SMOS L2 OS ATBD - ARGANS

38
ICM-CSIC
LOCEAN/SA/CETP
IFREMER
SMOS L2 OS
Algorithm Theoretical
Baseline Document
Doc: SO-TN-ARG-GS-0007
Issue: 3 Rev: 9
Date: 25 January 2013
Page: 38
Measurements rejected by the “Detect_outliers” (nominally not near to land, ice) and
Detect_measurement_outliers” filters (nominally Fm_L2_RFI_snapshot_out_of_range,
Fm_L2_RFI_high_snapshot_std, Fm_L2_RFI_high_snapshot_std_Stokes3,
Fm_L2_RFI_high_snapshot_std_Stokes4) are not used in outlier detection tests.
The standard deviation is computed over the whole dwell line and compared to 1.2 times of
the theoretical standard deviation (i.e. the sum, over all measurements included in the dwell
line, of the radiometric noise and the modelled error). The factor 1.2 corresponds to the 99%
confidence interval. In case there is a risk of outlier, if the difference between the measured
TB and the modelled TB are higher than 3 time of the radiometric noise, this measurement is
discarded from the retrieval.
Do not perform this test on a polarisation dwell line if the number of measurements is less
than a threshold (Tg_num_meas_RFI_outliers_min, nominally 16 measurements).
Step 1: identify for each grid point whether there is a risk of RFI outliers.
std_theory= (sum(radiometric_noise 2 + error_on_Tb_model 2 ) / number_meas)
risk_of_RFI = std(Tbmodel-Tbmeasurement) > RFI_std * std_theory
where RFI_std = 1.2 (estimated assuming we have 100 measurements, the 99%
confidence interval on std_theory computed from these 100 stds would be 0.18
std_theory so putting a factor=1+0.2 seems reasonable).
Step 2: if risk_of_RFI:
for each measurement
if abs(TBmodel[pol] - TBsmos[pol]) > RFI_nsig * radiometric_noise
set Fm_L2_RFI_outlier = true
where RFI_nsig = 3 (nsig is reduced from 5, non-RFI outlier detection threshold)
Step 3: remove false positives:
k=count(Fm_L2_RFI_outlier)
for each measurement
if k < Tg_num_RFI_outlier_max (nominally 3%)
set Fm_L2_RFI_outlier = false
3.5.2.4 Combining RFI detection methods
Fm_L2_RFI is set for all measurements returned by the “Set_RFI_flag_from_outlier_tests”
(nominally Fm_L2_RFI_outlier) or “Set_RFI_flag_from_snapshot_tests” (nominally
Fm_L2_RFI_snapshot_out_of_range, Fm_L2_RFI_high_snapshot_std,