Minimize Ocean re-tracking

Two different retrackers have been implemented in the Level-2 processor when operating over ocean:

  • LRM: Ocean-3, inherited from JASON-2 mission, is used to perform a fitting of the waveform with a 4-th parametric model estimation (epoch, composite sigma, amplitude and mispinting angle) based on a weighted least square error (LSE) minimization derived from a Maximum Likelihood Estimator (MLE).
  • SAR: Fully analytical re-tracker for SAR mode operation over open ocean and coastal zones inherited from SAMOSA project. It tries to fit the theoretically modelled multi-look L1B waveform to the real L1B SAR waveform, providing estimates of the epoch, composite sigma, amplitude and mispointing angle.

The main output of these two retrackers annotated in the Level-2 products are:

  • corrected "ocean" altimeter range
  • corrected "ocean" backscattering coefficient
  • corrected "ocean" significant waveheight
  • epoch (used to derive the corrected altimeter range)
  • composite sigma (used to derive the corrected backscattering coefficient)
  • thermal noise
  • square of the angle misspointing
 
 

PARAMETER

Ocean Retracking Outputs

NETCDF NAME

epoch_ocean_[x1]_[x2]
sigmac_ocean_[x1]_[x2]
sigmas_ocean_[x1]_[x2]
inverse_ms_slope_ocean_[x1]_[x2]
amplitude_ocean_[x1]_[x2]
max_power_ocean_[x1]_[x2]
thermal_noise_ocean_[x1]_[x2]
off_nadir_angle_wf_ocean_[x1]_[x2]
number_of_iterations_ocean_[x1]_[x2]
MQE_ocean_[x1]_[x2] where:
[x1]: 1 Hz or 20 Hz data rates
[x2]: Ku-band or C-band

DESCRIPTION

LRM mode: ocean re-tracking (ALT_RET_OCE_01 algorithm).
SAR mode: ocean/coastal re-tracking (ALT_RET_OCE_02 algorithm).
ALT_RET_OCE_01 algorithm performs the ocean re-tracking (called ocean-3 re-tracking from JASON heritage) on the waveforms. This algorithm estimates the altimetric parameters: epoch, composite Sigma, amplitude, square of the mispointing angle.The aim of the ocean-3 re-tracking algorithm is to make the measured waveform coincide with a return power model, according to weighted Least Square Estimators derived from Maximum Likelihood Estimators. Regarding mispointing, the return power model accounts for the second order terms.Moreover, within the Ku-band ocean-3 re-tracking, the square of the off-nadir angle is estimated together with the three standard altimetric parameters (epoch, composite sigma and amplitude).
The ALT_RET_OCE_02 algorithm performs the ocean/coastal zone re-tracking fitting the theoretical multi-looked waveform model to one 20 Hz Level-1B SAR waveform using the Levenberg-Marquardt method and retrieving geophysical variables as well fitting quality information.

UNITS

Epoch: 10-15 secs
Composite Sigma: 1015 secs
RMS height of the specular points: 10-4 m
Inverse mean square slope: 10-6 (scale factor)
Amplitude: 10-6
FFT power units Maximum Received Power: 10-4 Watts
Thermal noise: 10-6 FFT power units
Square of the mispointing angle: 10-4 degrees
Number of iterations: count Mean Quadratic Error (MQE) between waveform and model: 10-4 count.

DATA SET AVAILABILITY

Reduced: none
Standard: number of iterations and MQE between waveform and model only
Enhanced: all

OPERATING MODE AVAILABILITY

Epoch: LRM and SAR
Composite Sigma: LRM
RMS height of the specular points: SAR
Inverse mean square slope: SAR
Amplitude: LRM
Maximum Received Power: SAR
Thermal noise: LRM and SAR
Square of the mispointing angle: LRM and SAR
Number of iterations: LRM and SAR
Mean Quadratic Error (MQE) between waveform and model: LRM and SAR

DATA ORIGIN

1 Hz Ku/C-bands: none
20 Hz Ku-band: ALT_RET_OCE_01 (LRM) and ALT_RET_OCE_02 (SAR) algorithms
20 Hz C-band: ALT_RET_OCE_01 (LRM) and ALT_RET_OCE_02 (SAR) algorithms

ADDITIONAL INFORMATION

None

PARAMETERS RELATED

None

PARAMETER DETAILED DESCRIPTION

ESA's Global Land and Ocean Mission for GMES Operational Services

Table 1. Ocean re-tracking estimates

 

 

PARAMETER

Ocean Sea Surface Height Anomaly

NETCDF NAME

{ssha_[x1]_[x2]} where:
{[x1]}: 1 Hz or 20 Hz data rates
[x2]: Ku-band or C-band

DESCRIPTION

The sea surface height (SSH) above the sea level is only computed for Ku-band measurements. The equation used to compute SSH is:
SSH anomaly =
Orb_Alt - (Alt_Range_Ku + Alt_Iono_Cor_Ku + Model_Dry_Tropo_Cor + Rad_Wet_Tropo_Cor + SSB_Ku + MSS + Solid_Earth_Tide + Geo_Ocean_Tide_1 + Geo_Pole_Tide + X) where:
Orb_Alt: altitude of satellite
Alt_Range_Ku: Ku-band corrected ocean altimeter range
Alt_Iono_Cor_Ku: altimeter ionospheric correction on Ku-band
Model_Dry_Tropo_Cor: model dry tropospheric correction
Rad_Wet_Tropo_Cor: radiometer wet tropospheric correction
SSB_Ku: sea state bias correction in Ku-band
MSS: mean sea surface
Solid_Earth_Tide: solid earth tide height
Geo_Ocean_Tide_1: geocentric ocean tide height solution
Geo_Pole_Tide: geocentric pole tide height
X: for NRT products inverted barometer height correction. For STC and NTC products, MOG2D (inverted barometer height correction + frequency fluctuations of the sea surface topography).

UNITS

10-3 m

DATA SET AVAILABILITY

Reduced: 1 Hz Ku-band only
Standard: 1 Hz and 20 Hz Ku-band only
Enhanced: 1 Hz and 20 Hz Ku-band only

OPERATING MODE AVAILABILITY

LRM and SAR

DATA ORIGIN

1 Hz Ku-band: ALT_PHY_RAN_02 and ALT_MAN_INT_01 algorithms
20 Hz Ku-band: ALT_PHY_RAN_02 and ALT_MAN_INT_01 algorithms
20 Hz C-band: none

ADDITIONAL INFORMATION

None

PARAMETERS RELATED

None

PARAMETER DETAILED DESCRIPTION

ESA's Global Land and Ocean Mission for GMES Operational Services

Table 2. Sea surface height

 

 

PARAMETER

Corrected Ocean Significant Wave Height

NETCDF NAME

swh_ocean_[x1]_[x2]
[x1]: 1 Hz or 20 Hz data rates
[x2]: Ku -and or C-band

DESCRIPTION

LRM mode: ocean re-tracking.
SAR mode: ocean/coastal re-tracking.
The GEN_MEC_CON_01 algorithm derives the significant wave height from composite Sigma.
The ALT_COR_SWH_01 algorithm adds the following corrections: modelled instrumental errors correction (mod_instr_cor_swh_[x1]_[x2]), system bias.
The ALT_COM_SWH_01 algorithm computes a compressed estimate of the re-tracked significant wave height from elementary estimates.
The ALT_COM_SWH_02 algorithm computes a compressed estimate of the re-tracked composite sigma from elementary estimates.

UNITS

10-3 m

DATA SET AVAILABILITY

Reduced: 1 Hz Ku-band only
Standard: all
Enhanced: all

OPERATING MODE AVAILABILITY

LRM and SAR

DATA ORIGIN

1 Hz Ku/C-bands: ALT_COR_SWH_01 algorithm
20 Hz Ku-band: ALT_COR_SWH_01 algorithm
20 Hz C-band: ALT_COR_SWH_01 algorithm

ADDITIONAL INFORMATION

None

PARAMETERS RELATED

Ocean significant wave height validity flag (swh_ocean_qual_[x1]_[x2]). Flag indicating the use or not of the 20 Hz estimate of the ocean significant wave height in the computation of 1 Hz estimate.
RMS of the ocean significant wave height (swh_ocean_rms_[x1]_[x2]).
Number of valid points used to compute the ocean significant wave height (swh_ocean_numval_[x1]_[x2]).

PARAMETER DETAILED DESCRIPTION

ESA's Global Land and Ocean Mission for GMES Operational Services

Table 3. Significant wave heights