Outlier detection in L2OSPP- usefulness in RFI detection-

SMOS L2OS Progress Meeting – 21/22 February -BEC

Outlier detection in L2OSPP- On L1c measurements-

• Applied during pre-processing on L1c measurements• Done for each gridpoint and separately for each polarisation• Successive steps:

1°) Apply OTT to valid measurements2°) Separate by polarisation all measurements associated to one gridpoint 3°) For each polarisation:

- Computation of the median over all measured Tbs : Mmeas

- Computation of the median over all associated modelled Tbs : Mmodel

- Computation of the difference DA = Mmeas – Mmodel

4°) For each measured Tbs

- Correction of the modelled Tbs : Mmodel_corr = Mmodel + DA

- Test : abs(Mmeas - Mmodel_corr ) > N √(radiometric error² + modelled error²)Yes this measurement is flagged as outlier and

discarded from the retrieval

5°) Counting the outlier measurements - If % outliers > Tg_many_outliers gridpoint flagged

• In any case, gridpoint is processed

Inconstency with ATBD:

Median of the differences vs difference of the medians

Diff(median) vs median(diff)• Not a big issue when there are only a few outlier measurements or

when there are few very high measurements

• Real issue for some cases :

No longer outlier

Detected as Outlier

Usefulness for RFI detection

• RFI Too high Tbs measurements (> 500 K)

• L2OSPP Outlier detection

only gridpoints close too RFI are flagged

SSS retrieval near Greenland:

- Color scale from 30 to 40 pss - White spots = GP flagged as « many_outlier » - close to RFI: SSS > 80 or SSS <0

• Try to detect some characteristic behaviour of the TBs close to on RFI focuse on gridpoints contained in an affected snapshot:

Blue = Tbs close to 0Red = Tb close to 1000 K

Only a small area seems to be not affected by the RFI (i.e. realistic values of SSS)

Question : Are Out of range Tbs values sufficient to detect RFI effect ?

60

-40

90

-10

-Ok for gridpoints close to the RFI spot

- Not relevant for further gridpoints

Improvement of outlier detection ??

For each gridpoint, the maximum and minimum associated Tbs are observed

4 specific gridpoints : 2 with retrieved SSS > 40 pss 2 with retrieved SSS close to 0

Each of them not flagged as « many_outlier »

RFI spot = gridpoint with the maximal Tb over the whole snapshot

• close to RFI gp, SSS = 104.15 pss

• In VV pol, Alternation between Tbmeas >> Tbmodel and Tbmeas<< Tb

model = characteristic behaviour??

• No difference between the two « methods » for HH pol• Difference for VV pol for incidence <50

• Far from RFI gp, SSS = 50.62• Several outlier measurements close to modelled Tbs

• Most of the outliers are no longer flagged with the « ATBD » method Which influence on the SSS retrieval values ??

• close to RFI gp, SSS = 2.44 pss• For incidence >45°, same alternation in VV pol

• No significant change for HH or VV pol

• Far from RFI gp, SSS = 7.3088

• For VV pol :- all measurements with incidence > 54° and incidence <38 are no longer

flagged as outlier- several measurements with incidence [38, 54]° are now flagged as outlier

CONCLUSION :

• Change of outlier detection in L2OSPP :

-method «median of the differences » closest to modelled TBs

- influence on SSS retrieval?

• Outliers are mostly observed on VV polarisation

• Alternation between higher and smaller measurements than modelled TBs = carcateristical behaviour of affected gridpoints ?