The document summarizes a study analyzing radio frequency interference (RFI) in observations from ESA's Soil Moisture and Ocean Salinity (SMOS) mission. RFI is a major concern for L-band microwave radiometry. The study examines RFI properties in SMOS data, identifies artifacts caused by strong sources and system properties, and explores strategies to reduce artifacts to better understand the true RFI environment. Statistics of the artifact-reduced data still show significant RFI presence, which has implications for planning NASA's upcoming SMAP mission that will also use L-band radiometry.

1. Studies of Radio Frequency Interference in SMOS Observations IGARSS 2011 Joel T. Johnson and Mustafa Aksoy Department of Electrical and Computer Engineering ElectroScience Laboratory The Ohio State University Vancouver, Canada 29th July 2011 *SMOS data provided by the European Space Agency

3. ESA’s SMOS mission experiencing significant RFI since launch Nov 2009

4. RFI expected for NASA’s Aquarius (now in orbit) and SMAP (launch ~ 2014)

5. Important to understand RFI environment to plan for these missions

6. SMAP will include a digital backend to enhance RFI detection/mitigation, but need to assess expected performance

7. Examine properties of RFI in SMOS observations

8. Primarily with L1C data but selected L1B examples also shown

9. Some “artifacts” caused by strong sources and SMOS system properties

10. Try to reduce artifacts to see RFI “truth”

11. Compile statistical information, “low level” RFI of particular interest

13. SMOS Basics ESA’s Soil Moisture and Ocean Salinity (SMOS) mission has operated an L-band interferometric radiometer in space since Nov 2009 Provides multi-angular observations for each pixel (including SMAP’s 40o) SMOS measurement properties: native time resolution ~ 1.2 sec, a single frequency channel Tb’s (H, V, U, 4) give power levels (fullband, time averaged) and location Interferometric focusing can cause some sidelobes and aliasing of RFI sources ~ 40 degrees portion of SMOS swath 4

14. SMOS L1 Data Products L1A: calibrated visibilities (i.e. correlations): Not using at present L1B: calibrated Fourier components of brightness temperature Can construct brightness temperature image by applying appropriate window function then performing FFT L1C maps L1B data onto an Earth-fixed grid Reordered by grid point, multiple angles at each grid point Expected Tb accuracy and resolution also included for each angle at a given grid point Sidelobes ofAliased RFISources Can impact AF-FOV Alias-Free Field of View Plus extendedAlias-Free Field of View

15. Initial Examination of Statistics: 2/9/11-3/8/11, %Th>350 K, 40o+/-2.5o

16. Initial Examination of Statistics: 2/9/11-3/8/11, %Th>350 K, 40o+/-2.5o(log scale)

17. Initial Examination of Statistics: 2/9/11-3/8/11,Th Max, 40o+/-2.5o

18. Initial Examination of Statistics: 2/9/11-3/8/11, sqrt(Tu^2+T4^2) Max, 40o+/-2.5o

19. Where do “artifacts” come from? Sidelobes from strong sources spread into rest of snapshot Worse because these sources may be outside L1C field of view Sources appear multiple times outside of FOV due of aliasing Sources “sit” on Earth horizon: atmospheric ducting mechanism? Sidelobes ofAliased RFISources Can impact AF-FOV Alias-Free Field of View Plus extendedAlias-Free Field of View

20. L1B/L1C Movie Illustrating Artifacts

21. L1B/L1C Movie Illustrating Artifacts (Descending)

22. Reducing Artifacts Sources in forward part of swath more problematic Ascending more problematic when large sources are to the North Descending more problematic when large sources are to the South Use only descending observations for North America Outer parts of swath more susceptible to corruption Using only the AF-FOV helps some, but still some issues Reduce swath further using a limit on accuracy Try to throw out “bad” snapshots entirely Look at time series of boresite accuracy (related to mean image brightness) Use a “pulse” detection algorithm to discard outliers Examine RFI in remaining data: should have reduced artifacts Some large RFI sources not causing artifacts may also be discarded

23. Swath and Snapshot Reduction Examples Include only points whose accuracy is within10% of the boresite value (limits swath) Apply “pulse” detection algorithm and a threshold to boresite accuracy to detect“bad” snapshots and discard Portion of swathmeetingaccuracy limits “Bad” snapshots detected by pulse algorithm are entirely discarded

26. Higher sensitivity of polarimetric channels shows more sources, although likely some artifacts remainImage Before/After Artifact Reduction