satellite altimetry - possibilities and limitations by per knudsen kort & matrikelstyrelsen...
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Satellite Altimetry- possibilities and limitations
by
Per KnudsenKort & MatrikelstyrelsenGeodetic DepartmentRentemestervej 8DK-2400 Copenhagen September 2001
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Contents:• The system:
• Principle of altimetry• Sampling characteristics
• Applications:• Mean sea surface• Sea level variability• Sea level changes
• Operational issues• Data services• In Europe
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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.
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Altimeter Satellites:Satellites: Launch:Skylab 1973
GEOS 3 1974
SeaSat 1978
GEOSAT 1984
ERS-1 1991
TOPEX/POSEIDON 1992ERS-2 1995GFO 1998JASON 2001ENVISAT 2001
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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
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The Altimeter:
The waveform:
The waveform depends on
• Travel time: Distance / height
• Surface roughness sigma_0 - Windspeed
• The slope - Significant waveheight
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Sampling of the Sea Surface
Altimeter data are collected along the satellites ground track with a spacing of 7 km – 1 sec averages.
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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.
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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.
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Orbit Parameters
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°
The actual coverage of the sea surface depends on the orbit parameters such as the inclination of the orbit plane and repeat preiod.
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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.
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Mean Sea Surface from multi-mission altimetry
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Applications 2
Sea surface variability: • Statistics of variability• Sea level anomalies
Periodic signals Non-periodic signals
Examples:1. Variability (RMS) of sea surface w/o
tides2. Daily signals: Ocean tides3. Meso-scale: El niño 1997 event4. Seasonal: Annual cycle
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Sea level variability
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M2 ocean tides model
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M2 loop
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Seasonal Cycle
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El Niño - 1997
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El Niño - 1997
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Applications 3
Sea level changes: • Global coverage – open ocean• Uniform Geocentric reference
• About 10 years of data Spatial characteristics
• Calibration needed at tide gauges
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ERS
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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 )
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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
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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
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ConclusionsSatellite 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