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 types

The issues related to this objective are: To adress these issues experimental measurements are needed from both ground-based and airborne instruments

Questions and actions:

Recommendations to date:
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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   30% 
  • FAUGA Toulouse (J-C Calvet) long term Ground Based measurements (end 2001 - 2003 ) 
  • A. Van de Griend / L. Simmonds (Gr. based , 2001 ?) 
  • Grass lands  20%
    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 

  • Studies:  
  • INRA/CETP : (Wigneron, Pardé, ..): 'On-going study based on existing round-based data sets (wheat, corn, soybean, ...)' 
  • Forests
  • Valencia site (Spain) E. Lopez airborne measurements (2001 ?) 
  • Northern forests (M Hallikainen, HUT) airborne measurements (2001 ?) 

  • Studies:  
  • Univ. Roma (P. Ferrazzoli and L Guerriero) : 'On-going simulation study based on discrete modeling and ground-based data sets' 
  • Tundra areas 

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    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

    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 C. Prigent).
    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, ...)
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    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.
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    6. Topography

    Preliminary responses to this problem will be given by the ESA / ITT, in which topographic effects will be accounted for in some specific areas.

    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, submitted).
    Other validations of this study can be carried out based on future experimental campaigns (Avignon 01, Toulouse 02).