Algorithm Theoretical Based Document (ATBD) - CESBIO

SO-TN-ESL-SM-GS-0001
Issue 1.a
Date: 31/08/2006
SMOS level 2 processor
Soil moisture ATBD
In every of three cases ( (i) L2 RFI detection consistent with current RFI map, (ii) L2 RFI detection not consistent
with current RFI map, (iii) no L2 RFI detection although RFI is indicated in current map), the RFI current map
and/or statistics associated to it should be updated. (see § 3.2.5.2.1)
• BORDER_FOV and AF_FOV(=FALSE) are used to enhance corresponding radiometric uncertainties.
Similarly, SUN_TAILS and SUN_GLINT_AREA flags are used to enhance corresponding radiometric
uncertainties. Enhancing factors C_BORDER, C_EAF, C_SUN_TAILS, C_SUN_GLINT_AREA are provided
in UPF.
3.2.2.1.5 Filtering L1c pixels
Once the faulty or dubious L1c views have been filtered out from the initial number M_AVA0, the number M_AVA of
remaining views for a given SMOS grid point is estimated.
The initial validation index MVAL0 is a weighted sum of the number of available data which expresses roughly their
information content:
MVAL0 = C val DTB_F * sum(1/ DTBa) Eq 62
Where
• The sum is to be carried out over every view and polarizations TB X and TB Y .
• The DTBa are radiometric uncertainties over each TB at antenna level
• DTB_F is a scaling factor
• C val (=C val_2 or C val_4 ) is a coefficient depending on the polarization mode.
The scaling factor is adjusted in such a way that MVAL0 is equal to the maximum available number of views along the
track. Away from track, it decreases rapidly because the along track size of the FOV decreases as well as the range of
available incidence angles.
MVAL0 is next compared to the eliminatory threshold TH_MMIN0:
Numerical values for DTB_F, C val , TH_MMIN0 are provided in UPF.
If MVAL0 < TH_MMIN0, the L1c pixel is eliminated.
3.2.2.1.6 Computing TOA brightness temperatures
The brightness temperatures measured by SMOS in the antenna reference frame must be converted in components
relevant at the Earth surface, actually at top of the atmosphere (TOA). This takes into account a rotation of the electric
fields, due both to geometrical considerations and to the Faraday rotation induced by ionosphere.
Here the definition of angles is believed to follow the conventions described in Earth Explorer CFI Software Mission
Convention Document; we introduce the mathematical expressions for the angles to be used in the transport from
ground to antenna reference frames[94] [95]:
[ sin tilt sinθ
sinφ
+ cos tilt cosθ
]
θ
a
= Arccos
g g
g
for π/2 ≤ φ g ≤ 3π/2
⎡ ( −sin tilt cosθ
g
+ cos tilt sinθ
g
sinφg
) ⎤ (modulo 2 π):
φa
= − Arcsin⎢
⎥
⎣
sinθ
a
⎦ φ a to be replaced by π − φ a
⎡(cos tilt sinθ
g
− sin tilt cosθ
g
sinφg
) ⎤ ψ to be replaced by π − ψ
ψ = π − Arcsin⎢
⎥
⎣
sinθ
a
⎦
We define the rotation angle a = − φ a − ψ − ω Fa . Both φ a + ψ and ω Fa are expected to be supplied by L1c.
Angles φ a & ψ are defined above.
Theoretical description
The Faraday rotation is due to the effect of ionospheric electrons on the propagation of electromagnetic waves.
Mathematical description of algorithm
Eq 63
The Faraday angle ω Fa for each view is provided by L1c data (field #09 or #10 in tables 26 or 27, depending on
polarization mode), using auxiliary TEC n (Total Electron nadir columnar Content) values. Therefore the description
below need only be implemented when introduced in the direct model in the case where TEC n is retrieved.
.
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