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

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

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.

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

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

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

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

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.

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.

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°

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.

Mean Sea Surface from multi-mission altimetry

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

Sea level variability

M2 ocean tides model

M2 loop

Seasonal Cycle

El Niño - 1997

El Niño - 1997

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

ERS

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 )

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

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

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