SARAL Near-Real-Time Value-added Operational Geophysical Data Record Sea Surface Height Anomaly (XOGDR-SSHA)
------------------------------------------------------------------------------------------------------------

Date: November 19, 2013
Update: October 3, 2016 (To note transition from xovers using Jason-3 instead of Jason-2)

The SARAL XOGDR-SSHA is a value-added product intended to provide improved
accuracy near-real-time (NRT) sea surface height anomaly (SSHA) measurements
for the SARAL/AltiKa mission. It is based on the official OGDR-SSHA [1], but takes
advantage of several improvements, most notably for the orbit altitude, but also
for the sea-state bias and wind speed. The product is identical to the
OGDR-SSHA [1], with the addition of the following fields:

    1) ssha_dyn           - Sea surface height anomaly computed using improved orbit
                            altitudes that are derived from inter-satellite crossover
                            differences with SSHA measurements from the Jason-3 GPS-OGDR-SSHA
                            product from Cycle 102 pass 569 onward, (OSTM/Jason-2 GPS-OGDR-SSHA
                            product before that). ssha_dyn currently also uses an improve sea
                            state bias model (sea_state_bias_jpl) described below.
                            The two SSHA measurements on the product, ssha_dyn and ssha, have the 
                            following two differences:
                            ssha_dyn uses orbit altitudes from (alt_dyn - xover_corr) instead of alt.
                            ssha_dyn uses sea state bias from sea_state_bias_jpl instead of sea_state_bias.
    2) wind_speed_alt_jpl - Derived by calibrating SARAL sigma0 to that from OSTM/Jason-2.
                            The calibrated sigma0 values are then applied to the Jason-2
                            wind speed model from Collard to obtain the altimeter wind speed.
    3) sea_state_bias_jpl - A 4-parameter model derived from inter-satellite SSHA crossover
                            differences with OSTM/Jason-2, and depending on significant wave height (swh)
                            and the calibrated wind speed (wind_speed_alt_jpl).
                            This sea state bias model therefore also includes a representation
                            of the inter-satellite SSHA bias with respect to OSTM/Jason-2.
                            This has not been updated for Jason-3, since the crossover orbit adjustment
                            implicitly levels the SARAL product to the Jason-3 GPS-OGDR from cycle 102
                            pass 569 onward.
    4) alt_dyn            - Dynamic fit to DORIS-DIODE orbit.
    5) xover_corr         - Cross-over based correction to alt_dyn. alt_dyn and xover_corr provide
                            the orbit altitude that is used to generate ssha_dyn.

Together, the SARAL XOGDR-SSHA (ssha_dyn) and Jason-3 (or Jason-2) GPS-OGDR-SSHA (gps_ssha) products
provide high accuracy NRT SSHA measurements from the two missions that have already been leveled
to each other. This leveling leverages the 1-cm accurate GPS-based NRT orbits for
Jason-3 (and Jason-2) [2,3], and is accomplished through the use of inter-satellite SSHA crossover
differences betweeh the SARAL and Jason-3 NRT measurements to derived improved accuracy
orbit altitudes for SARAL.

The SARAL XOGDR-SSHA product is available with a typical latency of 7-9 hours
from the last available measurement on each data file, and the orbit altitude
used to generate ssha_dyn has accuracies of approximately 1.5 cm (RMS). The XOGDR-SSHA
product is available at:

ftp://podaac.jpl.nasa.gov/allData/saral/preview/L2/XOGDR-SSHA/

The SARAL XOGDR-SSHA product is a value-added research-grade product that is
generated on a best efforts basis by the Near-Earth-Tracking Applications
group at the Jet Propulsion Laboratory. More details on the SARAL XOGDR-SSHA product
are provided in Ref. [4].

As always, we are grateful to our CNES colleagues for providing us with
near-real-time access to the various data inputs needed to generate the
SARAL XOGDR-SSHA. Additional information on the SARAL mission can be found at
the CNES/AVISO web site:

http://www.aviso.oceanobs.com/en/missions/current-missions/saral.html

Please refer questions or comments to:

Shailen Desai (Shailen.Desai@jpl.nasa.gov)
Bruce Haines (Bruce.Haines@jpl.nasa.gov)

[1] http://www.aviso.oceanobs.com/fileadmin/documents/data/tools/SARAL_Altika_products_handbook.pdf
[2] Desai, S., and B. Haines, Marine Geodesy 33(S1), 419,434, 2010.
[3] Haines et al., J Astronaut. Sci. 58(3), 445-459, 2011
[4] http://www.aviso.oceanobs.com/fileadmin/documents/OSTST/2013/posters/Desai_Haines_NRTSSHPoster_2_.pdf

Sources of data on the SARAL XOGDR-SSHA product
-----------------------------------------------

The SARAL XOGDR-SSHA product is generated from:
1) The official project Operational Geophysical Data Record (OGDR)
   and DORIS-DIODE orbit ephemerides.
2) Orbit altitudes derived from SSHA crossover differences with
   measurements on the NRT Jason-3 (and OSTM/Jason-2) GPS-OGDR-SSHA product.
   This implicitly levels ssha_dyn, from the SARAL XOGDR-SSHA product,
   and gps_ssha, from the Jason-3 (OSTM/Jason-2) GPS-OGDR-SSHA product.