handbook was sent with the first SLA CD-ROM.                          
                                                                              
For more information, please use our web site:                                
http://www-aviso.cls.cnes.fr/English/SLA/Intro.html                           
                                                                              
Processing starts with quality control and validation of                      
the altimeter data and geophysical corrections.  Altimeter                    
measurements have many sources of error.  They need to be                     
corrected for instrumental errors, environment perturbations                  
(wet tropospheric, dry tropospheric and ionospheric effects),                 
the ocean wave influence (sea state bias), the tide influence                 
(ocean tide, earth tide and pole tide) and inverse barometer                  
effect. Together with the most recently updated version of                    
the GDR software, improved orbits and the latest state of                     
the art models and homogeneous corrections are used, such as                  
CSR3.0 tidal model, ECMWF for dry tropospheric and inverse                    
barometer corrections.  Orbit errors are then estimated.                      
Given the high accuracy of T/P orbits, no orbit error                         
correction is calculated for T/P.  The ERS data are fitted                    
to the more precise TOPEX/POSEIDON data using a global                        
minimization of ERS - TOPEX/POSEIDON dual crossover                           
differences (Le Traon et al., 1995 and Le Traon and Ogor,                     
1997).  This provides ERS orbits with accuracy similar to                     
T/P orbits: 2 cm rms. The ERS bias and any long-wavelength                    
error are estimated at the same time as the ERS orbit error                   
and included in the ERS orbit error correction. This provides                 
precise and homogeneous ERS and T/P CORSSH data sets.                         
                                                                              
Then the Sea Level Anomaly files are generated from several CORSSH            
files using the now conventional repeat-track                                 
analysis method. For a given track and for each cycle, corrected data         
are resampled every 7 km using cubic spline and differences relative          
to a given mean.                                                              
- For TOPEX/POSEIDON products, the mean is a 3-year mean                      
  from data, cycles 11 to 121, i.e. January 1993 - December 1995.             
- For ERS-1 and ERS-2 products,the mean is calculated from ERS-1 phases C     
  and G: a 3-year mean compatible with the TOPEX/POSEIDON mean.               
                                                                              
When refering to the products, please cite:                                   
"The SLAs products are supplied by the CLS Space Oceanography Division,       
Toulouse, France (AVISO/Altimetry, 1996; Le Traon et al. 1995,                
Le Traon and Ogor 1997). The ERS products were generated as part              
of the proposal "Joint analysis of ERS-1, ERS-2 and TOPEX/POSEIDON            
altimeter data for oceanic circulation studies" selected in response          
to the Announcement of Opportunity for ERS-1/2 by the European                
Space Agency (Proposal code: A02.F105)."                                      
                                                                              
References:                                                                   
                                                                              
AVISO/Altimetry,1996                                                          
--------------------                                                          
"AVISO User handbook for Sea Level Anomaly altimeter products",               
AVI-NT-011-312-CN, edition 2.0                                                
                                                                              
Le Traon, P.Y., P. Gaspar, F. Bouyssel, H. Makhmara, 1995                     
---------------------------------------------------------                     
"Using TOPEX/POSEIDON data to enhance ERS-1 data",                            
J. Atm. Ocean. Techn. 12, 161-170                                             
                                                                              
Le Traon, P.Y., F. Ogor, 1997                                                 
-----------------------------                                                 
"ERS-1 orbit improvement using TOPEX/POSEIDON: the 2 cm challenge" (submitted)