CRYOSAT WORKSHOPESRIN – 8 to 10 March 2005

Cryosat Data Processing in Near Real Time

for Oceanographic ApplicationsJ. BENVENISTE - ESA/ESRIN, Italy

OZ. ZANIFE, B. SOUSSI - CLS, FranceM. P. MILAGRO – SERCO, Italy

1) 1) IntroductionIntroduction

CryoSat is an Opportunity Explorer mission, dedicated to answer one science question. The focus of that science exploration is in the cryosphere. For many reasons, including for simplicity of the operations, CryoSat may acquire data systematically all over the planet in the classical altimeter mode, when it is not busy exploiting the special SAR and SARin modes. This means that classical altimeter data (in LRM mode) would be acquired over the ocean. The current CryoSat L2 LRM (Low Rate Mode) processing chain does not address specialised ocean processing, other than elevation, sigma C and sigma 0. Indeed ocean processing was not included in the original processing requirements. A review of the available processing documentation confirms that no specialised ocean processing is present other than the above retracker derived parameters. Specific additional processing steps needed to add ocean processing to the LRM L2 chain is to be encapsulated in a CFI software package developed by CLS, under an ESRIN contract.

2) Users 2) Users RequirementRequirementWell aware of the important user requirement for radar altimeter data over the ocean, today supplied by ERS-2, GFO, ENVISAT, TOPEX/Poseidon and Jason-1, ESA set out to see how ocean data could be exploited from the CryoSat mission and to develop the necessary processing chain to provide ocean data to users from the CryoSat archive. The user requirements are also for fast delivery CryoSat LRM data over the ocean, which are the same as for the ENVISAT RA-2 FDMAR and IMAR products (3 hours and 3 days latency, resp.). The main user requirement for fast delivery altimeter data is “Improved sampling in space and time” (Cotton et al 2004, in the GAMBLE report). The applications of fast delivery CryoSat LRM data over the ocean are twofold: sea state and sea surface height. The users of such a CryoSat FDMAR product are worldwide Met Offices, the Navies, Groups supporting offshore and coastal activities, Ocean circulation modelers using altimetry data through assimilation (the international GODAE Project). The required latency for assimilation in meteorological models and ocean battlefield models is 3 hours. The required latency for assimilation in the GODAE models is on the availability of the MOE orbit (2-3 days). This means that the more data that can be delivered in 3 hours the better. The 3 hour data and its complement disseminated several hours later will be used for offshore applications and assimilation in ocean models for ocean circulation forecasting using enhanced meteo fields and orbit solution. Today GODAE users are served by DUACS (CNES), including enhanced data from all flying radar altimeters.

3) 3) ProcessingProcessing

Ocean processing functions needed to be added to the original CryoSat ground processing to compute significant wave height, wind speed and the oceanographic corrections at 1 Hz. Operational requirements for the CryoSat Near Real Time ocean processing functionality, product generation, dissemination and archiving were specified. The ocean processor is an additional functionality of the IPF2 LRM processor component of the CryoSat Ground Segment, to be run in near real time. It was developed under contract with CLS (F), re-using and adapting the ocean retracker from ENVISAT RA-2 with aim to generate a CryoSat FDMAR product equivalent to the ENVISAT FDMAR product to be supplied to the ENVISAT FDMAR and IMAR users by ftp. After launch validation of the product is planned

Internal Buffer

L1b input

L2Output

Existing Algorithms Delta_Ocean_CFI

L1b L2 NRTIntermediateL2 NRT

TO COMPUTE AVERAGED ALTITUDE RATE AND DOPPLER EFFECTS

TO INITIALISE THE PARAMETERS

TO COMPUTE THE U AND V COMPONENTS OF THE MODEL WIND

TO COMPUTE THE TOTAL OCEAN TIDE

TO COMPUTE THE MEAN SEA SURFACE HEIGHT

TO COMPUTE THE GEOID HEIGHT

TO COMPUTE THE OCEAN DEPTH / LAND ELEVATION

TO COMPUTE LONG PERIOD EQUILIBRIUM TIDE HEIGHTS

TO COMPUTE THE SEA ICE FLAG

TO AVERAGE THE ELEMENTARY ESTIMATES

TO COMPUTE THE PHYSICAL PARAMETERS

TO COMPUTE THE 10 METERS ALTIMETER WIND SPEED

TO COMPUTE THE SEA STATE BIAS

TO COMPUTE THE WAVEFORM OFF NADIR ANGLE

INPUT DATA

OUTPUT DATA

(- Surface discriminator-Range and Corrections-Retracking-Slope correction-Elevation-Slope Doppler-MSS and Geoid)

From internal buffer

To internal buffer

4) Ocean NRT Product format4) Ocean NRT Product format

Delta Ocean CFI

Group Field Descriptor Unit

time Time stamp 1 Hz and 20 Hz TAI µ-seconds

Location Latitude, Longitude 1 Hz and 20 Hz µ-degree

Counter and MCD Source Packet Counter and MCD -

Orbit Altitude of CoG above reference ellipsoid 1 Hz and 20 Hz mm

Instantaneous altitude rate mm/s

Range Ku-band ocean range 1 Hz and 20 Hz mm

Ku-band OCOG range 1 Hz and 20 Hz mm

Associated standard deviation and number of valid points  

Range corrections Doppler correction mm

Delta Doppler correction mm

Model dry tropospheric correction mm

Model wet tropospheric correction mm

Inverted barometer height mm

High frequency atmospheric correction mm

Model ionospheric correction on Ku-band mm

Sea state bias correction on Ku-band mm

SWH Square of Ku-band Significant wave height 1 Hz and 20 Hz mm2

Ku-band Significant wave height mm

Standard deviation and number of valid points  

Backscatter Ku-band corrected ocean backscatter coefficient 1 Hz and 20 Hz dB/100

  Ku-band corrected OCOG backscatter coefficient 1 Hz and 20 Hz dB/100

Associated standard deviation and number of valid points  

Off nadir angle off nadir angle of the satellite from platform data deg/104

off nadir angle of the satellite from waveform data (TBC) deg2/104

Geophysical Mean sea-surface height mm

Geoid height mm

ocean depth/land elevation mm

Total geocentric ocean tide height (solution 2) mm

Long period tide height mm

Tidal loading height (solution 2) mm

Solid earth tide height mm

Geocentric pole tide height mm

Altimeter wind speed mm/s

U-component of the model wind vector mm/s

V-component of the model wind vector mm/s

Peakiness flags 20 Hz ku-band peakiness  

Flags Ku-band ocean retracking quality [20bits] flags

Corrections and geophysical flags flags

Altimeter surface type flag flags

Sea ice flag (TBC) flags

2) CryoSat operating modes 2) CryoSat operating modes mapmap

Blue: LRM (over oceans), Red: SAR (over sea-ice), Green: SARin (over ice-sheets)

4) CryoSat ocean 4) CryoSat ocean NRTNRT processing chain processing chain schemescheme

Existing LRM chain

Future enhanced LRM chain