FAQ

What are the HDR and DBL file ?
All the products are composed of two files, the HDR and DBL. The HDR is the HeaDeR file containing general information about the acquisition itself (orbit number, the acquisition time…) The DBL (Data BLock) is a binary file contening the data obtained over the half-orbit (pole to pole).
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Why are the polarisations noted X and Y ?
The polarisation are noted X and Y to avoid confusion. They correspond to polarisation at the antenna frame. Classical H and V polarisations are at surface level frame. Those can differ significantly: between them, there a geometric transformation and the faraday rotation correction. To help transforming X and Y to Hand V values, a tool is provided in the tools page.
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What is the difference between Dual and Full polarisation ?
The instrument is monitoring in Full pol mode (i.e., the four stokes parameters). In this case, SMOS records alternatively the X, XY, Y, XY polarisations. XY is a cross polarisation (two polarisations at the same time with the different SMOS arms). The Dual pol is simply acquired in the two basic polarisations: X and Y . The mode is specified in the name of the product (D or F). SMOS operated in either Dual or Full during the first 6 months and then routinely in Full polarisation mode). The actual periods can be found in the L2SM product version changes page.
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What is the browse product ?
To help visualise the data and to have some means to compare or to merge with other data sets, a browse product is made available. It consists of data acquired at a given view angle (42.5°). This angle was selected as it corresponds to the maximum number of views.
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How is interpolated the brightness temperature in browse product ?
As SMOS is not a scanning system, the different points are not necessarily obtained at 42.5°. In order to have an almost fixed angle data are interpolated to 42.5 degrees. This angle is obtained by linear interpolation of all TB acquired at an angle between 37.5° and 47.5°. Note that the L1C browse corresponds to measured SMOS data while in the level 2 it is modelled brightness temperature at 42.5° that correspond to the results of the retrievals
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What are the minimal and maximal acquisition angle ?
SMOS is a multi-angular instrument that means, each point is seen at several incidence angles. The possible view angles range from 0 to 65°. The actual range depends on the distance to sub track for a given point (i.e., the position of the point in the field of view).
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How can I know the polarisation of the brightness temperature ?
To identify the polarisation of brightness temperature, you need to read the two first bits of flags field from L1C product. Remember that it is in the antenna frame and has to be converted to surface for normal use.
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Where can I find flags information ?
For L1C products, flags information are in the field « Flags ». Flags provide information on different spurious effects and corrections (sun refection, moon, RFI,… ).
For L2 products, flag information can be found in the L2 UDP product. Flags are separated in three fields in the product. The two main are the following:
- Confidence flags, relative to the quality of the retrieved parameters (soil moisture, optical vegetation depth,…) and RFI.
- Science flags are relative to surface conditions (snow, frozen,…).
For both products, the number read should be translated into a binary sequence to be interpreted (dec2bin fonction in MatLab). For L2 products, the exact meaning of the flags is described in the table « Structure of the Confidence Flag in the DSR » from the product specification documents that can be found on the Docs page. For L1C flags, the meaning is provided in the « Level 1 C Dual Polarization Reconstructed Brightness Temperatures Swath Product Data Block » table from the product specification documents.
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What is the resolution of the products ?
The spatial sampling of both L1C and L2 is 15km and is provided in an equal area grid: ISEA 4h9. It is a non regular grid. This sampling should not be confused with the actual radiometer resolution which is of 43km in average.
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What is the difference between L2 UDP and DAP products ?
L2 UDP product contains basic retrieval information (such as retrieved soil moisture values and vegetation optical thickness as well as the DQX). DAP file is mainly used by scientists working on the L2 algorithm.
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Which information is provided by DQX field ?
The Data Quality indeX (DQX) is an index related to the quality of the retrieved parameter. It takes into account the error in the parameter (i.e., SM, Tau, etc..) retrieval as well as the L1C brightness temperatures accuracy. It is provided in the parameter units (volumetric soil moisture units, usually between 0 and 0.1 for soil moisture for instance).
When DQX is equal to fill value (-999), the retrieval has failed and the « FL_NO_PROD » flag is set to 1.
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What are the brightness temperatures values provided by L2 UDP product ?
These brightness temperature values are modelled ones, obtained from the retrieved parameters. These are computed at 42.5° incidence angle in H and V polarisations. You can find two types of brightness temperature, at Surface Level (ASL) and Top Of Atmosphere (TOA).
Be carefull they do not correspond to a SMOS measurements given in L1c, but to the retrieval output !
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What will the L3 product provide ?
L2 are so called “swath products” provided through ESA, whereas L3 are global spatio temporal composites provided by national agencies (CNES for the French products). The composites are global and obtained over periods ranging from 3 days to 1 month.

Do you Speak SMOS ?

SMOS Soil Moisture and Ocean Salinity
AACES Australian Airborne Cal/Val Experiment for SMOS
ADF Auxiliary Data File
ATBD Algorithm Theoretical Based Document
CATDS Centre Aval de Traitement et des Données SMOS
CCN Contract Change Notice
CDTI Centre for the Development of Industrial Technology
CESBIO Centre d’Etudes Spatiales de la BIOsphère
CMN Control and Monitoring Nodes
CNES Centre national d’Etudes Spatiales
DBL DataBLock
DGG Discrete Global Grid
DPGS Data Processing Ground Segment
DQX Data Quality indeX
DSR DataSet Record
EAF Extended Alias Free
ECM External Calibration Manoeuver
ECMWF European Centre for Medium-Range Weather Forecasts
ESA European Space Agency
ESAC European Space Astronomy Centre
FTT Flat Target Transformation
FTR Flat Target Response
FOV Field Of View
L1PP Level 1Prototype Processor
LAI Leaf Area Index
OTT Ocean Target Transformation
RFI Radio Frequency Interference
SAA South Atlantic Anomaly
SCAO Système de Commande d’Attitude et d’Orbite (en anglais AOCS)
SETSI State Secretariat for Telecommunications and the Information Society
SMC Soil Moisture Content
SML2PP Soil Moisture Level 2 Prototype Processor
SODAP Switch-On and Data Acquisition Phase
SSS Sea Surface Salinity
STR Star TRacker
TB Brightness Temperatures
TIR InfraRed Temperature
UDP User Data Product

SMOS DPGS Acronymes Document

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