Abstract's details
Can the Sentinel-3 Next Generation Topography Altimeter Mission's continuity with Sentinel-3 be assessed with a 4-Hour Tandem Phase?
Event: 2025 SWOT Science Team Meeting
Session: Oceanography: Calibration and Validation
Presentation type: Oral
Since 1993, satellite altimetry missions have consistently provided long-term records of Sea Surface Height (SSH) measurements. Tandem flight phases have been crucial in verifying and ensuring consistency between successive altimetry missions. These phases have played a key role in both reference missions (Topex/Poseidon, Jason-1, Jason-2, Jason-3, Sentinel-6 Michael Freilich) and complementary altimetry missions (such as Sentinel-3A and Sentinel-3B).
During a tandem phase, two consecutive altimetry missions closely follow each other on the same ground track with a time interval shorter than a minute. As they observe the same ocean simultaneously, measurement differences primarily reflect relative errors. These errors arise from instrumental variations in altimeter characteristics (e.g., altimeter noise), measurement processing (e.g., retracking algorithm), precise orbit determination, and mean sea surface. By averaging relative errors over periods exceeding 100 days, systematic instrumental differences can be accurately determined, enabling precise calibration of the two altimeters.
For the future altimetry constellation, Copernicus Sentinel-3 Next Generation Topography (S3NG-TOPO) will succeed Sentinel-3 (S3), supporting various Copernicus applications. The S3NG-TOPO mission aims at maintaining the continuity of existing S3 nadir-altimeter measurements while enhancing measurement capabilities and performance. A tandem phase between S3 and S3NG is expected to evaluate this continuity.
Due to design constraints, S3NG satellites will be launched into a sun-synchronous orbit with a Local Time of Ascending Node (LTAN) at 6 pm, differing from the current S3 constellation's 10 pm LTAN. As a result, the tandem phase between S3 and S3NG satellites will have a 4-hour separation instead of the current few seconds.
This study evaluates the impact of the 4-hour tandem phase on the assessment of S3NG-TOPO's continuity with S3. Using the exceptional distribution of crossovers measurements between SWOT KaRIn (swath) and S3 (nadir), we analyzed the variance of ocean variability and altimetry errors at 4-hours intervals. Additionally, we simulated a 4-hour tandem phase between S3A and S3B by adding random noise due to ocean variability and altimetry errors at 4 hours to the SSH differences of the real tandem phase between S3A and S3B (separated by a few seconds).
Our findings indicate that the 4-hour tandem phase offers intermediate continuity performance between a classical tandem phase and a non-tandem scenario. For example, at regional scales of about 1000 km, a 4-hour tandem phase of 6 months would achieve an offset uncertainty of +/- 5 mm (at 1-sigma), while a classical tandem phase gives a result close to +/- 2 mm (at 1-sigma), and a non-tandem configuration would be above +/- 20 mm (at 1-sigma). This finding highlights the 4-hour tandem phase's potential to meet continuity requirements, depending on user needs. Moreover, this study emphasizes the crucial role of SWOT data in characterizing oceanic variability and improving altimetry performance and continuity assessments.
Back to the list of abstractDuring a tandem phase, two consecutive altimetry missions closely follow each other on the same ground track with a time interval shorter than a minute. As they observe the same ocean simultaneously, measurement differences primarily reflect relative errors. These errors arise from instrumental variations in altimeter characteristics (e.g., altimeter noise), measurement processing (e.g., retracking algorithm), precise orbit determination, and mean sea surface. By averaging relative errors over periods exceeding 100 days, systematic instrumental differences can be accurately determined, enabling precise calibration of the two altimeters.
For the future altimetry constellation, Copernicus Sentinel-3 Next Generation Topography (S3NG-TOPO) will succeed Sentinel-3 (S3), supporting various Copernicus applications. The S3NG-TOPO mission aims at maintaining the continuity of existing S3 nadir-altimeter measurements while enhancing measurement capabilities and performance. A tandem phase between S3 and S3NG is expected to evaluate this continuity.
Due to design constraints, S3NG satellites will be launched into a sun-synchronous orbit with a Local Time of Ascending Node (LTAN) at 6 pm, differing from the current S3 constellation's 10 pm LTAN. As a result, the tandem phase between S3 and S3NG satellites will have a 4-hour separation instead of the current few seconds.
This study evaluates the impact of the 4-hour tandem phase on the assessment of S3NG-TOPO's continuity with S3. Using the exceptional distribution of crossovers measurements between SWOT KaRIn (swath) and S3 (nadir), we analyzed the variance of ocean variability and altimetry errors at 4-hours intervals. Additionally, we simulated a 4-hour tandem phase between S3A and S3B by adding random noise due to ocean variability and altimetry errors at 4 hours to the SSH differences of the real tandem phase between S3A and S3B (separated by a few seconds).
Our findings indicate that the 4-hour tandem phase offers intermediate continuity performance between a classical tandem phase and a non-tandem scenario. For example, at regional scales of about 1000 km, a 4-hour tandem phase of 6 months would achieve an offset uncertainty of +/- 5 mm (at 1-sigma), while a classical tandem phase gives a result close to +/- 2 mm (at 1-sigma), and a non-tandem configuration would be above +/- 20 mm (at 1-sigma). This finding highlights the 4-hour tandem phase's potential to meet continuity requirements, depending on user needs. Moreover, this study emphasizes the crucial role of SWOT data in characterizing oceanic variability and improving altimetry performance and continuity assessments.