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Paul/Vivek: The most likely explanation for the scatter seen in A2C2 calibration solutions is strong RFI near 6200 MHz that is compressing the 3-bit samplers. This signal is strongest in antennas near the ends of west (ea24) and north (ea17, ea18, ea22) arms. The likely source is one of the nearby hilltop transmitters, and the pattern of antennas affected has to do with direction to the source combined with antenna pointing.
Dave: There is a standard Point-Point 30MHz channel allocation centered at 6197.24 MHz which fits.
Vivek via vla-test-maint: https://science.nrao.edu/facilities/vla/observing/RFI/Oct-2021-B-configuration/C-Band_spectra_202110B
Dave update: I have not had a chance to make any measurements at the site yet but the interference appears to be consistent with what was identified in https://bugs.nrao.edu/browse/WO-7315 first opened in July, 2020. The interference is from a microwave link transmitting from Gray Hill to Smith-Bagley west of Datil. This link traverses the North Arm and almost directly across BN7. The interference detected on the West Arm is likely a sidelobe of the microwave antenna.
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2021-22 B config:
always - ea18 [N28], ea22 [N24], ea24 [W36]
sometimes - ea06 [N32], ea17 [N36], ea26 [W32]
2022 A config:
ea05 (W40), ea07 (E72), ea13 [W48], ea19 [E48], ea21 (E56), ea22 [N24], ea26 [W32], ea27 (E40)
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Easily seen in finalcals stage of pipeline, "final amp time cal" and "final amp freq cal" tabs.
example pipeline products here:
/lustre/aoc/projects/srdp/problem_data/C_band/
previews below:
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