Instrumental effects in geodetic VLBI

1. Instrumental effects in geodetic VLBI Brian Corey (with copious cribbing from Alan Rogers’s 1991 Chapman Conference paper on “Instrumentation Improvements to Achieve Millimeter Accuracy”) East Coast Geodetic VLBI Meeting

2. Topics • Antenna reflections and focus effects • Feed cross-polarization • Spurious signals • Baseband filters • Cable cal • Correlation/fringe-fitting errors • Correlation/fringe-fitting repeatability • Challenges to fringe-fitting algorithms East Coast Geodetic VLBI Meeting

3. Antenna effects • Unstable antenna structure (Ft. Davis effect) • Multiple reflections within antenna structure • Typical effect on MBD: • 1-10 ps bias • <1 ps variable with pointing and focus changes • Multiple reflections within feed • For 10% voltage reflection, effect on MBD measured over 720 MHz: • <22 ps bias • <1 ps variable with pointing and focus changes East Coast Geodetic VLBI Meeting

4. Antenna effects (cont’d) • Dispersion in a defocused paraboloid • For 1-cm change in feed position, group-phase delay dispersion = ~4 ps, independent of BW. East Coast Geodetic VLBI Meeting

5. Feed cross-polarization • Cross-polarization on unpolarized source • MBD effect depends on product of LCP responses and parallactic angle difference between antennas • -15 to -20 dB typical xpol for non-VLBA antennas • For -15 dB xpol, MBD error ~10 ps at X-band • Cross-polarization on linearly polarized source • For 10% polarized source and -15 dB xpol feed, X-band MBD effect ≤ 10 ps, depending on feed and source polarization position angles. East Coast Geodetic VLBI Meeting

6. Feed cross-polarization (cont’d) • Cures: • Apply xpol corrections to MBDs • Determine correction terms by • Measuring xpol in special dual-pol experiments, or • Estimating from geodetic MBD data directly, using dependence on parallactic angle difference • Will not work for polarized sources unless polarization source maps are available • Fix the feeds (like that’s going to happen…) or build new antenna/feed systems (VLBI2010!). East Coast Geodetic VLBI Meeting

7. Spurious phase calibration signals • Spurious signals interfere coherently with phase cal • -20 dBc spur can shift MBD measured over 720 MHz by 22 ps (worst case) • Causes: • Additive signals • Intermodulation • Images • Crosstalk • And on and on… East Coast Geodetic VLBI Meeting

8. Spurious phase calibration signals (cont’d) • Typical spur levels in current systems are -40 dBc, but there are exceptions (worse!). • Methods for reducing effects of spurious signals on MBD: • Clean up receivers and backends • Offset receiver LO from integer MHz • Stabilize receiver LO phase ( more of effect of spur gets soaked up in clock, provided spur amplitude and phase are stable) • Correct for spurs at analysis stage (e.g., phasedoctor) East Coast Geodetic VLBI Meeting

9. A new kind of spurious signal East Coast Geodetic VLBI Meeting

10. A new kind of spurious signal (cont’d) East Coast Geodetic VLBI Meeting

11. Example of S-band spur spectrum Onsala S-band pcal amplitude spectrum East Coast Geodetic VLBI Meeting

12. Example of S-band spur spectrum (cont’d) Onsala S-band spur power spectrum, relative to pcal East Coast Geodetic VLBI Meeting

13. Baseband filters • Delays through analog filters of same type may differ by ~5 ns at 8 MHz BW. • Delay may change with temperature by ~10%. • With single pcal tone at band-edge, delay change appears as phase change of ~1°, or 4 ps in MBD. • Cures: • Use phase cal tone near center of band. • Use multiple phase cal tones to measure changes in filter phase/delay characteristics (but beware spurious signals). • Go digital! East Coast Geodetic VLBI Meeting

14. Mismatched baseband spectra in the extreme • Impedance mismatch in Gilcreek samplerformatter cable causing severe distortion in baseband spectrum Gilcreek-Kokee channel X5 in RD0505: DRF & xpower spectrum East Coast Geodetic VLBI Meeting

15. Extreme baseband spectra (cont’d) RD0505 X-band closure MBDs for 4 stations: Gc (A), Ap (G), Ny (N), and Wz (V) SNR = 100-300 for all scans East Coast Geodetic VLBI Meeting

16. Cable calibration • Short-term stability of typical cable cal system = 1-3 ps rms Cable cal adds 1-3 ps noise to corrected delay. East Coast Geodetic VLBI Meeting

17. Correlation/fringe-fitting errors From Alan Rogers (1991): East Coast Geodetic VLBI Meeting

18. Correlation/fringe-fitting errors (cont’d) (Non)dependence of total X-band MBD on residual SBD – MBD differences from repeated processings using different clock models RMS MBD diff = 4 ps Residual SBD (ns) East Coast Geodetic VLBI Meeting

19. Correlation/fringe-fitting repeatability Distributions of R1113 X-band observable differences from repeated processings East Coast Geodetic VLBI Meeting

20. Correlation/fringe-fitting repeatability (cont’d) Dependence of R1113 X-band MBD differences from repeated processings on SNR • σ[ ΔMBD/ σ(MBD) ] = 0.12-0.16 • 0.14 is expected value if two sets of input data are disjoint by 2% • But input data for two processings are typically disjoint by ~1%  mildly inconsistent. East Coast Geodetic VLBI Meeting

21. Fringe-fitting challenges How do you handle a case like this?? East Coast Geodetic VLBI Meeting

22. Fringe-fitting challenges (cont’d) East Coast Geodetic VLBI Meeting