Science with the Korean Solar Radio Burst Locator (KSRBL)

1. Science with the Korean Solar Radio Burst Locator (KSRBL) Dale E. Gary & Gelu M. Nita Center for Solar-Terrestrial Research New Jersey Institute of Technology KSRBL Science Colloquium 2009 Aug 24

2. Outline • Overview of KSRBL • Topics for Study • Radio spectrum for study of solar activity • Burst location • Radio frequency interference mitigation • Radio effects on navigation and communication systems • Conclusions KSRBL Science Colloquium 2009 Aug 24

3. Yagi Feed (245 and 410 MHz) Spiral Feed (0.5-1 GHz) KSRBL Antenna Full Sun coverage => 2.1 m antenna Gives 33 arcmin at 18 GHz KSRBL Science Colloquium 2009 Aug 24

4. 17,500 frequency points at 1 MHz resolution Frequency Coverage • Frequency range • 0.5-18 GHz continuous coverage, plus 245 and 410 MHz (yagi feed) • Frequency resolution • Best resolution 244 kHz (for RFI excision) • Target science resolution 1 MHz (4:1 compression) • Instantaneous bandwidth • 4 x 500 MHz = 2 GHz KSRBL Science Colloquium 2009 Aug 24

5. Time Resolution • Full spectrum in 1 s • Time resolution for each sample is 25 ms, measured four times before tuning (100 ms for each tuning). - + + + - + + + • Takes 10 tunings to cover 18 GHz, hence 1 s to cover all bands. • Best resolution on a single band, nominally 25 ms, but can be changed (trade-off with data-rate and data volume). KSRBL Science Colloquium 2009 Aug 24

6. RFI Excision (this is a portion of a single band at full resolution) KSRBL Science Colloquium 2009 Aug 24

7. Solar Radio Burst (SRB) Spectra • The spectrum of SRBs reveals a great deal of information about plasma parameters (temperature, density, magnetic field strength, accelerated electron energy distribution). KSRBL is unique in its ability to combine high frequency resolution with broad frequency coverage. • At very high temporal and spectral resolution, additional features may appear, especially at decimetric (<3 GHz) frequencies. • Polarization is also important—KSRBL measures only RCP (required for burst location). A measurement of LCP is also needed—a second KSRBL?. KSRBL Science Colloquium 2009 Aug 24

8. Pk 1: B = 240 G Pk 2: B = 120 G Example: Gyrosynchrotron Spectrum Pk 1: Bt = 120 G Pk 2: Bt = 60 G KSRBL Science Colloquium 2009 Aug 24

9. Example: High-Resolution Bursts • Decimetric burst types, seen with similar frequency and time resolution as KSRBL KSRBL Science Colloquium 2009 Aug 24

10. alert! safe? possible concern? Burst Location • Motivation • Because of Parker spiral, particles from bursts east of central meridian are few and of low energy. • Particles from bursts on the west limb are of far more concern. • Knowing the location of the burst on the disk, especially for large flares, is important. • KSRBL can act as backup for spacecraft. KSRBL Science Colloquium 2009 Aug 24

11. Burst Location KSRBL Science Colloquium 2009 Aug 24

12. Board Implementation • CASPER iBOB-based 500 MHz Spectrometer • 2048 channels, 4 tap PFB (polyphase filter bank) • Accumulates power (S1) and power-squared (S2). • 1 GS/s (1 GHz clock), 500-1000 MHz IF • Settable dump times—use 25 ms • Output via fast-ethernet • Operate 4 boards in parallel, for 2 GHz total bandwidth ADC FPGA KSRBL Science Colloquium 2009 Aug 24

13. scale coeff bit shift P bit select 4096-pt FFT Scale RF In ADC PFB BRAM P RAM Serializer P2 P RAM Parallel- izer P2 Bit select P Accumulator P Multiplier P2 P Data Out (ethernet) Bit select P2 1 GHz clock accum length P2 bit select bit select Simplified Block Diagram • Multiple levels of scaling to ensure sufficient precision of P and P2. KSRBL Science Colloquium 2009 Aug 24

14. Spectral Kurtosis RFI Algorithm • We have described the SK Algorithm previously (Nita et al. 2007, PASP 119, 805). The SK estimator provides a way to distinguish whether a single accumulation (time-frequency bin) is consistent with Gaussian noise. • Bins with certain kinds of radio frequency interference (RFI) are typically not Gaussian, hence the SK estimator can be used to identify and flag bins containing such RFI. KSRBL Science Colloquium 2009 Aug 24

15. Spectral Kurtosis RFI Algorithm • The recipe for computing the SK estimator is very simple, and lends itself to real-time RFI excision using high-speed digital processing. To compute the SK estimator, one must accumulate sums of power and power-squared • The SK estimator for an accumulation over M samples is then • The variance of the SK estimator is so with a criterion of an accumulation in spectral channel k is Gaussian if it obeys the expression KSRBL Science Colloquium 2009 Aug 24

16. One instantaneous (25 ms) spectrum with no RFI Each spectral point is an accumulation of M = 6104 samples. Occasionally (~0.13% of time) exceeds 3s threshold SK (=1±3s ) Example: A Band with No RFI KSRBL Science Colloquium 2009 Aug 24

17. SK Estimator vs. Spectral Power Plot of SK vs spectral power (S1) (very useful plot, as we will see) Shows that SK estimator is independent of power level—a key property. Also, RFI decision is made based on statistics of a single accumulation of a single spectral channel. No “relative” comparisons needed. SK estimator for 150 instantaneous spectra KSRBL Science Colloquium 2009 Aug 24

18. Clean spectrum after frequency-binning to target resolution (512 spectral channels) Same spectrum after applying SK flags (2048 spectral channels) Full-resolution dynamic spectrum (2048 spectral channels) Average spectrum (3-3.5 GHz) Full-Resolution vs. Integrated Spectrum KSRBL Science Colloquium 2009 Aug 24

19. SK Estimator for Non-Gaussian Signals • The SK estimator for a Gaussian signal is very close to 1, but what is the SK value for non-Gaussian signals? • One type of RFI we have simulated is a CW signal of constant amplitude, which can be used to simulate transient RFI by considering its presence or absence with some duty cycle, d. • Consider M contiguous samples out of which only R are contaminated by RFI of signal to noise ratio hk. This leads to an RFI duty cycle of d = R/M. For this case, the expected SK estimator value is • Note several interesting properties: • For d = ½, (50% duty cycle), the estimator is always 1 • For d < ½ (highly intermittent RFI), the estimator is above 1 • For d > ½ (more continuous RFI), the estimator is below 1 KSRBL Science Colloquium 2009 Aug 24

20. A Key Plot for Understanding SK Armed with these ideas, let’s look at bands with RFI KSRBL Science Colloquium 2009 Aug 24

21. Clean spectrum after frequency-binning to target resolution (512 spectral channels) Same spectrum after applying SK flags (2048 spectral channels) Full-resolution dynamic spectrum (2048 spectral channels) Average spectrum (0.5-1 GHz) Example—A Band with a Lot of RFI Average of 150 spectra, each accumulated with M = 6104 samples. KSRBL Science Colloquium 2009 Aug 24

22. Why does this RFI survive? Previous plot was lower resolution than actual data. Zoom in at full resolution KSRBL Science Colloquium 2009 Aug 24

23. SK mimics Gaussian noise! • SK > 1 • highly intermittent • SK < 1 • more continuous SK Estimator vs. Spectral Power KSRBL Science Colloquium 2009 Aug 24

24. SK vs. Power Plot Features • Continuous RFI appears as discrete dots. • Intermittent RFI appears as “fountain” of points. • Curve of fountain likely reflects effective duty-cycle. “Accidental” 50% duty cycle can occur. • Multiscale SK moves points—can guard against 50% duty cycle problem. • Some RFI masquerades as Gaussian noise. • Let’s take a closer look to discover what characteristics the problem-RFI has. KSRBL Science Colloquium 2009 Aug 24

25. This RFI is untouched! Incompletely removed Turns out this is XM and Sirius (digital satellite radio) KSRBL Science Colloquium 2009 Aug 24

26. SK Estimator vs. Spectral Power 50% duty cycle Digital radio acts like Gaussian noise KSRBL Science Colloquium 2009 Aug 24

27. Southern California Edison digital data link KSRBL Science Colloquium 2009 Aug 24

28. 6093 MHz center frequency 30 MHz BW, pointing right at observatory. SK Estimator vs. Spectral Power Digital data links act like Gaussian noise KSRBL Science Colloquium 2009 Aug 24

29. Multiscale SK KSRBL Science Colloquium 2009 Aug 24

30. SRB Effects on Navigation System Observed by OVSA and FST at Owens Valley Solar Observatory 18 10 GHz 1 30 s near end of burst OVSA and FST data of this record solar burst. Zooming in reveals the burst as composed of millions of millisecond spike bursts (electron-cyclotron maser emission). The FST data at right shows 20 ms time-resolution data, switching between right-circular (RCP) and left-circular polarization (LCP) every 4 s. The spikes are essentially 100% RCP. System automatically switches between polarizations KSRBL Science Colloquium 2009 Aug 24

31. GPS Outage • GPS satellites broadcast at 1247 and 1575 MHz, and the signal is right-circularly polarized (RCP). • The burst reached record flux levels at both of these frequencies, and was also RCP. • The direct interference of the solar flux caused receivers on Earth to loose lock on the satellites. KSRBL Science Colloquium 2009 Aug 24

32. Conclusion • KSRBL is a unique instrument in its combination of high frequency and time resolution and broad frequency coverage. • It is a research instrument ideal for four types of study: • Solar radio bursts and solar activity • Burst location and space weather effects • Radio frequency interference mitigation • SRB effects on navigation and communication systems. KSRBL Science Colloquium 2009 Aug 24

33. Conclusions Re: RFI • KSRBL has the first FPGA implementation of Spectral Kurtosis for real-time flagging of RFI. • The method supplies an automatic way to flag the worst intermittent RFI. • The SK estimator vs. S1 plot is useful for characterizing types of RFI: • SK < 1 is intermittent RFI, easy to remove. A few points may get through by chance hitting near 50% duty cycle. • Typical continuous RFI appears as small clusters, sometimes near or overlapping with SK = 1 window. Multiscale SK can be applied to address this. • Digital radio, digital data links, and likely digital TV are “awful”—they are both band-filling and they appear to the SK algorithm as indistinguishable from Gaussian noise. • Further study of digital RFI is needed. KSRBL Science Colloquium 2009 Aug 24

34. Thank You KSRBL Science Colloquium 2009 Aug 24

35. Effect of Incorrect Precision • When S1 (S2) precision is too low (LSB truncated), the effect is to raise (lower) the SK estimator S1 truncated slightly (SK raised to 1.014) Excessive clipping of valid data KSRBL Science Colloquium 2009 Aug 24

36. Results in too many flagged points Mean of SK estimator in this case is 1.014 due to truncation of S1. Thus, it is important to manage dynamic range using settable parameters. KSRBL Science Colloquium 2009 Aug 24

37. scale coeff bit shift P bit select 4096-pt FFT Scale RF In ADC PFB BRAM P RAM Serializer P2 P RAM Parallel- izer P2 Bit select P Accumulator P Multiplier P2 P Data Out (ethernet) Bit select P2 1 GHz clock accum length P2 bit select bit select Simplified Block Diagram • Multiple levels of scaling to ensure sufficient precision of P and P2. KSRBL Science Colloquium 2009 Aug 24