Satellite Altimetry Permanent Service for Mean Sea Level

							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           1992
ERS-2                    1995
GFO                      1998
JASON                    2001
ENVISAT                  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
               Period     spacing       on

 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 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