Altimeter upstream data description
The L2 altimeter measurements used as input in the DUACS system consist in products from different missions, that are available under three forms, with different delays of availability:
- Fast delivery or Near Real Time products (OGDR or NRT). These products do not always benefit from precise orbit determination, nor from some external model-based corrections (Dynamic Atmospheric Correction (DAC), Global Ionospheric Maps (GIM)).
- The Intermediate or Short Time Critical products (IGDR or STC) that are the latest high-quality altimeter data produced in near-real-time
- Delayed Time or Non Time Critical product (GDR or NTC)
Details of the different L2 altimeter products sources and delay of availability are given in the following tables.
OGDR/NRT and IGDR/STC are both used in operational system while GDR/STC are involved in delayed time processing.
Table: Source and delay of availability of the different altimeter data used in input of DUACS system
| Altimeter mission | Type of product | Source | Availability delay |
|---|---|---|---|
| Sentinel-3A | NRT | ESA/EUMETSAT | ~3 h |
| STC | ~48 h | ||
| NTC | ~1 month | ||
| Sentinel-3B | NRT | ESA/EUMETSAT | ~3 h |
| STC | ~48 h | ||
| NTC | ~1 month | ||
| Sentinel-6A | NRT | ESA/EUMETSAT | ~3 h |
| STC | ~48 h | ||
| NTC | ~1 month | ||
| Jason-3 | OGDR | EUMETSAT/NOAA | ~3 to 5 h |
| IGDR | CNES | ~24h | |
| GDR | CNES | ~3 months | |
| Cryosat-2 | NOP | ESA/CNES | ~7 h |
| IOP | ~48 h | ||
| GOP | ESA | ~3 month | |
| Saral/AltiKa | OGDR | ISRO/EUMETSAT | ~3 to 5 h |
| IGDR | CNES | ~48 h | |
| GDR | CNES | ~2 months | |
| SWOT-nadir | OGDR | CNES | ~3 to 5 h |
| IGDR | CNES | ~48 h | |
| HaiYang-2C | IGDR | NSOAS | ~48h |
| HaiYang-2B | IGDR | NSOAS | ~48 h |
| GDR | NSOAS | Best effort | |
| HaiYang-2A | GDR | NSOAS | Best effort |
| Topex/Poseidon | GDR | CNES | Reprocessing only |
| Jason-1 | GDR | CNES | Reprocessing only |
| Jason-2 | GDR | CNES | Reprocessing only |
| Envisat | GDR | ESA | Reprocessing only |
| ERS-1 | GDR | ESA | Reprocessing only |
| ERS-2 | GDR | ESA | Reprocessing only |
| Geosat Follow On | GDR | NOAA | Reprocessing only |
L2 products with two different frequencies are used:
- The low frequency 1 Hz (~7 km). This frequency corresponds to the historical sampling used from the start of the DUACS production. It is used as input of the L3 1 Hz production when higher frequency is not available.
- The high frequency 20 Hz (~350 m). This frequency allows a better observation of the fine scale structures and near coast area. It however presents a higher level of measurement noise compared to the conventional 1 Hz and thus requires specific processing and/or corrections to extract the useful short wavelength signal. It is used for producing the L3 products with a final 5 Hz sampling (~1 km) frequency.
The following table gives relevant information about the characteristics of the different altimeters used.
Table : Altimeter missions characteristics and L2 products availability period
| Altimetric mission | Cycle duration (days) | Latitude range (°N) | Number of tracks in the cycle | Inter-track distance at equator (km) | Sun-synchronous | Dual-frequency Altimeter | Radiometer on board | Input (L2p) data availability Start-End dates |
|---|---|---|---|---|---|---|---|---|
| Sentinel-3A | 27 | ±81.5 | 770 | ~100 | Yes | Yes | Yes | 2016/03/01 (cycle 1) Ongoing |
| Sentinel-3B | 27 | ±81.5 | 770 | ~100 | Yes | Yes | Yes | 2018/11/23 (cycle 19; interleaved orbit) Ongoing |
| Sentinel-6A | 10 | ±66.0 | 254 | ~315 | No | Yes | Yes | 2020/11/30 (cycle 1) Ongoing |
| SWOT-nadir (Calval phase) | 1 | ±77.6 | 28 | ~1500 | No | Yes | Yes | 2023/01/17 (cycle 403) 2023/07/20 (cycle 588) |
| SWOT-nadir (science phase) | 21 | ±77.5 | 584 | ~140 | No | Yes | Yes | 2023/07/21 (cycle 1) Ongoing |
| Jason-3 | 10 | ±66.0 | 254 | ~315 | No | Yes | Yes | 2016/02/12 (cycle 1) 2022/04/07 (cycle 226) |
| Jason-3 Interleaved | 10 | ±66.0 | 254 | ~315 | No | Yes | Yes | 2022/04/25 (cycle 300) 2025/01/08 (cycle 400) |
| Jason-3 Interleaved Long Repeat Orbit | 9.86 | ±66.0 | 254 | - | No | Yes | Yes | XXX |
| Jason-2 | 10 | ±66.0 | 254 | ~315 | No | Yes | Yes | 2008/07/12 (cycle1) 2016/10/02 (cycle 303) |
| Jason-2 Interleaved | 10 | ±66.0 | 254 | ~315 | No | Yes | Yes | 2016/10/13 (cycle 305) 2017/05/17 (cycle 327) |
| Jason-2 Long Repeat Orbit | 9.86 | ±66.0 | 254 | - | No | Yes | Yes | 2017/07/11 (cycle 500) 2018/11/13 (cycle 612) |
| Cryosat-2 | 29 (sub cycle) | ±88.0 | 840 | ~98 | No | No | No | 2010/07/16 (cycle 7) 2020/07/31 (cycle133) |
| Cryosat-2 New Orbit | 25 (sub-cycle) | ±88.0 | 724 | ~98 | No | No | No | 2020/08/01 (cycle 200) Ongoing |
| Saral/AltiKa | 35 | ±81.5 | 1002 | ~80 | Yes | No | Yes | 2013/03/14 (cycle 1) 2015/03/31 (cycle 24) |
| SARAL-DP/AltiKa | 35 | ±81.5 | 1002 | - | Yes | No | Yes | 2015/03/31 (cycle 24) Ongoing |
| HaiYang-2A | 14 | ±81.0 | 386 | ~210 | Yes | Yes | Yes | 2014/04/12 (cycle 67) 2016/03/15 (cycle 117) |
| HaiYang-2A geodetic | 9 (sub cycle) | ±81.0 | 248 | - | Yes | Yes | Yes | 2016/03/30 (cycle 118) 2020/06/09 (cycle 289) |
| HaiYang-2B | 14 | ±81.0 | 386 | ~210 | Yes | Yes | Yes | 2019/11/15 (cycle 27) Ongoing |
| Topex/Poseidon | 10 | ±66.0 | 254 | ~315 | No | Yes | Yes | 1992/09/25 (cycle 1) 2002/04/23 (cycle 353) |
| Topex/Poseidon Interleaved | 10 | ±66.0 | 254 | ~315 | 2002/04/24 (cycle 354) 2005/10/08 (cycle 481) |
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| Jason-1 | 10 | ±66.0 | 254 | ~315 | No | Yes | Yes | 2002/01/15 (cycle 1) 2009/01/26 (cycle 259) |
| Jason-1 Interleaved | 10 | ±66.0 | 254 | ~315 | No | Yes | Yes | 2009/02/10 (cycle 262) 2012/03/03 (cycle 374) |
| Jason-1 Geodetic | 10.91 | ±66.0 | 280 | - | No | Yes | Yes | 2012/05/07 (cycle 500) 2013/06/21 (cycle 537) |
| Envisat | 35 | ±81.5 | 1002 | ~80 | Yes | Yes (S-band lost after cycle 65) | Yes | 2002/05/14 (cycle 6) 2010/10/19 (cycle 94) |
| Envisat-New Orbit | 30 | ±81.5 | 862 | - | Yes | Yes (S-band lost after cycle 65) | Yes | 2010/10/26 (cycle 95) 2012/04/08 (cycle 113) |
| ERS-1 | 35 | ±81.5 | 1002 | ~80 | Yes | Yes | Yes | 1992/10/23 (cycle 15) 1993/12/20 (cycle 27) & 1995/03/24 (cycle 41) 1996/06/02 (cycle 53) |
| ERS-1 geodetic | 168 | ±81.5 | 1002 | - | Yes | Yes | Yes | 1994/04/10 (cycle 30) 1995/03/21 (cycle 40) |
| ERS-2 | 35 | ±81.5 | 1002 | ~80 | Yes | Yes | Yes | 1995/05/15 (cycle 1) 2003/08/11 (cycle 86) |
| Geosat Follow On | 17 | ±72 | 488 | ~165 | No | No | Yes | 2000/01/07 (cycle 37) 2008/09/07 (cycle 222) |