Physics of Measurements
Yann Kerr, CESBIO Toulouse
JP Wigneron, CESBIO Toulouse
1. Main Issues:
Validating two- or three- parameters retrievals
from multi-angular observations over a variety of scenes and vegetation
The issues related to this objective are:
To adress these issues experimental measurements are needed from both ground-based
and airborne instruments
Correcting for the effects of vegetation:
Characterizing the dependence of optical depth t and
w as a function of view angle, polarization,
and vegetation type.
Accounting for within-pixel heterogeneity (mixed pixel include open water
surfaces, forests, agricultural areas, ...)
forest signature at L-band
snow at L-band
special events (frozen soils, dew effects, rain interception ...)
Correcting for the effects of topography (invariant with time) and surface
Improving knowledge of surface roughness effects, are they significant
at SMOS scale?
Testing methods to account for topography
Testing methods on actual 2-D interferometric observations (accounting
for actual sensor capabilities and performances)
Questions and actions:
Recommendations to date:
Radiometers and data sets exist already: Any new action should capitalise
on these so as to optimize scientific outputs.
for instruments : coordinate actions on required instrument characteristics
vs what exists
for data sets: pool existing measurements and design future campaigns accordingly
(i.e., types of vegetation not well covered, adequate instrumentation,
campaigns: coordinate actions over different vegetation types
Pool all existing data sets over soil and vegetation surfaces and document
Coordination of future experiments (campaign design , measurement protocols,
Define standard calibration procedures
2. Experiments which are most needed:
An accurate characterization of the microwave signatures of a variety of
vegetation covers are needed for two main reasons:
(1) for the vegetation cover types where SM retrieval is feasible (grass,
crops, ...), it is necessary to improve the retrieval algorithm over this
specific vegetation cover, by accounting for the dependence of optical
depth on polarization and incidence angle.
(2) for some other vegetation canopy types (dense forests, ..),
SM retrievals amy be questionable, but these canopies will be included
in SMOS pixels. For instance, forest canopies will be included in most
SMOS pixels in Europe.
Thus, an accurate characterization of the microwave signatures of forests
in required in order to perform SM retrievals from time variations of the
average pixel emissivity (in relation with time variations of emissivity
in non-forested areas).
Five main types of vegetation cover categories were experiments are
most needed, were defined:
|| % Coverage
|| Scheduled Experiments & existing or near future Study
|Arid / Semi Arid areas
FAUGA Toulouse (J-C Calvet) long term Ground Based measurements (end 2001
- 2003 )
A. Van de Griend / L. Simmonds (Gr. based , 2001 ?)
|Agricultural crops mainly:
Avignon 2001, (JP Wigneron , INRA / TUD) 4 -month, Gr. based, on corn,
wheat, bare soils polarimetric measurements
Univ of Reading (L Simmonds) 2001. 'multiple angle measurements over crops'
Orgeval site (C. Loumagne, C Ottley), airborne meas. (2002?), wheat
INRA/CETP : (Wigneron, Pardé, ..): 'On-going study based on existing
round-based data sets (wheat, corn, soybean, ...)'
Valencia site (Spain) E. Lopez airborne measurements (2001 ?)
Northern forests (M Hallikainen, HUT) airborne measurements (2001 ?)
Univ. Roma (P. Ferrazzoli and L Guerriero) : 'On-going simulation study
based on discrete modeling and ground-based data sets'
3. Calibration / Validation site
Collaboration with USA have to be developed , in particular to merge with
the AMSR team as requirements are very similar to those of SMOS
Possible sites over the land surfaces for validation are:
-> It is necessary to build list including detailed
information with specifications
Valencia site (agricultural, matorral, mediterranean forests) (E. Lopez,
Univ. of Valencia) .At the present time, this site is the main site considered
for SMOS validation studies in the near future. Other sites are:
Africa (Botswana, site used in previous studies of A. Van de Griend et
4. Dielectric constant
For instance, the effect of salinity on the dielectric constant of soils
is not well-known, as shown from studies with SSM/I (A. Van de Griend or
Measurements of both soil and vegetation dielectric constants (?) are needed
Also, measurement devices would be very useful to monitor e
during experimental campaigns.
Some works are being made by C. Prigent (Obs. P Meudon) to improve
our knowledge of soil permittivity (pb of salinity, ...)
5. Within-pixel heterogeneity
Several studies will investigate the effects of within-pixel heterogeneity
on TB over land surfaces, in the near future.
Two main works (main contacts) are
1) ESA ITT : AO/1-3652/00/NL/DC, 'Study on soil moisture
retrieval by a future space-borne earth observation mission'. (2 years
- accepted Nov 2000), Resp. L Simmonds (Univ of Reading) in relation
will several european scientists. Task 2 and 3 concerning direct modeling
and retrieval studies will investigate the microwave emission from SMOS
pixels (at spatial scale ~ 40 km) and will account for within-pixel heterogeneity
(using land cover maps at scale of ~ 5 km)
In complement of the simulation studies, airborne measurements at different
flight altitudes could be carried out over heterogenous scenes (including
agricultural areas , grassland and forests ...) in the S/W of France.
2) NASA Proposal, 'Remotely sensed soil moisture from SMOS in land
data assimilation systems with heterogeneous land cover', J. Shuttleworth
and E. Burke (Univ of Arizona)
Develop and test methods that will allow patch specific estimates of
soil moisture in the NASA-Goddard and NCEP Land data assimilation system
(LDAS) from multiple measurements of pixel-average TB data.
Other studies are to be carried out in Europe:
For all these works, there is an urgent need for a simulator (to account
accurately for SMOS measurement configuration) and for a demonstrator for
carrying out the airborne campaigns.
A. Vand de Griend & L Simmonds : simulation study of within-pixel
heterogeneity effects, by combining microwave signatures of several crop
JP Wigneron & M Pardé (Alpilles site): simulation over
an actual agricultural site (Alpilles) during one year, by aggregation
of TB for each field, obtained by coupling the INRA crop growth model (STICS)
with TB models.
M Hallikainen, Scandinavia site (HUT): airborne measurements at
different flight altitudes could be carried out to study heterogenous scenes
(including frozen soils, snow, forests, ...). Funding will be required
for this airborne campaign.
It seems that based on existing data sets and expertise, it is possible
to correct for this effect, since,
topography characteristics are invariant with time (in general !)
Digital elevation Model (DEM) can provide accurate information on
topography at rather fine resolution
Preliminary responses to this problem will be given by the ESA / ITT, in
which topographic effects will be accounted for in some specific areas.
We do not know well what is the effect of topography on the accuracy of
the soil moisture retrievals.
7. Roughness effects
In order to study these two main aspects, it is necessary to develop simple
modeling approach of soil roughness effects (mostly over agricultural areas).
A study based on the PORTOS-93 data sets (over 7 types of roughness conditions
and a large range of SM values) was recently made. A simple and tractable
modeling approach was described an validated (Wigneron et al., IEEE-GE,
Probably, simple modeling approach including both topography and
small scale roughness effects can be developed for the SMOS retrieval algorithm.
Roughness effects are mostly invariant, except over agricultural areas.
Over such areas, seasonal change in the soil roughness characteristics,
in relation with the agricultural practices and rainfall events, may have
an impact on the land surface emissivity.
Other validations of this study can be carried out based on future
experimental campaigns (Avignon 01, Toulouse 02).
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