Satellite Altimetry - possibilities and limitations

1. Satellite Altimetry- possibilities and limitations by Per Knudsen Kort & Matrikelstyrelsen Geodetic Department Rentemestervej 8 DK-2400 Copenhagen September 2001

2. Contents: • The system: • Principle of altimetry • Sampling characteristics • Applications: • Mean sea surface • Sea level variability • Sea level changes • Operational issues • Data services • In Europe

3. Principle of altimetry – A space borne tide gauge: S: The distance is measured by the onboard altimeter, H: The position and height of the satellite is computed, SSH = H – S: The sea surface height above a reference ellipsoid is obtained. The System:

4. Altimeter Satellites:

5. The pulse and its return: The pulse is generated by a 13.5 GigaHz oscillator, transmitted at 1000 Hz, and averaged at 1 sec.s The return of the pulse depends on Distance Surface roughness Significant wave height The Altimeter:

6. The waveform: The waveform depends on Travel time: Distance / height Surface roughness sigma_0 - Windspeed The slope - Significant waveheight The Altimeter:

7. Sampling of the Sea Surface Altimeter data are collected along the satellites ground track with a spacing of 7 km – 1 sec averages.

8. Sampling of the Sea Surface One revolution of the satellites orbit takes about 100 - 115 min.s depending on the altitude of the satellite (800 – 1350 km). Hence, the satellite completes 13–14 revolutions per day.

9. Sampling of the Sea Surface The number of tracks, i.e. the track density, depends on the repeat period, e.g. 3 days. The coverage depends on the inclination of the orbit plane.

10. Orbit Parameters The actual coverage of the sea surface depends on the orbit parameters such as the inclination of the orbit plane and repeat preiod.

11. Applications 1 Mean sea surface: Reference surface Geo-centric Consistent with GPS / GNSS Example: Mean sea surface from a merge of about 10 years of altimetric sea surface heights.

12. Mean Sea Surface from multi-mission altimetry

13. Applications 2 Sea surface variability: Statistics of variability Sea level anomalies Periodic signals Non-periodic signals Examples: Variability (RMS) of sea surface w/o tides Daily signals: Ocean tides Meso-scale: El niño 1997 event Seasonal: Annual cycle

14. Sea level variability

16. M2 ocean tides model

17. M2 loop

18. Seasonal Cycle

19. El Niño - 1997

20. El Niño - 1997

21. Applications 3 Sea level changes: Global coverage – open ocean Uniform Geocentric reference About 10 years of data Spatial characteristics Calibration needed at tide gauges

23. ERS

24. Altimeter data services Satellite altimetry is made available through the space agencies: • ESA • NASA, CNES, • US-navy Upgraded / value added products are available through • Space agencies ( NASA/JPL, CNES/CLS ) • Research institutions ( CSR, DEOS, KMS, GFZ )

25. Operational issues Satellite altimetry is becoming available in near-real time to contribute to the monitoring of present changes in sea level to contribute to programmes such as • GOOS, and • El Niño forecasting Problems: • Not real-time (4-6 hours) • Insufficient sampling locally – more satellites

26. Operational issues Potential data products: • Mean sea surface • Statistics of variability, extreme sea level • Ocean tides • Seasonal cycles • Sea level anomalies • Sea level trends – decadal variations

27. Conclusions Satellite altimetry has proven its value in studies of • Mean sea surface • Ocean dynamics Satellite altimetry has shown its potential in studies of • Sea level changes Satellite altimetry has, furthermore, a potential in • Hydrography / surveying – Bathymetry - GPS • Operational sea level services