1. Recent Activities of Ocean Surface Topography Virtual Constellation (OST-VC)
Remko Scharroo (EUMETSAT)

2. OST-VC — Background Satellite altimeter launches since 2016
Fully operational
Jason-3 (Jan 2016)
Sentinel-3A (Feb 2016)
Sentinel-3B (Apr 2018)
In commissioning
HY-2B (Oct 2018)
CFOSAT (Oct 2018)
Long-term (25+ years) altimeter record
Cross-calibration and reprocessing
In-situ data
Various initiatives and projects

3. OSTST
Ocean Surface Topography Science Team (OSTST)
Is expanding to become a multi-mission international working group on all altimeter missions
Wide range of scientists and operational users with interest in Jason / altimetry missions
Sponsored by all Jason Partner agencies (CNES, NASA, NOAA, EUMETSAT) and ESA
Decision making on data processing algorithms and standards, reprocessing, validation protocols
Commissioning data releases to OSTST PIs for review prior to release to wider community
Mission-long cal/val activities

4. S3B minus S3A SSHA in SAR Mode
Excellent consistency
Std of difference only 3.5 cm
SSHA Bias is -7 mm (S3B below S3A)
Slight dependency of the difference with the sea state
Basin Scale Errors ? (waiting for better orbits)
Tracks Visible

5. Decision making by Joint Steering Group (Partner Agencies)
Jason-2 End-of-life
Small group of experts made recommendations to optimise between constellation needs and science output
Decision making by Joint Steering Group (Partner Agencies)
Currently in geodetic orbit with spatial resolution of 4 km (2 km planned)
To improve detail and accuracy in mean sea surface topography as reference for future missions
To improve knowledge of ocean bottom topography and map tens of thousands of unknown sea mounts

6. Jason-3 overall status
Reference altimeter mission
Jason-3 is now the reference mission for the monitoring of global and regional sea level rise
Cross-calibrated with Jason-2 to 1-mm level

7. Sentinel-3 Missions (1)
In-orbit verification, calibration and validation
Conducted largely by Partner Agencies (ESA, EUMETSAT, CNES)
Decision making on data release to S3VT and wider community
Sentinel-3 Validation Team (S3VT)
Wide range of scientists and operational users with interest in Sentinel-3
Meet approximately yearly to present cal/val results and report on data quality/availability and areas of improvement
Suggestions for future evolutions of algorithms/auxiliary data and reprocessing

8. Sentinel-3 Quality Working Group (S3QWG)
Sentinel-3 Missions (2)
Sentinel-3 Quality Working Group (S3QWG)
Consists of select group of key scientists or operational users (e.g. CMEMS) on various aspects of satellite altimetry applications:
Sea level, sea ice, land ice, winds and waves, climate, geodesy
Reports on quality analysis
Recommends evolutions to Partner Agencies

9. Sentinel-3 Missions (3) CEOS OST-VC Highlight
At time of Sentinel-3A launch it was planned to launch Sentinel-3B directly into its interleaved orbit
So no tandem mission
ESA and EUMETSAT with the help of OST-VC recommended to management to have a tandem mission first
Common to previous altimeter missions for long-term record
Unique for optical instruments
Paid off in detecting small inter-satellite (processing) differences

10. S3A/S3B Tandem Mission
Cross-calibration of SSH measurements
3.5 cm RMS
–7 mm mean (to climate quality level)

11. Sentinel-6/Jason-CS Status Satellite under integration and testing
System passed Critical Design Review
Processors under development
Organisation
Mission Performance Working Group (EUMETSAT, ESA, NASA, NOAA, CNES) reviewing product specifications and (in future) conduct cal/val and analyse product quality and evolutions
Integration in OSTST
Agencies are preparing call for participation of OSTST members in Sentinel-6 Validation Team
Team will have early access to commissioning data to do additional cal/val within their expertise and recommend evolutions
To be continued throughout mission lifetime

12. FRM4ALT: Fiducial Reference Measurements for Altimetry
International Review Workshop (April 2018)
Critical review of methodology
Cal/Val using ground-based measurements
Define requirements, establish standards
Calibration such that sea level can be monitored under international standards
Roadmap
Establish reference frame
Minimum set of observations
Harmonised approach with continuous capability
Requirements on in-situ equipment
Transponder, tide-gauges, radiometer

13. SAOO Phase 0 study
CNES Phase 0 study for new (swath) altimeter constellation
Based on requirement to resolve 50 km & 5 days resolution
Leads to either D altimeters or 2-3 swath altimeters
Still needs “traditional” 1-D altimeters for reference
ESA Phase 0 study for Swath Altimeter for Operational Oceanography (SAOO)
To lead to establishment of User Requirement Document on SAOO
Study can be expanded to white paper on future overall altimeter constellation
Based on up-to-date requirements on
Spatio-temporal coverage
Accuracy
Long-term stability
Observations of sea ice, continental ice, and inland waters
Accounting for merging of wide swath with nadir altimeter constellation
Intended as OST-VC activity for next 2 years

14. Contributing missions
CryoSat-2
Providing ocean topography data since 2010
Has been essential to the understanding of SAR altimetry in preparation of Sentinel-3 and Sentinel-6
SARAL/AltiKa
Orbital altitude not maintained since 2016, relying on accurate mean sea surface for SSH anomaly measurements
CFOSAT
Nadir information will be useful for developing higher level products
Hai Yang 2
HY2A: Moved to geodetic orbit in 2016
HY2B: Under commissioning

15. Summary
Best constellation of surface topography missions ever
Best constellation ever

16. Challenges ahead
Wide swath and conventional altimetry
Integration of conventional altimetry and wide swath to optimum temporal/spatial resolution and operational needs
Future of reference missions
What after Sentinel-6B?
Noting that Sentinel-3A/B/C/D will have more than 15 years of overlap with reference missions