Study of the LO calibration frequency impact on SSS

SMOS QWG-4 – ESRIN 7-9 March 2011 1

Study of the LO calibration frequency impact on SSS

C. Gabarró, V. González, J. Martínez, M. Portabella, J. Font and BEC team

SMOS Barcelona Expert CentrePg. Marítim de la Barceloneta 37-49, Barcelona SPAINE-mail: [email protected]: www.smos-bec.icm.csic.es


Decimation study 57 full-pol semiorbits acquired at 2min and decimated at

4min, 6min, 10min (24-25 March 2010) Processors used (best available):

L1PP 3.5 L2PP 3.17

All the comparisons are relative to the 2 min LO calibration orbits. Results are not absolute values.

Example: descending Pacific orbit 20100325T040821 Global L1 data set L1 statistics over Pacific and North Atlantic Available SSS data after applying L2PP Statistics on SSS data Conclusions


Example: TB comparison @ 38-40º

20100325T040821 - descending

TBH_10min-TBH_2min TBV_10min-TBV_2min


TBH_6min-TBH_2min TBV_6min-TBV_2minExample: TB comparison @ 38-40º


TBH_4min-TBH_2min TBV_4min-TBH_2minExample: TB comparison @ 38-40º


TB differences for 1 orbit EAFTB10min-TB2min TB6min-TB2min

TB4min-TB2min


TB differences AF ONLYTB6min-TB2min AF

Very similar results for EAF or AF

TB10min-TB2min AF

TB4min-TB2min AF


Data set: TB H @ inc=38-40º


TB H @ inc=38-40º L1 RFI mask


TB V @ inc=38-40º


TB V @ inc=38-40º L1 RFI mask


TB histogram PACIFIC area

PACIFIC dataLat=[-50,10] Lon=[-180,-100]


TB differences in PACIFIC

% TB_Nmin -TB_2min> 0.5K

TB_Nmin -TB_2min> 1K


N=10min 2.11% 0.27% 0.05%

N=6min 0.08% 0.03% 0.01%

N=4min 0.03% 0.01% 0.001%

PACIFICLat=[-50,10] Lon=[-180,-100]


TB histogram N. ATLANTIC area

N ATLANTIC dataLat=[30,90] Lon=[-70,0]


TB differences in N. ATLANTIC

% TB_Nmin -TB_2min> 0.5K



N=10min 17.54% 8.76% 3.87%

N=6min 5.93% 1.96% 0.46%

N=4min 1.42% 0.29% 0.053%

North ATLANTICLat=[30,90] Lon=[-70,0]


SSS data set 24-25/3/2010 57 orbAttention:Almost no clean datain N. Atlantic (L2PP filter)


SSS 24-25 march 2010 57 orbSSS 10min – 2min






Histogram SSS WORLD

Global L2OS output

When filtering byFg_poor_retrieval=1 STD (SSS10-SSS2)= 0.48 psu


Histograms SSS - PACIFIC

OnlyFg_poor_retrieval=0


Histograms SSS – N. ATLANTIC

OnlyFg_poor_retrieval=0


SSS differences: best & worst cases

% SSS_Nmin -SSS_2min> 0.2 psu

SSS_Nmin -SSS_2min> 0.5 psu

SSS_Nmin -SSS_2min>1 psu

N=10min 30.67%12.03%

9.59%2.87%

2.07%0.55%

N=6min 9.67%1.85%

1.42%0.05%

0.22%0.01%

N=4min 2.85%0.42%

0.38%0.03%

0.11%0.01%

Lat < -50º PACIFICLat=[-50,10]Lon=[-180,-110]


System requirements

R-4.5.1-008 The systematic radiometric error (including: bias, drift, periodic and other errors such as thermo-elastic effects) on the retrieved brightness temperature maps shall be lower than 1.5 K RMS at the antenna boresight and lower than 1.5´1.64 = 2.5 K RMS within 32° from boresight, after calibration

G-4.7.2-007 In order to support vicarious calibration, the system shall ensure a stability of the systematic component of the radiometric error (including: bias, drift, periodic and other errors such as thermo-elastic effects) better than 0.02 K/day.


Conclusions

The present LO calibration frequency has a clear impact on TB (larger in V-pol) and SSS.

TB differences between LO-10min and LO-2min have a global STD of 0.47 K (1.1K in N. Atlantic, 0.21 K in Pacific)

Differences in SSS (filtered by good retrieval flag): global STD 0.48 psu (0.94 psu in N. Atlantic, 0.20 psu in Pacific)

Accuracy requirement for L2 SSS is 1.2 psu. We don’t know yet the actual retrieval error, but the low calibration freq can represent in average 0.5 psu of additional error!!

SMOS SSS requirements will likely never be met with LO being calibrated at 10 min intervals


Remembering SSS requirements

R-4.6.2-004 The accuracy of the ocean salinity data products with a 50 km spatial resolution shall be <= 1.2 PSU.Note : This OS Level-2 accuracy requirement corresponds to :· snapshot images (100) of one pixel taken during one single pass,· usage of the narrow swath,· full spatial coverage,· ocean temperature of 277 K

G-4.7.2-005 The accuracy of the ocean salinity maps outside coastal areas, after averaging over 200 km x 200 km x 10 days space-time domain, shall be <= 0.1 PSU, after corrections by using vicarious calibration, namely by periodical imaging of one or more well-known reference sources.


On statistics During 10 minutes 500 SMOS snapshots are acquired Calibrating LO every 10 min means loosing 5 snapshots,

then the error in building an L3 map by averaging L2 products (wrt no calibration) increases by sqrt (500) /sqrt(495) = 1.005

Calibrating every 2 min, 25 snapshots are lost, the L3 error increases by sqrt(500)/sqrt(475) = 1.026

Going back to one orbit, the increase of usable snapshots @10 min compared to @2 min implies an error reduction of sqrt(495)/ sqrt(475) = 1.02

This 2% reduction on the required SSS error means 0.02 psu

We showed that calibrating @2 min instead of @10 min implies an error reduction by 0.2 psu (clean Pacific), 0.5 psu in global average

SMOS Barcelona Expert Centre (SMOS-BEC)Pg. Marítim de la Barceloneta 37-49, E-08003 Barcelona, SPAINTel. (+34) 93 230 95 00; Fax. (+34) 93 230 95 55URL: www.smos-bec.icm.csic.es


RFI PROBLEMS DETECTED ???

C. Gabarró, J. Martínez, V. Gonzalez, M. Portabella and BEC –TEAM

SMOS Barcelona Expert CentrePg. Marítim de la Barceloneta 37-49, Barcelona SPAINE-mail: [email protected]: www.smos-bec.icm.csic.es


RFI detected by LO cal study?

PLOT ONLY TB10min – TB2min >2K


RFI in the LO cal ?(TBH_10min –TBH_2min)>2K

(TBV_10min –TBV_2min)>2K


RFI in detector ?

Orbit ASC 20100324T0717Snapshot 20470787


RFI in detector ?

TBV LO 10min TBV LO 2min

Snapshot 20470787 Orbit ASC 20100324T0717

If TB>220 -> set to 0


RFI in the LO cal ?MASK (TBV_10min –TBV_2min)>2K

RFI TAILS??

Documents

Study of the LO calibration frequency impact on SSS