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
RFI map designates here a DGG map. No angular dependence is considered in the baseline. Provisions to take angular
information into account might prove useful. Polarisation may be useful as RFI could only affect one polarisation. The
table includes both the total number N SNAP of times the DGG node has been observed, and the total number N RFI-p of
cases where an anomalously high TB has been unambiguously detected for polarisation p, as the results of any of 3
tests: TB range testing, L2 RFI test (see 3.2.2.1.4), post retrieval outliers.
For each snapshot, the counter N SNAP is incremented; the N RFI-p counter is updated as well when RFI is detected.
Over any point, if the DGG RFI_MAP indicates potential RFI while the tests are negative, the radiometric sensitivity is
degraded by a factor depending upon the ration R RFI-p =N RFI-p / N SNAP using Eq 61 (see § 3.2.2.1.4).
This algorithm is activated or deactivated by the global switch UPDATE_RFI defined in UPF. The use of RFI map to
flag the input TB is activated or deactivated by the global switch USE_DEFLT_RFI defined in UPF.
The map would thus contain for each node N SNAP , N RFIp for each polarisation requiring as SNAP will grow quickly 4
bytes.
3.2.5.2.2 Updating current optical thickness TAU_NADIR LV and FO map
This external post-processing action still requires confirmation and detailed explanations.
The basic concept is when the retrieval of optical thickness is possible and successful; this optical thickness updates the
evolving map. The idea is that the new tau nadir will be more accurate/ up to date than the LAI derived one. It might
also be affected by spurious events such as a heavy rainfall. The new TAU_NADIR is for a whole node but actually
only for the class for which the retrieval process was carried out (e.g., low vegetation or forest).. It is also considered
that for forests we will only update for fine tuning as the relevant variable is linked to the branch volume and thus
should remain fairly constant. Any variation should be linked to changes in other variables (interception, litter water
content…)
So, when the retrieval of optical thickness is possible and successful, the TAU map class to be updated is the fraction
for which TAU_nadir is retrieved, as provided by UPF (TGRD).
The CURRENT_TAU_NADIR_LV is only updated following LV class retrievals:
• Current TAU is updated with the retrieved TAU value,
• Current DQX TAU is updated with a specific RSTD, obtained as indicated in 3.2.4.4.2 but with a matrix
COV Z _CURRENT computed assuming that TAU is completely free (that is, practically, a very high ASTD).
This quantity will be stored in the DAP
• Current date is updated with swath acquisition date.
•
The CURRENT_TAU_NADIR_FO is only updated following FO class retrievals:
• Current TAU is updated with the retrieved value,
• Current DQX TAU is updated with the DQX of the retrieved value
• Current date is updated with swath acquisition date.
No update is allowed in the following cases:
• Risk of interception
• Presence of snow cover, or freezing conditions (vegetation is almost transparent and tau nadir is abnormally
low)
If no Tau_ nadir is retrieved, or update not considered as valid, the date of the last acquisition is checked. If the lag is
larger than the allowed TH_LAG_TAU_LV, a default or missing value is put.
This algorithm is activated or deactivated by the global switches UPDATE_TAU_NADIR_LV and
UPDATE_TAU_NADIR_FO defined in UPF.
The use of DGG_CURRENT TAU_NADIR_LV map and DGG_CURRENT TAU_NADIR_FO map in the retrieval is
activated or deactivated by the global switches USE_DEFLT_TAU_NADIR_LV and USE_DEFLT_TAU_NADIR_FO
defined in UPF.
3.2.5.2.3 Updating current H roughness HR map
.
88