Algorithm Theoretical Based Document (ATBD) - CESBIO

More documents

Recommendations

Info

SO-TN-ESL-SM-GS-0001 Issue 1.a Date: 31/08/2006 SMOS level 2 processor Soil moisture ATBD • Sqrt(4 x FOOTPRINT_AXIS1 x FOOTPRINT_AXIS2) > TH_SIZE or • FOOTPRINT_AXIS1 / FOOTPRINT_AXIS2 > TH_ELON On Figure 7 is shown also the impact of a margin over the alias free zone. It is necessary to account for such a margin; to this end, use will be made of the L1c BORDER_FOV flag 3.2.2.1.4 Filtering L1c views In this subsection and the following one, the "TB" notation refers to brightness temperatures at antenna level, i.e. the BT in Table 10 (L1c inputs). • Views must be first identified from the remaining L1c records. Views are available provided every necessary polarisation (either 2 or 4; flags #03 inTable 10 B & C) is provided for adjacent integration times. It is assumed that it will be possible to use the continuity of the SNAPSHOT_ID index to verify this condition. The order among polarisations is not relevant. The TB are screened in the order in which they are stored (referring to the COUNTER loops in Table 10 B & C). It is assumed that the first one then corresponds to the highest incidence angle. If the adjacency condition is not filled, the TB are ignored until a full L1c view can be identified from adjacent integration times. Incomplete L1c views may be met at the end of the COUNTER loops and will then be discarded. • TB range testing: A first elimination criterion for full L1c views is introduced by comparing each antenna TB (fields #04 or #04 and #05 in Table 10 B & C) to their expected ranges: defined by TB X _MIN, TB X _MAX and similar brackets for the other polarization axis components. Full polarization is handled in the same way but ranges are provided for real parts, imaginary parts and also for the TB XY cross-polarization.. Range limits are provided in the UPF. Currently the range is fixed. It might be necessary to have a range defined as per tuned to actual node characteristics. Further criterions are based on L1c flags. Such flags are taken into account to either eliminate a full L1c view, or enhance radiometric uncertainties, as soon as they appear in at least one of the antenna L1c TB polarizations. According to L1 documentation ([AD 4] § 3.4), the following flags are entered in the L1c products: AF_FOV, EAF_FOV, BORDER_FOV, DIAGONAL_FOV (the latter one to be deleted); SUN_FOV, SUN_POINT, SUN_TAILS, SUN_GLINT_FOV, SUN_GLINT_AREA, RFI flags • The SUN_POINT flag eliminates the view. • An RFI L2 flag eliminates the view. TB range testing (see above) should take care of highest RFI contamination cases: TB values beyond the upper TB range limits are understood as RFI L2 flags. There is one per polarisation. 2 2 X T Y T + greater than a limit should be a better indicator that a monopolarized (H or V) RFI at earth surface that could pass the test on Tx, Ty range because of the rotations of the TB vector. A short additional modulus aimed at refining L2 RFI detection is proposed for the L2 processor. This procedure takes advantage of incidence angle continuity whenever possible, and of the fact that, while TB X and TB Y vary strongly with incidence angle, this is not the case for the first Stokes parameter. It consists in: • considering only pixels for which M_AVA is larger than a given number TH_AVAR (this condition ensures at the same time a significant range of incidence angles); • computing the mean value 〈TBS1〉 of the halved first Stokes parameter TBS1 = 0.5*(TB X + TB Y ); • eliminating views for which (TBS1 - 〈TBS1〉 ) > CA_TBS1 + CB_TBS1 x DTB X , where CA_TBS1 and CB_TBS1 are constant (provided in UPF), and DTB X the radiometric uncertainty on TB X. • Finally, if data is acquired in full pol, there is a possibility to detect RFI using the 4 th Stokes paratmeter. IF ST4 > TH_RFI_ST4 then the point is considered as contaminated and the view is eliminated • In cases where, although the current view is identified as a potential RFI view by the LUT map, no anomaly is detected in L2, the view is not eliminated; rather, the corresponding radiometric uncertainties are enhanced by a factor C RFI-p depending on the RFI statistics R RFI -p A suggested formula would be: C RFI-p = C1 RFI R RFI-p^C2 RFI / (1 + R RFI-p^ C2 RFI ) Eq 61 . 63
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. . 64
Page 1 and 2:
SO-TN-ESL-SM-GS-0001 Issue 1.a Date
Page 3 and 4:
Page 5 and 6:
Page 7 and 8:
Page 9 and 10:
Page 11 and 12:
Page 13 and 14:
Page 15 and 16:
Page 17 and 18:
Page 19 and 20:
Page 21 and 22:
Page 23 and 24: SO-TN-ESL-SM-GS-0001 Issue 1.a Date
Page 73: SO-TN-ESL-SM-GS-0001 Issue 1.a Date
Page 123: SO-TN-ESL-SM-GS-0001 Issue 1.a Date
show all

Algorithm Theoretical Based Document (ATBD) - CESBIO

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?