Transcript Overheadskabelon

Satellite Altimetry
- possibilities and limitations
by
Per Knudsen
Kort & Matrikelstyrelsen
Geodetic Department
Rentemestervej 8
DK-2400 Copenhagen
September 2001
Contents:
•
The system:
•
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:
Satellites:
Launch:
Skylab
1973
GEOS 3
1974
SeaSat
1978
GEOSAT
1984
ERS-1
1991
TOPEX/POSEIDON
ERS-2
GFO
JASON
ENVISAT
1992
1995
1998
2001
2001
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.
Repeat
Track
Inclinati
Satellite
spacing
on
Period
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:
1. Variability (RMS) of sea surface w/o
tides
2. Daily signals: Ocean tides
3. Meso-scale: El niño 1997 event
4. 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 nearreal 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