RFI Blanking

The WIDAR Station Boards provide a capability to excise or "blank" impulsive radio frequency interference on microsecond timescales, thus avoiding the corruption of the longer visibility integrations that are typically accumulated over a few seconds. This mode is particularly effective at identifying and blanking interference from radar transponders in L, S, and X Bands, especially those used for aeronautical navigation between 1.0 and 1.2 GHz. The blanking applies to the voltage sample time-series recorded for each subband on a per station (i.e., antenna) basis before cross-correlation. The voltage samples are recorded at 256 megasamples per second (~4 ns) and sample values that exceed a dynamically set threshold are flagged as bad data for a fixed dwell time and ignored. Thus by blanking in time, the whole subband is effectively blanked in frequency. Optimal values identified for the RFI Blanking mode are plus or minus 5 standard deviations in voltage and a dwell time of 10 microseconds. Typical fractions of blanked data are approximately 0.1% of data per integration per subband.

The RFI Blanking mode is currently enabled in the NRAO default continuum resources for L, S, C, X, and Ku Bands in the "NRAO Defaults" resource catalogue. Continuum resources with the feature disabled are also available in the "Alternatives" catalogue under "NRAO Defaults." The mode is disabled by default for user-generated resources, but may be enabled using a radio button toggle under the "Special Modes" tab (see Figure 3.23). For user-generated resources, the default state is "No" to disable blanking for all sub-bands. Selecting "Yes" enables the feature for all subbands. Selecting "Yes" applies a threshold of 5-sigma and a duration/dwell time of 10 microseconds. When enabled, the blanking parameters will appear on the "Validation" tab under the columns "RFI Det. Level" and "RFI Blank. Dur." (see Figure 3.24). When disabled, these columns are hidden. To configure these parameters on a per-subband basis, please contact NRAO science support staff through the NRAO Science helpdesk.

Figure 3.23: The RFI Blanking feature may be enabled or disabled under the "Special Modes" tab.

Figure 3.24: When the RFI Blanking feature is enabled, the blanking parameter values appear as new columns in the "Validation" tab.