Monday, August 8, 2011
We are pleased to announce the availability of near-real-time (NRT) sea surface height anomaly (SSHA) measurements from the ENVISAT mission. These along-track (1-Hz) NRT SSHA measurements are posted with a typical latency of 7-9 hours from the last available measurement. The orbit altitudes used to derive the SSHA measurements have typical accuracies of 3 cm (RMS).
These ENVISAT measurements are provided on a new data product called the FGDR-SSHA (for Fast-delivery Geophysical Data Record Sea Surface Height Anomaly). The ENVISAT FGDR-SSHA has a similar format to the existing Jason-1 and Jason-2 NRT products (OSDRSSHA and GPS-OGDR-SSHA respectively). The primary difference is the name of the data field containing the highest-accuracy NRT SSHA measurements:
ssha_dyn - on the ENVISAT FGDR-SSHA product
ssha_dyn - on the Jason-1 OSDR-SSHA product
gps_ssha - on the OSTM/Jason-2 GPS-OGDR-SSHA product.
The ENVISAT FGDR-SSHA product is derived from a combination of:
- Altimeter and radiometer data provided on the ENVISAT Fast Delivery Geophysical Data Record (FDGDR).
- Forecast pressure fields from the National Center for Environmental Prediction (NCEP).
- Orbit altitudes derived from SSHA cross-over differences with NRT SSHA measurements from the Jason-2 GPS-OGDR-SSHA product.
We note that the "ssha_dyn" on the ENVISAT product has been globally leveled to "gps_ssha" on the OSTM/Jason-2 product. The ENVISAT "ssha_dyn" and Jason-2 "gps_ssha" thus have a common bias of approximately 17 cm, as measured by the absolute calibration sites. This, however, implies that they are approximately unbiased with respect to one another, simplifying simultaneous application of the ENVISAT, Jason-1 and Jason-2 NRT SSHA products for operational oceanography.
Note that the ENVISAT FGDR-SSHA product is a research-grade product that is generated on a best efforts basis by the Orbiter and Radio Metric Systems Group at JPL.
The FGDR-SSHA product is available via ftp at the Physical Oceanography Distributed Active Archive Center (PO.DAAC):
We thank our colleagues at the European Space Agency (ESA) and Centre National d'Etudes Spaciales (CNES) for providing us with near-real-time access to the Envisat FDGDR data product and the DORIS-DIODE ephemerides, as well as additional assistance that they have provided. This FGDR-SSHA product would not be possible without their support and cooperation.