Minimize Ice sheet re-tracking

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

  • LRM: Maximum Likelihood Estimation of the 4th order (MLE4) is used to perform a fitting of the waveform with a 4-th parametric model (amplitude, arrival time/epoch, rise time/leading edge, and trailing edge decay).
  • SAR: The method is htat used for waveforms received in the SARin degraded case for the CryoSat-2 mission, where the phase and coherence information is unavailable. The retracking algorithm has been designed to exploit least squares fitting of a semi-analytical model of the echo. The echo model is a modified gaussian form. It corresponds to a six parametrizable function with 5-section modelling.

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

  • corrected "ice sheet" altimeter range
  • corrected "ice sheet" backscatter coefficient
  • elevation of echoing points
 

PARAMETER

Corrected Ice-Sheet Elevation of Echoing Points

NETCDF NAME

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

DESCRIPTION

LRM mode: ice-sheet retracking using the ALT_RET_ICE_04 algorithm.
SAR mode: ice-sheet margin retracking using the ALT_RET_ICE_05 algorithm.
ALT_RET_ICE_04 algorithm merges MLE4 retracker results into the ice chain. In the ALT_RET_ICE_05 algorithm, a model of the SAR mode waveform is fitted to the received power. The amplitude and retracking offset are derived from analysis of the fitted waveform. The method is that used for waveforms received in the SARin degraded case for CRYOSAT where the phase and coherence information is unavailable. The ALT_PHY_GEN_01 algorithm computes the elevation and location of echoing points.
The instrumental corrections already included are: USO drift correction (uso_cor_[x1]_[x2]), internal path correction (int_path_cor_[x1]_[x2]), distance antenna-COG (cog_cor_[x1]_x2]), Doppler slope correction (dop_slope_cor_[x1]_[x2).

UNITS

10-4 m. An offset of 700 000 m to be added to the value read from the netCDF file.

DATA SET AVAILABILITY

Reduced: None
Standard: 20 Hz Ku band only
Enhanced: 20 Hz Ku band only

OPERATING MODE AVAILABILITY

LRM and SAR

DATA ORIGIN

1 Hz Ku/C-bands: None
20 Hz Ku-band: ALT_PHY_GEN_01 algorithm
20 Hz C-band: not computed to date

ADDITIONAL INFORMATION

None

PARAMETERS RELATED

None

PARAMETER DETAILED DESCRIPTION

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

Table 1. Elevation

 

 

PARAMETER

Corrected Ice-Sheet Altimeter Range

NETCDF NAME

range_ice_sheet_[x1]_[x2]
[x1]: 1 Hz or 20 Hz data ratesV [x2]: Ku-band or C-band

DESCRIPTION

LRM mode: Ice-sheet re-tracking using the ALT_RET_ICE_04 algorithm.
SAR mode: ice-sheet margin re-tracking using the ALT_RET_ICE_05 algorithm.
The ALT_RET_ICE_04 algorithm merges MLE4 re-tracker results into the ice chain. In the ALT_RET_ICE_05 algorithm, a model of the SAR mode waveform is fitted to the received power. The amplitude and re-tracking offset are derived from analysis of the fitted waveform. The method is that used for waveforms received in the SARin degraded case for CRYOSAT where the phase and coherence information is unavailable.
ALT_COR_RAN_02 computes the general Doppler correction.
The instrumental corrections already included are: USO drift correction (uso_cor_[x1]_[x2]), internal path correction (int_path_cor_[x1]_[x2]), distance antenna-COG (cog_cor_[x1]_x2]), doppler slope correction (dop_slope_cor_[x1]_[x2]).

UNITS

10-4 m. An offset of 700 000 m to be added to the value read from the netCDF file.

DATA SET AVAILABILITY

Reduced: None
Standard: 20 Hz Ku/C bands
Enhanced: 20 Hz Ku/C bands

OPERATING MODE AVAILABILITY

LRM and SAR

DATA ORIGIN

1 Hz Ku/C-bands: None
20 Hz Ku-band: ALT_COR_RAN_02 algorithm
20 Hz C-band: not computed to date

ADDITIONAL INFORMATION

None

PARAMETERS RELATED

None

PARAMETER DETAILED DESCRIPTION

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

Table 2. Ice sheet altimeter range