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

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

3. The System: 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.

4. Altimeter Satellites:
Launch:
Skylab
1973
GEOS 3
1974
SeaSat
1978
GEOSAT
1984
ERS-1
1991
TOPEX/POSEIDON
1992
ERS-2
1995
GFO
1998
JASON
2001
ENVISAT

5. The Altimeter: 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

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

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 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.
Satellite
Repeat
Period
Track spacing
Inclination
Geosat
17 days
163 km
108°
ERS
35 days
80 km
98°
T/P
10 days
315 km
66.5°

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

20. El Niño

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