NRT altimeter standards

The following table list the standards used in the NRT DUACS L3 1Hz and 5Hz products.

Parameter

Version used

Altimeter standards

 
Product standard ref
 
  • Sentinel-6A : L2P products (Aviso+, 2022)); HR mode used
  • Jason-3 and SWOT-nadir : GDR-F
  • HaiYang-2B : NSOAS
  • Cryosat-2: NOP/IOP baseline
  • SARAL-DP/AltiKa: GDR-T version, Patch 2 up to 2020/01/27; GDR-F after
 
Retracking
 
  • Sentinel-3A/B : SAMOSA
  • Sentinel-6A : NR
  • SWOT-nadir : MLE4
  • HaiYang-2B : MLE4
  • Cryosat-2: “Ocean MLE4” retracking. MLE4 fit from 2nd order Brown analytical model: MLE4 simultaneously retrieves the 4 parameters that can be inverted from the altimeter waveforms: Epoch (tracker range offset)⇒altimeter range; Composite Sigma⇒SWH; Amplitude⇒Sigma0; Square of mispointing angle; “Ice OCOG” retracking.Geometrical analysis of the altimeter waveforms, which retrieves the following parameters: Epoch (tracker range offset)⇒altimeter range; Amplitude⇒Sigma0
 
Orbit
 
  • Sentinel-3A/B : MOE for STC; Navigator for NRT
  • Sentinel-6A : MOE for STC; Navigator for NRT
  • Jason-3: MOE-F mixt (DORIS+GPS)
  • SWOT-nadir : MOE-F
  • HaiYang-2B : MOE-F
  • Cryosat-2: NOP: DORIS Navigator; IOP: DORIS Preliminary Orbit-GDR-E
  • SARAL-DP/AltiKa: MOE-F

Environemental corrections

 
Sea State Bias
 
  • HaiYang-2B : NSOAS
  • Cryosat-2: Empirical solution from Cryosat-2 Ocean Baseline-B data in LRM Mode
 
High Frequency Adjustment 
 
  • Sentinel-3A/B , Sentinel-6A, HaiYang-2B, Cryosat-2, SARAL-DP/ALtika : none
 
Ionospheric
 
  • Sentinel-3A/B , Sentinel-6A, Jason-3 : Dual-frequency altimeter range measurements; filtered
  • SWOT-nadir, HaiYang-2B, Cryosat-2, SARAL-DP/AltiKa: GIM model computed from vertical Total Electron Content maps (Chou et al., 2023) rescaled on the orbit altitude with IRI95 model (https://irimodel.org/ )
 
Dry troposphere
 
All missions: Model computed from ECMWF Gaussian grids (new S1 and S2 atmospheric tides are applied)
 
Wet troposphere
 
  • Sentinel-3A/B, Sentinel-6A : From Sentinel Microwave radiometer
  • Jason-3: AMR radiometer 
  • SWOT-nadir, HaiYang-2B , Cryosat-2: Model computed from ECMWF Gaussian grids
  • SARAL-DP/AltiKa: ALTIKA_RAD radiometer GDR-T patch2 up to 2020/01/27; GDR-F patch-4 after

Geophysical corrections

 
DAC
 
All missions: MOG2D High Resolution forced with ECMWF pressure and wing fields (S1 and S2 were excluded) + inverse barometer computed from rectangular grids. (Carrère and Lyard, 2003)
 
Ocean tide
 
All missions: FES2014b (S1 and S2 are included) (Carrere et al., 2015) before 2024/07/07; FES2022b (Carrère et al., 2023) after
 
Internal tide
 
All missions: (Zaron, 2019) HRETv8.1 tidal frequencies: M2, K1, S2, O1)
 
Pole tide
 
All missions: (Desai et al., 2015) & Mean Pole Location 2017
 
Solid earth tide
 
All missions: Elastic response to tidal potential (Cartwright and Edden, 1973; Cartwright and Tayler, 1971)
 
Loading tide
 
All missions: FES2014 (Carrere et al., 2016)
 
GIA
 
All missions: The DUACS products are not corrected from Glacial Isostatic Adjustment (GIA) effects

Reference surfaces

 
Mean Profile/ Mean Sea Surface
 
All missions: Hybrid MSS (SIO,CNES/CLS15,DTU15) before 2024/07/07; Hybrid MSS (SIO; CNES/CLS22; DTU21) (Laloue et al., 2024) after; Referenced to the 1993-2012 period
 
Mean Dynamic Topography
 
All missions: Before 2024/11/06: MDT_CNES_CLS18 (Mulet et al., 2021) combined with MDT_CMEMS_2020_MED and MDT_CMEMS_2020_BLK (Jousset et al., 2022; Jousset and Mulet, 2020) over the Mediterranean and Black Sea. Consistent with the 20-year reference period used for the SLA. 

From 2024/11/06: Global ocean: MDT_CNES_CLS22 (Jousset et al., 2023), combined with MDT_CMEMS_2020_MED and MDT_CMEMS_2020_BLK (Jousset et al., 2022; Jousset and Mulet, 2020) over the Mediterranean and Black Sea. Consistent with the 20-year reference period used for the SLA; Europe area: MDT Europe CMEMS_2024 (Jousset et al, in prep)