Conclusions and Recommendations drawn from the Crete Workshop in Chania, 20-21/Jan/2011, are as follows:
- All satellite calibration/validation world sites should aim at two objectives: a) To report on the satellite altimeter bias and their byproducts, as users will directly apply these to get unbiased altimetry products; b) to try new corrections-algorithms to improve future altimetry products and provide more insight into the origin of errors and biases.
- All satellite altimeter Cal/Val sites in the world (Corsica, Gavdos, Harvest, Bass Strait) should make an effort to homogenize solutions and results. Homogenisation may entail the following actions:
- All Cal/Val sites should apply common processing strategies and loading models (tides, atmosphere etc,) for their GNSS data to produce comparable results in a common world reference system using the same world GNSS/IGS reference sites;
- The coordinates and velocities of their Cal/Val reference sites should be tied to the same global frame (i.e., center of mass of the earth);
- GLONASS measurements as well as GPS data is recommended to be used in their GPS reference site solutions;
- Different software, such as Bernese, GIPSY, GAMIT should be applied for precise positioning solutions of the reference sites;
- For the ionosphere corrections, GDR dual ionospheric corrections should be used, instead of GPS ionosphere modelling;
- The wet troposphere corrections, based on local GPS, should serve for comparison and to cross-validate satellite results. Radiometer corrections used over the whole ocean, must be used for continuity. Investigations using local GPS give insight on the error budget in coastal areas. Development of new algorithms (e.g., S. Brown) and/or new instruments is strongly encouraged to avoid land contamination.
- Retracking is also an important issue when satellites are approaching the coasts and should be applied;
- Looking at 18/20 Hz to assess quality of waveforms (that can impact estimates of ranges);
- Open ocean SSB models are not valid in coastal regions. The SSB correction may require further investigation, for example development of local models, use of instruments to measure wave, swell, new parameterisations to capture increased non-linearity of waves, etc. The SSB is intimately linked to waveform retracking, e.g., coastal wind sheltering will cause specular waveforms that affect retracking performance;
- Cal/val can benefit of advances in coastal altimetry, especially exploiting coastal-GDRs (from PISTACH, COASTALT, CTOH initiatives).
- When calibrating satellite radar altimeters with GPS buoys, then:
- GPS heights should be determined with the antenna radome on and off to determine biases in heights and systematic errors before deploying the buoy on the sea surface. These tests can be conducted on ground for long period (at least 1 day);
- In a GPS-buoy campaign, accelerometers may be installed to determine the significant wave heights and thus the sea-state bias; the SWH can be also determined by the raw movement of the buoy after applying a low-pass filtered time series.
- Atmospheric loading effects are of the order of sub-cm;
- Geoid heights and local measuring conditions may be shared by all Cal/Val sites;
- Digital Elevation Models are useful to have in altimetry calibration. Phillipa Berry may help in that direction for ACE2 and details.
- TIGA processing and analysis should be re-activated and all Cal/Val sites should be re-processed.
- The IGS does not maintain a public archive for all GNSS sites. More precise positioning solutions are required by the Cal/Val sites.
- Transponder calibration algorithms and procedures should be investigated for the new altimetric missions as well.
- Workshop Proceedings are planned to be published in an "Advances in Space Research" Special Issue.