John Wilkin

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Remote Sensing of the Atmosphere and Ocean

Active microwave systems(1) Satellite Altimetry
John Wilkin
jwilkin_at_rutgers.eduIMCS Building Room
214C609-630-0559 (g-voice)
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Active microwave instruments

• Scatterometer (scattering from surface roughness)
• ocean vector winds
• Synthetic Aperture Radar (SAR)
• sea ice
• high resolution wind speed over water
• land mapping surface roughness and 3-D terrain
• surface currents and swell
• CODAR
• coastal ocean surface vector currents
• Altimeter

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Active microwave instruments

• Altimeters (nadir pointing radar)
• sea surface height (long wavelengths 50 km)
• mesoscale currents, eddies, fronts
• thermal expansion
• significant wave height
• wind speed
• gravity and bathymetry
• ice sheets

http//www.aviso.oceanobs.com
http//topex-www.jpl.nasa.gov
http//earth.esa.int/brat/html/general/overview_en
.html
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Microwave energy is largely unaffected by the
atmosphere It has almost 100 transmission
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Radar systems operate in the microwave region of
the EM spectrum
Ku-band 13.6 GHz C-band 5.3
GHz Poseidon dual-frequency altimeteron Topex,
Jason-1 and Jason-2
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Key Components of any Radar System Microwave
transmitter electronic device used to generate
the microwave EM energy transmitted by the
radar Microwave receiver electronic device
used to detect the microwave pulse that is
reflected by the area being imaged by the
radar Antenna electronic component through
which microwave pulses are transmitted or
received (usually shared on satellite systems)
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• The relationship betweenpower received P and
  power transmitted PT is given by the radar
  equation
• (1)
  (2) (3)
• Power of EM wave at range R. G gain
  of antenna
• Radiant intensity in the direction of the radar
  produced by scatter from a surface with a
  scattering cross-section s (which depends on area
  of target, fraction of incident radar pulse
  absorbed and scattered)
• Ae is antenna effective area1/(4pR2) is
  isotropic spreading over range R in both
  transmittedand received signal

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• Satellite Altimeters
• altimeters are nadir-pointing satellite-based
  radars used to measure the height of the surface
  of the Earth
• transmit a radar pulse that is reflected from
  the Earths surface
• measure the time it takes for the pulse to
  travel to Earth and back, t
• range from satellite to surface is R ½ ct
  where c speed of light
• Precision Orbit Determination (POD) systems
  measure the altitude of the satellite above a
  reference ellipsoid

• c 3 x 108 m/s
• satellite altitude 1200 km
• t 2R/c 0.008 s 8 milliseconds
• Poseidon uses 1700 pulses per second

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ERS-1 1991-2000
History of Altimetry
OSTM/Jason-2 2008 -
Envisat 2002 -
Cryosat-2 April 8, 2010 -
Jason-1 2001 -
http//www.youtube.com/watch?vcSaYIIBZk10
ERS-2 1995 -
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Sea surface HEIGHT (SSH)

• Sea Surface Height is satellite altitude minus
  range
• It comprises two contributions geoid and
  dynamic topography
• Geoid
• The sea surface height that would exist if the
  ocean were not moving. This surface is not
  strictly flat because of gravity variations
  around the planet due to mass and density
  differences associated with the seafloor. The
  geoid is a geopotential surface.
• Major bathymetric features deform sea level by
  tens of meters and are visible as hills and
  valleys of the geoid
• Dynamic topography
• The ocean circulation comprises a permanent mean
  component linked to Earth's rotation, mean winds,
  and density patterns
• and a highly variable component (wind
  variability, tides, seasonal heating, mesoscale
  eddies)

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Altimetry How it works
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Altimetry How it works
Reference ellipsoid Satellite position is
determined relative to an arbitrary reference
surface - an ellipsoid. This reference
ellipsoid is a raw approximation of Earth's
surface, a sphere flattened at the poles. The
altitude of Jason above the reference ellipsoid
is measured to within 3 cm.
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Geoid height (meters)
80
-80
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Sea surface HEIGHT (SSH)

• Sea Surface Height is ssh altitude minus
  range
• Geoid and dynamic topography
• To derive the dynamic topography,D, the easiest
  way would be to subtract the GEOID height from
  SSHD ssh geoid alt range geoid
• This is feasible for long wavelengths 1000 km
  along the Jason groundtrack the geoid is know
  well enough there.
• But the geoid is not yet known accurately enough
  at shorter wavelengths. So for many applications
  the time mean sea level is subtracted instead.
  This yields the variable part of the ocean
  signal, but removes the mean ocean circulation.

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The slope of the sea surface relative to the
geoid is directly related to the geostrophic
current that balances the pressure gradient (due
to the sea surface gradient) and the Coriolis
force The long-term mean ocean circulation has
an associated mean dynamic topography that is a
permanent component of the time-mean orbit range
as a function of position.
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Jason-1 satellite AVISO Web site http//www.aviso.
oceanobs.com/en/missions/current-missions/index.ht
ml
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Jason launch movies
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Satellite orbit and tracking

• The critical orbital parameters for satellite
  altimeter missions are altitude, inclination and
  period
• Topex/Poseidon and Jason satellites (same orbit)
• altitude 1336 km
• relatively high less drag and more stable orbit
  
• inclination of 66 to Earth's polar axis
• it can "see" only up to 66 North and South
• the satellite repeats the same ground track every
  9.9156 days
• the ground-tracks are 315 km apart at the equator
• track repeat precision is about 1km
• ground scanning velocity is 5.8 km/s, orbit
  velocity 7.2 km/s

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Where is Jason now?
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Jason-1 ResearchGround segment
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OSTM/Jason-2 OperationalGround segment
Tracking
Operational real-time products ground station
redundancy archive
Delayed-mode reanalysis for research quality
datasets
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Geostrophic current computed from altimeter sea
surface height gives only the component
perpendicular to the ground-track. To get
surface geostrophic current vectors we need to
map the SSH field in two dimensions. The high
alongtrack resolution (20km) is then lost because
of the large separation of the ground-tracks (315
km at Equator)
Where is Jason now?
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(a) (b)
(c)
Grid of sea surface height measurements by T/P,
ERS-2 and GFO in the Northeast Atlantic over (a)
10 days, (b) 7 days, and (c) and 3 days. There
are gaps in coverage of 200 km and more over 3
days. Combining data from all three missions
increases coverage. gt Multiple satellites are
required to resolve mesoscale current patterns
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Tropical Cyclonic Heat Potential computed from
altimetry on 28 August 2005, with Hurricane
Katrina's trajectory and intensity overlaid.
Katrina's intensification seems to coincide with
its crossing over the Loop Current.
Altimetry data in combination with historical
hydrographic observations are currently used to
estimate synthetic upper ocean temperature
profiles. These profiles are then used to
(empirically) compute the integrated vertical
temperature from the sea surface down to the 26C
isotherm. This quantity is referred to as the
Hurricane or Tropical Cyclone Heat Potential
(TCHP) Leipper and Volgenau, 1972 and
represents the amount of heat in the upper ocean
available for tropical cyclone intensification.
Leipper, D. and D. Volgenau. Hurricane heat
potential of the Gulf of Mexico, J. Phys.
Oceanogr., 2, 218-224, 1972.
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Altimetry How it works
For altimeter observations to be useful for
oceanography, range accuracy of order 2 cm is
required.
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• The challenges to achieving 2 cm accuracy are
• making range corrections for the atmosphere
• density of atmosphere, water vapor
• accounting for the aliasing of tides
• knowing the shape of a reference gravitational
  potential surface, or geoid, that defines a
  surface along which gravity is constant(and
  therefore dynamically level)
• computing the satellite position accurately

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Precision Orbit Determination

• The Jason-2 satellite is tracked in 3 ways
• GPS Payload (GPSP)
• continuously tracks up to 16 GPS satellites
• dual-frequency receiver
• estimates position to better than 50 m and time
  to 50 nanoseconds
• Laser Retroflector Array (LRA)
• an array of mirrors on the satellite that provide
  a target for laser-tracking measurements from
  ground stations
• round-trip time of the laser is another range
  measurement
• accuracy is a few mm, but only 10 to 15 stations
  are in operation
• DORIS
• receivers on the satellite measure Doppler shift
  of signal from ground-station beacons (at 2
  frequencies)
• gives satellite velocity
• a dynamic orbit model integrates the velocity and
  position data, drag, solar forces on satellite,
  to continuously compute the satellite trajectory

Where is Jason-1 now?
Where is Jason-2 now?
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Doris beacons transmit signals at two separate
frequencies (2036.25 MHz and 401.25 MHz) to the
satellite. The receiver onboard the satellite
analyzes the received signal frequencies to
calculate its velocity relative to Earth.
This velocity
is fed into orbit determination models to
derive the
satellite's position on orbit to within 2 cm on
the radial
component.
Doris receiving antenna under the satellite. It
receives signals from the terrestrial beacons
network (its length is 42 cm).
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The LRA is a totally passive reflector designed
to reflect laser pulses back to their point of
origin on Earth. It is used for the calibration
of the Precise Orbit Determination system on the
spacecraft.                                       
    LRA is an array of mirrors that provide
a target for laser tracking measurements from the
ground. By analyzing the round-trip time of the
laser beam, we can locate very precisely where
the satellite is on its orbit. The LRA is used to
calibrate the other location systems on the
satellite (Doris, GPSP) with a very high degree
of precision.