2005 October 17East Coast Geodetic VLBI Meeting1 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”)

2005 October 17East Coast Geodetic VLBI Meeting2 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

2005 October 17East Coast Geodetic VLBI Meeting3 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

2005 October 17East Coast Geodetic VLBI Meeting4 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.

2005 October 17East Coast Geodetic VLBI Meeting5 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.

2005 October 17East Coast Geodetic VLBI Meeting6 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!).

2005 October 17East Coast Geodetic VLBI Meeting7 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…

2005 October 17East Coast Geodetic VLBI Meeting8 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)

2005 October 17East Coast Geodetic VLBI Meeting9 A new kind of spurious signal

2005 October 17East Coast Geodetic VLBI Meeting10 A new kind of spurious signal (cont’d)

2005 October 17East Coast Geodetic VLBI Meeting11 Example of S-band spur spectrum Onsala S-band pcal amplitude spectrum

2005 October 17East Coast Geodetic VLBI Meeting12 Example of S-band spur spectrum (cont’d) Onsala S-band spur power spectrum, relative to pcal

2005 October 17East Coast Geodetic VLBI Meeting13 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!

2005 October 17East Coast Geodetic VLBI Meeting14 Mismatched baseband spectra in the extreme Gilcreek-Kokee channel X5 in RD0505: DRF & xpower spectrum Impedance mismatch in Gilcreek sampler  formatter cable causing severe distortion in baseband spectrum

2005 October 17East Coast Geodetic VLBI Meeting15 Extreme baseband spectra (cont’d) RD0505 X-band closure MBDs for 4 stations: Gc (A), Ap (G), Ny (N), and Wz (V) SNR = for all scans

2005 October 17East Coast Geodetic VLBI Meeting16 Cable calibration Short-term stability of typical cable cal system = 1-3 ps rms  Cable cal adds 1-3 ps noise to corrected delay.

2005 October 17East Coast Geodetic VLBI Meeting17 Correlation/fringe-fitting errors From Alan Rogers (1991):

2005 October 17East Coast Geodetic VLBI Meeting18 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 Residual SBD (ns) RMS MBD diff = 4 ps

2005 October 17East Coast Geodetic VLBI Meeting19 Correlation/fringe-fitting repeatability Distributions of R1113 X-band observable differences from repeated processings

2005 October 17East Coast Geodetic VLBI Meeting20 Correlation/fringe-fitting repeatability (cont’d) Dependence of R1113 X-band MBD differences from repeated processings on SNR σ[ ΔMBD/ σ(MBD) ] = 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.

2005 October 17East Coast Geodetic VLBI Meeting21 Fringe-fitting challenges How do you handle a case like this??

2005 October 17East Coast Geodetic VLBI Meeting22 Fringe-fitting challenges (cont’d)