Satellite altimetry - icisequynhon.com€¦ · 1 Satellite altimetry - Measurement principle and...
Transcript of Satellite altimetry - icisequynhon.com€¦ · 1 Satellite altimetry - Measurement principle and...
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Satellite altimetry
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Measurement principle and main characteristics
Frédéric Frappart email: [email protected]
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1) Principle of radar altimetry measurement
2) Height estimate and corrections to the altimeter range
3) Orbits: accuracy and coverage
4) Overview of the altimetry missions and their characteristics
5) Acquistion modes: from LRM to InSAR
Contents
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Principle of altimetry measurement
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Principle of radar altimetry measurement
http://aviso.altimetry.fr
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Radar echo – Altimetry waveforms
@ CNES
Smooth homogeneous surface (ideal ocean surface – no wave)
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Radar echo – Altimetry waveforms
@ CNES
Rough homogenous surface (real ocean surface)
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Radar echo – Altimetry waveforms
P0: thermal noise
P: amplitude of the useful
signal τ: epoch at mid-height or
tracking point
Leading edge slope :
related to significant wave height (SWH) Leading edge curvature:
skewness
ξ: related to mispointing
antenna
http://www.altimetry.info
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Height estimate and correction to the altimeter
range
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Height estimate
h=H-R-Σcorr
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Corrections applied to the range
@ D. Dettmering
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Corrections applied to the range
Computed combining radar altimeter measurements acquired at two separate
frequencies over the ocean
Estimated using Total Electronic Content (TEC) models
http://www.altimetry.info
Ionosphere correction: refraction due to the electron content
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Corrections applied to the range
Approximated by :
where Po: sea level pressure (mbars) and φ: latitude (Saastamoinen, 1971)
A surface pressure error of 5 mbar corresponds to a range error of ~ 1 cm
00.002277 1 0.0026cos(2 )
dryR P
http://www.altimetry.info
Dry Troposphere correction: refraction of the dry gases of the atmosphere
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Corrections applied to the range
Wet Troposphere correction: refraction of water in the atmosphere
q: specific humidity (kg/kg), T (K)
An error of 10 m in the thickness of the atmosphere column
corresponds to an error of 1 mm in the correction.
31.116454x10 17.66543928 1 0.0026cos(2 )P P
qsurf surfR qdp dpwet P P Tsat sat
Computed from radiometer measurements or meteorological
model outputs
http://www.altimetry.info
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Orbits: accuracy and coverage
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Orbits: accuracy
http://www.altimetry.info
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Groundtracks
Topex/Poseidon/Jason-1/2/3 nominal orbit
Inclination: 66°, Repeat period: 9.92 days, Eq. cross-track: 315 km
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Groundtracks
ERS-2/ENVISAT/SARAL nominal orbit
Inclination: 98.55°, Repeat period: 35 days, Eq. cross-track: 80 km
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Groundtracks: multi-mission
Cryosat-2 ERS-2/ENVISAT/SARAL Sentinel-3A
Sentinel-3B Topex/Jason-1/Jason-2/Jason-3
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Overview of the radar altimetry missions and their
main characteristics
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Radar altimetry missions: past, current and future
https://podaac.jpl.nasa.gov/Altimetric_Data_Information
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Altimetry missions – main characteristics
Frappart et al., Encyclopedia EEE, 2017
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Frequency bands
Several different frequencies are used for radar altimeters. The
choice depends upon regulations, mission objectives and
constraints, technical possibilities and impossibilities. Each
frequency band has its advantages and disadvantages
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Frequency bands
Ku band (~13.6 GHz)
most commonly-used frequency
best compromise between the capabilities of the technology
(relating to power emitted), the available bandwidth (determined
by international regulations for specific applications), sensitivity to
atmospheric perturbations, and perturbation by ionospheric
electrons.
C band (~5.3 GHz)
more sensitive than Ku to ionospheric perturbation
less sensitive to the effects of atmospheric liquid water
Mainly used for correction of the ionospheric delay
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Frequency bands
S band (~3.2 GHz)
Similar use as C-band (only on ENVISAT)
Ka band (~35 GHz)
much less affected by the ionosphere than Ku-band
greater performance in terms of vertical resolution, time
decorrelation of echoes, spatial resolution and range noise
near-continuous altimetric tracking over all kinds of surface with
adapted tracker algorithm coastal regions
Ka-band EM waves are sensitive to rain
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Acquisition modes: from LRM to InSAR
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Low Resolution Mode
http://www.altimetry.info
Fatras et al., Remote Sens., 2018
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Synthetic Aperture Radar mode
http://www.altimetry.info @ R.K. Raney, Johns Hopkins University Applied Physics Laboratory
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Interferometry SAR mode
Frappart et al., Encyclopedia EEE, 2017
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Cryosat-2
@ ESA
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Surface Water and Ocean Topography
@ NASA
KaRIn, altimeter-interferometer
(large-swath) in Ka-Band
Measurements over a 120 km
wide swath with a ±10 km gap
along the nadir direction
• Coverage > 90 % of the Earth surface.
• Will be able to resolve 100 meter wide rivers and lakes of 250 m²
• Water stages with an expected accuracy of 10 cm and a slope
accuracy of 1 cm/km
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Surface Water and Ocean Topography
h, h/x, h/y at 100 m of spatial resolution in the water mask, 1 km outside
(Biancamaria et al., Rev. Geophys., 2016)
Frappart et al., Encyclo. EEE (2017)
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