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SATELLITE ALTIMETRY

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   INTRODUCTION
   HOW IT WORKS
       Surface Height
   ALTIMETRY : DATA FLOW
       Data Acquisition:
       Data Processing:
        o   Altimeter measurements
        o   Geophysical corrections
        o   Instrumental errors
        o   Reference surfaces
        o   Flags-
        o   Retracking-
       Data Qualification: ”Calval”
        o Calibration DATA phase
        o Data exploitation

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   MISSIONS-
     Topex / Poseidon satellite
   APPLICATIONS
   CONCLUSION
   REFERENCES.




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 A technique for measuring height.

 Measures the time taken by a radar
pulse to travel from the satellite
antenna to the surface and back to
the satellite receiver.


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   Emits a radar wave signals
    at high frequencies
    (over1700pulse per sec) and
    analyses the return of the
    same
   Analysis includes SURFACE
    HEIGHT ,AMPLITUDE
    ,WAVEFORM of the signal



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Surface height (corrected) =
 Altitude - ( corrected) Range


 For oceans ,the sea surface height
  integrates effects such as:

• GEOID : sea surface height exists
  with out the influence of any
  factors ( wind , currents , tides
  etc.)
• DYNAMIC TOPOGRAPHY or ocean
  circulation : permanent circulation
  linked to Earth's rotation,
  permanent winds.
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   Step 0 – Data acquisition : data from the
    satellites are downlinked to the ground
    stations.
   Step 1 – Row telemetry ( level 0) and
    level 1data : row telemetry is forwarded
    to quality control and processing units
    and processed to obtain level 1 data( ie
    timed and located data with appropriate
    units, and checked for quality)



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   Step 2 – Level 1 data and level 2
    geophysical data: corrections for
    different errors are done , level 1data
    for instrumental and atmospheric
    errors. Precise orbit determination also
    performed for accuracy .
   Step 3 – Data validation and
    qualification: geo physical data validated
    for quality assurance . Validation involves
    precise quality control and monitoring the
    instrumental drift.


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   Step 4 – Level 3 and level 4 data :”
    value added products”: level 3 data
    validated, cross calibration between the
    missions done .level 4 are multi satellite
    gridded data




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   Data acquisition from the altimetry
    satellites is performed by each agency.
   Several ground stations around the
    world (wallops for NASA, Sweden for
    ESA)
   Collects raw data from satellites and
    send them to the control and processing
    centres



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   Altimetry data processing performed by
    ground segment includes :

     i.     Altimetry measurements
     ii.    Orbit Determination
     iii.   Geophysical corrections
     iv.    Instrumental errors
     v.     Reference surface
     vi.    Retracking
     vii.   High-level processing


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   Altimetric range -
   Significant wave height –is computed
    from the slope of the return radar pulse.
   Backscatter coefficient – from the
    power of the altimeters return pulse.
   Wind speed – backscatter coefficient
    can be related to wind speed. Emperical
    models have established relation ship
    b\w wind speed ,back scatter and
    significant wave height.

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Following corrections are computed:
 Geophysical corrections :
    •   Ocean tides
    •   Solid earth tides
   Propagation corrections:
    • Ionosphere : correction for path delay in the
      radar signal due to the atmospheres electron
      content
    • Wet troposphere : correction due to liquid water
      in atmosphere.
    • Dry troposphere : due to the presence of dry gas
      component in the atmosphere.
   Surface corrections:
    • Inverse barometer: correction for variations in
      sea surface height due to atmospheric pressure
      variation.
    • Electromagnetic bias : due to errors formed by
      varying reflectivity of wave crust and troughs.

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   USO correction ( Ultra – stable
    Oscillator): correction for drift of
    onboard oscillators used for sending
    radar pulses.
   Centre of gravity: due to the variation of
    satellite’s Centre of gravity due to fuel
    consumption solar panel orientation .
   Wave form : corr for effects due to
    filters used to eliminate certain
    frequencies in the return of radar signal.
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   Reference ellipsoid : is a rough
    approximation of the Earths shape,
   Geoid : is the shape of sea surface in the
    absence of perturbing forces.
   Mean dynamic topography: is the
    permanent stationary component of ocean
    dynamic topography
     MSS=GEOID +MDT



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•   Used to indicate some data such as –
    what type of surface beneath the
    satellite ,which instruments are on ,
    which frequencies are used.


•   Retracking altimetry data is done by
    computing the departure of the
    waveform's leading edge from the
    altimeter tracking gate and correcting
    the satellite range measurement
    accordingly.
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•   Calval (CALibration / VALidation ) is what
    determines data quality.
•   Involves a series of quality controls designed
    to ensure a continuous supply of data.
•   Calval operations consist in determining data
    quality and keeping a check on instrument
    drift.
•   Quality control is achieved through statistical
    data analysis, analyses at crossover points, and
    comparisons between orbit cycles.
•   Data are also compared with other missions.


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•   Absolute calibration is the comparison
    of the engineering measurement with an
    independent measurement (e.g. buoy,
    tide gauge) of the same parameter
    (range, sea surface height, etc.).
•   Absolute calibration provides one
    reference point for the complete
    altimetric time series



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•   In relative calibration, two altimetry
    systems are compared through their
    global geophysical data products. Due to
    the huge number of globally-distributed
    measurements processed, relative
    calibration is significantly more precise
    than local absolute calibration
•   Ensure consistency.



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•   After the Calibration Phase, the Routine
    Exploitation phase begins, and continues
    until the end of the altimetry mission.
•   The commissioning phase group is then
    required to formulate further
    recommendations at the completion of the
    phase .
•   During the routine phase these
    recommendations are taken on board, and
    new instrument or processing anomalies are
    investigated and solved.

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   Selecting a proper
    inclination, we should taken
    into account that the
    precession of satellite orbit
    should not coincide with the
    harmonics of tidal cycle.
   As a result TOPEX /
    POSEDON AND JASON – 1
    satellites are formed, have
    altitude 1300km and
    inclinaton 66 degree.



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    Topex/posedon is based
     on three earlier earth –
     orbiting satellite missions.
1.   GEOS-3
2.   SEASAT
3.   GEOSAT




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   It’s primary instrument
    for measuring ocean
    topography is an
    altimeter
   A second instrument
    called a radiometer is
    used to correct the
    influence of water in
    atmosphere




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   The TOPEX/POSEIDON
    spacecraft uses a high –
    precision radar
    altimeter to take
    measurements of sea –
    surface height over 90%
    of the world’s ice – free
    oceans .
   Approximately 10 days
    to complete 127 orbits.
   Has improved
    instrumentation and
    has most accurate orbit
    determination
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   Its altimeter has a
    measurement
    precession of
    approximately 2.5 cm
   After correction
    resulting sea level is
    accurate to 5 cm.




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1.   GEODESY & GEOPHYSICS-
2.   OCEANOGRAPHY-
3.   CLIMATE-
4.   LAND APPLICATIONS –
     etc.




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   Precise satellite altimetry missions have transformed the
    way we view Earth and its oceans. Highly accurate
    altimetry measurements give us the ability to observe
    sea surface height systematically.
   A wealth of applications are possible using radar
    altimetry measurements, involving most geoscience
     fields and practised by more than a thousand teams
     of users around the world.
    From the 'historical' applications (geodesy, general
     ocean circulation) to the developing ones (solid
     Earth and coastal applications, etc.) and the ones
     that have become classic (ocean variability, ice
     topography, hydrology), altimetry has shown over
     and over that it is a very productive technique.

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1.   Satellite Altimetry, The Marine Geoid, And
     The Oceanic General Circulation : Carl
     Wunsch And Detlef Stammer Department Of
     Earth, Atmospheric, And Planetary Sciences,
     Massachusetts Institute Of Technology,
     Cambridge Geophysical Applications of
     Satellite Altimetry David t . Sandwell
     Geological Research Division, Scripps
     Institution of Oceanography, La Jolla, CA
     92093 3.Bruce C. Douglas (1997). "Global
     Sea Rise: A Redetermination". Surveys In
     Geophysics 18: 279–292.

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2. Bruce C. Douglas (1997). "Global Sea Rise:
  A Redetermination". Surveys In Geophysics 18
  : 279–292.
3. .Radar Altimetry TutorialApril 2009
   V. Rosmorduc, J. Benveniste, O. Lauret,
   C. Maheu, M. Milagro, N. Picot
4. Recent Progress In The Application Of
   Satellite Altimetry To Observing
  The Mesoscale Variability And General
  Circulation Of The Oceans Jet
  Propulsion Laboratory, California Institute
  Of Technology Lee-Lueng Fu.

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THANK YOU
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