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]

XVth Rencontres du Vietnam - 8-14/09/2019 2nd VSEO Remote Sensing of Inundation, Flooding and Wetland

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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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@ CNES

Rough homogenous surface (real ocean surface)


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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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Computed combining radar altimeter measurements acquired at two separate

frequencies over the ocean

Estimated using Total Electronic Content (TEC) models


Ionosphere correction: refraction due to the electron content


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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


Dry Troposphere correction: refraction of the dry gases of the atmosphere


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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



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Orbits: accuracy and coverage


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Orbits: accuracy



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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


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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